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Author:tara
ID Date Author Type Category Subject
  1631   Wed Apr 13 16:45:18 2016 taraHowToRefCavOptical Contact Tutorial

I showed Antonio how to do optical contact (refcav-mirror setup).  The videos and pictures are posted on Google photo

 I used a blank plane mirror (from coastline optics, intended for AlGaAs coating) and bonded it on the spare 1.45" refcav. It took me 4-5 tries and I scratched one mirror (I put it back and marked the box) before I could get a reliable bond. In the videos, I added comments for each failure. Mostly I think the problem is only cleanliness of the tools and the solvent. 

 

After that I removed the mirror from the refcav. First I tried to use a setup to push the mirror along the mirror surface. To my surprise, the mirror did not pop out, but just slid off of center and still stuck on the surface of the refcav. 

So I used a razor blade to wedge in between the mirro and the refcav, add some isopropanol. With little effort, the mirror popped out nice and easy. The edge of the HR surface of the mirror is beveled, so it is easy to wedge in a razor blade without scratching the refcav or mirror's surfaces.

To sum up, before trying to do the optical bond we should

  • Prepare enough gloves, lens cleaning paper
  • Use spectochromatography grade isopropanol, or other solvents
  • Clean the area carefully.
  • Have a bright light source to inspect the surface and the bond. 

To do the bond

  • clean the surfaces, make sure there is no dust
  • Put the two surfaces together, give them a little push.
  • Observe the fringe to disappear as the two surfaces get together ( if your objects are transparent), So if you bond silicon cavity, you can't see it, but you should be able to tell if the bond is good or not by just try to pull them apart.
  • NOTE: I have done only dry bonding. The two surfaces are dry when I put them together.

Trouble shooting: If you can't form the bond

  • If you see the fringe, there are dust, debris on the surface, clean the surface again.
  • If you don't see the fringe. The two surfaces seem to have a contact, but still pop out with little force. You might want to push the mirror and the spacer together and see if the "color" of the bonding area changes or not. To have a good bond the contact surface should be completely transparent, not cloudy. You can tell by adding more pressure and see the cloudy area becomes more transparent, then there is some thing on the surface. It is most likely that the solvent has gone bad, or there is contamination in the solvent you use to clean the surface.
  • For a good bond, ususlly you cannot pull the mirror/rafcav off by your hands.
  1627   Fri Apr 8 09:44:49 2016 taraDailyProgressPMCtest on new PMC

I checked the VME crate, and made sure that the PMC cards/ LO were functioning.

There was a problem with the VME crate and we couldn't turn on the kepco power supply for +/- 24 V for the PMC servo cards and 21.5 MHz LO on the crate. There were wires that connected to the power supply and the loose ends shorted together. I fixed that by taping over the wires. Now the power is up +/- 24V.

Summary of the crate status:

  • the crate is powered up with +/-24V. The current from each power supply is ~0.8 A. 
  • The power cables to TTFSS in the crate (+/- 17 and 24 V) are disconnected.
  • Both PMC servo cards, are working. Though we cannot adjust the gain/ switches.
  • The 21.5 MHz LO is working. But without the computer, we cannot adjust the power and phase (currently ~ -35 dBm). The signal is not very sinusoidal. (I think I reported this before)

 

Quote:
  • Note: V input for RFPD should be +/- 15 V (schematic)
  • Power for driving 21.5 MHz resonant EOM, can be up to 20 dBm , PSL:791 (seems that we have used only 9 dBm from the LO driver card, PSL:1092 )
  • power for mixer (LO) , 23 dBm ( Minicircuit Ray-3, see the schematic)
  • Schematic for LO for PMC , DCC

 

  1626   Thu Apr 7 21:52:43 2016 Antonio and TaraDailyProgressPMCPMC number 2

Today we have tested the second PMC that we have assembled.

 

We have obtained the 00 mode;

We measured the finesse (~300);

We have scanned the PMC PZT and we see 3 FSR;

 

We also noted again the presence of a second 00 mode and we confirmed that it was due to laser mode hopping. This has been checked

by injecting a voltage (0.2V) in the temperature input of the laser. Once the voltage was applied the second mode disappeard.

 

We think that it is enough PMC testing for now. I am tempted to assemble a third PMC, while we should try to fix the glass PMC.

We need to work on the lab organization too.

  1624   Wed Apr 6 15:02:50 2016 taraDailyProgressPMCtest on new PMC
  • Note: V input for RFPD should be +/- 15 V (schematic)
  • Power for driving 21.5 MHz resonant EOM, can be up to 20 dBm , PSL:791 (seems that we have used only 9 dBm from the LO driver card, PSL:1092 )
  • power for mixer (LO) , 23 dBm ( Minicircuit Ray-3, see the schematic)
  • Schematic for LO for PMC , DCC
  1623   Tue Apr 5 16:32:36 2016 taraDailyProgressPMCtest on new PMC

We are hacking the PMC card, to try to lock the PMC without using a computer.

Aidan helped me try to lock the PMC with the current equipment (LIGO PMC card, the schematic can be found here). Since, the computer for controlling the card is not here and we cannot put it back together within one week I have left, we are trying to use the card and send in signals to control the card manually. For today test, we used a spare PMC left on the crate (I think Frank used it to drive the PZT for shaking the table, long time ago. It might be modifed from what we saw on the schematic.)

We tested the control signal for the high voltage output using the following procedures.

  • Power up the card with +/- 24 V with a power supply
  • Use Acromag to send Vin ( I think it can send in +/- 10 V, but we used only [-6 , 2] V) .
  • Use HV from the power supply on the rag with 0-200 V. 
  • Measure Vout at J6 (where it will be connected to the PMC's PZT)

The HV drive is working and linear with the input control [HV = -24.1*Vin + 40] (Vin = [-6,2] V, HV = [4,185] V). At first, we injected the signal at INOFFSET2 (through R4), but we couldn't see the signal out. I'm not sure why, but we are working on it.

To do:

  • Setup RFPD on the table for the PMC
  • Add the EOM on the path (may be temporary, as the mount is not ready)
  • Connect the LO to EOM and the PMC card
  • Check Error signal
  • Lock the cavity
  • Note: the mysterious mode I reported in previous log may be mode hopping (as suggested by Johanness during Pizza meeting) I'll check that by operate the laser at different slow offset and see if there's any change or not
  1620   Thu Mar 31 15:11:27 2016 taraDailyProgressPMCtest on new PMC

The measured finesse for the new PMC is ~ 300. The previous calculation was wrong because of the wrong calibration.

 

Finesse measurement revisited I rechecked the Finesse measurement and found that I got the same result, so I realized that the calibration might be wrong. So here is the explanation

  • The calibration of 3 MHz/V is wrong.  The measurement is reported in PSL:182. It is for the laser in the South path. The laser has been in the original setup since 2009. The more recent calibration (4.4 MHz/V) wasd done by Evan,  see PSL:1478.
  • The calibration of 4.5 MHz/V is for laser in the North path, which is the path we use in the PMC test. I wrote down the calibration in PSL:1393.
  • The correct calibration with the measurement yields the Finesse of 308 +/-7 (the design is 310).  It seemed our assembly process was quite ok, no serious extra loss observed. The error is mostly from the uncertainty in the FWHM measurement ( 0.90 +/- 0.01 mV), and the error in the calibration (say 4.5 +/- 0.1 [MHz/V]). The error in FSR is smaller ~ 1249 +/- 1 MHz, assuming that the epoxy changes the mirror position by 0.1 mm, so it does not contribute much. 

PZT range revisited:   I mentioned in the previous entry that I can find several (4-5) TEM00 within 0-100 V applied to the PZT. This number should be wrong because we can expect only ~ 6 resonance over 200 V (3 um), or ~ 3 resonance over 100 V. It turned out that what I saw was not real TEM 00, but something else that made me think I saw some hysteresis.

  • The PZT range is 3.3 um with 200 V input, so it is roughly 16 nm/V. We should be able to scan over 1 FSR if the PZT stretch out by lambda/2. And this is what I found. TEM00 occured at every 30V from 0 - 100 V, which corresponded to 30 V* 16 nm/V =0.5 um (lambda/2). So PZT is quite linear and works well.
  • The problem is, between each resonance I saw other resonance modes  1) Another TEM00, but with ~ 6 V apart from the main TEM00. The shape was similar to TEM 00, but the transmitted power is ~ 20 % less. 2) I also saw are other TEM00 like modes that were quite bright, but their powers were negligible (less than a few percent of the main TEM00). It appearred and went away fast, I could not measure the corresponding voltage carefully. These unexpected resonance made me think it was hysteresis in PZT. 
  • For the TEM00 with 5 V apart (~ 80 nm or ~ 10 MHz), I thought it was from some polarization effect. I changed the polarization using half wave plate, but I did'nt see any change in the power ratio of the two modes.
  • Antonio suggested that it might be from the internal back reflection of the mirror because both surfaces are parallel. I'll need to think of a way to check that.

To Do Next: Anyway, the PMC/PZT are good. We will try to lock it tomorrow.

  • Need to find the EOM base to install an EOM from PMC in the North path ( I remembered I had a spare but couldn't find it).
  • Turn on the machine for controlling PMC card. (Need to ask Aidan how to do that).

 

 

  1617   Wed Mar 30 08:29:15 2016 taraDailyProgressPMCtest on new PMC

We tested one of the two new PMCs. finesse is ~ 500 and the PZT works fine.

We assembled two new PMCs for testing how the new designed PMC would be. They looked great. The PZT wires were connected to BNC connectors.

We temporarily put the new PMC in the north path. The Faraday isolator (as an extra back reflection protection) was removed, and replaced by the new PMC. (The original marking for the PMC is still visible on the table, where the waist radius is 370 um). We used a post to clamped the PMC on the 2" block.

As a quick test, we have not mode matched the beam into the PMC yet. We just maximized the coupling by using two steering mirrors in front of the PMC.

Finesse measurement

  • Once we got TEM00 mode coming out, we scanned the laser and mesaured the transmission power.
  • The measured FWHM was 2.4 +/- 0.1 MHz. (The calibration for laser fast mod was taken from PSL:182)
  •  The FSR of the cavity = c/L ~ 1.2 GHz. So the Finesse = FSR/FWHM ~ 500 +/- 20.  

PZT test

  • We checked if the PZT works or not by applying high voltage (0-100 V) and see how many resonance of TEM00 we could find.
  • The slow offset (0.275 V) was used to set the first resonance at 0V, then the other resonance occrued at (16 , 32 , 41, 55, 71 V). They were not very linear, but it was likely to be the drift of the laser. And we were turning the knob bak and forth to find the resonance, so there might be some hysteriasis like behavior here. I will check it again.
  • According to the datasheet(add link), the pzt range is 3.3 um, with 200 V input. We should not have a problem with locking both PMC and refcav simultaneously anymore. By applying ~ 100 V, we can shift the PMC by 5 FSR (~ 6GHz), we can expect it to cover ~ 10 FSR (12 GHz) for the range of 200 V and refcav's FSR is 4GHz apart. So The PMC whole range (10 GHz) should be able cover at least 2 refcav modes (8GHz).

To Do:

  • Antonio will design a permanent mount for the new generic PMC, and check HOM in order to determine the sideband frequency we can use for locking.
  • We will try to lock the new PMC using the existing equipment, to determine loss and see if we can bring the PMC system back.
  • Test the 2nd PMC cavity.
  • We may fix the glass PMC, as Evan found out that it was broken long time ago, see PSL:1182
  • I need to look up the numbers for PMC (cavity length, mirror's reflectivity, ROC, designed finesse, spotsize, for comparing with the measurement and mode matching).
  • If time permits, assemble and characterize the rest of the PMCs (might be a project for a SURF student?). 
  1615   Sat Mar 26 13:36:09 2016 taraDailyProgressPMCEpoxy glue on PMC mirror and PZT

 

The glue is dry and the back PMC mirror is steady. we are ordering the o-ring for clamping flat mirrors on the pmc. See more pictures here.

Assembly work

  • We checked with the epoxy left in the mixing cup. It was hardened nicely.
  • When I bent it, it was britten. So I think the position should be fixed nicely because the epoxy is rather hard, not gummy
  • The PMC parts  (body, mirror clamps, screws) were cleaned with soap water and isopropanal. 
  • However, the O-rings planned to put between the window clamp and the mirrors are too small. Antonito is ordering a bigger one.
  •  

To do next:

  • If the oring- size match with the window, we can proceed. If not we will use Kapton tape like Zach did with his PMC see ATF:1543
  • Test the PMC range, The max-min for PZT range should cover ~ 4 GHz so it can follow the short cavity (FSR = 4 GHz). We had some problems locking PMC and the ref cav last time because PMC could not span the short cavity's FSR.
  • If possible, check the Finesse.

f

 

  1614   Thu Mar 24 19:07:38 2016 taraDailyProgressPMCEpoxy glue on PMC mirror and PZT

We ordered fixtures for gluing mirror-pzt-end cap. Waiting for the glue to settle.

  1613   Tue Mar 22 16:29:04 2016 taraDailyProgressPMCEpoxy glue on PMC mirror and PZT

[Antonio,Tara] We glue PMC back mirror to PZT and wait for it to settle.

Since we have all parts for the steel PMC (body/PZT/Mirrors) we start putting it together. The first step is using epoxy to glue the back mirror to the PZT.

There is a little problem. The soldering tin on the PZT gets a bit to the edge of the PZT, see picture. When we put the PZT on the steel end cap, one side of the soldering tin electrically connects to the end cap.

So we decide to put a kapton tape as an insulator between the pzt and the end cap. (We thought about resoldering the tin, but we were not sure if we would make it worse).

It is quite hard to center the mirror-pzt on the end cap, so we plan to use a fixture to keep the mirror center at the end cap. It will be something similar to the plastic piece we used for cavity-mirror assembly.

We want the mirrors' positions to be as precise as possible to reduce clipping loss, and beam alignment problem.

See the rest of the picture in Picasa

 

Note about epoxy: The epoxy we use is EP-30 (suggested by Rick Savage, provided by Callum). There's a test procedure to check if we mix the epoxy correctly or not.

After mixing the epoxy, you have to put a small amount of epoxy in an oven at 200 F for 15 minutes to check if the mixture is good or not.

If the epoxy in test came out smooth and hard, it is good. You can proceed with the work. We tested that, and our mixture was good.

  1520   Mon Sep 15 18:36:32 2014 taraNotesVacuumtransmission of vacuum windows

 We measured the transmission of the vacuum windows. The total transmission through two windows is 0.975 +/- 0.002.

 

  •   This measurement is for checking how much the power is transmitted through the vacuum windows. It can be used for calibration of photo thermal measurement for better accuracy of absorption estimation.
  •  The beam is incident normal to the two windows. The power before the vac tank and after the vac tank were measured. The incident level was set ~ 4 mW. I used a Thorlab power meter with ND filter off to measured the power.
  •  
  Pin [mW] Pout [mW]
1 3.9 +/- 0.02 3.81+/- 0.02
2 3.96 3.87
3 3.98 3.88
4 3.99 3.88
  Tavg 0.975

 

If we assume that both windows have the same transmission, the transmission for each window will be 0.988.

  1517   Mon Sep 15 04:32:21 2014 taraNotesBEATnote for tonight beat

 RCAV transPD_DC :0.54 V

ACAV transPD_DC: 0.16 V (loop might oscillate when DC level was measured, need to double check)

  1516   Mon Sep 15 02:24:48 2014 taraNotesTempCtrlepic for slow control is disabled

Perl scripts for controlling the vacuum tank and slow feedback to the two lasers are acting weird. Usually we can run three scripts simultaneously, but I just notice that only two can be run at the same time. When I restarted the script for acav feedback, the vac temp control stopped. When I restarted the vac temp control, acav slow feedback stopped.  I'll check this later.

  1513   Thu Sep 11 21:43:32 2014 Tara, EvanDailyProgressISSSouth ISS

We turned on both ISS loops today.

Here is an in-loop characterization of the south RIN with and without the ISS.

Attachment 1: RINsouth.pdf
RINsouth.pdf
Attachment 2: southRIN.zip
  1510   Tue Sep 9 23:22:41 2014 Tara, EvanDailyProgressFSSSouth 200 kHz oscillation; cavity pole

We locked the south cavity using the north TTFSS. 200 kHz oscillation is still present, so whatever this is probably doesn't reside in the TTFSS box.

Tara and I also took cavity pole measurements using the EOAM and two PDA10CSs, one placed before the cavity (monitoring the light rejected from the post-EOAM PBS) and one after the cavity (on the ISS breadboard). The HP4395A was used to drive the EOAM, and we then took the transfer function which takes the pre-cavity PD voltage to the transmission PD voltage. I will fit these later, but the poles appear to be consistent with the values tabulated in ctn:1475.

Attachment 1: algaasFinesse.pdf
algaasFinesse.pdf
Attachment 2: algaasFinesse.zip
  1506   Sun Sep 7 20:37:50 2014 Tara, EvanDailyProgressopticSome lens tweaks

Some minor maintenance/improvements to the optical setup:

  • We replaced the existing lens before the beat PD (RoC = 51.5 mm) with a slightly faster lens (RoC = 38 mm) in order to reduce the spot size on the diode.
  • Tara improved the clamping of mode-matching lenses before the south cavity (they weren't tightened down enough before)
  1505   Sun Sep 7 19:33:40 2014 Tara, EvanDailyProgressopticNorth photothermal TF

Tara and I took another photothermal TF of the north cavity today. Relevant parameters:

  • Power incident on cavity: 10 mW (up from the usual 1 mW)
  • Beat frequency: 1.2 MHz, drifting to 650 kHz (we are hoping it will swing through 0 Hz overnight and settle above a few megahertz by tomorrow)
  • DC voltage on north ISS PD: 2.39(5) V
  • DC power transmitted through cavity: 3.77(2) mW
  • PLL actuation coefficient: 50 kHzpk / 1 Vrms
  • PLL UGF: 80 kHz (measured)
  • EOAM drive: 5 Vpp from 20 kHz to 300 Hz, then 3 Vpp from 300 Hz to 0.2 Hz

In the attached data I have already converted the raw data (in V/V) into hertz of beat frequency per watt of circulating power. For this I use the conversion factor (50 kHz / 21/2 V) × (2.39 V / 3.77 mW) × π / F, with F = 16 700. Since the TF (again) appears to be junk above 1 kHz, I haven't bothered undoing the CLTF of the PLL.

The attached plot shows the expected photothermal TF in terms of hertz of beat frequency per watt of absorbed power per mirror. Therefore, the scaling factor that makes our measurement (given in hertz per watt of circulating power) overlap with the expected TF (given in hertz per watt of absorbed power per mirror) should be the average absorption of each mirror. I find that this scaling factor is 6 ppm, which seems surprisingly low, especially given our earlier finding that we have at least 120 ppm of scatter + absorption loss. So I will double check for missing factors of 2, 4, π, etc.

At any rate, the shape of the measured transfer function appears to be in good agreement with the expectation up to 100 Hz. If we believe that the coating/substrate photothermal crossover happens around 10 Hz, and we believe our measurement from 10 Hz to 100 Hz, then this seems to indicate that the thermo-optic cancellation has been somewhat successful.

Attachment 1: photothermalTF.pdf
photothermalTF.pdf
Attachment 2: northPT.zip
  1503   Sat Sep 6 12:54:05 2014 Tara, EvanDailyProgressopticNorth photothermal TF

Tara and I took an SR785 measurement of the north photothermal transfer function.

Clearly there's something wrong with the measurement above 1 kHz.

Attachment 1: nPT.pdf
nPT.pdf
Attachment 2: npttf.zip
  1502   Thu Sep 4 19:39:36 2014 Tara, EvanDailyProgressFSSTTFSS OLTFs

With the cavities locked, Tara and I took OLTFs of the PDH loops.

Below 100 kHz, we used the SR785 with a 70.7 mVpk excitation. Above 30 kHz, we used the HP4395A with a 22.4 mVrms excitation (these are the "HF" traces on the attached plot).

Before taking these TFs, we turned the loop gains up as high as possible without making the loops saturate.

  • For north, the gains were 900 common and 900 fast, and the incident power was 1.26(5) mW.
  • For south, the gains were 632 common and 770 fast, and the incident power was 1.05(2) mW.

We injected the excitation on common EXC. We then measured the TF which takes common OUT2 to common OUT1. This is almost the OLTF of the loop, except for an AD829 between OUT2 and OUT1 which has a gain of −4 V/V.

Currently I'm not sure how to explain the magnitude discrepancy between the SR785 and HP4395 measurements. Both OUT1 and OUT2 have a 50 Ω output impedance, so I would expect the impedance difference between the SR785 and HP4395 would cancel out in this measurement.

Attachment 1: pdhTFs.pdf
pdhTFs.pdf
  1500   Wed Sep 3 16:34:37 2014 Tara, EvanDailyProgressBEATBeat, mode-matching

Quote:

Tara added some more juice to the north cavity heater last night. Now we can lock both cavities to TEM00 and get a beat within the bandwidth of the 1811.

  • North laser slow: 5.020 V
  • South laser slow: 0.722 V
  • Beat frequency: 49.3 MHz

Beat frequency drifted to 61 MHz over the course of a few hours. We need to wait for the cavity temperatures to settle.

I improved the mode-matching a little bit on the south cavity; it's about 50% (the theoretical max is 71%). The south lenses are now on translation stages.

I've attached a beat spectrum. Nothing is floated, RAM is not optimized, etc.; this is just a rough indicator of where things stand.

Here is what I think should happen next, in rough order of importance:

  1. Float chamber
  2. Measure RIN
  3. Measure photothermal TF (I also need to recheck my photothermal code — I don't believe the coating TE part)
  4. Put photothermal noise on noise budget
  5. Reduce RAM.
  6. Measure residual frequency noise.
  7. Measure PLL noise. Use ATF DAQ and make spectral histogram.
  8. Measure seismic noise (with Guralp or T240), with table floated and unfloated. Use ATF DAQ and make spectral histogram.
Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1499   Wed Sep 3 12:02:19 2014 Tara, EvanDailyProgressBEATBeat found

Tara added some more juice to the north cavity heater last night. Now we can lock both cavities to TEM00 and get a beat within the bandwidth of the 1811.

  • North laser slow: 5.020 V
  • South laser slow: 0.722 V
  • Beat frequency: 49.3 MHz
  1484   Mon Aug 25 03:56:17 2014 taraHowToNoiseBudgetoptimization for ETM with a-Si/SiO2 coatings

 I used optimization codes for ETM. The optimization reduce the PSD of Brownian noise by ~ 3/4 (in units of [m^2/Hz]) from QWL structure.

 Since we have not had all the material parameters for aSi:H at 120K with 1550nm, the optimization here is for room temperature with 1550 nm (for Brownian noise only). 

 opt2_aSi.png

opt2_RT.png

fig1: optical thickness for ETM with minimized BR noise. The transmission is 5.4 ppm and the reflected phase is ~ 179 degree.

Parameters/configuration used in the optimization:

  • T = 300 K   (room temp)
  • wavelength = 1550 nm;
  • Si substrate, n = 3.5;
  • Low index material : fused silica, loss = 0.4e-4, n = 1.444;
  • High index material: aSi:H, loss = 1e-6, n = 3.48; 
  • The coating has SiO2 cap (air-coating surface) for protection
  • Spot radius = 6 cm.
  •  This optimization is only for Brownian noise, we can do another optimization once the thermo-optical properties are known (thermal expansion, dn/dT)

It is remarkable that 5ppm transmission can be achieved with just 17 layers of coatings due to the largely different values between nL and nH. This makes the total thickness down to ~ 3 um.

BR noise from the optimized coating is  3.3x 10^-42 [m^2/Hz] at 100 Hz. This is converted to the strain of ~ 5x10^-25 [1/sqrt Hz] for 4 km interferometer. 

Note: for QWL structure, with 14 layers + half wave cap of SiO2 (total of 15 layers), the transmission is ~5.2 ppm and the coating Brownian noise is 4.2x10^-42 [m^2 /Hz]. So the optimization reduced the PSD of BR noise by ~ 25%. 

  1483   Sun Aug 24 20:07:57 2014 taraNotesVacuumion pump is on

I turned the ion pump for vacuum chamber on. The initial current is 7.3mA ( the value before opening the chamber was 7 uA)
The turbo pump was turned off.

  1481   Wed Aug 20 12:38:06 2014 Tara, EvanDailyProgressVacuumChamber pumping down

We put on the CF gasket and closed the transmission side of the chamber. Now we are pumping down.

Tara did some work last night to ensure that the window reflections on the input side of the chamber are not overlapping with the cavity reflections. The south window reflection appears to be clipping on the bottom periscope mirror, but we can fix this later.

Next steps:

  • Mode matching (including adjustment of the input lenses)
  • Locking
  • Realignment of transmission optics
  • Re-establishing beat
  1477   Tue Aug 19 03:55:36 2014 Tara, EvanDailyProgressRefCavSouth cavity OK so far

Quote:

Quote:

Tara has successfully formed the AlGaAs cavities. The configurations are as follows:

  • Spacer 95: to the left of the ATF logo is mirror 114, and to the right of the ATF logo is mirror 143.
  • Spacer 96: to the left of the ATF logo is mirror 141, and to the right of the ATF logo is mirror 132.

Mirror 137 has not been contacted.

Redid optical contacting on south (for a second time) to try to get rid of scattering defects.

Spacer 95: left of ATF logo is 143, right is 137. 143 is on transmission side of chamber.

We redid the mode-matching into south, and judging from CCD images it appears to be free of gross scattering effects.

In the process of moving the seismic stack around, we found that the two rubber noodles on the transmission side had fallen over (so they were being compressed transversely instead of longitudinally). We stood them upright again, but one of them broke, so we had to swap it with a spare. (We tried for a while to make a new one by coring out a cylinder, but they seem to be very brittle. Tara suspects that they're old and broken down.)

Next step is to adjust the stack as necessary to avoid reflections from the windows.

At some point I would like to do the following:

  • Birefringence measurement: temporarily swap QWP before periscope with HWP, record swept transmission as a function of HWP angle
  • Redo NPRO PZT calibration: record swept transmission with 14.75 MHz sidebands on, and thereby infer voltage-to-frequency coefficient
  1476   Mon Aug 18 17:58:57 2014 Tara, EvanDailyProgressRefCavOptical contacting

Quote:

Tara has successfully formed the AlGaAs cavities. The configurations are as follows:

  • Spacer 95: to the left of the ATF logo is mirror 114, and to the right of the ATF logo is mirror 143.
  • Spacer 96: to the left of the ATF logo is mirror 141, and to the right of the ATF logo is mirror 132.

Mirror 137 has not been contacted.

Redid optical contacting on south (for a second time) to try to get rid of scattering defects.

Spacer 95: left of ATF logo is 143, right is 137. 143 is on transmission side of chamber.

  1474   Fri Aug 15 15:01:37 2014 Tara, EvanDailyProgressRefCavInserting AlGaAs cavities

Executive summary

  1. We replaced the northeast air spring on the vacuum chamber, because it was leaky.
  2. We opened the transmission side of the vacuum chamber, removed the silica/tantala cavities, and inserted the AlGaAs cavities. The configuration is as follows:
    • SN 00095: south. Logo readable when standing on north side of table.
    • SN 00096: north. Logo readable when standing on north side of table.
  3. We scanned the modes of the north cavity and did some rough mode-matching to TEM00. All modes (including TEM00) appear to be doubled. Is this birefringence?
  4. We scanned the modes of the south cavity. We we able to match into TEM(10)0, then TEM90, TEM80, etc., with relative ease (albeit with the same doubling as observed in the north cavity). However, as we got closer to TEM00, we noticed the presence of two bright scattering centers near the mode axis. These scattering centers appear to be hosing the buildup of the TEM00 mode in the south cavity.
  5. Tara thinks we cannot proceed with the south cavity as is. We'll have to take off and reclean at least one of the mirrors.

Details

At various times, we put the transmission of the north cavity on a PDA100A and monitored the voltage on a scope while sweeping the laser PZT. For the two TEM00 modes of the north cavity, the observed splitting was 11.5 ms when the PZT was driven with a 4 Vpp, 5 Hz triangle wave. Tara has previously measured the south laser PZT actuation coefficient as 3.1 MHz/V (ctn:182). This gives the frequency of the splitting as 1.4 MHz. Since the expected FSR of these cavities is 4070 MHz, this corresponds to a cavity length difference of 180 pm.

The FWHMs of the two peaks (again as seen on the scope) were 1.16 ms and 1.30 ms. With the FSR given above, this gives the finesses as 29 000 and 25 000. That's higher than what should be possible given the measured transmissivities of the mirrors [we expect a finesse 2π/(300 ppm) = 21 000], but this was a quick and dirty measurement that relies on a PZT calibration that's a few years old.

  1462   Wed Jul 30 17:47:56 2014 taraDailyProgressopticBRDF of AlGaAs mirror 143

 I used the setup to measure scattered loss from an REO mirror (mirror for iLIGO refcav, the one we measured coating thermal noise) and get 6 ppm. This number agrees quite well with the previous Finesse measurement.

 

  Finesse measurement from REO mirrors = 9700 , see PSL:424 The absorption loss in each mirror is ~ 5 ppm ( from photo thermal measurement, see PSL:1375). The measured finesse infers that the roundtrip loss is ~ 24 ppm, see here. So each mirror has ~ 12 ppm loss. With ~ 5ppm absorption loss, we can expect ~ 6-7 ppm loss for scattered loss.  So this measurement roughly says that our scattered light setup and calibration is ok.

 

  1456   Thu Jul 24 02:00:23 2014 taraDailyProgressopticsetting up scattered light measurement

I rechecked the CCD response vs exposure time and power. The results are linear.

 

After some adjustments (strain relief on the camera's cables, clamping down the camera properly), I made sure that the camera is more stable and repeated the measurement. The CCD response is linear with the incident power on the sample (this is under the assumption that the scattered power is directly proportional to the incident power). 

power.png

Fig1:  CCD response vs incident power. The camera response is linear.

== AlGaAs Samples==

I prepared the sample for measurements. All the samples are quite dirty, especially on the flat sides. So I wiped all of them. I still cannot get rid off some water marks on the annulus of the mirror. It might cause some problems when I optical contact the mirrors. I'll try to clean them later.

photo-1.JPG

fig2: one of the AlGaAs mirrors before cleaning. 

I put one of the samples in the scattered light setup. The transmitted beam has a lot of diffused light behind the mirror. The amount of the diffused light changes with the beam direction. I'm not sure exactly why. I'll try to investigate it more. But the scattered light from the sample is very small. Most of the light is from debris on the surface, not the micro roughness of the sample. The amount of scattered light significantly changes with the beam position on the mirror.

photo-2.JPG

fig3: diffused light behind the mirror. It might come from the reflection inside the substrate because the incident beam is not normal to the surface.

  1455   Wed Jul 23 00:28:14 2014 taraDailyProgressopticsetting up scattered light measurement

I'm checking the linearity of power and exposure on the camera. The ccd counts are quite linear with the exposure setup, but I have to check the power again.

 

==ccd count vs exposure setup==

  The exposure time on the camera can be set to adjust the brightness of the image. Since we might have to adjust it to make sure that the images won't be saturated, it is necessary to check if the ccd count response linearly to the exposure setup or not.

I used a silver mirror as a test sample. The incident power is constant, and the camera position is fixed. Then adjust the exposure from 5k to 30k. I'm not sure if it is in nano second or microsecond unit. [Edit, 20140725: according to page 18 of the manual for the Prosilica GC750, the available exposure options are 30 µs to 60 s, in 1 µs increments. —Evan] But from fig1, the ccd count is quite linearly proportional to the exposure value.

exp_vs_light.png

It turns out that when I try to calibrate a sample, the incident power on the sample has to be more (so the power meter can measure some scattered power) and the camera can be saturated. The exposure value has to be around 1000, and I have not checked the response at this level. I might have to remeasure it.

 

==ccd count vs power== 

 This measurement is similar to the above. But this time the incident power (to the sample) is varied. The result is not linear. I check the images and see that the bright spot moves. The camera might move during the measurement. I'll repeat this again. It will be complicated for the calibration if the ccd count is not linear with the power.

power.png

== To do==

  • check the ccd count for exposure value down to lowest setting.
  • check the ccd count for different power incident.
  • check the ccd count with different ND filter in front of the camera.

 

  1452   Thu Jul 17 18:57:54 2014 taraDailyProgressopticsetting up scattered light measurement

I'm testing the setup and a code for extracting scattered light from the images. 

 

I used a red laser pointer to test the scattered light setup. Then took a picture with no light (fig1) and a picture with the incident light (fig2). The scattered light can be extracted by subtract fig1(background) from fig2.

The snapshots saved by SampleViewer are in .bmp file. When it is read by MATLAB, the file will contain 480x752x3 matrix element, Each are varied between 0 and 255. The values are proportional to the brightness (how many photons hit the cell). 480x752 is the resolution of the image, x3 are for R G B color. In our case, the image is greyscale and the values are identical. The code can be found in the attached file.  

 dark1.png

fig1: The test mirror without incident beam taken as a background image. The image is enhanced by a factor of 5 (by matlab).

 

light.png

fig2: The test mirror with a red incident beam around the center. The image is enhanced by a factor of 5.

 

residual.png

fig3: the image is created by subtracting data of fig1 (background) from fig2 (scattered light) and enhanced by a factor of 100. The scattered light on both surfaces can be seen clearly around the center.

 ==To do next==

  • From fig 3, the background can be seen even after subtraction, so some black curtains and beam dumps should be added behind the mirror.
  •  A room light filter should be installed in front of the camera.
  • I'll see if we can find a sample with known scattering loss, so that we can compare how accurate the measurement is.
Attachment 4: scattered_.zip
  1450   Tue Jul 15 23:40:16 2014 taraNotesPMCPMC heater

How hot do you need to heat it? if the thermal expansion of aluminum/steel is much higher than that of fused silica, then just heating the end cap might be a better idea. Thermal conductivity is also better.

Quote:

Quote:

I took some hard yellow foam, made it into a U-shape, and wrapped it with a combination of aluminum and duct tape.

This insulation fits snugly over the PMC and its copper shield. In retrospect, the foam is probably a little too thick. I had to temporarily move the beam dump at the input of the Faraday isolator.

Putting 20 V across the 105 Ω heater produces a change of 5 V on the PMC PZT (when locked). So we need better insulation or more heating.

The CTE of fused quartz is something like 0.5×10−6 K−1, and the CTE of steel is more like 15×10−6 K−1. So I suspect there's not much point in heating the glass spacer if I'm going to leave the steel end cap open to air.

A possible solution is to put a heater on the end cap, but I worry that the differential expansion of steel vs. glass will cause the end cap to pop off the spacer (it looks like it's only held on by epoxy).

A better solution is to improve the insulation on the back end of the PMC. I'll do that next.

 

  1449   Tue Jul 15 18:26:20 2014 taraDailyProgressopticsetting up scattered light measurement

I'm setting up a scattered light measurement for AlGaAs samples. The methods are summarized below.

 

I discussed with Manasa about the setup and how to do the measurement. The goal is to measure  scattered losses from AlGaAs samples from a normal incident beam. The setup is shown below.

 

==setup==

The setup is in the ATF lab, on the unused optical table. It is too crowded on CTN table. So I will need a to borrow a 1064 laser from somewhere. 

The incident beam will have to be slightly angle from the normal angle in order to dump the beam properly. 

The arm holds the camera, it can rotate to change the angle to cover the measurement from around 10 degrees to ~70 degrees.

 photo.JPG

==calibration method==

  • We can take a picture from a diffuser plate, make sure it is not saturated. 
  • Then use a power meter, measure the power fall on the camera.
  • compare the output of the camera and the measured power
  • I have to think about how to make sure the solid angle of the camera aperture and the power meter are the same.

==measurement and data analysis==

  • For each position, take one picture without the beam on the mirror and one with the beam on the mirror. The first one will be used for subtracting the ambient noise from a picture when the beam is on the mirror. 
  • Make sure that no pixel is saturated
  • For each pair of picture, we will use Matlab to count the output, then use the calibration to convert to power.
  • integrate over half the sphere. I have to think about this to make sure I get it right.

 

  1444   Wed Jul 2 20:05:32 2014 taraNotesNoiseBudgetaLIGO Noise budget with uncertainties in material parameters

The calculation for LIGO astronomical reach with uncertainties are updated. See the details below.

       I got the updated GWINC from Nic. When I run the nomm file, the BNS range is 189.5 Mpc. The nominal value of the refractive index for nH in the code is 2.06 which is the value for the pure Ta2O5. The refractive index for Ta2O5 doped with TiO2 25% (Ta2O5:TiO2) is 2.119 (see Harry et al 2007 paper). So when I changed nH to 2.119, the BNS range became 192 Mpc due to the thinner coating. 

 

     The ring down measurements from Harry 2007 paper measure parallel loss of the coating from multilayer coating, then extract PhiH from the measurement. Again, this is done by assuming the knowledge of phiL, YL and YH. The loss angle of silica (phiL) used in the paper is 1e-4 (from Crooks 2006 paper) while the value in GWINC is 0.4e-4. In this calculation, I use phiL = 1e-4 because of a couple reasons:

  1. The value of 1e-4 is determined in crooks paper. Their samples and the samples used in Harry 2007 paper are for fused silica with the same heat treatment (annealing temp ~ 400-600 C). I think 0.4e-4 is from higher heat treatment (up to 700-800 C) (see Penn 2010 LVC march presentation, here).
  2. In Harry 2007 measurements, sample 1 is a multilayer structure of fused silica and Ta2O5: TiO2(6%). Sample6 is a single layer of Ta2O5: TiO2(6%). Sample 6 can determine the parallel loss of phiH directly without the knowledge of fused slilca's material parameters (thin form). When the loss angle of Ta2O5:TiO2(6%) as measured from sample6 is combine with the loss angle of fused silica = 1e-4, the result from the measurement from Sample1 and the calculation agree

So the value phiL = 1e-4 is only used for extracting phiH as a function of YH, while phiL = 0.4e-4 is used as a nominal value in noise budget calculation.

 

Running the code

The calculation is done in the code name BNS_score.m (see the attached zipped file). The file calls on IFOModel_rnd2.m that generates random material parameters by normrnd command. The plot can be made by running plot_hist.m file.

 strain2.png

fig1: histograms and gaussian fits for coating BR noise and total noise at 100Hz. 

 

BNS2.png

fig2: The astronomical range for BNS, in MPC unit.The histogram is from 20k samples. The average is at 189.2 Mpc, while the mode is around 188 MPc.

Attachment 1: BNS2.png
BNS2.png
Attachment 4: strain2.fig
Attachment 5: BNS2.fig
Attachment 6: gwinc_2014_07_01.zip
  1441   Tue Jul 1 19:27:25 2014 taraDailyProgressComputersInstalling prosilica gc750

 Manassa is helping me installing a camera for scattering measurement. The work is in progress.

I'm borrowing a Prosilica gc750 from the 40m. It will be used for scattering measurement on AlGaAs samples. It is a good idea to have a setup that can quantitatively measure scatter loss on mirrors. 

First I tried to install it on the small Acer laptop used with win cam, but it did not work. I'm not sure if the ethernet card of the laptop does not support the camera or not. Now I'm trying to install it on my mac book instead, since Manassa claimed that it worked on her macbook. 

I'll write a step by step installation guide once we succeed.

 

==note about the AlGaAs samples== 

I used a green laser pointer to check scattering loss on one sample. I couldn't see any green spot of the laser with my eyes. This means that the scattering is probably less than 100 ppm (according to Josh). Once we use the camera to measure it and it turn out to be smaller. We will probably go to Fullerton to have the samples measured there for better accuracy. 

  1436   Thu Jun 26 03:41:31 2014 taraNotesNoiseBudgetaLIGO Noise budget with uncertainties in material parameters

Quote:

 

 That GWINC link is more than a year old. You're best off just updating your CVS checkout of the code, or getting a new zip file from someone else if your CVS is broken. When I run gwinc with nomm.m, I get R_BNS = 189.5 Mpc.

 I just saw you comment. I'll find an update version for GWINC.

Anyway, I have a code to plot the result. I will use it on an updated code.

Some material parameters in the calculation are:

  •  The concentration of TiO2 is 25 % based on Harry 2010 paper (Class. Quantum Grav. 27 (2010) 084006).
  • Young's Modulus of Ta2O5 doped with TiO2 (25%) is ~ 140 +/- 6 GPa (one sigma).  (Abernathy et al. 2014). 
  • Loss angle of TiO2 doped Ta2O5 is  (2.3 +/- 0.4) x 10^-4  [Martin 2008, Martin2009] (@300K, heat treated) 
  • Loss angle of SiO2 is (6.3 +/- 2.6) x10^-5   [LMA2014, Crooks2006, Crooks2004]

In the IFOModel_rnd.m file which is a copy of IFOModel.m for material params, I use normrnd(mean,sigma) to generate the random value of the material parameters.

Note, for loss angle of SiO2, I have to use abs command to make sure that all the generated values are greater than zero.

This is because the mean is comparable to the standard deviation, and sometime it gives negative values.

Note: I have not taken the coherent between the loss and Young's modulus of Ta2O5 into account yet. I have to read how they measure this more carefully.

Here are prelim results from the above numbers.

BNS.png

above: a histogram of BNS range, due to uncertainties in loss angles/young's moduli of the coatings.

The mean of the histogram is slightly less than the nominal value from GWINC because the mean values of loss angles for fused silica (6e-5) is slightly higher than the original value (4e-5) used in the code.

 

strain.png

above: histograms and Gaussian fits for BR noise (blue/green) and total noise and its Gaussian fit (red/cyan) at 100 Hz in the strain unit.

 

Attachment 2: strain.png
strain.png
  1432   Mon Jun 23 17:13:06 2014 taraNotesNoiseBudgetaLIGO Noise budget with uncertainties in material parameters

I'm estimating the BNS range of aLIGO. Here is a quick note about the calculation.

  •  The range can be calculated by GWINC, more details about the calculation can be found in T090049
  •  This is for NS-NS (1.4M_solar) CBC.
  •  The change in sensitivity due to material parameters results in a different LIGO detection range.
  • One sigma here means one standard deviation. +/- one sigma cover 68% of the normal distribution. 

For example, the normal configuration for aLIGO will have BNS in spiral range equal to 178.29 Mpc (based on the current code available on gwinc.

  1428   Tue Jun 10 17:00:18 2014 taraNotesNoiseBudgetaLIGO Noise budget with uncertainties in material parameters

 I'm working on estimating aLIGO sensitivity when material uncertainties are taken into account. I have a result for a reference cavity, uncertainty due to Ta2O5's Young's modulus might have smaller effect than we previously expected. All plots and code are attached below.

 

 ==Intro==

  GWINC does not take any uncertainties in material parameters into account, so its noise budget does not have any error bar. We want to know how the noise budget might change due to imprecise knowledge of the material parameters. One particular issue is coating thermal noise that is dominating around 30 - 200 Hz, so we want to know how its level will change with material parameters. Some import ant parameters are loss angles and Young's moduli of each material. 

  In Hong et al 2013 paper, there is a plot of the calculated coating Brownian noise vs Ta2O5's Young's modulus (YH). The calculated coating BR noise is calculated with the corresponding YH while other parameters are fixed. This would be ok if each parameters were independently measured. In reality, loss angles are measured from ring down measurements, and YH and YL are used to calculated the material loss angles (phiH/phiL), see Penn et al. 2003. So to make the calculation reflects the real situation, we should take the correlation between phiH/phiL and YH/YL into account when we calculate coating BR noise. So the goal is to estimate coating BR noise for aLIGO with some uncertainties from loss angles and Young's moduli of the coatings.

==calculation== 

calculate BR noise vs YL and YH  (see PSL:1408 for the original code) using numbers from our setup (these can be changed later when we want to apply for aLIGO calculation). The code calculates BR noise with phiL = 1e-4, phiH = 8e-4. the numbers are from our measurement and another ring down measurement. This does not take the correlation between loss any YL/YH into account.  I do this to compare my code to Hong's result, and they agree. YL and YH are varied between 80% and 120% of their nominal values (YL = 72 GPa, YH = 140 GPa).

 

BRnoise_YLYH.png

above: Fig1:Thermal noise level of 28 Layer QWL structure, spot size = 180 um as a function of YH and YL

BRnoise_YH.png

above:Fig2: three slices from the 3-d plot for different values of YL, Y_L min and Y_L max are 80% and 120% of the nominal value. 

BRnoise_YL.png

 above:Fig3: three slices from the 3-d plot for different values of YH, Y_H min and Y_H max are 80% and 120% of the nominal value. 

 
From the plots, we see that the uncertainty in Y_L does not change the noise level that much compare to Y_H. This is because phiH is about a factor of 8 larger than phiL, the effect from the lower loss material does not show up much. If the losses of the two materials are comparable, then the uncertainties of their Young's moduli will equally change the BR noise.
 
We can see from the plot that, in the estimated range of YH, the higher YH leads to the higher BR noise level.

Next, let's assume that the values of SiO2 are well measured and the error is much smaller than those of Ta2O5, so we can fix phiL and YL. Then recalculate BR noise when phiH and YH are correlated. I use a calculation from ring down measurement (see PSL:1412 or Harry 2002 or Penn 2003). The equation is

constant = phi_parallel = (YL*dL*phiL + YH*dH*phiH) / (YL*dL + YH*dH)   

from this equation, we can write phiH as a function of YH assuming that other parameters are constant. Currently, I'm using numbers from CTN setup.   

BR_YH_correlate.png

above: Coating BR noise as phi_H is varied along Y_H (green) compared with the previous calculation (Blue) from fig 2. The two traces cross at YH = 140 GPa.  Note that in this plot, YH is varied between 50% and 200% of the nominal value. We see that the uncertainty of coating noise due to YH becomes smaller compared to the previous calculation done in Hong paper

==what to do next==

  • Apply my calculation to GWINC: Right now it is a stand alone code, but I plan to write a code that uses parameters and function mostly from GWINC because it will be easy if we want to plot it later for aLIGO noise budget. 
  • Find out the uncertainties of YH and phiH  for Ta2O5 doped with TiO2, check the amount of TiO2 for aLIGO coating.   
  • Plot aLIGO noise budget , and estimate the inspiral range due to the sensitivity.

==Note==

From fig3, uncertainty in YL does not change the BR noise level that much, but this calculation assumes no correlation between YL and phiL. I have not been able to include uncertainty in YL and see the effect on phiL yet, because that will need more constraint equation. But I should check if it will greatly change phiL and affect the total BR noise calculation or not.

 

Attachment 5: Youngs_dep.m.zip
  1426   Sun May 18 10:45:42 2014 taraNotesopticCoating TO opt for Adv LIGO

I did an optimized structure for ITM and plotted the estimated noise budget of AdvLIGO using optimized AlGaAs coating on ETM and ITM. More details will be added later.

 

 dOpt_ITM1.png

Above: Optimized structure of ITM

 

AdvLIGO_AlGaAs.png

Above: AdvLIGO with Optimized AlGaAs coatings on SiO2 substrate, room temp. The plot is generated by GWINC.

Attachment 2: dOpt_ITM1.fig
Attachment 4: AdvLIGO_AlGaAs.fig
  1425   Sat May 17 22:01:28 2014 taraNotesopticCoating TO opt for Adv LIGO: ETM

Here is a prelim result for AlGaAs TO opt for ETM coating.

 

The optimization is named opt_ETM5 in .mat file. The structure is in optical length unit ( the physical thickness = (opt length) * 1064e-9 / n). The first layer is the air-coating GaAs layer . For the current optimization (opt_ETM5.mat) the transmission is 5.4 ppm, the reflected phase is off by about 2 degrees. 

dOpt_ETM5.png

opt_ETM5_nb.png

opt_ETM5_RT.png

ETM parameters used in the optimization

  • Spot radius (1/e^2 power) = 6 cm
  • Substrate radius =19 cm (GWINC)
  • Substrate thickness = 20 cm (GWINC)
  • Target transmission = 5 ppm

Note about optimization:

  •  The current optimization is just a prelim result for demonstrating that it is possible to optimize coating structure for ETM.
  • I have not included any uncertainties in the result.
  • The cost function used for this optimization is a only for nH = 3.48, nL = 2.977. So the coating properties might change rapidly with the uncertainties of the refractive indices see PSL:1356. I have not used the cost function with multi-value refractive indicesbecause this optimization is intended for parameter configuration (adjusting the target TO noise level, weight functions for each parameters, numbers of layer). 
  • The optimization was done for room temperature (300 K) @1064nm.
  • All the codes are attached in the zip file.

 To run the code:

  • run optETM.m with the current parameters. The result structure will be called xout in matlab workspace.
  • use CoatingThermalNoiseCalcETM.m to plot the noise budget.
  • I might miss some functions in the zip file, one is multidiel_rt.m , those are available on svn directory
Attachment 1: ETM_TO_OTP.zip
Attachment 2: opt_ETM_2014_05_17.mat
Attachment 6: dOpt_ETM5.fig
Attachment 7: opt_ETM5_nb.fig
  1424   Fri May 9 17:10:25 2014 taraNotesNoiseBudgetnoise comparison

Add the measurement from AlGaAs coating, and Silicon refcav (see CRYO:1045). The source file, figs, and eps files are attached in the zip file.

df_compare2.png 

dx_compare2.png

Attachment 3: EXP_compare2.zip
  1423   Wed May 7 12:27:25 2014 taraNotesNoiseBudgetnoise comparison

I plotted our measurement together with other experiments. The source file and fig files are attached below.

Details about each experiment (cavity length, wavelength), are included in the source file.

 

 df_compare.png

 

dx_compare.png

Attachment 3: df_compare.fig
Attachment 4: dx_compare.fig
Attachment 5: Exp_compare.zip
  1421   Thu Apr 24 18:54:44 2014 taraDailyProgressDAQmDV in ATF

Quote:

I'm trying to record beat measurement for a few days. The data will be taken from ATF using mDV. There are a few issues about mDV right now, I'm looking into it and asking around.

There is a problem with gps.m that converts the string to gps second. It is used in get_data where we specify the start time. I tried enter the gps second manually but it returns an empty time struct, and the get_data cannot be used.

 A reminder entry: psl:978

Zach helped me re-setting up the channels for PSL lab.  The two channels are: 

  • channel (#29 on the panel): C2:ATF-PSL2_OUT_DAQ
  • channel (#30 on the panel): C2:ATF-PSL3_OUT_DAQ

The sampling rate is set to 10kHz (8192 Hz).  

 Anti-alias, provided by foton, cheby2, low pass at 4096 Hz, is induced in both channels.

calibration 2^15 count for 20 V -> 20V/ 2^15  = 6.1035e-4 V/count 

==Note about start configuring MDV==

Run this from the terminal on ws2:
 eval `/cvs/cds/caltech/apps/linux/ligotools/bin/use_ligotools`

 Then, start matlab  and run:
 run /cvs/cds/project/mDV/mdv_config

(there are some error messages about the paths that can't be added (matapps_SDE, matapps_path , frame cache, ligotools_matlab, home_pwd.) ,but they are irrelevant.  

 

Now gps and get_data commands are working. We checked with the test signal and see both time domain, and frequency domain. The anti-alias filter is working fine.  

 

 

 

 

  1417   Sat Mar 29 23:32:50 2014 taraNotesNoiseBudgetextracting phiL phiH

Heat treatment after coating changes loss angles of both SiO2 and Ta2O5. Our coating might really have higher loss (maybe because of the low temp annealing), regardless of the actual values of coatings Young's moduli.

I went through the paper by LMA2014 that measured loss in SiO2 and Ta2O5 using interferometry on a cantilever blade. They could also extract Young's moduli from thin film SiO2 and Ta2O5, their results are ~ 70GPa and 118 GPa respectively.

One interesting result is that losses are reduced with heat treatment after coating process.

SiO2 loss before heat treatment is ~ 6e-4,  and it goes down to ~ 0.6e-4 after the annealing (from broadband measurement).

From ring down measurement, SiO2 before heat treatment is ~ 4e-4. no result for the measurement after annealing.

For Ta2O5,from broadband measurement, the loss after heat treatment is ~ 4.7e-4, no result from the before heat treatment is reported.

From ring down, the loss is ~ 11.4e-4 before annealing, and down to ~ 4.9e-4 after annealing. 

 

Their annealing process is described in the paper.  I should find out more how losses of both materials change with different heat treatment i.e. time/ temperature/cooling, then see if any information about our mirrors can be retrieved from REO or not.

Right now, we have only the information about phiH and phiL as phiH = a*phiL + b. I still need another relation to get phiH and phiL individually. My plan is finding information about heat treatment vs loss, like the picture below (I still need to find for Ta2O5). Otherwise, it is hard to say anything about the loss from each material.

 

Most reports have different annealing temp, (I'm not considering time/ heating rate/ cooling rate right now, but they might be important) So I can compare loss vs annealing temp.

  • LMA2014 without annealing and annealing @ 500C, for SiO2 and Ta2O5, thin film.
  •  G1000356: Thin SiO2 at 300C, 600C 800C, 900C. This shows that loss decreases as annealing temp increases, see the picture below. Notice that loss at 600C is comparable to what report in LMA (0.6e-4, @500C).

PennLVCmarch2010.png

It is hard to extract the similar plot as above for Ta2O5 from Martin2010 paper. I'll try to ask Ian Martin if he can give me the raw data. 

From the loss vs annealing temp I found out below, it seems that the annealing temp for our mirrors will be less than 300C. Since at 300C, silica loss is ~ 1.5e-4, tantala loss is ~ 4e-4. These numbers give the estimated BR noise below our measurement. 

 

Note:

  •  find out if Young's moduli change with annealing process. 
  • Coating layer thickness increases after heat treatment effect. This directly increases thermal noise. Find out how much it could be.

ref: silica loss with vs different heat treatment temperature. https://dcc.ligo.org/DocDB/0010/G1000356/001/PennCoatingMarch10.pdf

Martin2010: Class. Quantum Grav. 27 (2010) 225020 (13pp): Tantala loss with different heat treatment temperature

  1416   Wed Mar 26 21:53:50 2014 taraNotesNoiseBudgetextracting phiL phiH

 I used results from ring down measurement in Penn 2003, without assuming the values of YL,YH. If the actual Young's moduli of both materials are about 60% of their nominal values, the calculation of BR noise will match our measurement within 3%. 

 

Penn2003_tab2.png

I used ring down drumhead mode from sample C2 and F2 since the phi_coating as reported in the paper is about the same as the phi_coating obtained from the analytical result (see previous entry). With these two eqs, I can write

Ysub * D/3 * phitot_1  = phiL*YL*dL_1 + phiH*YH*dH_1-------(1)  (see previous entry, last eq).

Ysub * D/3 * phitot_2  = phiL*YL*dL_2 + phiH*YH*dH_2-------(2)  .

phi tot_1 and _2 are 1/Qtot from the two samples. D is the thickness of the substrate (0.25 cm). dL and dH are the physical thickness of siO2 and Ta2O5 in each sample.

For any fixed values of YH and YL, the two eqs will solve for a pair of phiL and phiH.

First, I checked the validity of these two ring down measurements by using YL = 72 GPa, YH= 140GPa. The results are 

PhiL = 1.29e-4, phiH = 4.13e-4. These numbers agree with the reported values.

Then, I varied YH from 0.5*YH_0 to 2*YH_0 and YL from 0.5*YL_0 to 2*YL_0 ( YH_0 = 140GPa, YL_0 = 72GPa), and solved for the corresponding phiL and phiH. Then with all 4 parameters, BR noise can be calculated.

Below is a plot of ratio of BR calculation and our measurement, vs YH. Each trace represents different value of YL.

 PSL1416_fig1.png

Each point on the plot will have information about phiL and phiH. If YL = 43 GPa (0.6*72GPa) and YH = 84 GPa (0.6*140GPa), the loss angles extracted from the ring down measurements are phiL = 2.15e-4 and phiH = 6.9 e-4. All these four parameters give the estimated BR noise comparable to our measurement to 2% (in PSD unit). 

 

==Conclusion==

 I'm trying to explain why our measurement is larger than the estimated calculation using numbers from literature. But we have good reasons to believe that the measurement is really BR coating since 

  • The data has a correct slope, 1/f in PSD
  • Scale with 1/w^2 in PSD, (BR noise from substrate/ spacer will have different scaling)
  • Agree with Numata2003.

It is possible that loss angles in our coating is lossier than usual. But there are still other possible explanations. The results from ring down measurements rely on the values of Young's moduli of the coating materials. If the actual values divert from the nominal values, the losses will be changed as well. So I used the result from the ring down measurement, without assuming any values of YH and YL, then extracted values of phiH and phiL using different combinations of YH and YL and  calculated the coating noise according to each set of parameters.  If YL and YH have lower Young's moduli than their nominal values, coating BR noise will be higher and agree with our measurement. 

One might argue that 0.6 YL and 0.6 YH are too low.  Ta2O5 was measured with nano indentation to be ~ 140 GPa (Abernathy). Other references measured Ta2O5 ~ 100 GPa (see ref 16, 20 in Crooks2006 paper). So, uncertainty around 40% might be possible.  

In addition, this calculation also assume phi_bulk = phi_shear.  But the different value of phiB/phiS can also change the calculation between 0.5*S_0 to 1.6*S_0, for different values of phi bulk/phi shear ratio is varied by a factor of 5(see Hong2013). These values also change the noise level significantly.

So with the uncertainties in Young's moduli, the loss angles from ring down measurements can be changed significantly.  If the Young's moduli of the coatings are smaller than the nominal values, the loss angles calculated from a ring down result will be higher, and  it resuls in a higher level of coating BR noise calculation.

==Note== 

I'm surprised that for the value of 0.6*YL_0 and 0.6*YH_0 used above, with the loss angles of phiH = 6.89e-4 and phiL = 2.15 e-4, the calculated BR noise is almost the same as when I use the nominal value of YH,YL with the same loss (2.15 and 6.89e-4) see, PSL:1408.  I double checked the result, but I did not see anything wrong in the calculation. It turns out that the BR calculation is not very sensitive to YL, YH, but it is directly proportional to phiH, phiL. However, the values of phiH, phiL obtained from a ring down measurement are very sensitive to YL and YH  as we can see from the plot above.  

 

Attachment 3: PSL1416_fig1.fig
Attachment 4: phi_result.m.zip
  1414   Tue Mar 25 14:37:54 2014 taraNotesNoiseBudgetextracting phiL phiH

Quote:

 Ta2O5 Young's modulus is quoted to be 140 GPa from this paper Martin1993,  but that is the value of Ta2O5 deposited on Silicon substrate cf fig5, top plot. The deposition technique is IAD. I'm not sure if it is the same as ion beam sputtering or not. I'm looking into it.

Anyway, the Young's modlus of Ta2O5 can be down to 70 GPa for IAD technique on glass substrate, as the paper says in the conclusion section. 

 Note that Crooks2006 mentions other papers measure YTa2O5 to be around 100-110 GPa as well. I'm looking into it.

 I just talked to Matt and learned that:

  • The measurement from the above paper (that everybody quotes) is wrong, they just measured the Young's moduli of the substrate. However, the actual value of Y_Ta2O5 is ~ 140 GPa (use +/- 30% for conventional uncertainty) (Abernathy's from 500 nm sample) and the measurement for Y_SiO2  is ~ 70 GPa ( from 2 um sample)
  • The term ion beam sputter (IBS) for our coating is actually IAD.
  •  Note about coating loss in Penn2003
  • Tphi_Y.JPG

The measurement from Penn extracts phi1 and phi2 from

 phi_c = (Y1 *d1*phi1 + Y2*d2*phi2) / (d1*Y1 + d2*Y2).

Phi_c is calculated from the total phi of the ring down system.

 

The dissipated energy comes from two part, the substrate and the coating. With the assumption that phi sub is much smaller than phi coat, we can write 

phi_total (measured from ring down) =  |energy in coating| / |Energy in substrate|  * coating loss, and the ratio Ec/Es can be obtained from FEA.  For drum head mode it is ~ 1500 (From Penn paper), see the picture, top panel.

This Ec/Es also depends on the Young's moduli, so the calculated phi_c also has Y as a parameter.  The calculation I did before takes phi_c from the reported values, so it is not correct.

To get the correct phi_c, the ratio of Ec/Es has to be changed with Y. Crooks PDH thesis has an analytical expression for the drum head mode of a cylindrical substrate. The analytical result is comparable to the FEA result used in Penn2003 within 5%. Note that the young's modulus of the coating is the volume average (Yc tc = y1*t1 +y2*t2) where tc is the total thickness, tx = thickness of material x. See the middle panel in the picture above. 

For the next step, instead of using the report value of phi1,phi2 and Y1,Y2 to reconstruct phi_c (Penn2003). I will use the measured phi_tot (for drum mode) then use that as a constraint on Y1, Y2, phi1, and Phi2 instead, see bottom panel in the picture. This should give a correct dependent among these variables.

 

 

  1413   Mon Mar 24 20:56:35 2014 taraNotesNoiseBudgetextracting phiL phiH

 Ta2O5 Young's modulus is quoted to be 140 GPa from this paper Martin1993,  but that is the value of Ta2O5 deposited on Silicon substrate cf fig5, top plot. The deposition technique is IAD. I'm not sure if it is the same as ion beam sputtering or not. I'm looking into it.

Anyway, the Young's modlus of Ta2O5 can be down to 70 GPa for IAD technique on glass substrate, as the paper says in the conclusion section. 

 Note that Crooks2006 mentions other papers measure YTa2O5 to be around 100-110 GPa as well. I'm looking into it.

  1412   Sun Mar 23 17:32:42 2014 taraNotesNoiseBudgetextracting phiL phiH

I'm checking how loss angle of Ta2O5 is related to its Young's modulus (as used in ring down measurements), then I use that relation in error calculation for BR noise in coating. The uncertainties in Young's moduli of SiO2/Ta2O5 might lead to errors in loss angles and BR noise in coating.

 

==Background==

Many ring down measurements (see Penn2003, Crooks2004, Crooks2006), observed loss from a disc substrate with multilayer coatings of Ta2O5/SiO2. The loss in the coating (ring down mode) is written as  

phi_c = (phi1*Y1*d1 + phi2*Y2*d2 )/ (Y1*d1 + Y2*d2)    --------(1)

Where phi_c is determined from the measurement. Y is the young's modulus, phi is loss angle of material, d is physical thickness of the material.

Then phi1 and phi2 is determined with the assumption that Y1 and Y2 are known.  

So, the reported value of phi Ta2O5 is directly related to its Young's modulus. The uncertainty calculation of BR noise where Y, phi are varied independently might not reflect the real situation.

For example, I recalculated phi_c (of QWL structure) using phiH phiL of  4e-4, 1e-4. YH = 140 Gpa, YL = 72GPa. Then I rearranged eq(1) so that phiH can be written as a function of YH to see how the loss angle of Ta2O5 (H) will change with its Young's modulus assuming that YL and phiL are fixed.

phi_young_ta2o5.png

fig1: How phi Ta2O5 changes with Young's modulus.

 

==BR calculation with loss parametrized by Young's modulus==

With the loss angles parametrized by the Young's modulus, I calculate the estimated thermal noise compared to our measurement (using Hong2013) 

BR_Y_ta2o5.png

fig2: ratio of BR calculated and our result. 

It is interesting that, even with the lower phiH as YH increases, the total BR noise increases. And the nominal value that we have been using (YTa2O5 = 140 GPa) yields almost the minimum value of BR noise calculation. 

 

==next step== 

  So far, the calculation is done assuming phiL = 1e-4, YL = 72e9.  The next step is to varied phiL, YL, phiH, YH all together ( with the constraint given by eq1) and see how BR noise changes.

  I'm also checking how large the errors are in the measurements for Young's modulus (both SiO2/Ta2O5). Crooks2006 reports the value of Young's modulus of Ta2O5 with the assumption that Y_SiO2 is 72e9. This might give another constraint. 

 

 

 

 

 

Attachment 3: BR_Y_ta2o5.fig
Attachment 4: phi_young_ta2o5.fig
  1411   Sat Mar 22 19:17:31 2014 taraNotesNoiseBudgetextracting phiL phiH

 I checked Brownian coating noise level with uncertainties in coating parameters. The measured result is barely at the edge of the confident interval. 

Hong2013 look into coating noise level when materials' parameters are changed. One example is the Young's modulus of Ta2O5. With the assumption phi bulk = phi shear, if Y_Ta2O5 is varied between 70e9 to 280e9 (nominal value = 140e9), coating thermal noise can be changed by a factor of 0.9 to 1.5 from the nominal value (in PSD m^2/Hz unit). It seems that the range is quite large compared to the numbers measured by various groups, (see PSL895 for error in material parameters). I used a smaller range, but I varied other parameters as well.

==Note about uncertainties in calculation==

I used rand command in Matlab to generate random values. The reasons are 1) for reported loss angles, say 4+/- 2e-4, if I use Gaussian dist, with sigma =2, mean = 4, sometimes the generated value will be negative, and 2) since we are only trying to see the possible range of the estimated noise level, not the real statistic value, rand should be ok at this point.

==1:fixed loss angles==

  First I checked how much the parameters effect the calculation if the loss angles are fixed (phi silica = 1e-4, phi tantala = 4e-4). Y tantala is chosen between (70-280 GPa), Y silica is varied between72e9 +/- 10%,  Poisson's ratio are varied between 10percent for coating materials. All substrate parameters are fixed, since they should be relatively well measured compared to that of the coatings.  The result is around 0.5-0.85 of the measurement (in PSD m^2/Hz).

  For a more conservative value of Ta2O5 ( 140+/-40 GPa), the result is a factor of 0.5-0.64 of the measurement.

 ==2:varied loss angles==

In this study, I varied loss angles of phi_silica = [0.8-1.2] x10^-4, phi_ta2O5 = [3,5]x10^-4, these numbers are reported from several measurement. Then I change the uncertainties range of Y_Ta2O5 in my calculation

  •  Y_ta2o5 = [100,180] GPa, the result is around [0.4,0.88] of the measurement.
  •  Y_ta2o5 = [70,280] GPa, the result is around [0.4,1.09] of the measurement. A histogram of the ratio between noise level and the measurement is shown below (from 1e4 runs). The measurement value (Sx/Smeasured = 1) is barely at the edge of the confident interval (and not from Gaussian distribution either).
  • hong_hist.png

==note and comment==

 Both Hong's and Harry's calculation provide quite the same value (within 3%). So I show only histogram obtained from Hong's calculation. I don't know why the study shown in Hong paper choose the value of Y tantala between 70-280 GPa, most of the measurements report smaller uncertainty. But with that higher value of Y_Ta2O5, it can explain the measured noise level from our measurement. However, I doubt that this argument is valid, since most of the ring down measurements to evaluate phi_Ta2O5 assume Y_tantala ~ 140GPa. Then the loss angle of Ta2O5 should carry some information about Y_Ta2O5 in it and cannot be treated as an independent parameter like this calculation. I'll look into the ring down papers to see how much Y_Ta2O5 affects the extraction of its loss angle.

 

Attachment 2: hong_hist.fig
  1408   Thu Mar 20 18:09:33 2014 taraNotesNoiseBudgetextracting phiL phiH

 Since we measured thermal noise from the coating(QWL, SiO2/Ta2O5), we want to extract loss angles of each materials.  The losses are about a factor of 2 higher than the numbers reported in the literature.

So far, there are 3 calculations we have been using for coating noise estimation.

  • Nakagawa , this calculation assumes Young’s moduli , Poisson ratio of the coating and the substrate are the same. The coating is s a single thin layer.  Sx ~ phiC . If we fit this calculation to our result, the coating loss is about 4.15e-4. This number agrees with the result from Numata2003, and agree with both our measurement from short and long cavities.
  • Harry2002:  Sx is function of ( phi_perp, phi_para), both phi_perp and phi_para are functions of( Y,Yc,sigma,sumac, phiL, phiH)
  • Hong2013,  Sx is function of phi_bulk and phi_shear. If we assume that phi bulk and shear are the same for each material phi bulk L = phi shear L = phi L, Sx ~ const1 phi L  + const2 * phi H,,  

In essence, both Harry's and Hong's result can be written as a linear combination of phiL and phiH.  I used Harry result to compare with Hong to see if there is any differences in the result or not, but both gave me the same answer.

==calculation==

The calculation is attached below. I made sure that the calculation from Hong and Harry are correct by choosing the elastic properties of the coatings to be the same as that of substrate and checking the the results agree with Nakagawa's. So the code should be correct.

Then, I varied phiL and phiH to match the measurement. The measurement is represented by the prediction by Nakagawa with the fitted loss (phiC = 4.15e-4).

Both calculations gave the similar relation between phiH (phi tantala) and phiL (phi silica) to match the measurement: 

 phiH = -1.4 phiL + 9.7  (Hong)

 phiH = -1.44 phi L + 9.77 (Harry) (assume sigma1 = sigma 2 = 0) 

The problem is if we use the nominal numbers from various reports, phiL ~ 1e-4, phiH ~ 4e-4.  The result will be off by almost a factor of 2. For example, for phiL = 1e-4, this means phi H has to be 8.4e-4.  Or if phi H is chosen to be 4e-4, phi L will be ~4e-4 as well. It seems that our result is higher than the predictions (under some assumptions).

 

Table1 below shows some possible values of phiL and phiH extracted from our result and the calculation.

phiL x10^-4 phiH x10^-4
0.1 9.6
0.5 9.05
1 8.33
1.5 7.61
2 6.89
3 5.35
 

 But we have good evidences from Numata and short/long cavities (spot size dependent) to believe that the measurement is real coating thermal noise . The reason why the prediction is smaller than the measurement could be that the losses is actually higher in our coating. Most ring down measurements were done after 2002 while our coatings were fabricated around 1997. Coating vendors might become more careful about loss and improved their process. But the result from Numata was out in 2003, and it is about the same as ours, so I'm really not sure what can we say about this.

==numbers from literature==

Penn2003: (disc ring down)  phiL = (0.5 +/- 0.3) x10^-4 ,  phiH = (4.4 +/- 0.2) x10^-4

Numata2003 (direct measurement)    phiC = 4.4e-4;

Crooks2004(disc ring down)    phiL = 0.4+/-0.3  x10^-4,   phiH = (4.2+/-0.4)x10^-4   (the frequency dependent part is ignored)

Crooks2006: (disc ring down)   phiL = (1.0 +/- 0.2) x10^-4     phiH = (3.8 +/- 0.2) x10^-4    (small change in TE calculation from previous paper)

Martin2009 : (blade)                                                      phiH = 3+/- 0.5 x10^-4  (at 300K)

Martin2010: (blade)                                                       phiH = (2.5-5) x10^-4  ( heat treated at 600C, several frequencies)

 LMA2014: (blade)   phiL = (0.43+/-0.02) x10^-4        phiH = (2.28 +/- 0.2) x10^-4 

 

 

Attachment 1: Hong_coat_BR.zip
  1407   Tue Mar 4 18:53:48 2014 taraNotesRefCavcomsol simulation for cavity suspension

I tried to estimate the coupling from seismic to displacement noise. With the common mode rejection taken into account, the coupling from vertical acceleration to differential length between the two cavities is about 6x10^-12.

 

  • With random errors add in the position (+/- 1mm) of the support position, I used COMSOL to find out the coupling from acceleration to strain noise to a single cavity. Note that I did not use Gaussian distribution because sometimes the support position will be place away from the cavity. so I used random error with confined limit to +/- 1mm.
  • Then I histogram it. The result is shown in the figure below.

hist_small_mount_err.png 

  • Take some common mode rejection into account. Since most of the time, the coupling will fall between 0.98 x10^-10 to 1.04 x10^-10  [s^2/m].  We can assume an upperbound that one cavity has the coupling of 1.04x10^-10 and another has 0.98x10^-10. Thus, the effective coupling for the differential strain is (1.04-0.98)x10^-10 = 6x10^-12. 
  • I have to admit that the result is very weird. With the ideal support position, the coupling for a single cavity is also about 6x10^-12. I did not expect that by changing the positions slightly, the coupling for each cavity can be worse by 200 times. I suspect that COMSOL might add some errors as well, see psl:1056. The cavity does not bend down symmetrically, even with the perfectly symmetric support position, but I did not think it will be this huge.
Attachment 2: hist_small_mount_err.fig
  1405   Tue Feb 4 01:11:01 2014 taraNotesRefCavcomsol simulation for cavity suspension

 I'm trying to estimate the coupling between seismic to displacement noise of the cavity using COMSOL. 

From the design, the strain due to the seismic noise is about 2e-11.  But we want to see what happen if the support positions are moved away from the specified points a bit. This time the model is a whole cavity, not just 1/8 as I did before. This is to see results of the mis-positioned support points. However, COMSOL has some problems

  • The solution for the full sized cavity is not symmetric even with the optimum support points. I think this is because the mesh size is not physically symmetric, but the result gets better (the sagging becomes more symmetric) when the mesh size is smaller. However, the calculation time is quite long (~3.5 minutes for each run).
  •  I cannot adjust the angle, I can only adjust the beam line position. I'm not sure why COMSOL has this problem. It is not a syntax problem. I ran the iteration and it gave me two results , then the error message poped out. So I only changed the support positions along the beam line direction 
  • The random I used is white, where the error is +/- 0.5 mm. I could not use Gaussian, because sometimes the support positions were out of the spacer. I think It should be fine for now to see how the result will be

Right now I have ~30 data points after 4 hrs of running the simulation. I'll get a bit more data and will see how it goes when I histogram it.

  1403   Mon Feb 3 23:56:07 2014 taraPhotosopticpacking mirror

 I"m packing the mirrors so that they are ready to be shipped to G. Cole. The mirrors are packed properly, see picasa.

  1401   Wed Jan 29 21:36:53 2014 taraSummaryNoiseBudgeterror in spacer brownian noise

Spacer in BR noise

 

== COMSOL vs result from Kessler etal 2012==

     The analytical result from kessler2012, assume the force acts on whole surface of the spacer (with bore hole), I check this with COMSOL by comparing the result, similar to what I did in PSL:1075. The result agrees well within 2%. This verifies that COMSOL model is correct

 

==thermal noise level vs annulus thickness==


Typically, the contact surface between the spacer and the mirror is only a thin annulus, see psl:1199  . And the noise level is dependent on the actual area of contact. So I run the simulation to see the dependent of the stored energy (U) vs the annulus thickness. The annulus thickness is about 2 mm +/- 0.2 mm. The displacement noise is proportional to sqrt(U).

spacerBR_comsol.png

fig1: The stored energy as calculated by COMSOL, fitted with cubic polynomial.

The error from the contact area, the simulation result are small ~3% and 2%. These are smaller than the uncertainty of loss in bulk fused silica (can be from 10^-6 to 10^-7). The effect is still small in the total noise.

Attachment 2: spacerBR_comsol.fig
  1400   Wed Jan 29 05:44:51 2014 taraSummaryNoiseBudgetNoise budget fitting: need uncertainties

I looked into the uncertainty in coating thickness of the QWL SiO2/Ta2O5 coating The thickness of  4.53 +/- 0.07 um (~1.5%)seems to be appropriate.

The thermal noise level is directly proportional to the coating thickness, so we want to estimate its uncertainty. The error in the thickness is from

  • The uncertainties in nL and nH: since the physical thickness is lambda/(4*n), the error in n goes to the error in d.
  • Manufacturing process.

The errors in nL and nH are quite small, nL ~ 1.45 +/ 0.01, nH ~ 2.06+/- 0.01. (From the literature). I also looked around the error in IBS thickness control, they are usually better than 0.1 nm, IBS, but that is the current technology. In literature around 2000s, 2% error seems to be the number estimated for the thickness control (Sullivan 2000, Badoil 2007). As a quick check, I used the same assumption for error propagation similar to that of AlGaAs coating. The result gives ~ 4.53 +/- 0.07 um for coating thickness.

Note that the error here is smaller than the difference in coating thickness for the coatings with or without half wave cap.

For 28 Layer (with cap), the coating thickness is 4.53 um,  for 28 layer QWL, the coating thickness is 4.35 um. But after digging up all the information from REO, and peter king they agree that it is 28 QWL with half wave cap.  I tried to compare the calculation and the photothermal TF measurement, but the effect is too small to be conclusive about the structure. So the biggest error might come from the fact that the coating has cap or not. The error is about 4%.

  1399   Fri Jan 24 21:13:13 2014 taraDailyProgressDAQmDV in ATF

I'm trying to record beat measurement for a few days. The data will be taken from ATF using mDV. There are a few issues about mDV right now, I'm looking into it and asking around.

There is a problem with gps.m that converts the string to gps second. It is used in get_data where we specify the start time. I tried enter the gps second manually but it returns an empty time struct, and the get_data cannot be used.

 A reminder entry: psl:978

  1393   Tue Dec 31 19:33:47 2013 taraSummaryNoiseBudgetbeat measurement

I got a chance to measure beat measurement. The noise budget is updated and contains all dominant noise traces.

 

== Beat measurement ==

beat_2013_12_24.png

1) at DC to 10Hz, the contribution is mostly from RIN driven Photothermal noise and a bit of seismic noise, a small peaks around 10Hz is probably from the stack, not the cavity sagging. The hump from DC to ~ 50Hz disappear when it is quiet. I think it is mostly scattered light associated with the seismic noise, not displacement noise due to the vibration.

2) 10Hz to 1kHz is pretty much Coating Brownian noise.

3) At 1kHz and above, it is PLL readout noise and residual frequency noise from the laser, where the gain cannot suppress enough noise. This is mostly from ACAV. The residual frequency noise = free running noise / (1+ OLGTF). The measurement of the open loop gain is explained below.

 

==TTFSS Loop characterization==

The OLG TF of TTFSS is measured up to 10MHz and compared with the calculation. The schematic explaning how TTFSS actuates on the laser is shown below.

TTFSS.png

The freqeuncy discriminator can be measured from the slope of the error signal (from Common out1) while scanning the laser. For RCAV Dv = 1/ (194 kHz/V) and 1/(164kHz/V) for ACAV. with 1mW input power.

The adjustable gain stage can be tuned by turning the dial knob. At 400, gain=1, and the gain changes by 10dB with every 250click.

The PZT actuator has a gain of  4.5MHz/V (measured), and the EOM actuator is 15mRad/V (or 15mHz/f  Hz/V) (taken from the spec sheet).

OLG measurement is taken:  RCAV OLG is measured and plotted against the theoretical approximation, see the below figure.

RCAV_OLG.png

above: RCAV OLG TF. Note: The calculation and the measurement do not include the integrator with corner frequency at 4.6kHz.

 

There are some problems with ACAV loop and I could not increase the gain up as much as it used to be and the UGF is around only 200kHz , but the measurement matches the calculation. Right now RCAV servo has a better loop performance.

ACAV_OLGTF.png

 The calculated OLG TF trace(green) should go down at 1MHz or above because of the opamps' bandwidth. I used ideal Op Amps in the simulation because I don't have some op amps in my liso library. I'll see if I can fix it.

  1392   Wed Dec 18 21:05:28 2013 taraNotesoptic photothermal noise in AlGaAs: thickness resolution

We heard back from G. Cole about the thickness resolution in the AlGaAs coating manufacturing process will be around 0.5 A. So I'm checking how the noise budget will change by rounding up the physical thickness in opt V4 to the next 0.5A. The design will still work. The round up thickness is added in the google document (for opt v4 only).

The estimated growth rate of the crystal is 4.8A/s and shutter speed is assumed to have 0.1 sec time step. This means the smallest step of the thickness control is ~0.5A. So I round up the physical thickness to the next 0.5 A and calculate the coating properties.

1) Rounding up to the next 0.5 Angstrom. The truncating process acts like a random thickness variation in the optimized coatings with maximum error ~ 0.25 Angstrom. The averaged layer thickness is ~ 800 Angstrom.

 05Atrancate_err.png

 

2)Results when the layers physical thickness are round up to the closest 0.5 A. The noise budget does not change much.

05Atruncate_nb.png

05Atruncate_T.png

05Atruncate_err_ana.png

The coatings properties still hold, even with random error in parameters, thickness.

 

Note: For the error calculation I did before I used 1 sigma to be 1% for AlGaAs, and 0.5% for GaAs. The thinnest layer is AlGaAs at 35 A, so its sigma is about 0.35 A. The average thickness is 90 Angstrom, so the average error is about 0.9 A. The estimated error in the calibration process is already larger than the error from the truncation(0.25A). That's why the error analysis results are still valid.

Attachment 5: 05Atrancate_err.fig
Attachment 6: 05Atruncate_err_ana.fig
Attachment 7: 05Atruncate_nb.fig
Attachment 8: 05Atruncate_T.fig
  1390   Mon Dec 16 15:14:43 2013 taraNotesElectronics EquipmentOLG of RCAV TTFSS

 open loop gain transfer function of RCAV is measured. 

 1) how to measure OLG TF

  • see, PSL:592 about how to measure the OLG from TTFSS.
  • current schematic [add fig]

2) setup

  • 1mW input to the cavity
  • gain common/fast = 730/950
  • Mod depth =
  • Boost off 
  • U3 -> 11.84 dB

Result:

openloopTF.png

The requirement assumes that the residual frequency noise is 5% or less in the total noise. The servo performance is definitely ok for 1.45 inch cavity.

Attachment 2: openloopTF.fig
  1389   Mon Nov 25 15:28:12 2013 taraNotesRefCaveigenmodes of 1.45" refcav

I realized that we have not checked the eigenmodes of 1.45" cavity yet, so I used comsol to find out several modes. The lowest mode is ~ 46kHz, and the first longitudinal mode is about 60kHz. The frequencies are high enough so that the thermal noise calculation in dc- 10kHz frequency band can be done with quasi-static assumption.

 

1) I tried a simple cylindrical shape, with the dimension of the spacer. The result for the first longitudinal mode is 74KHz, the analytical result is ~ 77kHz, see PSL:1135. It seems that COMSOL's result and the analytical results are comparable.

spacer_only_z.png

 

2) Then I simulated the whole reference cavity. The lowest body mode is ~ 47kHz. The body expand-contract radially, and should not change the cavity beamline length that much. The first longitudinal mode is ~ 60kHz. The color on the surface shows the rms displacement from all direction.

 

spacer_br_8_edge.png

 

spacer_br_8_edge_z.png

Attachment 1: spacer_eigenmode.mph
  1388   Wed Nov 20 18:19:01 2013 taraDailyProgressNoiseBudgetphotothermal noise in SiO2/Ta2O5

I compared our beat measurement with results from Numata2003 and TNI. They agree well. I'm quite certain that we reach Brownian thermal noise from coatings.

 

 To make sure that what we measure is real Coating Brownian noise (It could be something else, i.e thermal noise in the support, spacer , or optical bond), we should compare our result to previous measurements to make sure that the numbers agree.

 Numata etal and TNI reported coating thermal noise measurement from suspended cavities (no spacer). They adjusted loss in the coatings to fit the measurement.  Phi coatings as reported in Numata is 4e-4 while TNI gives phi perp = phi_para = 2.7e-4.  Both agree with our result, see the plot below.  This means that our result is comparable with what they measured. It should be an evidence to support that we see real coating thermal noise, not contribution from something else (spacer, optical bond between the mirrors and the spacer).

beat_compare.png

Another evidence is from our previous measurement from 8" cavity.

  • The measurement also agrees with Numata's 2003 result, with phi coatings = 4e-4, see PSL:1018.
  • And the signal scales correctly with a factor of ~ 9 (from shorter cavity, and from smaller spotsize^2), seeT1200057. Had it been noise from optical bonding/ spacer (independent from spotsize), the scale factor would have been 8/1.45 ~ 5.5. The scale from substrate Brownian will also be different because of 1/w_spot dependent. Thermoelastic/ thermoopitc will have different slope.

So It is clear that our beat measurements from both 8" and 1.45" cavities are coating Brownian noise limited (around 50Hz-1kHz).

 

Attachment 2: beat_compare.fig
  1387   Tue Nov 12 15:27:32 2013 taraNotesDocumentationthesis on ctn

I created an svn folder for my thesis on CTN measurement.

It can be found here

 

 

  1386   Mon Nov 11 19:37:13 2013 taraDailyProgressopticredo- PMC path

PMC path is back, I aligned the polarization of the input beam to the BB EOM for TTFSS. The visibility of PMC is now ~ 80%.

  1385   Fri Nov 8 03:36:44 2013 taraDailyProgressopticredo- PMC path

I'm re-arranging the optics in PMC path a bit. The work is in progress, so ACAV path is still down.

I'm investigating why ACAV TTFSS performance is worse than that of RCAV. One thing is that ACAV has the PMC. This area has not been optimized for awhile, so I'm checking everything.

  1384   Thu Nov 7 05:08:13 2013 taraDailyProgressNoiseBudgetphotothermal noise in SiO2/Ta2O5

I add the photo thermal noise effect in the noise budget. With ISS, photothermal noise should be sufficiently small.

 

What I did

  • Measure beat
  • Measure RIN after ACAV and RCAV
  • Measure TF between TRANSPD and beat, compare the result with Farsi's calculation to determine the absorption (8ppm, with Finesse = 1e4) [add more details]
  • Apply the measured RIN to Farsi calculation to get the conversion from RIN to frequency noise ( I did not use the measured TF because I have not measured the whole range yet, and the calculation matches the measurement quite well).

beat_2013_11_07.png

Comment about the beat

  • At DC -30 Hz, the noise seems to be a combination of photothermal noise, and seismic induced scattered light. Air spring might not help as much as I thought.
  • Above 2kHz, it's not clear if it is gain limited on ACAV loop or not, but this is likely. We can check by measure the PSD of the error signal and convert it to frequency noise.
  • Frequency stabilization of ACAV is significantly inferior than that of RCAV. I don't know if it is the result from PMC or not. More investigation is needed.

Note about RIN measurement

  • RIN (measured behind the cavities) depends considerably on the TTFSS gain, luckily, at optimum gain level, RIN is pushed down enough.
  • RIN from ACAV is almost a factor of 10 worse than that of RCAV @ the optimum gain setting
  • There might be coupling from BB EOM to RIN (due to the mismatches E field between the EOM and the beam). This may explains why RIN is getting worse if common gain is increased a bit before the loop oscillate. Will check that.

 

Note about loss angles: For  SiO2 and Ta2O5 loss angles = 1e-4 and 7.5e-4 (a factor of 3 above the regular number), the noise budget matches the measurement well. I'll see if it is the same for the data from 8" cavities or not.

Attachment 2: beat_2013_11_07.fig
  1383   Wed Nov 6 01:14:58 2013 taraDailyProgressElectronics EquipmentTTFSS

We made a mistake by choosing the input power to the cavities to be 0.25 mW, so today I turned them back to 1mW and measure the beat.

beat_2013_11_05.png

Setup:

  • input power: 1mW
  • TTFSS gain (C/F): RCAV (760/980), ACAV (630/650)

Note about the measurement:

  • The noise at error point from ACAV is pretty high (~up to 100 nV/rtHz around 6 kHz). Better characterization will be done later to see if the suppression is enough or not. I made sure that this measurement is good up to ~2kHz, (this was done by changing the gain level a bit and beat level did not change).
  • Table was floated, but the air springs was not activated. I hope we will get better signal around 20-100 Hz once the air spring is re-installed.
  •  Intensity noise->photothermal around a few hundred mHz, cause the PLL to drift away from the input range. This effect becomes worse when the input power is increased.
  • RIN from ACAV is about a factor of 10 higher than that of RCAV.
  • I measured the beat with ISS on/off on ACAV, nothing were significantly different. So maybe it is not a problem for now.

To do next:

  • I'll compare this measurement with the one from 8" cavity to see if the results agree or not. They are different mirrors, but from the same coating run. I suspect that the loss in these mirrors are higher than what we estimate (Ta2O5=2.5e-4, SiO2 1e-4).
  • Think about the error in the measurement (calibration, spotsize etc).
Attachment 2: beat_2013_11_05.fig
  1381   Tue Nov 5 01:11:15 2013 taraDailyProgressElectronics EquipmentTTFSS

Quote:

 

 Do you guys have a plot that shows the required loop gain and the achieveable loop gain with this TTFSS on the same plot?

 Not yet, we will add this later. but we measured the noise at error point before and it is well below the estimated coating noise.

 

Plan for this week

Mon: (See Evan's entry for more detail)

  • minimize RFAM,
  • bring back the beat signal, optimized gain setup. A lot of improvement around 10 - 50 Hz.
  • measured error noise from each PDH loop, with slope of the error signal to calibrate it to frequency noise
  • measure RIN

Tue:

  • measure photothermal coupling from RCAV (may remeasure ACAV)
  • measure cavity pole, to check the cavities' finesses
  • check calibration on PLL

Wed

  • replace the broken air spring with a new one.
  • fix harmonic lines in the beat
  • update the calculation for the noise budget ( electronic noise, etc)
  • Turn on ISS

Thur

  • beat measurement
  1379   Fri Nov 1 00:22:40 2013 taraDailyProgressopticmore optimization

I'm putting EOAM back on ACAV path. The setup is roughly optimized.

(14.75 MHz) EOM , EOAM, quarter waveplate and PBS in ACAV path are put back together. I used a half waveplate in front of the EOM to adjust the beam to S- polarization. Right now all the polarizations optimization (to all EOMs, both ACAV/RCAV path) are adjusted to S-polarization with respect to the table. We may have to fine tune it later to match the E field in the EOMs.  The EOAM setup is optimized. With +/-4 V, the output power can be adjusted to 1mW +/- 0.09 mW (+/- 9%). The performance is comparable to RCAV EOAM. (10%) . I have not add another half waveplate before the EOAM yet. We can add it back later if we need to adjust the input polariztion to the EOAM.

I checked scattered light in the area between PMC and ACAV.  There is a reflection from EOAM back to EOM, but I cannot really block it with an iris. It probably bounces of the case of the EOM or going back to the crystal. Anyway I'll block the beam around this path later.

I have not aligned the beam to the cavity yet, since the temperature was changing because I removed the insulation  caps to patch them with black out material.

 I put black out material (R @1064 ~0.4-0.6%)on the vac tank insulation caps to minimize any possible scattered light source inside the tank that might leak out.  It also keep the surface cleaner from all the foam dust.

foam1.jpg

foam2.jpg

  1377   Thu Oct 31 00:02:17 2013 taraDailyProgressElectronics EquipmentTTFSS

Evan found that when common gain is changed, DC offset also changes as well. I'm still looking into the problem.

 

D040105C.png

a part of schematic, the driving signal was sent in through test port (the switch was flipped from off to test), so the signal came through PD line in this page.

 We still cannot lock RCAV with TTFSS, so I'm checking the box 2009007 (#7).

  • The modifications I did two days ago were 1) adding a push switch for gain reduction and 2) replacing one resistor. The rests were changed before we got the TTFSS.
  • I checked the TF between TP1 and TP5, it works as it should be (20log( 390/100) ~ 12 dB). So this stage does not have any problem. I checked both TF and time domain signals. So the modifications are ok.
  •  Note, when I measured the TF of TP1/TP4 or TP5/TP4, the signals oscillated and became very noisy. I don't understand why, but this problem disappeared when I used TP4 and out2. Both boxes (#5,#7) have this same problem.

Common Gain - DC offset problem

  • When Common gain is increased (CG signal to U2A chip), there is an offset observed in TP4 which is after the variable gain stage. Both boxes behave similarly. <- This surprises me, as we haven't seen this (or haven't noticed this) before.
  • I checked if the offset varied with the input drive or not. I changed the input from 20mV to 40mV, with constant gain = 1000(25dB). The behavior is nonlinear (see the plot below). I checked this only on box#5.

offset.png

DC offset vs input drive. DC offset is calculated from (Vmax + Vmin) /2  from a sinusoidal signal input. The signal was taken from TP4. The behavior is very non linear and it is impossible to make a table for an appropriate offset level vs common gain setting.

 What to do next?

  • This seems to be an important clue about why the loop behaves badly when common gain is increased. From today test, both boxes behave the same, so I think it might be the chips' problem.
  • Fortunately, we can use the offset adjustment(OS)
  • to cancel the offset introduced by the common gain, but we might need to add a port some where (we might be able use fast mon channels during laser scan).  So that when we increase the gain, we can adjust the offset accordingly.
  • Box#7 that I modified should be working. I don't know why I could not lock the laser before, more checking has to be done.

 

  1376   Wed Oct 30 01:56:38 2013 taraDailyProgressoptictable work

I'm optimizing the setup, and clearing the table a little bit.

  • Self homodyne setup in ACAV path is removed. This is from Erica's setup and it is not used. The input part is left, since I might use it for fiber distribution system
  • optics on RCAV path, all polarization are optimized. This includes, the input and output polarization for EOAM, and quarter wave plate before the periscope. The input polarization for sideband EOM is left intact after the last adjustment, and it should be good. With+/- 4V input, I can change the power by +/-10%, (1.0 +-0.1 mW is the current setup). For Evan: Do not touch anything before discussing with me!!!
  • I replaced a new PBS for PDH locking in RCAV path. The old one is bad. The surface between the prisms is milky, see the pictures below for comparison. There is also beams from multiple reflection within the cube. The new one is much better. There is no ghost beam anymore.
  • I blocked all the scattered light I could find in RCAV path with Irises and beam dumps. For ACAV, I just blocked the scattered lights from the laser to the PMC. I will finish the whole setup later.
  • I rechecked the height of the beam through EOMs/EOAMs. Since it is a little tricky to center the beam through the openings. The EOMs in RCAV path are all checked. For ACAV, only those between the laser and the PMC are checked(BB for phase locking and 21.5 for PMC sideband). The 14.75Mhz sideband and EOAM will be done later. The EOAM and wave plates are removed temporarily.
  • I modified the TTFSS for RCAV to have a gain reduction switch to help locking the laser. I tried to lock RCAV, but I cannot turn up the gain. I'm not sure what I did wrong but this has to be investigated.

To do lists

  • put optics back in ACAV path and optimize them (alignment + polarization).
  • fix RCAV TTFSS . Check by measuring the TF of the modified stage/ scanning laser + checking error signal

oldPBS.jpg

above: old PBS, bad inter surface can be seen.

newPBS.jpg

above: new PBS: all surfaces are clear

  1374   Sun Oct 27 20:12:25 2013 taraNotesoptic photothermal noise in AlGaAs

I revised the calculation for photo-thermal noise in AlGaAs coatings, the photo thermal noise should not be a limiting source.

==review==

photothermal noise arises from the fluctuation in the absorbed laser power (RIN + shot noise, mostly from RIN) on the mirror. The absorbed power heats up the coatings and the mirror. The expansion coefficient and refractive coefficients  convert thermal change into phase change in the reflected beam which is the same effect as the change of the position of the mirror surface.

Farsi etal 2012, calculate the displacement noise from the effect. The methods are

  • Solving heat equation to get temperature profile in the mirror.
  • Use elastic equation to calculate the displacement noise due to the temperature change (thermoelastic)
  • For TR, the effect is estimated from effective beta (from QWL stack) and the temperature at the surface ,as most of the TR effect comes from only the first few layers

When they solve the heat equation, the assume that all the heat is absorbed on the surface of the mirror. This assumption is ok for their case ( SiO2/Ta2O5) with Ta2O5 at the top surface, all QWL, as 74% of the power is absorbed in the first four layers (with the assumption that the absorbed power is proportional to the intensity of the beam, and all absorption in both materials are similar).

However, for AlGaAs coatings with (nH/nL) = (3.48/2.977) The E field goes in the coatings more that it does in SiO2/Ta2O5, see the previous entry. So we might want to look deeper in the calculation and make sure that photo thermal noise will not be a dominating noise source.

==calculation and a hand waving argument==

 The plot below shows the intensity of the beam in AlGaAs Coatings, opt4, and the estimated intensity that decreases with exponential square A exp(-z^2/z0^2). X axis is plotted in nm (distance from surface into coatings). The thickness of opt4 is about 4500 nm. To simplify the problem, I use the exponential decay function as the heat source in the diff equation. But I have not been able to solve this differential equation yet. Finding particular solution is impossible.  So I tried to solve it numerically with newton's method, see PSL:284. But the solution does not converge. I'm trying green function approach, but i'm still in the middle of it.

Int_cotings.png

 

However, the coatings optimized for TO noise should still be working. Evans etal 2008 discuss about how the cancellation works because the thermal length is longer than the coating thickness. The calculation (TE and TR)  treat that the temperature is coherent in all the coatings ( they also do the thick coatings correction where the heat is not all coherent, and the cancellation starts to fail at several kHz). So the clue here is that the cancellation works if the heat (temperature) in the coatings change coherently.

For photothermal calculation, if we follow the assumption that all heat is absorbed at the surface (as in Farsi etal), we get the result as shown in psl:1298, where most of the effect comes from substrate TE . In reality, where heat is absorbed inside the coatings as shown in the above plot, heat distribution in the coatings will be even more coherent, and the effect from TE and TR should be able to cancel each other better. Plus, higher thermal conductivity of AlGaAs will help distribute the heat through the coatings better.

This means that  the whole coatings should see the temperature change more coherently, thus allowing the TO cancellation in the coatings to work. The assumption that heat is absorbed on the surface should put us on an upper limit of the photothermal noise.

This means that photothermal noise in the optimized coatings should be small and will not be a dominating source for the measurement.

 

Attachment 2: Int_cotings.fig
  1370   Tue Oct 22 04:34:12 2013 taraDailyProgressSeismicnew table legs installed

After installing the table legs, I have been trying to measure the beat. However, there is an unknown scattered light noise up to 400 Hz. I'm still trying to fix that.

  Here are some bullets about what happened, I'll add the details later.

  • Extra noise that looks like scattered light goes up to 400Hz ( was around 100 Hz before, not from floating the table)
  • One of the air spring supporting the vac tank has a leak. But it is unlikely to be the source of the extra noise mentioned above.
  • The finesse of the cavities may be less than the designed value (10 000) because of the not so clean isoprop I used on the mirrors.

Note: check if the beams in the tank is blocked by wires or not.

  1367   Mon Oct 14 21:02:00 2013 taraNotesopticcoating optimization for AlGaAs:variation in x

I checked the dependent of coatings properties with the uncertainty in x (amount of Al in Al_x Ga_(1-x) As). The effect is already within the uncertainties in materials parameters we did before and will not be a problem.

G. Cole told us about the variations in Al contents in the coatings. Right now the values are 92% +/- 0.6%. 

(92.10, 91.43, 91.34, 91.57, 92.73, 92.67).  Although the deviation is small, the Al content does not always hit 92%, but 92+/- sigma%. So I decided to check the effect of x on the optimization.

The materials properties that change with x are heat capacity, alpha, beta, heat conductivity and n. The values of those as functions of x can be found on ioffee  except n. So I looked through a couple of sources ( rpi, sadao)  to get n as a function of x, (Note: E0 and D0 are in eV, they have to be converted to Joules when you calculate chi and chi_so).  GaAs (nH) has a well defined value ~ 3.48+-0.001, nL has a bit more uncertainty, but it is within the approximated standard deviation of 0.03 . The table below has numbers from the sources. For RPI, I use linear approximation to get nL for x = 0.92 @ 1064nm.

source nL(x=0.92) nH
G.Cole 2.977 3.48
RPI 3.00 3.48
Sadao 2.989 3.49
     

The dependent of n on x is about -0.578 *dx. The numbers from RPI and Sadao are about the same. This means that for the error of 0.6% in Al. nL can change by 0.578*0.006 = 0.0035. The number is almost a factor of ten smaller than the standard deviation of nL and nH I used in previous calculation (0.03). For examples,

  • x = 0.914, nL = 2.993,
  • x=0.92,     nL = 2.989
  • x=0.926    nL = 2.986  (From Sadao's fit)

This means that the uncertainty in nL/nH (+/- 0.03) we used are much larger than the effect coming from uncertainty in x. This is true for other parameters as well.

  1365   Fri Oct 11 15:23:54 2013 taraNotesopticcoating optimization for AlGaAs:electric field in coating layer

Electric field in coating layer is calculated. This will be used in loss calculation in AlGaAs coatings.

 

  • In each coating layer, there are two E waves, transmitted and reflected  waves. The two interfere and become an effective field.
  • The averaged electric field will depend only on the transmitted beam inside each layer, see the calculation.
  • The effective transmissivity can be calculated, for coatings with N layers between air and substrate, there will be an N+1 vector representing the effective transmission, called tbar in the code. This tbar(n) is the transmissivity in the nth layer, similar to rbar in Evans etal calculation.
  • The ratio of E field/ E input in nth layer will be tbar(1)*tbar(2)*...tbar(n)
  •  |E field/ E input |^2 of the final transmitted beam is the transmission of the coatings.  The numbers from this calculation agrees to the calculation from before.

==supplementary information==

1) average E field in layer is the transmitted E field in the layer.

avgE.jpg

 I attached a short matlab file for a simulation of the combined field. Ein in each layer will be the transmitted beam through the layers. For a value of r close to 1, we get a standing wave. Try changing the value of r in test_refl.m to see the effect

 

2) Calculation for the transmitted field in each layer

transE.jpg

I borrow the notation from Evns etal paper (rbar), the calculation code multidiel_rt.m is attached below. Note: the final transmission calculated in the code is the transmission from the coating to the substrate. To calculate the transmission to the air, multiply the last transmission by 2*n_sub/(n_sub + n_air) which is the transmission from sub to air. Since the thickness of the substrate is not known with the exact number, it will not be exact to the transmision calculated in GWINC or Matt A's code (which do not take the sub-air surface into account), but they will be close, because the reflected beam in the last interface will be small compare to those in the coatings.

 

==result==

Efield.png

The penetration of E field for QWL and different optimized coatings are shown here. The transmissions in the legend are calculated from MattA./GWINC and the values in the parenthesis are calculated from multidiel_rt.m which include the effect from the substrate-air surface. This makes the values in the parenthesis smaller (as more is reflected back and less is transmitted).

Attachment 3: test_refl.m.zip
Attachment 4: multidiel_rt.m.zip
Attachment 6: Efield.fig
  1363   Thu Oct 10 01:59:24 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

I recalculated the coatings properties, with the values of nH and nL to be 3.48 and 2.977. Note about each optimization is included here. Transmission plots are added in google spread sheet. I'll finish the calculation for E field in each layer soon.

Note about each optimized coating version: different versions were obtained from different cost functions, and different number of layers.

opt1

  • 55 Layers
  • T = 210 ppm
  • TO noise and transmission is too sensitive to the change in nH and nL
  • 1/4 cap of nH. I did not fix the cap thickness for other coatings. Since there is no reason to keep the thickness of the cap constant.
  • TO noise and transmission of this one changes a lot with uncertainty in nH/nL

 

opt3

  • 57 Laayers
  • T = 150 ppm
  • Transmission is still too sensitive to the change in nH and nL
  • TO noise/ transmission is less susceptible to change in nH/nL.
  • First layer is 0.1 lambda thick (~285 nm) I'm not sure if this will be a problem for a cap or not.

 

opt4

  • 57 Layers
  • T = 150 ppm
  • TO noise and Transmission are less sensitive to nH and nL
  • less amount of nL material, should be less sensitive to error in thickness control

 

opt5

  • 59Layers
  • T= 144 ppm
  • TO noise and Transmission are less sensitive to nH and nL
  • reflected phase is more sensitive compared to opt4
  • use less nL material
  • 0.1 lambda thick

Judging from TO noise level, Transmission and reflected phase, I think opt4 is the best choice for us. The structure consist of thick nH layers and thin nL layers. This is good for us in terms of thickness control.

 

  1360   Mon Oct 7 19:53:53 2013 taraDailyProgressSeismicnew table legs installed

New legs were installed.  The table is floated. The cables for signals/ power supplies will be reconnected later

 Evan and I replaced the old legs. I made sure that the leak was not in the connections and the tube. After the legs replacement, the air pump can reach 25 psi within ~25 minutes and the table can be floated.

The regulating valves are adjusted and the table is leveled.

newleg.jpg

 

  1359   Thu Oct 3 10:34:32 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

The new optimization is less sensitive to the values of refractive indices, but the overall error will not change much if other material parameters have the uncertainties as we estimate.

Summary: see update of error analysis in PSL:1356. The issues from the previous entry are cleared

  • I made sure that the monte carlo tests were correct
  • The new optimization (called opt4, and opt5) will make the TO noise level/Transmission less sensitive to nH and nL values. But with the current estimate of uncertainties in other parameters, the performance will be about the same to that of the original optimization (called opt1).

 

1) show error analysis

  1356   Thu Sep 26 23:25:40 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

I'm trying to find another optimization that is less sensitive to change in nH and nL. Here is a few thought and a few examples.

 ==problem==

We have seen that uncertainties (withing +/- 1%)in nH and nL result in higher TO noise (up to 10 time as much) in the coating. So we are trying to see if there is another possible optimized structure that is less sensitive to the values of n. We estimate the value of nH to be 3.51 +/- 0.03, and nL to be 3.0 +/-0.03. (The numbers we have used so far are nH/nL = 3.51/3.0,  while G.Cole etal use nH/nL = 3.48/2.977.

==Optimization method==

The algorithm is similar to what I did before[PSL]. But this time the cost function is taken from different values of refractive indices. The values of nH and nL used in this optimization are

  • nH = 3.48, 3.51, 3.54
  • nL = 2.97, 3.00, 3.03.

The cost function is the sum of the TO noise level at 100Hz, Transmission, and reflected phase, calculated from 9 possible pairs of nH and nL values. The weight number from each parameters (which parameter is more important) are chosen to be 1, as a test run. I have not had time to try other values yet, but the prelim result seems to be ok.

[Details about the codes, attached codes]

Note about the calculation,

The calculation follows these facts:

  • The nominal values of nH/nL are 3.51/3.00
  • The optical thickness is designed based on the above nH and nL
  • The optimized design is reported in optical thickness which is converted to physical thickness with the nominal values of nH/nL
  • The procurement of coatings control the physical thickness (with error in thickness discussed before PSL:)
  • If the values of nH/nL changes from the nominal values, this will affect in the coatings properties because of the change in optical thickness.

 ==results from  QWL (55layers) and 4 other optimized coatings.==

  1. Left plot shows  TO noise at 100Hz in m^2/Hz unit,
  2. Middle plot:Transmission [ppm]
  3. Right plot: reflection phase away from 180 degree.

Each plot has three traces (blue, black, red) for different values of nH (3.48, 3.51, 3.54). nL is varied on x-axis from 2.97 to 3.03. The first result is from QWL coating, with 55 layers. This serves as a reference, to see how much each property changes with the uncertainty in nH and nL.

   I tried to change the cost function in the optimization code and numbers of layer to see if better optimized structure can be done. The optimized structure (V3,4,5) seems to be less sensitive to the values of n, see below.

 n_check_QWL.png

Above: from QWL coatings, 55 layers. nominal transmission = 100ppm.  We can see that the transmission of QWL coatings is still quite sensitive to uncertainties in nH and nL.


n_check_opt0.png

Above: First optimization reported before, TO noise is larger by a factor of 10 in certain case, and transmission can be up to 500 ppm. This coating is very sensitive to the change in refractive indices.


n_check_opt3.png

Above: opt3, obtained from the code using the new cost function discussed above.  55 layers, nominal transmission = 150ppm. The TO noise is less dependent on nH and nL, but the transmission is still quite high.


n_check_opt4.png

Above: opt4, the weight parameter for transmission is changed to 3, 57 layers.


n_check_opt5.png

above opt5,the weight parameter for transmission is changed to 50, Lower/Upper thickness bound = 0.1/0.5 lambda, 59 layers


n_check_opt6.png

 Above: Opt6, the weight parameter for transmission is changed to 500, Lower/Upper thickness bound = 0.1/1.2 lambda, 59 layers


From the results, optimized structure # 3,4,5 seem to be good candidates. So I ran another monte carlo error analysis on opt1 (as a reference), opt3, opt4, and opt5, assuming errors in both material properties and coating thickness. Each one has 5e4 runs. Surprisingly, the results from all designs are very similar (see the plot below). It is possible that, by making the coatings less sensitive to changes in nH/nL, it is more sensitive to other parameters (which I have to check like I did before). Or the properties are more dependent on coating thickness, not material parameters (this is not likely, see psl:1345). Or perhaps, there might be a mistake in the monte carlo run. I'll check this too.

compare_error_ana.png

 

I'll update the coating structure and forward it in google doc soon.

Attachment 2: compare_error_ana.fig
  1351   Mon Sep 23 18:50:05 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

Quote:

 

If that's true, then it means that a 1% deviation in the index of refraction of the low index material can by a 10x increase in the TO noise. Is this really true?

 That surprises me too, but, that's what the calculation gives me. It is also strange that deviation in nH has smaller effect on to TO noise than nL does. I'm checking it. I ran the code one more time, and still got the same result.

Note: when I calculate the error in refractive indices, I assume that the physical thickness is constant = x * lambda/ n_0. Where x is the optical thicknesss. But if the the actual refractive index is not n_0, it means the optical length is not x, but x*n/n_0. I think this is a valid assumption, if they control the physical thickness during the manufacturing process.

 

update:Tue Sep 24 02:09:28 2013

compare_indices.png

The TO noise level does really change a lot when nL is nL + sigma (nL=3.0+ 0.03), dark green trace. Most of the change comes from TR noise level (not shown in the plot). TE noise remains about the same level.  It might be worth a try to find another optimization that is less sensitive to the change in value of n. I'll spend sometime working on it.

Attachment 1: compare_indices.png
compare_indices.png
Attachment 2: compare_indices.fig
  1345   Fri Sep 20 19:26:45 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

I'm using Matt's code to do error analysis for AlGaAs coatings. This time I vary materials' parameters and compare the thermo optic noise, reflected phase and transmission. There is no alarming parameter that will be critical in TO optimization, but the values of refractive indices will change the transmission considerably.

Eric, Matt and I discussed about this to make sure that even with the errors in some parameters, the optimization will still work.

Parameters in calculation and one sigma estimated from Matt

% Coating stuff
betaL = 1.7924e-4 +/- 0.07e-4; %dn/dT
betaH = 3.66e-4  +/-0.07e-4 ;
CL = 1.6982e6   +/- 5%  ; % Heat Capacity per volume
CH = 1.754445e6   +/- 5%;
kL = 69.8672   +/- 5%   ; % Thermal Conductivity
kH = 55           +/- 5%;
alphaL = 5.2424e-6 +/- 5%; % Thermal expansion
alphaH = (5.73e-6 ) +/- 5%;
sigmaL = 0.32      +/- 10%; % Poisson Ratio
sigmaH = 0.32     +/- 10% ;
EL = 100e9    +/-20e9; % Young's modulus
EH = 100e9    +/-20e9;
nH = 3.51  +/-0.03   ; % Index of refraction
nL = 3.0     +/-0.03 ;

 

* Note: when I change nH and nL value, I keep the physical thickness of the layers constant. This is done under the assumption that the manufacturing process controls the physical thickness. The optical thickness in the calculation will be changed, as the actual dOpt = physical thickness * actual n / lambda.  And averaged values of coatings will depend on physical thickness.

 This is fixed in Line 120-180

== Effect on TO cancellation from each parameters==

 First, I calculate the TO cancellation when one of the parameter changes. Some parameters, for examples, Poisson ratios, Young's moduli, are chosen to be the same for both AlAs and GaAs. In this test, I vary only one of them individually, to see which parameters are important. The numbers indicate the ratio between the PSD of TO noise with change in the parameter and the optimized TO noise . Not the standard deviation of the parameters.

params +sigma -sigma Note
BetaL 1.02 1.12  
BetaH 1.03 1.15  
Young L 8.0 1.77  A
Young H 8.3 1.8  A
Young HL 28.3 4.7  B
       
alpha L 1.54 1.2  
alpha H 1.19 1.53  
kappa L 0.979 1.023  
kappa H 0.975 1.028  
CL 0.99 1.0143  
CH 0.99 1.0137  
sigmaL   20.6  C
sigmaH   21.7  C
sigmaHL   84.14  B
nH 1.168 1.004  
nL 11.15 6.507

 

 

  • A) + value for Young modulus is 142 Gpa, and - value is 83 Gpa, the value in the section below is 100 +/- 20 GPa
  • B) Young's moduli and Poisson's ratios for the two materials are the same value in the calculation, Young HL row calculate the TO noise when both materials have the same value of Young's modulus, while YoungH and Young L row calculate the TO noise under the assumption that only nH material or nL material has different Young's mod.
  • C) + value for Poisson is the nominal value, and - value is 0.024  the value in the section below is 0.32 +/- 10%

 Turns out that the change in Young's moduli and Poisson's ratios are quite important.

==Effect on TO cancellation, from all paramerters==

 Then, I calculate the TO noise when all parameters vary in Gaussian distribution, similar to what I did before,all parameters are uncorrelated. The histograms from 1000 runs are shown below.

error_check_params.png

  1. Top, ratio of PSD of TO noise at 100Hz. The cancellation should still work well.
  2. Bottom left, reflected phase. It is still close to 180 degree.
  3. Bottom fight, transmission. The design is 200ppm, the result spread out in a big range from  10-500ppm.

I'll try more run overnight. Each run takes about 1 second.

== combined effect from errors in layer thickness and material parameters==

Since the comparison does not need to calculate the thermal fluctuations and finite size correction all the time, I cut that calculation out and save some computation time.  Now I compare errors from

  1. Error in both layer thickness and materials parameters (red)
  2. Error in layer thickness only (green)
  3. Error in materials parameters only (blue)
  4.  Error in refractive indices only (cyan)

Each simulation contains 5e4 runs.   The Transmission varies a lot depending on the material parameters ( mostly refractive indices,  see the cyan plot).

error_thick_params_compare.png

The cancellation seems still ok. Most of the time it will not be 10 times larger than the optimized one. Only the transmission that seems to be a problem, but this is highly depends on refractive indices. It's weird that the error makes the mean of the transmission smaller.

Attachment 2: error_check_params.fig
Attachment 4: error_thick_params_compare.fig
  1344   Thu Sep 19 20:38:17 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

Details for AlGaAs coatings order

  • Coating structure can be found in http://nodus.ligo.caltech.edu:8080/PSL_Lab/1340, 55 layers, T = 197ppm.
  • Coatings for 4 mirrors plane/concave, 1” diameter, 1/4” thick, with radius of curvature = 1.0m.
  • AlGaAs coatings will be applied on the concave side of the mirror.
  • Flat side is already AR coated
  • absorption loss 6-10ppm / scattered loss 3-4ppm
  • Spot radius (1/e^2 power) will be 215 um.
  • The mirrors have an annulus on the rim for optical contact with thickness ~ 3mm. This area should be kept clean.
  • The coating wafer should be inside the mirror sagitta to make sure that it will not obstruct the optical bond area. By calculation, the wafer with 8mm diameter, 4.5um thick should be ok. The maximum diameter that makes the coating to be above the sagitta is about 16mm, for 4 um thickness.
  • Required coating diameter = 5-8mm, Power loss due to clipping is less than 0.1 ppm, see below figure.

power_vs_mirror_size.png

Above, plot of ratio of power due to finite size mirror P(r) / P0,  P(r) is the power of the beam at radius r from the center. G Cole said that the wafer can be made to 8mm diameter. diameter between 5-8 mm should be good for us.

  1343   Thu Sep 19 18:09:18 2013 taraSummaryNoiseBudgetCoating Thermal Noise Calculator

Quote:

 

Tara noticed an accidental re-definition in my old code. I fixed it, and updated the svn. This fixes most of the discrepancies, but shifts the difference in thermo-optic to the low-frequency region.

Attachment 1 is the comparison from case 3 between mine and Tara's calculations of his optimized coating structure.

Attachment 2 is the comparison from case 2 between mine and Tara's calculations of a 55-layer 1/4-wavelength stack.

 I discussed the calculation with Matt. The error in TO noise is large because it is a fraction of something small. Mostly it comes from TE part. The error in TO noise appears large (10%-20%) because the TO level is small.  Otherwise, the rests are in good agreement, and I think we should be able to order soon. 

 Below, summary of the calculation, dTE is alpha_effective * coating thickness, dTO is beta effective * lambda. 0.2% difference in dTE and 0% difference in dTR can cause error upto 40% in dTO when dTE and dTR cancel each other really well. But this will be insignificant, since the final TO levels are still in the same magnitude.

  Matt Gwinc
dTE 8.161e-11 8.141e-11
dTR -8.11e-11 -8.11e-11
dTO (dTE+dTR) 4.87e-13 2.88e-13

 

The summary of the TO cancellation is in wiki page AlGaAs

  1342   Thu Sep 19 14:55:11 2013 taraSummaryNoiseBudgetCoating Thermal Noise Calculator

Quote:

Quote:
...
...
The attached figures show that the two techniques agree to better than 20% of the GWINC output, with most of the mismatch at higher frequencies. I'm not yet sure why that is. It could be due to my improved integration in calculating Evans' thick coating correction, it could be due to my using a different form of the equation for Braginsky's finite substrate correction for the thermo-elastic noise, or it could just be due to some minor differences in the precision of some of the input values. ..

 

 

 I checked the calculation. I think most of the discrepancies are from the thick coating correction calculation (from Evans etal paper). The error is frequency dependent, and the calculations that involve frequency dependence are temperature fluctuation and thick coating correction. The temperature fluctuations are the same from our results. So it is most likely the thick coating correction. I checked and the corrections did differ at high frequency.

 I need to take a closer look to tell exactly where the errors are. Since the error is small and only at high frequency (around the shot noise limit, 10kHz),  I don't think it will be a problem for us.

Tara noticed an accidental re-definition in my old code. I fixed it, and updated the svn. This fixes most of the discrepancies, but shifts the difference in thermo-optic to the low-frequency region.

Attachment 1 is the comparison from case 3 between mine and Tara's calculations of his optimized coating structure.

Attachment 2 is the comparison from case 2 between mine and Tara's calculations of a 55-layer 1/4-wavelength stack.

Attachment 1: ThermalUpdateEmbed.pdf
ThermalUpdateEmbed.pdf ThermalUpdateEmbed.pdf
Attachment 2: ThermalUpdateEmbed2.pdf
ThermalUpdateEmbed2.pdf ThermalUpdateEmbed2.pdf
  1340   Wed Sep 18 21:55:11 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

 

Optimized coatings structure.

 

Attachment 1: opt_coatings.mat
  1338   Tue Sep 17 19:43:45 2013 taraSummaryNoiseBudgetCoating Thermal Noise Calculator

Quote:
...
...
The attached figures show that the two techniques agree to better than 20% of the GWINC output, with most of the mismatch at higher frequencies. I'm not yet sure why that is. It could be due to my improved integration in calculating Evans' thick coating correction, it could be due to my using a different form of the equation for Braginsky's finite substrate correction for the thermo-elastic noise, or it could just be due to some minor differences in the precision of some of the input values. ..

 

 

 I checked the calculation. I think most of the discrepancies are from the thick coating correction calculation (from Evans etal paper). The error is frequency dependent, and the calculations that involve frequency dependence are temperature fluctuation and thick coating correction. The temperature fluctuations are the same from our results. So it is most likely the thick coating correction. I checked and the corrections did differ at high frequency.

 I need to take a closer look to tell exactly where the errors are. Since the error is small and only at high frequency (around the shot noise limit, 10kHz),  I don't think it will be a problem for us.

  1332   Tue Sep 10 04:53:34 2013 taraDailyProgressNoiseBudgetRIN induced TO noise in beat

The cause of the peaks around 1kHz in RIN is solved, PMC is the reason. After damping it, the peaks disappears.

Short notes from tonight measurement:

  • RIN in ACAV is better after PMC is damped, no peaks around 1kHz anymore.
  • The peaks in the beat measurement also disappear, so we really see photo thermal noise.
  • PD for RIN measurement behind RCAV is added, RIN is measured.
  • I Will add the effect from RIN and compare with beat measurement soon.

need to buy:

  •    new PBS for PDH locking. the one for RCAV is not good because there is an unwanted reflected beam going to the RFPD.
  •  other optics for EOAM for RCAV
  1331   Mon Sep 9 21:19:24 2013 taraDailyProgressNoiseBudgetRIN induced TO noise in beat

I'm trying to understand the measured RIN in the setup. The evidence suggests that the measured RIN in 100Hz- 6kHz, is real intensity noise and not associated with alignment + jitter.

==Problem==

As seen in PSL:1329 that we might be limited by RIN at high frequency, I tried to figure out what cause the shape of the RIN around kHz to be mechanical -like peaks. So the problem can be minimized, and does not have to rely on ISS that much.

==Assumption==

My assumption was that they were from mirror mounts, because

  1. the frequencies were close to mechanical resonances of the mounts, see PSL:818, PSL:824 for examples. The power coupled into the cavity would reduce, and thus causing the peaks in the RIN measured behind the cavity. And
  2. the shape changes during the time of measurement. During the day, the shape is like a big bumb, while during the night, around 2-3am, the level is smaller and the individual peaks shows up instead (may be because of the lower seismic)

==Measurement==

So to test this, I measured RIN before and after ACAV (NOTE:ACAV path has PMC in it),  when

  1. The beam was well aligned to the cavity (DC from REFL PD =94mV, total level ~ 1.6V)
  2. The beam was misaligned a bit (DC from REFL PD = 175mV)

front_ACAV.jpg

above, beam path in front of ACAV, before the beam enters ACAV. The PD for RIN measurement is circled in blue.

benid_ACAV.jpg

above, beam path behind ACAV.

If the measured RIN was from the jitter, RIN after the cavity should change with the alignment, and RIN before the cavity should not change much. I made sure that the spotsize on both PDs are significantly smaller than the PD to make sure that any jitter in front of the cavity should not change the power level that much.

RIN_ACAV.png

==comments about the result==

  • The result agrees with the assumption at low frequency (DC - 30Hz),
  • However, from 100Hz and above, the measured RIN from four cases are very similar.
  • The measured coherence between the two PDs are similar in both cases (aligned and misaligned), I plotted the one from the misaligned beam. It shows that at low frequency, RIN behind and after the cavity are not caused by the same mechanism, the one behind PD might suffer more from jitter. However, at 100Hz and above, anything observed before the cavity is seen behind the cavity as well. This rules out the assumption that the alignment change due to the motions(resonant peaks) from optics.
  • As a comparison, I plotted RIN measured around 3am in brown trace, see PSL:1329, the level is smaller than those measured in the evening. It still makes me think that it is related to seismic but not alignment. I have to think about what other seismic driven mechanism might cause this intensity noise.
  • The level of RIN seems to change as well, I looked at Evan's measurement, earlier today[/PSL:1330]. The bump around 1kHz is ~ 2e-6 1/rtHz, while for my measurement, it is close to 1e-5.  I'll try to investigate more to find out what change the level of RIN.

==To do next==

  • Pick up the beam some where before PMC to check the RIN level and see where the peaks occur
  • Install PD behind RCAV, see RIN from RCAV
  • update noise budget: add RIN induced noise from ACAV and RCAV.
Attachment 2: RIN_ACAV.fig
  1329   Mon Sep 9 02:27:46 2013 taraDailyProgressNoiseBudgetRIN induced TO noise in beat

The measured RIN is measured and converted to frequency noise via photo thermal effect then compared to beat. The effect seems to be significant now since we lost the common mode rejection.

I measured RIN after ACAV (there is only one PD behind ACAV right now. we will add another one for RCAV soon). The magnitude is comparable from what we measured before but the peaks seem to change, see PSLPSL:1326, :PSL:1308(8"cavity) PSL:742 .

ACAV_RIN.png

The peaks around kHz  are more clear. I'm not sure where they are from, but I think it is associated with vibration on mirror mounts that causes beam jitter. Because the peaks look like mechanical peaks, and this time the cavities are shorter, the beamsize is smaller from 8" cavities, the same beam misalignment will cause the power coupled into the cavities to change more compared to that of 8" cavity. We can check that by mis-aligning the input beam a bit, and see if RIN becomes larger or not.

The coupling from RIN to frequency noise is discussed in PSL:1328

I applied that to the measurement and here is the result. Note, only the effect from one cavity (ACAV) is taken into account.

beat_2013_09_06.png

The peaks seems to match up, especially around 20-30Hz and around 1kHz, see the zoomed in picture below. This makes me think that we might be limited by RIN noise now.

rin_noise.png

 

To Do next:

  • Install ISS system on RCAV (PD behind the cavity / EOAM)
  • Re-measure the coupling from RIN to frequency noise of the cavity
  • Measure RIN and apply it to the noise budget.
  • Find out what causes RIN to change
Attachment 2: rin_noise.fig
Attachment 4: ACAV_RIN.fig
Attachment 5: beat_2013_09_06.fig
  1327   Sat Sep 7 04:29:32 2013 taraDailyProgressNoiseBudgetbeat

Short note from tonight measurement:

1) scattered bump from dc to 100Hz is mostly from seismic. It is worse during the day. It gets smaller at around 3-4 am. Unless we have a better seismic isolation, we might not be able to see anything below 100Hz.

2) RIN shape from RCAV changes, reasons still unknown. (DC level 0.7 V)

3) I might see the effect from RIN induced TO noise at frequency ~ 1-3 kHz. (compare RIN and beat).

I'll get into details tomorrow.

  1322   Mon Sep 2 18:31:46 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

Coating optimization and error analysis are updated, see PSL:1320.

  1321   Mon Sep 2 03:38:27 2013 taraDailyProgressNoiseBudgetbeat

It's a quiet night, so I went down the lab to measure the beat signal. We are getting close. I think I have to review my noise budget calculation and estimate the error in the measurement carefully.

So after a few things Evan and I did a few days ago:

  1. rotate the stack to get rid off the reflected beam from the window
  2. fix the insulation so the beam is not clipped on the opening.
  3. add more modulation depth to RCAV path with the EOM driver (tuned to 14.75MHz)
  4. Minimize some RFAM, by rotating the half wave plate in front of the sideband EOM

Then I measured the beat signal.

We reduce some noise from scattered light at frequency below 100 Hz, we are limited by some white noise at high frequency ~ above 1 kHz.

beat_2013_09_02.png

fig1: measurement vs noise budget

zoom_beat_2013_09_02.png

fig2: zoom in. The slope of the measured signal agrees well with the slope of thermal noise.

ToDo

  • Estimate/measure shot noise PD noise and electronic noise in the setup. See if they match up with the measurement.
  • Review the noise budget calculation. Looking at the slope of the signal around 1kHz, I think the calculated brownian noise is lower than what it should be.
  • noise hunting, seems like scattered light at frequency below 100Hz. There are many mechanical peaks, and harmonic lines at higher frequency.
  • add the contribution from RIN induced TO noise in the nb.
Attachment 1: zoom_beat_2013_09_02.png
zoom_beat_2013_09_02.png
Attachment 3: nb_short_cav.fig
  1320   Sun Sep 1 18:38:37 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

I updated the optimization and error analysis. The error in optimized structure is comparable to that of a standard quarter wave length structure.

      After a discussion with Rana, Garrett, and Matt, I fixed the thermo-optic calculation, and the error analysis done in PSL:PSL:1315.  The modifications are

       1)  fix the TO calculation (Yamamoto TR correction): There is a modification for TR correction that is not in Evans etal 2008, paper. I contacted M. Evans to ask about the details of this correction which is done in GWINC.  

       2)  Try another optimized coatings with the correct TO calculation:  After the correction, I ran doAlGaAs.m code, cf PSL:1269  using fmincon function , to find another optimized structure. The result is shown below.

2013_09_01_opt_nbv2.png

above) layer structure in optical thickness, the .fig and .mat file are attached below. Note .mat file contains 54 layers, you need to add 1/4 cap to the first entry to calculate the noise budget.

  2013_09_01_opt_nb.png

above) noise budget of the optimized coating.

       3)  Repeat the error analysis : This time I used the following assumptions (from G Cole)

  • the error is not random among each layer
  • the error is constant in each layer type, ie all the layers from the same material (nH or nL) have the same percentage of error,
  • error from nH and nL have the same sign. If one is thicker, another one is thicker, but the magnitude are uncorrelated.
  • nH (GaAs) has better thickness control with 2sigma = 1percent, while nL(AlGaAs), has 2sigma = 2 percent.

error_dist.png

Fig1: Above, percentage of error distribution between the two materials used in the calculation. nH(red) has 2 sigma = 1% and nL(blue) has 2sigma=1%.The same error distributions are used for both optimized layers and QWL layers in comparion, see fig2.

The section below is the algorithm used to distribute the error, this one makes the error between the two materials to be the same sign. The whole code can be found on svn.

mu1 = 0;
sigma1 = 0.5;  % 2sigma is 1percent;
mu2 = 0;
sigma2 = 1;

run_num = 5e4; % how many test we want

errH = normrnd(mu1,sigma1,[run_num,1]);  %errH in percent unit
 
errL = normrnd(mu2,sigma2,[run_num,1]);  %errL in percent unit   
errL = abs(errL).*sign(errH);                        %make sure that errH and errL have the same sign

dOpt = xout;             % xout from doAlGaAs (optimized layer)
dOpt = [ 1/4 ; dOpt];    % got 54 layer no cap from doALGaAs, need to add the cap back

dOpt_e = zeros(length(dOpt),1);


  for ii = 1:run_num;

dOpt_e(1:2:end)= dOpt(1:2:end)*(1+ errH(ii)/100 );
dOpt_e(2:2:end)= dOpt(2:2:end)*(1+ errL(ii)/100 );

 

 

===Result==

This time I calculated the change in reflection phase (TOP left), the ratio between TO noise from the coatings with error and the coatings with no error(top right), transmission (bottom left), and ratio of BR noise ( bottom right). The result from the optimized coating(blue) is compared with the QWL coating (black).

 error_compare_opt0901v2.png

Fig2: Error analysis, in 5e4 run. Blue: from optimized coatings Black:from 55 QWL coatings, from 5x10^4 runs.

Reflection phase: The reflection phase can be away up to ~6 degree. The power at the surface will be ~Finesse/pi * Power input * sin^2 (6degree) ~ 50mW. Seems high, but this is about a regular power used in the lab.

Ratio of PSD TO/TO_0 : At worse, it seems the PSD TO noise will be ~ a factor of 10 higher than the design. However, this will be still lower than BR noise. I plotted only the error distribution for optimized coatings because for QWL coatings, the ratio will be about the same, since TO is dominated by TE.

Transmission: Most of the results are within 197-200 ppm. The optimized coating has transmission ~ 197ppm. The QWL with 55 layers has transmission ~100ppm.

Ratio of BR: not much change here.

 

Attachment 2: error_compare_opt0901v2.fig
Attachment 6: 2013_09_01_opt_nbv2.fig
Attachment 7: 2013_09_01_200ppm_54v2.mat
  1319   Thu Aug 29 13:25:49 2013 taraDailyProgressVacuumpumping down the chamber

The turbo pump is removed, and the ion pump is on. The initial value is ~7mA.

I removed the turbo pump and turn on the ion pump, see the procedure on wiki page. The initial value on the ion pump is ~ 7mA, similar to the last time we opened the chamber although this time I left the turbo pump on 4 days instead of 2 days. So I think this is the limit of this turbo pump.

 

  1318   Wed Aug 28 21:21:38 2013 taraNotesopticGWINC for TO calculation: recap

Here is a summary for how I verify the codes for TO calculation.

So far, we have been using a set of modified GWINC codes to calculate TO noise, but I have not mentioned how did I make sure that the codes were reliable. So I'll try to explain how I check the codes here.

==What do we compute?==

  For the TO nosie calculation and the optimization, we are interested in:

  • effective dn/dT (TR coefficient) of the coatings
  • effective alpha (TE coefficient) of the coatings
  • total reflectivity of the coatings (including the phase), and transmissivity

==Beta calculation check==

For TR coefficient we can compare GWINC with an analytical result (see Gorodetsky,2008, and Evans 2008) (when # of layers ~ 50 or more), see psl:1181. I tried the solution with nH, 1/4 cap and nL, 1/4 and 1/2 cap. All results agree.

==Alpha calculation check==

 There is no complication in this calculation. The effective alpha is just the sum of all layers. This calculation is quite straight forward.

==reflectivity check==

   This was done by reducing the coating layers to one or two layers and comparing with an analytical solution by hand. I checked this and the results agreed.

 

So I think the calculations for TO noise is valid, the noise estimated from the optimized coatings is done with some error check (previous entry). I think we should be ready to order.

  1317   Wed Aug 28 20:19:44 2013 taraNotesEOMEOM driver: modification

We modified the EOM driver, so that the resonant frequency is now~ 14.75MHz. The full test will be done later.

As mentioned in PSL:1311, the resonant frequency on the EOM driver was not at 14.75MHz. Evan and I discussed about how to modify it and decided tof change L4 from 1.4uH to 3 uH, see the schematic here.

 

driver1.jpg

driver2.jpg

above, the driver after the inductor was replaced. The new one has a shield to reduce any magnetic field leakage. The legs are not fit with the footprint on the PCB, so I had to solder it to another wire to reach the footprint.

 

driver_TF.jpg

above: the TF of the driver measured between the drive and the mon output. Red trace shows the TF before the modification. Yellow  trace shows the TF after the modification, notice the peak is at 14.75MHz, the Q is about the same.

  1315   Tue Aug 27 16:11:26 2013 taraNotesopticcoating optimization for AlGaAs:error analysis

Since the optimized layer structure is designed, I'm checking how the coatings properties change with error in layer thickness.

G.Cole said that they can control each layer thickness within 0.3%. So I tested the optimized coatings properties by adding some random number within +/- 0.5% on each layer thickness. The results are shown below for 10 000 test.

The error check does the following:

  • start from the optimized coating structure reported in PSL:1291.
  • add random thickness to each layer, within 0.5% of each layer
  • calculate the values of interest, then histogram them.

The figure below is an example of the varying layer thickness added by rand command. They are confined within 0.5%.

layer_error.png

 1) result from the error in thickness control

error_analysis_0.5percent.png

Above: histograms of the important values. top left, reflected phase. top right, ratio between PSD of Brownian noise and Thermo optic noise at 100 Hz. Bottom left, transmission. Bottom right, total coating thickness error.

 

 comments: this test is chosen for 0.5% error which is almost a factor of 2 worse than what they claimed (0.3%), so the actual result should be better. I assumed 0.5% errof because of the irregular layer structure of the optimized coatings, there might be some more error in the manufacturing process.

  • Reflected phase: we want the reflected phase to be close to 180, so that the E-field at the coating surface is close to 0. more than 50% of the results are within 179.5degree, this means that the power build up will be ~ Finesse/pi * Power input * sin^2 (0.5degree)  ~ less than 0.4 mW, so there should be no problem about burning at the surface.
  • ratio between PSD of Brownian/Thermo optic noise. This plot imply how well the cancellation works. Since Brownian noise will almost not change (both materials have the same loss, total thickness varies less than 1%), the ratio of Br/TO noise (at 100Hz) tells how much TO cancellation is. From the histogram we are quite sure that cancellation will work most of the time.
  • Transmission is good around 200+/- 10ppm this is ok with the requirement.
  • total physical error is ~5nm while the coatings thickness is ~ 4um. so the total error is <0.1% Brownian noise calculation will not change much.

2) result from different calculated Beta values:

Here I checked what happen if the beta calculation was wrong, and the error is still within 0.5% in each layer.

In Evans paper, the effect from "Thermo-refractive" comes from the phase changes of the wave travels in each layer. So it includes the effect from dn/dT and dz. The effective beta for each layer is given as

evanB8.png[evan B8],

where alpha bar is

evanA1.png[evans A1]

Where s denotes substrate, k denotes the material in each layer (high or low indices).

So my, calculation & optimization have been using these equations.

However, in the original GWINC code for TO calculation, the calculation [B8], alphabark( used in dTR) is not the same as A1, but rather.

alphaH * (1 + sigH) / (1 - sigH)

see getCoatLayerAGS.m.  Line 16-17.

This is used in the calculation for beta effective in getCoatTOphase. Line73-74. Notice that for dTE, the alpha_bar_k is the same as used in Evans. (line 77).

the comment says "Yamamoto thermo-refractive correction". I emailed kazuhiro yamamoto, but never got a response back. So I keep using the same formula as in Evans because I don't see the reason why the expansion contribution should be different between TE and TR.

So this is the nb plot for TO noise from the optimized coating, if using yamamoto TR correction.

yamamoto_TR_correction.png

Above: nb from the optimized coatings, using Yamamoto TR correction. The cancellation becomes worse, but TO is still lower than other noise.

 

Finally, I repeat the same error analysis for random noise in the thickness (+/- 0.5%).

 yamamoto_error.png

Most of the parameters behave similarly, except the cancellation (upper right plot). Now BR is only ~ x12 larger than TO noise because of the worse cancellation. Good news is, it still below Brownian noise, the cancellation still somehow works.

 

=summary=

  • From the optimized coating structure (T=200ppm), thickness control within 0.5% in each layer will make the coating work as expected.
  • The yamamoto TR correction is still an unresolved issue, but the optimized coating will still work.
  • we should be ready to order soon.
Attachment 2: error_analysis_0.5percent.fig
Attachment 6: yamamoto_TR_correction.png
yamamoto_TR_correction.png
  1313   Sat Aug 24 15:42:54 2013 taraDailyProgressVacuumpumping down the chamber

I closed the chamber. The turbo pump is on and pumping down.

 I realigned the beams so the visibilities for both cavities were 80% or more. This made sure that the beams' path would be close to the optimized path.

Now, the window reflection won't overlap with the cavity reflection, and can be dumped properly.

Note about a few things to do:

  • The beam holes on the foam might have to be fixed, the beams slightly clip at the openings. I have to check if the beams are clipped at the periscope or not.
  • modification of the seismic stack as suggested by Koji. The teflon pieces at the bottom plate are not screwed down to the stack making it hard to put the stack in the chamber. I think this should be fixed after the SiO2/Ta2O5 measurement is done and we have to reopen/ installed AlGaAs cavities.
  • There is some strayed beam from the PBS in RCAV path. This is from left over beam in S-light that reflected off, and bounced back at the PBS surface before going to the PD. This might have to be fixed too.

FYI for torque wrench setting for CTN cavity:

Quote:

cf_torque2.pdf

 The CTN cavity is 10" OD,  the Torque required is 190 InchPound.

  1311   Thu Aug 22 20:31:28 2013 taraDailyProgressNoiseBudgetinstalled EOAM

The bump at 2kHz in the beat signal that I saw before was also from RFAM. By adjusting the 1/2 waveplate in front of the sideband EOM, the bump disappears. I still don't understand why adjusting the EOAM can reduce the bump from RFAM.

 As I planned to add the eom driver to the BB EOM for sideband in RCAV path, I wanted to see the improvement without worrying about the EOAM optimzation. So I removed the EOAM, but I still saw the bump I observed before. This time it came from the RFAM. By adjusting the wave plate to match the polarization of the input beam to the EOM axis, the bump is gone.

RFAM.jpg

above: From right to left, 1) laser for RCAV, 2)&3) 1/2 and 1/4 wave plates, 4) lense, 5) Faraday isolator, 6) 1/2 wave plate, 7)BB EOM for frequency locking, 8) BB EOM for side band, the EOM driver is attached to the side (in aluminum foil wrapped box). RFAM is minimized by adjusting (6) 1/2 wave plate.

I added the EOM driver, however it was not yet modified for 14.75 MHz, so the amplification is small, see PSL:1234 . After adjusting the phase of the demodulated sigmal, the error signal slope is increased by a factor of 2. Then I remeasured the beat signal, and the beat was better by ~ a factor of 2 at high frequency. So I think now the signal is gain limited (in RCAV loop) at high frequency. This makes me confused why the error noise from RCAV loop does not match the beat signal in PSL:1307. I have to re check my work.

 nb_short_cav.png

The next few things to do are:

  • minimize RFAM (by temp control on both EOMs )
  • re-install EOAM in RCAV path, think about alignment
  • now scattered light at low frequency might come from seismic noise as well. I'll order the new floating table legs soon.
  • check other noise limit to make sure that it will not be dominating (shot noise, electronic noise)
  • modify the EOM driver, so that we have more gain in RCAV path.

 

 

Attachment 2: nb_short_cav.fig
  1310   Thu Aug 22 13:36:19 2013 taraDailyProgressNoiseBudgetTransfer Functions (RIN to Frequency noise via photothermal)

 I went through all the code with Evan and found another mistake. This time the code should be correct, and the result is close to what we measured a year ago.

 The calculation in PSL:1014 is wrong. There should be no square root for the absorption power (Finesse/pi * absorption).  With that correction, and an assumption of absorption of 18ppm in the mirrors (9ppm on each) with Finesse of 7000, see PSL:425. The result matches with the calculation quite well.

fasi_2013_08_22.png

The validity of this result depends on the absorption factor and cavity finesse. The finesse was measured, but the absorption measurement has never been done. So it might be good to think about how to measure that.

We did the same measurement with the current ACAV 1.45" cavity. Evan will post the result later.

Attachment 2: Farsi_compare.fig
  1309   Thu Aug 22 00:29:32 2013 taraDailyProgressNoiseBudgetinstalled EOAM

I installed an electro-optic amplitude modulator (EOAM) in RCAV path. Better optimization will be needed to reduce extra noise.

 EOAM.jpg

above, the setup for ISS actuator, the first 1/2 wave plate rotates p-beam to s-beam, EOAM, 1/4 Wave plate that tuned so that the output beam is 45 degree so the power transmitted through the final PBS is reduced in half.

 

After the EOAM was added, I checked the beat noise and saw a bump at ~ 2 kHz, see the figure below(blue plot). This was from the EOAM even though there was no input drive. It disappeared after I changed the EOAM position by rotating it a bit( yellow plot). I have not finished with optimizing it yet. I'm thinking about what kind of mechanism that causes the noise here.

 

nb_short_cav.png

  1307   Tue Aug 20 20:10:01 2013 taraDailyProgressBEATnoise hunting

Noise hunting is in progress. Today I identified that scattered light from the window is one of the problem.

I spent sometime making sure that all the beams in the input optic and the beat areas were dumped properly. I also tightened all the screws on the optics and the mounts on the table.

I mentioned in the previous entry that for RCAV, the reflected beams from the cavity and the vacuum window overlapped a little bit. The window beam was much smaller and actually closer to the edge of the main beam, so I used an iris to remove the outer path, and let only the beam in the center area go through to the RFPD. With that I could increase the gain in RCAV loop to Common/Fast = 624/750, where they used to be ~ 600/600 before. The iris might introduce some extra scattered lights, since it clips a part of the beam.

The scattered noise around DC to 100 Hz is reduced a bit, see the below figure. However, not much improvement in the flat region (100Hz and above). Plus, some mechanical peaks around 1kHz appear with higher level than before.

 beat_2013_08_20.png

I expected the scattered noise will be even lower if the cavities are tilted a bit to avoid the beams overlapping. At higher frequency, it might be the gain limit from RCAV loop where the modulation depth is very small.

Next thing to do is to increase more power in the modulation depth for RCAV.

==Note==

I found out that the sidebands in the beat signal mentioned in the previous entry changed with the gain of the TTFSS (both ACAV and RCAV). With higher gain, the sidebands are suppressed more. It might have to do with the PZT resonant of the NPRO. 

  1305   Fri Aug 16 22:05:27 2013 taraDailyProgressBEATnoise hunting

 Noise hunting is in progress, I checked the error noise from ACAV and RCAV loops and compared them to the beat. The beat is about an order of magnitude higher than the sum of error noise.

 NOte: slope of error signal RCAV = 1.57 MHz/V (13 dBm from Marconi, throug 4-way splitter, to BB EOM, 1mW input power).

 nb_short_cav.png

ABOVE: beat signal in comparison with noise at error points from ACAV and RCAV loops. The beat signal is about an order of magnitude higher than the error noise.

 

I'm working on optimization and noise characterization of the setup. Before measuring the beat I have to make sure that:

  • The beams to the cavities are aligned
  • The power input is 1mW for both cavities
  • I aligned the polarization of the beams into EOM for side band ( minimizing RFAM)
  • The gains for TTFSS are adjusted and recorded
  • Beams in the beat setup are aligned, and dumped properly.
  • The PD is not saturated.
  • PLL is setup properly.

I think the gain in the TTFSS is the problem. For ACAV, the scattered light from the window interferes with the main beam and causes the loop to oscillate when the gain is up. For RCAV, the EOM is a broadband one and does not have enough gain. The bump in the frquency lower than 100Hz is probably the contribution from scattered light. I have not properly dumped all beams yet.

 

Also I noticed that the beat signal has weird sidebands at +/- 100kHz, 200kHz, and 300kHz, see the figure below. I'm not sure why, I have not seen it before. I might saturate the PD making it distorted from a perfect sine wave. I'll investigate this.

beat_psd.jpg

Attachment 2: nb_short_cav.fig
  1300   Fri Aug 16 04:35:58 2013 taraDailyProgressNoiseBudgetTransfer Functions (RIN to Frequency noise via photothermal)

I estimated the requirement for laser RIN for AlGaAs coatings. The result is a factor of 5 more stringent from what we need for SiO2/Ta2O5 cavity.

See some calculation about RIN requirement PSL:1270.

I estimated the RIN induced TO noise in AlGaAs cavities. Due to the TO optimization, the effect will be small and we will see only the effect from the substrate, see RIN induced noise estimate.

RIN_req_algaas.png

 This will be quite serious, if we do not have a good ISS, since we will not have common mode rejection like what we had with the single laser setup anymore. I'll look up what was the RIN performance we had before.

Attachment 2: RIN_req_algaas.fig
  1298   Tue Aug 13 21:45:51 2013 taraDailyProgressNoiseBudgetTransfer Functions (RIN to Frequency noise via photothermal)

I rechecked the TF between power fluctuation and frequency noise in beat measurement that I did last year. The estimated result agrees more with the measured result. This can be used to estimate the requirement for ISS for SiO2/Ta2O5 and AlGaAs coatings.

The calculation is taken from Farsi etal 2012 (J. Appl. Phys. 111, 043101), and compared with the measurement from 8" cavities, SiO2/Ta2O5 QWL with SiO2 1/2 wave cap. The code I wrote before has several mistakes, so I fixed them.

Mistakes in the original code:

  1.  Beta effective was for 1/4 cap of nL: I changed it to the right one (1/2 cap of nL). This can be done by GWINC or an analytical result.
  2.  Cut off frequency ws, wc in the paper, I divided by a factor of 2*pi make them in Hz.
  3. Missing a factor of imaginary in thermoelastic in coatings calculation.
  4. r0 in the paper is where the power is dropped by 1/e, so r0 = w0/sqrt(2) where w0 is the radius of the beam when the power is dropped by 1/e^2.

farsi_2013_08_13.png

Above: Measurement(purple) from SiO2/Ta2O5 coatings and analytical result (cyan) in comparison. Finesse = 7500 (old ACAV), absorbtion = 5ppm.  The slope at high frequency seems to be real TO noise. Notice that phases from TE and TR have different sign and cancel one another.

 

==for TO optimized AlGaAs coatings==

RIN_TO_algaas.png

Above: Calculation for RIN induced thermo noise for optimized AlGaAs coatings in Hz/Watt unit. The calculation is for 200 ppm transmission,-> Finesse ~14 000. 1.45" cavity. The cancellation in coatings will reduce the noise. The estimated effect is plot against the measurement from 8" cavity, T=300ppm, SiO2,Ta2O5 cavity.

We might have to make sure that RIN is small enough, since this time we will have no common mode rejection like what we had with just a single laser. I'll add the estimated requirement later.

Attachment 2: farsi_2013_08_13.fig
Attachment 4: RIN_TO_algaas.fig
  1293   Sat Aug 10 00:49:20 2013 taraNotesNoiseBudgetThermo-refractive noise in substrate

I wrote a code to calculate thermo-refractive noise in a finite-sized cylindrical substrate as given in Heinert etal 2011. The noise is very small ~10-7 [Hz/rtHz] compared to other noise in the cavity ( no surprise here). The code can be used to estimate the TR noise in fiber optic. The calculation should be correct as I double checked with the calculation by Koji and Deep.

 

I followed the calculation for TR noise in cylindrical substrate [Heinert etal 2011] for our setup (1" diameter , 0.25" thick, fused silica). The result is in [m/rtHz].

To convert it to frequency noise of the laser:

  1. Convert the displacement noise to phase noise in the beam first,  Sphi = Sx * 2*pi*n/lambda_0  (n is index of refraction).
  2. Sf = Sphi * f   (f is fourier frequency), multiply by 4 to get the contribution from 4 mirrors.

 TR_sub.png

above: TR noise in substrate. It just so small compared to other noise sources in the noise budget(~ 10-3 - 10-1 Hz/rtHz level that I don't see the need to add it in the complete noise budget.

Since we will use the same substrate, the noise level will be the same for short and long cavities. The different in beamsize will vary the noise level a bit.

Note: this calculation is for a Gaussian beam profile in a cylindrical substrate, to use this calculation for  fiber optic TR noise, some assumption about the mode of the beam is required.

Attachment 1: getTRsub.m.zip
Attachment 3: TR_sub.fig
  1291   Fri Aug 9 17:58:01 2013 taraNotesopticcoating optimization for AlGaAs

Better TO optimized coatings calculation is done. Now the Transmission, phase reflection, and TO noise are optimized.

From previous elog, these are explanation about the optimization codes.

Quote:

The codes for optimizing Thermo-optic noise in coatings are up on svn.

I adopt some codes that have been on svn for awhile and modified them for AlGaAs coatings. There are two main codes

.......

    2) optAlGaAs.m

        This file is the modification of optETM.m found in ../iscmodeling/coating/AlGaAs/optETM.m .It calculates the reflectivity and the TO coefficients from the given layer structure. The modifications are:

  • (Line41-45) Layer structure, the cap start with nH. The material for substrate is SiO2 with nsub = 1.45.
  • (Line60) Desired transmission, as a test, I chose 200 ppm.
  • (Line88) Calculation for TO coefficients (StoZ), I switched from getCoatThermoOptics.m to getCoatThermoOPticsAGS.m. Codes with AGS suffix in /GwincDev folder are fixed for AlGaAs coatings structure. This code calls many functions in /GwincDev folder.

.......    

 So optAlGaAs.m calculates a parameter y which is the cost function that is minimized in fmincon in doAlGaAs.m code.  Originally the cost function y includes the difference between the expected transmission and the transmission from the given layer, and the level of TO noise which are:

y = [(T - <T>) / <T>]^2   + sTO (f0).   The goal is to minimize y.   Where

  • T = transmission of the mirror with the optimized layers
  • <T> is the required Transmission
  • sTO(f0) is TO noise at f0
  • Each effect is weighted differently

This cost function does not care about the total phase of the reflected beam. T is the absolute value of the transmission, so the information about the phase is removed, and the optmized coatings calculated from this cost function won't have phase close to 180 degree. The previous result showed 180-1.2 degree.

So I added the phase of the reflection in the cost function, with appropriate weight, and ran the optimization.

==Phase calculation==

rCoat is the reflectivity of the coatings, by using atan(imag(rCoat)/real(rCoat)), we obtain the phase of the reflectivity. I tried to you atan2(y,x) to get the phase of 180, but it does not work well with the optimization. I'm not sure why. So I use atan function, and check the value of rCoat after the optimization to make sure that rCoat is close to -1 + 0i. The result is shown below.

TO_opt_200ppm_layer.png

above: the layer structure, optimized for 200ppm, y axis is in unit of lambda in the layer. The first layer is the 1/4 wave cap, the last layer is the layer just before the substrate.

TOoptimized_2013_08_09.png

above: noise budget for the optmized structure, the reflection phase is 180- 1e-6 degree.

 The layer structure is attached below in .mat format. Note: the structure does not include 1/4 cap on top.

== summary of the modifications of optAlGaAs.m==

  • (line 90 - 95) add calculation of the phase of the reflectivity
  • line 97 the cost function includes phase of the reflectivity that is close to 180 degree (r is close to -1 + 0i). The weigh functions  from TO noise/transmission/phase are chosen so that each factor are about the same, and the result looks reasonable ( coating thickness ~0.1 - 0.3 lamda, correct reflectivity, correct transmission).
Attachment 2: TOoptimized_2013_08_09.fig
Attachment 4: TO_opt_200ppm_layer.fig
Attachment 5: 2013_08_09_TOopt_200ppm.mat
  1290   Fri Aug 9 16:00:14 2013 taraNotesNoiseBudgetnoise hunting

I measured the slope of the error signal for ACAV path to be 200 kHz/V. This will be used for calibration the error point noise to frequency noise.

 See some details about the error signal's slope and calibration in psl:562.

THe setup for ACAV path is

  • Input power to the cavity 1 mW.
  • RF power on marconi for the driver = 13dB, to 4-way splitter then 14.75 MHz resonant EOM.
  • The error point noise was measured at COMMON channel out1 on RCAV TTFSS.

Next: Measure the slope at RCAV path, measure error noise from both loops, compare to beat signal.

 


Plan for opening the chamber:

I'm certain that the beam reflected from the window that overlaps with the reflected beam from the cavity going to the RFPD causes a lot of noise. This should show up in the error noise. So to avoid the reflection from the window, I have open the chamber to turn the cavity axis a bit. I need to:

  • calculate how much the cavity has to be turned if we will dump the beam at the lens for the RFPD.
  • see if the beam path is still ok for the rotated cavities.
  • Replace the cavity mount wall. The current one is too short due to the mistake in the design. I needed to use  nuts to raise the height, see pic.  Without the gap on the side, temperature control between the two cavities will be better due to smaller coupling. The walls will be ready on Monday, I might need a day or two to clean and bake them before the installation.
  • use screws to hold the cavities down firmly, instead of resting on four point supports.
  • I don't plan to replace the AD590s on the thermal shield. This will take too much work to remove the feed through, fix the cable. Otherwise, we can just slide the stack half way out of the chamber for replacing the wall and rotating the cavities. Plus, we can use beat signal as an error signal for Temp control.

 

  1276   Fri Aug 2 12:38:41 2013 taraDailyProgressfiber opticinsulating foam

I turned off the hepa fans over the table over the night. I came back this morning and the temperature (measured on the vacuum tank) was very stable(within 2mK) over 2 hrs.

chmbrtmp.JPG

above:BLUE Temperature measured on the can, the Y scale is in degree C. The temperature variation is within 2mK over 150 mins.

 

So I looked at the PD for Erica's fringe measurement, the fringe wrapping was slow, so with better temperature insulation, we should be able to hold the fringe for at least a minute.

fringe.JPG

above: The fringe signal from PD, the cursors show the max/min signal from the fringe. The signal drifts from min to max over ~ 60 seconds compared to ~10seconds as before.

 

So the drift we saw before was very likely to be from the temperature drift (1mK per second for 20second fringe wrap). More thermal insulation on the optic should reduce the temperature drift.

  1275   Fri Aug 2 12:18:16 2013 taraDailyProgressBEATsearching for beat

Found the beat @ 116 MHz. RCAV SLOW =5.762V, ACAV SLOW = 1.209 V.

beat_2012_08_02.JPG

 

beat 1kHz input range, calibration  = 718 Hz/V

nb_short_cav.png

above, beat signal with 1kHz input range on Marconi.

Plenty of things that I need to optimize and add:

input optics (ACAV/RCAV):

  • beam alignment
  • optimizing quarter wave plates in front of the cavities.
  • block all the reflected beams properly
  • fixing the back reflection from vac window for ACAV.
  • measure error point noise from both servos and compare them with beat
  • optimizing TTFSS servo gain

Beat setup:

  •  mode matching lens
  • power on beat PD
  • optimizing PLL servo
  • implementing ISS

Seismic isolation

  • new table legs ( I have not ordered the new set yet). The current set is broken
  •  
  1274   Thu Aug 1 21:19:57 2013 taraDailyProgressBEATsearching for beat

I locked both cavities and trying to search for the beat signal, I have not succeeded yet.

I used lenses that could get the two transmitted beam to be close and small enough for the beat PD (new focus 1811) (we ordered  what we need but they are not here  yet).

I locked ACAV at a fixed SLOW DC level (1.207 V), and varied RCAV's SLOW DC level from 1.199V, 0.33V, -0.554V, -1.477V (1FSR ~ 4GHz is about 1 V). The slider for RCAV slow is set to +/- 2V so I have not tried other values yet. It can be changed to -2V to 9 V, but I have to restart the crate which will disturb the temperature servo, so I'll try to adjust RCAV slow value using a voltage calibrator instead.

I talked to Evan about the beat measurement in GYRO lab, the SLOW DC for both lasers can be different up to 6 V (for ~100MHz beat). see gyro1832

I varied RCAV's SLOW DC first because this path does not have a PMC, so I don't have to worry about locking the PMC.

From PSl:1124 ,the beat frequency should be ~60-100 MHz, without the heater on any cavity.  I'll try the same method to check the beat frequency between the two cavities one more time. If it is still ~ 100 MHz, I'll increase the range of SLOWDC, and see if the beat will show up of not.  The setpoint was not changed that much (31.2 to 31.25), So I expect the beat frequency should still be close.

If the beat still not show up, I'll try to realign the beam.

 

Current setup

Vac chamber Setpoint = 31.25

Vheat for RCAV =  0

Vheat for ACAV = 0

 

 

  1269   Wed Jul 31 00:31:39 2013 taraNotesopticcoating optimization for AlGaAs

The codes for optimizing Thermo-optic noise in coatings are up on svn.

I adopt some codes that have been on svn for awhile and modified them for AlGaAs coatings. There are two main codes

      1) DoAlGaAs.m

         This file is modified from DoETM.m found in .../iscmodeling/coating/AlGaAs/doETM.m . The optimization method is using Matlab's fmincon function to search for coatings structure that minmize TO noise. Some modifications include:

  • (Line16-18 )Number of layer. For AlGaAs, the number of layer will be odd number (start with GaAs, end with GaAs), I fixed the layer structure to be odd number.
  • (Line74) Cap. During the optimization, the first cap is kept constant. For a cap made with high refractive index material (nH), the layer thickness is 1/4 lambda, see previous entry.

This code calls on  optAlGaAs.m when running fmincon.

    2) optAlGaAs.m

        This file is the modification of optETM.m found in ../iscmodeling/coating/AlGaAs/optETM.m .It calculates the reflectivity and the TO coefficients from the given layer structure. The modifications are:

  • (Line41-45) Layer structure, the cap start with nH. The material for substrate is SiO2 with nsub = 1.45.
  • (Line60) Desired transmission, as a test, I chose 200 ppm.
  • (Line88) Calculation for TO coefficients (StoZ), I switched from getCoatThermoOptics.m to getCoatThermoOPticsAGS.m. Codes with AGS suffix in /GwincDev folder are fixed for AlGaAs coatings structure. This code calls many functions in /GwincDev folder.

     2.1) multidiel1.m

       This code is used in optAlGaAs.m it calculates the reflectivity and impedance of the given coatinns structure. There is no modification to it. The code can be found in .../coating/coating_optimization_new/


    To run the codes

    check out .../iscmodeling/ folder from the svn. The optimization is in .../iscmodeling/coating/AlGaAs_TO_opt_CTN/ folder, but you need other functions in other folders.

    Once you run DoAlGaAs.m, the optimized layer will be in matlab workspace called xout. This is the layer structure withtout 1/4 cap. Check if there is a layer with thickness of 0.002 or not. I ran the code several times, sometime it shows up. Just rerun the code and get the layer that is around 0.1 or thicker. The 0.002 is just the lower bound used in fmincon search in doAlGaAs.m.

  Plotting noise budget

 The noise budget of the optimized layer can be plotted with /coating/AlGaAs_Refcav/nb_algaas.m . Currently, at line 38-39, the code will take xout  and create a layer structure with 1/4 cap on top of it. The reflectivity of the coatings is in rCoat workspace item after running the noise budget code. It should be close to -1 + 0i

  1263   Mon Jul 29 22:30:34 2013 taraNotesopticcoating optimization for AlGaAs

Since we are trying to optimize a layer structure for AlGaAs coatings. It is a good idea to summarize some notes about all the coatings details. Thanks Koji for the discussion about the coaitngs.

==some background about SiO2/Ta2O5 QWL with 1/2 wave cap coatings==

 For quarter wave layer stack (QWL) SiO2/Ta2O5 coatings, SiO2 and Ta2O5 are the material with low (nl) and high refractive indices (nh), respectively. Due to the stronger structure of SiO2, we usually have a cap of SiO2  as a protective layer on top. This cap has thickness of 1/2 wave length. The reason is that the reflected beam from the interface between the cap and the next layer will be in phase with the first reflected beam at the air-coating surface, see the figure below (top).

If the SiO2 cap is 1/4 thick, the reflected beam from the interface between the cap and the next layer will destructively interfere, causing the reflectivity to go down (see the picture below, middle). 

However, if the cap is Ta2O5 (nH) material, it can be QWL thickness, and the phase from every reflected beams still interferes constructively (picture below, bottom).

multilayer1.JPG

Note: As we can see, the incoming beam and the reflected beam are 180 degree out of phase. It means that the E field at the coatings surface will always be zero. This will prevent the burning on the surface of the coating. With this, the standing wave in the cavity will always have zero E field at the coating surface, see below picture.

This is not AR coat, since all the reflected beams interfere constructively. The reflected beams from AR coating will destructively interfere among each layer.

multilayer2.JPG

To sum up for the SiO2/Ta2O5 coatings:

  • SiO2 is stronger than Ta2O5, so we use it for the end cap.
  • Because SiO2 has lower n than that of Ta2O5, the cap thickness has to be 1/2 wave thick so that all the reflected beams interfere constructively.
  • We want the reflected phase to be 180 degree away from the incident beam so that the surface won't get burnt from the building up E field. (If the E field is non zero, it will be amplified by a factor of Finesse/pi).  My previous optimization for AlGaAs that used 1/8 cap was wrong because the reflection phase was not 180. This means that by adjusting the cap thickness to optimize the TO noise is not a good method, since the reflection phase is not close to 180 anymore. The optimization has to take the phase into account.

 

==AlGaAs coatings==

 For GaAs/Al0.92Ga0.08As (AlGaAs) coatings, the situation is a bit different from SiO2/Ta2O5. The cap has to be GaAs (nH) because Al0.92Ga0.08As will oxidize and change its material properties. Now that the cap will be nH, the thickness has to be 1/4 wavelength.  The last layer next to the substrate has to be GaAs (nH) too (I think because of both the better reflectivity and the fabrication process).

==optimization code==

 There is an assumption about the layer structure used in the optimization code that the cap is nL(SiO2), 1/2 layer. The coatings layers are even number ( doublets of SiO2/Ta2O5). I'm making sure all the assumptions in the code are fixed. Here is a preliminary result.

 

opt1_2013_07_29.png

above: Layer structure, the first layer (cap) is GaAs (nH). In the optimization, I keep the cap thickness to be 1/4, and vary the rest.

nb_opt1_2013_07_29.png

above: Noise budget of the optimized layer. TO noise is below BR noise from DC up to 1kHz.

The reflectivity of the coatings is -0.9997 + 0.0209i  (reflection phase = 180 - 1.2 degree). I'm not sure if this is good enough, maybe better optimization can be done.

Note: My layer structure is really different from what rana did in T1200003. For my structure, the layers near the cap vary a lot before getting close to 0.25 when the layers are close to the substrate. The result from 1200003 is the opposite. The layers near the cap are about 0.25, and start to diverge when the layers are close to the substrate.

T1200003_refcav.png

above:  Optimized coatings result from T1200003. The optimization probably assume the cap of low index material, but the following layers evolution are opposite of what I got. That's why I'm not sure about my optimization.

 

I'll upload my codes soon so that people can check my optimization.

  1239   Mon Jul 15 10:55:40 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

 After a discussion with Eric and Matt, here I'll summarize about thermo optic(TO) noise calculation plus some other important noise sources.

1) goal

     We aim to measure the limiting noise in AlGaAs coatings. If we order just 1/4 quarter wave stack, no optimization, the limiting noise source will be from TO noise due to high values of thermo elastic(TE) and thermo refractive(TR) coefficients of the materials. However, by optimizing the coatings structure to cancel TO noise we can:

  • Probe thermo-elastic (TE) noise in SiO2 substrate at low frequency and coating Brownian noise(BR) at higher frequency
  • Prove that TO cancellation can be done (according to Evans etal).

We can tell what kind of noise from the slope.  BR, TO noise or TE noise in substrate have different slopes at the interested band, see fig 1.

 

2) Is the calculation correct?

alGaAsnb.png

fig1: noise budget with some fundamental noise sources. The noise budget is for AlGaAs coatings on a mirror with ROC=1m. The cap is GaAs (high index material) with 1/8 lambda thickness. See explanation below for more details.

    The fundamental noise sources in our setup (1.45" cavity, 1m roc mirror, optimized AlGaAs coatings) will be:

==BR in coatings==:   

  • The calculation is taken from Harry2002, for half infinite mirror.
  • The result is compared with Somiya&Kazuhiro2009 for finite size mirror calculation (see solid blue line and dashed cyan line). The difference is small due to our small spotsize, so using either calculation is ok for us, but Harry's calculation is less time consuming.
  • The analytical result should be valid as it was verified by Numata and TNI measurements.

==BR in substrate==:

  • The calculation is taken from Levin1998, with finite size correction by Liu&Thorne(LT2001). 
  • The loss angle for bulk fused silica is frequency dependent ~ 10-11 x f0.8(Penn2006). This loss is much lower than conservative constant loss (10^-8) (number from DCC LIGO-T0900161) from dc upto 10kHz.
  • In this calculation, for constant loss of 10^-8, BR noise in substrate is still ~ a factor of 3 lower than BR in coatings.

==TE noise in substrate==:

  • BGV1999 gave a result for adiabatic limit (most of the heat flow is in 1-D heat diffusion length is much smaller than beamsize, sqrt(kappa/C * 2pi*f)<<r0 )for half infinite space mirror, Liu Thorne2001 verify the result. I used comsol to simulate the noise (with adiabatic assumption) and it agreed with the analytical solution.
  • However for our setup with a small spot size the assumption beaks down. Cerdonio2001, computed the noise that valid for low frequency and small beamsize which is a  case for our setup (cut off frquency ~ 10 Hz). All the factors and corrections are summarized in TNI2004 measurement and Nawrodt2012.  The calculation will be valid for our setup.

==TE and TR noise calculation:

  •   The temperature fluctuation sensed by the beam is taken from BGV1999 using Langevin approach, and Mike Martin Thesis (this takes care of the fluctuation at low frequency where adiabatic assumption breaks down. The calculation assume that coating thickness << thermal diffusion length. For AlGaAs, because of its high thermal conductivity, this assumption is still hold at the bandwidth of interest.
  • The thick coating calculation is given in Evans 2008.  It is important at high frequency and coatings with low thermal conductivity. This means that TE and TR effects won't be coherent in the coatings. This is not a problem for AlGaAs due to its high thermal conductivity.
  • TE and TR coefficients calculations are treated coherently in Evans2008.  The cancellation only depends on coating structure. With a cap of GaAs (nH) 1/8 lambda thickness, the cancellation is very good reducing the TO noise below other noise upto a few kHz.
  • The cap thickness has to be withing +/- 20Angstrom so that the TO is about a factor of2 below coating BR. G. Cole mentioned that each layer thickness varies about 0.3% or less which is about lambda/(4*n) * 0.3% = 2Angstrom. So the cancellation should be ok.

TR coefficients are calculated numerically (GWINC) and analytically (Gorodetsky2008). The results match up well (less than 1% difference), if all the parameters/ averaged values are from Evans.

In GWINC there is one correction noted as "Yamamoto thermo-refractive correction", this changes the Beta eff ~ 10% causing the cancellation to be not as good (still ok up to 1kHz). I emailed Kazuhiro Yamamoto asking him if he has anything to do with this. Otherwise all the calculations and optimization are in good shape.

Attachment 2: alGaAsnb.fig
  1233   Thu Jul 11 00:19:52 2013 taraDailyProgressopticboth cavities are locked

Both cavities are locked (not optimized yet). Since it has been awhile that both are locked, here is a picture.

photo_2013_07_10.JPG

 Rcav is locked by Fast feedback only. I still have to check the polarity for PC feedback.  I adjusted the phase between the LO and PD for RCAV loop to get a nice error signal. I noticed that there is an offset in the error signal, I will try to adjust the polarization of the beam in front of the EOM to see if I can reduce this offset from RFAM.

To do:

  • lock rcav with both fast and PC feedbacks
  • optimize the setup ( reducing RFAM, minimize back reflection)
  • setup the beat path (mode match + alignment)
  • setup the ISS path
  • check the beat frequency
  • re organizing the wiring on the table.
  • replace the current SMA cables with the semi-rigid ones, once all the equipments are in place.

 

 

 

  1231   Tue Jul 9 19:35:20 2013 taraDailyProgressopticbeat board is back

I installed the beat board back behind the cavities. I still have not finished aligning both beams to the 1811.

  • Note about ACAV ( this path has PMC on it): After new mode matching with more visibility (from 80% to 95%), I can increase more gain and the error noise is getting lower. However, there is a problem with the beam reflected from the window of the tank. It overlaps with the main beam and cannot be blocked. I think this is the reason why we cannot suppress the error noise down to what we had before. I still need to convert the error noise back to frequency noise to see if it is below the estimated coating noise or not. If not, we have to reopen the chamber and tilt the cavity a bit. Rcav does not have this problem, the back reflection is away from the main beam and can be dump properly.
  • Note about RCAV: Erica and I plan to finish the EOM driver test tomorrow. After that I'll use it to drive the broadband EOM for locking RCAV to the cavity. The plan is to use one marconi to drive two EOM at the same frequency (14.75 MHz). We use a 4-way splitter for 2 EOM and 2 demodulations. I don't know how using same frequency for EOM will turn out (cross talk problem), but I want to see the first beat measurement within this week.
  • Note about beat setup: Evan calculated the mode matcing for beat setup, but I had to modify it. The first lenses were moved out of the board and mounted between the vacuum tank and the board due to space limitation. This might add some extra resonant peaks in the beat setup due to the long posts for lenses. The spot diameter on the PD is about 130um, which should be fine because 1811's diameter is ~300 um.
  1228   Mon Jul 8 21:52:04 2013 taraDailyProgressopticmode matching to refcavs

 I redid the mode matching for both refcav, the visibilities are up to ~ 93% and 95% for RCAV and ACAV.

  • For RCAV (refcav with PMC), the visibility was ~ 80% before, now it is ~95%. (The numbers are measured from the reflected beam on the RFPD)
  • For ACAV (refcav without PMC), the visibility is now ~ 93%. This is pretty good, compared to ~ less than 85% from previous setup when we used an AOM.

I'll add the new layout for the current situation soon.

 

==Note==

  • We care about mode matching because we already saw that any light that was not coupled into the cavity was reflected back to the laser and caused extra noise.
  • By changing the lens, the beams for fiber optic (both for Gyro and Erica's experiment) have to be re calculated. I'm sorry about that .
  1216   Fri Jun 28 14:52:11 2013 taraNotesECDLlaser noise requirement for locking to a refcav

I estimated some requirement for an ecdl such that it is possible to be locked to a high finesse refcav. For 1.45" cavity, finesse = 1e5, the frequency noise of the ecdl has to be less than 400Hz/rtHz (assuming flat noise from 1kHz to 1MHz).

 ==background==

 We are developing an ecdl, however, we have to check if it can be locked to a high finesse refcav. If so we can use an ecdl in CTN/ cryo style experiments where an ecdl is locked to a refcav. Cryo had some problems with locking a laser to their cavities because of the noise at high frequency, see CRYO elog)

==calculation==

 For a good error signal in PDH locking, the laser linewidth of the laser measured in 1ms - 1us should be smaller than the cavity width (2xcavity pole).

  • Linewidth of the cavity = FSR/Finesse, for the current cavity FSR = 4GHz, Finesse = 1e5-> cavity linewidth = 400kHz.
  • linewidth^2 = integrate frequency noise from 1kHz-1MHz ~ frequency noise PSD[Hz^2/Hz] x 1e6 [Hz] , so S has to be ~ 400 Hz/sqrtHz or lower. (watch out for the unit).

==comments==

The requirement of 400 Hz/rtHz or below seems to be do able, see Chloe's calculation. However, this number is from Finesse = 1e5, with 1.45" cavity length. If we use different cavity with different FInesse, the number will change as well.  The frequency noise requirement (assuming flat from 1kHz to 1MHz) is 400 x [FSR/4GHz] x [ 1e4/ Finesse] [Hz/sqrtHz]

  1208   Mon Jun 24 21:27:02 2013 taraNotesRefCavsetup for 2nd refcav

I'm in the process of locking the 2nd cavity. The work is in progress. 

  •  The 2nd TTFSS is working fine. I tested it by using the 2nd TTFSS to lock the first refcav. The error signal was similar to what I got from the first TTFSS.
  •  Mode matching was revised (See Erica's entry).
  • Heatsink for the 2nd laser was ready, I added it on the laser.

To Do:

  • prepare for the 2nd EOM, I need to think about an oscillator driving and EOM. Since there is no resonant EOM, I'll use Rich's EOM driver on a BB EOM for sideband.
  1204   Fri Jun 21 21:02:40 2013 taraNotesDrawingslayout for the new setup

The mode matching between the laser to PMC for the 2nd path was not very good before, so I fixed it.

 I used 3 lens to mode match before. The new one will use only 2, see figure. I move the laser by two inch. The new setup is shown in the figure.

With a more careful lens setup, I should be able to couple more light to the cavity.

=note=

I assume that the spotsize in the PMC is 370 um, similar to the current one. I'll revise the PMC drawing and have it made next week. The required optics are ready.

=next=

 I'm preparing the power supply for the second TTFSS. It requires +/- 180, +/- 24 and +/- 17V input. I can use high V power supply from the electronic rack for +/- 180V, and +/-24 V. I'll find a commercial power supply for +/-17V.

  1199   Tue Jun 18 11:53:17 2013 taraNotesNoiseBudgetnoisebudget for 8" SiO2/Ta2O5 cavity

After careful checks, the estimated result is still below the measurement. For our geometry, the result from SK2009 is similar to that from Harry2002.

 

 So here are the results

  • The calculation for coating Brownian noise from SK2009 (finite size mirror) is similar to Harry2002 result (half infinite mirror).
  • I double checked my code by changing my parameters to those used in SK2009 paper and got the same result. Apparently, their spotsize is very big compare to ours (1/3 of the mirror radius vs 1/40 of the mirror radius).
  • I revisited my comsol model again to check Brownian noise from substrate and spacer when the mirror is curve (more realistic model). The difference is small. (add fig).spacer_br_8_edge.png

The optical bonding area in this model is similar to the real cavity, compared to what is done before where the bonding area is everywhere on the mirror beside the bore hole. So it is quite certain that it is the noise from the substrate/spacer.

Since the measurement has 1/f slope, it is very likely to be Brownian thermal noise (Thermoelastic/ TO will have different slopes). It might be that the 1998 mirrors have high loss. We will see that with shorter cavity measurement.

  1198   Fri Jun 14 02:36:22 2013 taraNotesNoiseBudgetnoisebudget for 8" SiO2/Ta2O5 cavity

I'm checking the result for the calculation.  I think it is too early to celebrate.

Nic suggested that I should use comsol to estimate the coating Brownian noise. There are a few problems:

  • For 3D model, I cannot mesh the geometry properly yet. The layer is too small for the rest of the mirror.
  • For 2D model, I cannot integrate the elastic energy from the coating layer. There is no choice to select the domain I want to integrate. I'll find out what happen.

I double checked the calculation code. I changed m to 65, and stepsize to w0/4000, the elastic energy (U) calculated is still ~ 1.5e-10 J. It did not change much from my last calculation. However, what I do not understand is the analytical result for half-infinite mirror (as given in SK2009 but different from GWINC), the number does not change that much.

I found a missing 2pi factor that causes the estimated noise to be higher and to match the measurement.  I'll check the calculation carefully again, but it might be that it's still not coating noise.

 

  1194   Tue Jun 11 16:46:52 2013 taraNotesNoiseBudgetnoisebudget for 8" SiO2/Ta2O5 cavity

note about the calculation for coating Brownian noise in a finite size mirror .

==Coatings Parameters==

Young's modulus, Poisson's ratio, and loss angle are taken from the volume averaged value of the coatings (Yavg = d/ ( d1/Y1 + d2/Y2) , sigma avg = 1/2 (sigma1+sigma2 ). These are used for "perpendicular" direction in Harry2002 formula.

Loss angles

  • SiO2 loss angle  = 1e-4
  • Ta2O5 loss angle = 2.3e-4
  • coatings loss = 1.32e-4

Young's moduli

  • SiO2 Young's modulus = 72e9  Pa
  • Ta2O5                         =140e9  Pa
  • Coatings Young's modulus = 93e9 Pa

Coatings structure

  • 1/2 lambda cap of SiO2
  • 26 layers
  • 300 ppm transmission

 ==calculation codes==

  • I got the file for finding zeroes of the bessel function from Matlab exchange.
  • The code for calculating Br noise is attached below.
  • For the finite size bdy condition, the solutions include all the besselj function of all orders (m=1 to inf). I used m from 1 to 55 in the calculation since it converged quite fast after that.
  • For the integration to calculate all the elastic energy, I used Riemann sum, with stepsize of ~0.15 um. The result does not change much (less than 3%) if I go from 0.8*a to a where a is the radius of the mirror. This is important to note because our coatings do not cover the whole surface of the mirror. There is an annulus edge with ~3mm width for optical contact area. The result means that the elastic energy is still localized in the spot area.

 ==Implication to AdvLIGO coatings==

As noted in SK2009, the estimated values for half infinite and finite size analyses are about the same (~2.5% difference) (I have not verified this). Then, the result from GWINC using Harry2002 formula is still accurate.

==note/comments==

  • The calculation in SK2009 uses an overall loss angle of the coatings, while calculation in Harry2002 separates the elastic energy in two directions,parallel and perpendicular to the surface, and also loss angles in the associated directions.  I use the perpendicular average under the assumption that most energy/deformation occurs in that direction.
  • The result matches the measurement quite well. This reassures us that other noises introduced by the setup (i.e. noise in optical bonding/ noise from supporting structure/ thermoelastic/ brownian noise in the spacer) are not higher than coating thermal noise.

 

Attachment 1: getCoatBrownian2.m.zip
  1193   Tue Jun 11 00:45:48 2013 taraNotesNoiseBudgetnoisebudget for 8" SiO2/Ta2O5 cavity

Quote:

Note:

  1. Somiya paper also include Brownian noise in Coatings with finite size substrate/coatings (see fig2) which is not done in Harry etal 2002. Finite size effect increases the noise level by a lot, I think this might explain why the beat result we measured from 8" cavities is a bit higher than the estimated noise using the result from Harry etal. I'll check that later.

 Here I applied Somiya&Kazuhiro (SK)2009 coating brownian noise calculation to the previous 8" cavity setup. The estimated noise matches up with the measured result well.

 

sk2009nb.png

The result for coating Brownian noise presented in SK is for finite size mirror. They emphasize that the estimated noise diverges from Harry2002 result (half infinite mirror) in the case of a thin mirror (thickness is less than mirror radius) which is our case (radius = 0.5inch, thickness = 0.25 inch). 

I'll attached the calculation and explain some differences between the two calculations later. Here are some notes about the parameters:

  • Loss angle of the coatings used in the calculation is phi perpendicular (1.326 e-4), from GWINC, (phi parallel - 1.4e-4);
  • Young's modulus of the coatings is 93 GPa
  • Poisson's ratio  = 0.2
  • m = 45; (# of zeroes for besselj(1,x))
  • stepsize for radial integral = wspot/1000

 

Attachment 2: sk2009nb.fig
  1192   Thu Jun 6 22:28:46 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

Here is an outline for TO calculation. I tried to summarize it and make it as simple to follow as possible.

  •  Use Levin's direct approach to calculate thermal fluctuations seen by the beam.
  • Apply power injection at the coating surface, with proper boundary condition, take coating into account. (Evans2008 see thick coating correction, Somiya2009)
  • To calculate the loss due to the dissipated heat, we need to solve heat equation. The loss associated with the injected heat is proportional to (gradient of temperature)2
  • The calculation for gradient of temperature has to be calculated in both longitudinal and transverse direction, as thermal length is comparable to the beam size [Cerdonio 2001]. Other papers usually approximate grad T = dT/dZ, which is 1-D treatment [Evans2008, Somiya2009]. The effect from Heat flow in transverse direction shows up at low frequency, where the noise level becomes lower.
  • When solve heat diffusion equation, apply boundary condition for finite size mirror (somiya2009).
  • Once we have thermal fluctuations, ST, we convert it to displacement noise with TE and TR coefficients. Sx = ST *(TE + TR)
  • TE and TR coefficients can be calculated from the layer structure. The cancellation will occur only at lower frequency where temp fluctuations in coatings are uniform. At higher frequency the effect from TE and TR will sum up in quadrature (if heat equation is solved in coatings), see thick coat correction section in Evans2008.

This means that for TO optimized coatings, we have to make sure that TE and TR coefficients are comparable for maximum cancellation. The calculation for TE and TR are quite well defined, [Fejer2004, Evans2008, Gorodetsky2008]. This part is independent from temperature fluctuation calculation outlined above. So we can choose the optimized design and then calculate the total TO noise level later. The proposed optimization can be found in psl:1183. (Here is the result for 1/8 cap of nH).

eighth.png

Note:

  1. Basically most of the calculations outlined above are done in Somiya2009, except transverse heat flow. If we consider transverse heat flow in coatings and substrate, the result will be valid at low frequency as well.
  2. The decision for G Cole etal to use substrate parameters in temperature fluctuations as suggested by Rana seems to be ok, since their calculation also include the thick coat correction (Evans2008), it means that temperature fluctuations in coatings are taken into account.  However, the cutoff frequency might be  off a bit, since the equation for transverse flow is only in substrate (BGV1999, cerdonio2001). I think the real cutoff frequency should be higher because kappa is larger in the coatings, and transverse heat flow becomes more significant at higher frequency. 
  3. Somiya paper also include Brownian noise in Coatings with finite size substrate/coatings (see fig2) which is not done in Harry etal 2002. Finite size effect increases the noise level by a lot, I think this might explain why the beat result we measured from 8" cavities is a bit higher than the estimated noise using the result from Harry etal. I'll check that later.
  4. I'm not quite sure about The TO calculation in Somiya. The injected heat from TO and TE are added independently, however, the result is similar to that of Evans (with half infinite limit). I'm checking it.
  1191   Wed Jun 5 22:25:28 2013 taraNotesNoiseBudgetTO calculation review

Since we have to review the calculation for Thermo-Optic noise (TO), I'll sketch an outline and some remarks here.

==TO noise overview==

To calculate TO noise, we have to calculate temperature fluctuations, then multiply by Thermoelastic (TE) and Thermorefractive (TR) coefficients to convert temperature fluctuation to displacement noise. Usually, in the frequency of interest, thermal length is much larger than coating thickness. Thermal fluctuations in coatings are uniform making the whole coatings expand/ contract uniformly. This assumption is important for cancellation between TE and TR. As TE effect comes from the whole coating thickness, while TR comes from only the first few layers (most of the power is reflected from these top layers). Modifying the first few layers can change TR effect significantly.

 ==Temperature fluctuations==

can be obtained from direct method (Levin 2008), by injecting heat with Gaussian beam profile. Example are done in Levin 2008, Evans etal 2008.

A few issues about these calculations:

  • heat flow in 1-D, under the assumption that temperature gradient is mostly in z direction coating thickness d << thermal length << beam radius. Where thermal length is ~ sqrt (  kappa/ (rho*C* 2pif) )  This is not true for AlGaAs coatings where kappa is ~ 60 W/mK which gives thermal length to be~ 2370 um  [sqrt (1Hz/f)], beam radius is ~ 200um.  Cerdonio 2001, and Mike Martin's thesis have the calculation in 3-D, however, heat diffusion in coatings is not taken into account.
  • Heat diffusion in coatings, is done in Fejer 2004, Somiya2009. (It is ignored in BGV1999/Liuthorne2000/cerdonio 2001)

At this point, I think Somiya paper is very good for us to look through. The calculation includes TE and TR. However, I don't quite get it yet. The calculation solve heat equation in 1-D, but has results for finite test mass. I need to spend more time on the paper.

 Heinert 2011, has calculation for TR in finite size substrate. I'm not sure how to connect the results to our setup yet. Plus, for our setup, the actual coatings will be ~ 8mm in diameter, with 1" diameter substrate, the boundary conditions will be non-trivial for us.

 ==TE +TR coefficients==

  [ coming soon]

  1190   Tue Jun 4 18:02:26 2013 taraDailyProgressRefCavbeam directed to 2nd refcav

The beam is sent to 2nd cavity (RCAV). The beam is mode match roughly, since there is no PMC, the exact beam size is hard to measure. The laser resonance when RCAV_SLOWOUT @ 0.2383V. There is enough transmitted beam for alignment the beat setup behind the cavity.

  1187   Fri May 31 13:21:37 2013 taraDailyProgressComputersTemporary south refcav autolocker

Quote:

 Special bonus settings: the common gain on the TTFSS is 404 clicks, the fast gain is 426 clicks, and the offset is 967 clicks. Tara pointed out that the frequency loop would catch lock easier if the gain settings were around these low values rather that what they were previously (~600).

 The offset should be ~ 500. I turned it back down.

  1183   Fri May 24 23:57:27 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

 

 The noise budgets below show noise from coating brownian, TO noise and TE in substrate. The three plots are from 52,54 and 56 Layer coatings.

 All the designs have 1/2 cap of nL, with nH ending on the substrate surface.  There are no significant differences in the noise level at low frequency, since TE noise in substrate starts to dominate. I used the substrate

parameters in thermal fluctuations, so the cut off frequency for TO calculation is low (~ 3 Hz instead of ~ 200 Hz). The design can go for 56 layers.

52lay.png

54Lay.png

56lay2.png

 

I'm thinking about another solution, where the top layer is nH, followed by 1/4 layers. If the first nH is 1/8 lambda thick, TO can be cancelled nicely (for 56Layer + nH cap). The transmission is 140 ppm , which is in the chosen range (100-200ppm). But I feel that the 1/8 cap is not good for a high reflectivity mirror, since the phase of the reflected light within  that layer is not really inphase or out of face with the light reflected at the air surface. I'll think about it more to see if it would be a good solution or not.

 

Attachment 4: 52lay.fig
Attachment 5: 54Lay.fig
Attachment 6: 56lay2.fig
  1181   Fri May 24 04:04:58 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

Quote:

  • The approximated Beta_eff, for 1/4 high reflective coatings, which is reported in BGV 2000, and Evans 2008 is given by B_eff ~ (nH^2 *BL + nL^2*BH) / (nH^2 - nL^2) (which was used in Cole's paper to calculated their TO noise). BGV gave a sketch of this calculation in their paper (which I have not yet thoroughly understood). One problem is that, the result for B_eff obtained from this formula is the same whether the coatings start with nH or nL. This should be wrong, since most of the TR effect comes from the very first layers. The order of nH/nL should matter.

 Beff ~ (nH^2 *BL + nL^2*BH) / (nH^2 - nL^2) is valid only if the top layer is 1/4 layer of nL, [Gorodetsky, Phys Lett A 372 (2008)].  The complete calculation for general case is given in the reference. If the layer starts with nH, beta eff is = (BetaH + BetaL) / (4x(nH^2 - nL^2) ).  So, GWINC and analytical approximation agree, Yay! .

The effective beta reported in Cole's paper is 5e-4, but it should be ~ 5e-5 for coatings start with nH. The real thermo optic noise for their setup will be lower ( because TE is about the same level as TR). Their real TO noise should be a factor of 5.5 below the reported one (in Hz^2/Hz unit).

Note: There are still issues about the thermal fluctuation and the cut off frequency. These will greatly change the shape of the TO noise and the total noise level. I'm still investigating it.

The 1/2 wavelength cap with nL does reduce the TO noise. But we need to know exactly how thick the nH film on top will be, so the real TO effect can be estimated accurately.

 

 

 

 

  1180   Wed May 22 00:04:48 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

I found out why the calculated values of the coatings' effective beta from GWINC and Cole etal paper are different. The order of Low/High refractive index material have something to do with the beta effective calculation.

Here are some facts about the coatings and calculation:

  • The AlGaAs coatings used in the paper have no 1/2 wave cap. The structure is consisted of only 1/4 wave layers. Start with nH on top, and end with nH at the substrate (SiO2).
  •  The PSD from thermo-refractive is SdT x Beta_eff x lambda. Where SdT is temperature fluctuation, Beta_effective is the overall dn/dT of the coatings,see the entry below for more details.
  • In GWINC, Beta_eff is calculated numerically, taking each layer and calculating the reflectivity, then sum all the effect together. The result for Beta_eff is different, if the first layer (the top one) is changed between nH or nL. ( 5e-5 and 5e-4, cf PSL1178).
  • The approximated Beta_eff, for 1/4 high reflective coatings, which is reported in BGV 2000, and Evans 2008 is given by B_eff ~ (nH^2 *BL + nL^2*BH) / (nH^2 - nL^2) (which was used in Cole's paper to calculated their TO noise). BGV gave a sketch of this calculation in their paper (which I have not yet thoroughly understood). One problem is that, the result for B_eff obtained from this formula is the same whether the coatings start with nH or nL. This should be wrong, since most of the TR effect comes from the very first layers. The order of nH/nL should matter.
  • Computed values of B_eff from Gwinc code and the simplified formula agree if both start with nL. This makes me think that there is some assumptions in the simplified B_eff formula that the first layer is nL (which is customary, in SiO2/Ta2O5 coatings ).

So, I believe that the calculation for TO noise I have right now is correct. And for 100 ppm transmission (56 layers) with 1/2 wave cap, the TO noise is significantly reduced (add plot).  We should be able to finalize what we want for the AlGaAs mirrors soon.

  1178   Tue May 21 01:06:43 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

I checked all the discrepancies in the calculations between GWINC and that of Cole. The issues are almost cleared, only the value of effective beta, (dn/dT) that still remains.

  The PSD of TO noise in [m^2/Hz] is given by Sx(f)= ST (f) x (dTE + dTR).  See Evans etal Phys Rev D 78, 102003.  Where:

  • ST (f) = Temperature fluctuation as sensed by a Gaussian beam
  • dTE = dx/dT, or rate of change of mirror position with respect to temperature change due to thermoelastic mechanism.
  • dTR = rate of change of mirror position with respect to temp change due to thermo-refractive mechanism

ST(f) can be calculated analytically, see BGV, Phys Lett A 271 , (2000) 303-307 eq9, this also assumes adiabatic approximation. In Mike Martin's thesis, the temp fluctuation is generalized  to all frequency (by contour integral, I'll show the details later). The parameters for calculating ST(f) are taken from that of substrate (in GWINC), but Cole's paper and Mike's thesis use that of the coatings. That makes Cole's result about a factor of 7 higher than that from GWINC. Matt and I discussed this with Mike, he thought that the calculation should use the substrate's properties since the thermal length in the frequency of interest is much larger than the coating thickness.

The issue with which parameters should be used might be a less serious problem if (dTE + dTR) can partially cancel out making the whole TO noise much smaller. Basically dTE is ~ alpha* coatings thickness, where alpha is the thermal expansion coefficient of the coatings. dTR is ~ beta_eff * lambda.  The calculations for dTE from GWINC and Cole are about the same (1.1 x 10^-10) [m/K], where the effective beta are different by about and order of magnitude. Cole reports the value of beta effective to be -5.5 x10^-10 , meanwhile GWINC gives me 0.5x10^-10.

This means that the TE and TR,as calculated from GWINC are more comparable, and the TO result is reduced significantly. While the TO result from Cole is mostly TR. I calculated the TR following the 1/4 stack approximation in Evans paper and got the same result as in Cole. I'm checking what happen in GWINC code for TR calculation.

  1177   Wed May 15 20:07:12 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

We checked the half wave cap solution for  minimizing TO noise. WIth a half wave cap of nl, the TO noise is smaller by ~ a factor of 2 in Hz^2/Hz unit.

Matt and I checked the  calculate the TO noise for a half wave cap solution. The noise goes down by a factor of 2. 

A few issues that we still have to investigate:

  • A thin layer of nh: we talked to Mike, he said that to prevent the oxidation that occurs on GaAs layer (nL), a thin layer of AlGaAs(nH) has to be applied on top. We are not sure how thick the layer will be, we should ask G Cole, so that we can estimated the effect before hand.
  • The TO noise with  half wave cap may already be lower than substrate thermoelastic (TE) noise. I'm checking the TE calculation and find out that the value for thermal expansion of fused silica is 3.9e-7 in Gwinc, but 5.5e-7 elsewhere (add sources). If it is really 5.5e-7, this will be higher than the current TO noise already. I'll look into it.
  • A factor of 2 : This comes from double sided PSD or either 2 mirrors. I'll change that to our standard here (1-sided PSD, with single mirror).
  •  The cancellation might change for different numbers of doublet. Since we plan to have ~ 100-200 ppm, the actual TO noise may be different than this calculation (2ppm). I try using 56 layers (1/2 lambda cap of nL included) which give us 100ppm, and TO noise is below coating brownian from DC to 200 Hz. This is a pretty good result which should be expected. Since we reduce the number of doublet, the effect from TE becomes smaller, (still larger than TR). Thus the different between the two (the total TO noise) is smaller.
  • Different cap thickness may bring down TO noise more than half wave cap does. I just try the cap with 0.1 wavelength of nL (for 40 doublet stack), and TO noise goes down by another factor of 2. This might apply for 56 stack as well. I'll check.

TO_compare_cap.png

Attachment 2: TO_compare_cap.fig
  1176   Tue May 14 02:06:15 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

[matt,tara]  We compared the TO result using GWINC, our results are similar (see PSL:1170). However, it still not agrees with result in Cole etal paper.

The result from GWINC and Cole etal's result are different in the following ways:

  • TO noise from GWINC is higher their result. This might be due to different values of the effective alpha, and effective beta in the calculation. We will check this next.
  • The calculated transmission for 81 layers is ~ 1.8 ppm, while they reported 10ppm. We are not sure what happen here.
  •  Half wave cap solution for TO noise cancellation is not shown in GWINC.
  • Thermal fluctuation as observed by a Guassian beam, SdTTO = const x kBT^2/ r0^2 sqrt(kappa x heat cap x 2*pi*f) depends on substrate parameters in GWINC, but their result use coatings' parameters. With coatings parameters, the thermal fluctuation will be lower, thus lower TO noise. It means that our TO result should be larger by an order of magnitude. However the results are about the same.   We think that the subsrate parameters should be used in the calculation, because thermal length in the coatings from dc up to 170kHz is smaller than the coatings thickness (~6 um).
  • The calculation in GWINC assumes adiabatic assumption. However, the assumption breaks down at 270 Hz for AlGaAs coatings, and 6 Hz in substrate. That explains why the TO noise in Cole paper is almost flat from DC to 100 Hz. Mike Martin's thesis explains the TO noise at all frequency, but I haven't yet quite understood all the equations.

 

  1173   Fri May 10 01:24:01 2013 taraNotesTempCtrltemp sensor on heat shields

AD590s on both thermal shields are not working. I was wrong when I checked them at the first time.

The temp sensors in the vacuum tank for monitoring temperature on heat shields are wired as shown in the picture. The resistor,R, is 30k ohms. According the the datasheet, the current from AD590 should be ~ 300uA, (30kx300uA = 9V). But what I read from the voltage across the readout R was 20V which was over the input range of EPICS (+/-10V). This happened on both of them. I compared the readout with a left over AD590, and got ~ 9.3 V readout which was expected at room temp.

At first I thought it might still be working linearly and useable if I just switched to lower R. However, with R=12 k, the readout voltage was 18V (I expected 20x(12/30) =8V). So certainly, this is not working.

I think the reasons they are broken is that they were overheated when I soldered them. I tried to be careful, but, apparently, that was not enough.

I'll check if there are spare AD590s in the lab or not, otherwise I'll order some more.

  1172   Wed May 8 01:11:09 2013 taraNotesDAQslow feedback to laser via EPICS is on

Slow feedback for 2nd laser is ready.

EPICs channel:

  • C3:PSL-RCAV-FMON was created for fast mon to laser.
  • C3:PSL-RCAV_SLOWOUT was created for SLOW feedback. The channel was originally named C3:PSL-FSS_VCOMODLEVEL J9 input 11 and 12, VMIVME-4116, C2 S4.

The output of EPICS channels have capacitors installed in parallel for low pass filter.

 

 

  1170   Mon May 6 03:11:44 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

I checked the calculation for TO noise in Cole etal people and found a few problems that I didn't understand.

  • In the paper, they have two solutions for TO-noise, at low and high frequency. The solution for high frequency is similar to that in Evan etal paper, but I'm not sure where the solution for low frequency are from.  I don't see this kind of calculation in Evans etal paper.
  •  I repeated and plotted the TO noise calculation as used in Cole etal's paper. The TO noise plotted in their paper mostly came from the low frequency part.
  • Some parameters reported in the paper might not be accurate, for example their beam radius is 250 um. However, with their 35mm spacer, 1.0 m RoC mirrors, the spotradius on the mirror should be 212 um. I haven't checked how much their materials parameters and what I used in my codes differ.
  • For low frequency solution (solid blue line), with the materials parameter given in the paper, it is a factor of 1.5 higher than their result (I got 3e-3, they report ~2 e-3 around 1-10 Hz).
  • For high frequency solution (solid yellow line), with the materials parameters given in the paper, the result is about a factor of 10 higher than that from Gwinc code (dashed blue line). The formulas are the same, but I used different material parameters. The two lines should be close, but they are a factor of 10 apart, just because of the material parameters. We should really make sure that the numbers are correct. Before trying to do the optimization.

 

RefCav_AlGaAs_TOnoise.png

 

My GWINC code for TO calculation can be found here. (other modified functions are in /GwincDev/ ).The main code is plotTO_algaas.m. This code uses getCoatThermoOpticsAGS.m which calls out other other functions in /gwincdev/

  1. getcoatTOposAGS.m (calculated effective alpha and beta in coatings.) This function uses getcoatLayers.m to generate the layer structure. The original one started with nL, I modified it to start with nH, and end with nH.
  2. getcoatThickCorrAGS.m, which computes the correction factor (gamma TO).
  3. getcoatavgAGS.m, this code compute the average material parameters in coatings.
  4. in /coating/AlGaAs_Refcav, I created a database file for material parameters called algaasmodels.m.

 

  1169   Thu May 2 23:40:46 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

I used GWINC code to calculate TO noise in AlGaAs coatings, with some modifications to the code I can get the result that is comparable to Cole etal's result. However, there seems to be some minor details that I have to check. The half wave cap solutions for TO cancellation is not verified by the current calculation yet.

 

 What I modified and checked in the code:

  • The variable called thermaldiffusivity in the code is actually treated as thermal conductivity in the calculation, so all calculations in the past are still correct.
  • The layer structure in the code was originally for Ta2O5/SiO2. The first layer started with SiO2 (low index material,nL), and ended with Ta2O5(high index material,nH) at the substrate surface. However, the AlGaAs coatings start with nH and ends with nH. I changed the calculation for effective thermal expansion accordingly. With the correct layer structure and materials parameters from Matt, the TO nosie is closer to JILA's result. However, the shape is still not the same, what reported in JILA is almost flat across 1-100 Hz. The calculated transmission from the layers is 1.8 ppm, but the paper says 4ppm. I'm looking into this.

RefCav_AlGaAs_TOnoise.png

cole.png

Above figures: top plot is the result from GWINC. Its title should be Al0.92Ga0.08As coatings, not SiO2/Ta2O5, bottom picture is taken from Cole, etal. TO noise crosses coating brownian noise around 3 Hz for both plots, however the slope is very different. NOTE: the y axes are in Hz^2 / Hz.

As a quick check for the proposed half wavelength cap solution to reduce TO noise, I modified the layer structure and computed TO noise. Since they did not mention what kind of material for the cap I tried:

  1. 81 layers, starts with nH, ends with nH, the first layer is 0.5 lambda thick. This is not working.RefCav_AlGaAs_TOnoise.png
  2. 82 layers, starts wit half wave nL, followed by the original 81 layers. This also does not work. Both cases have comparable TO noise, but transmissions are different. RefCav_AlGaAs_TOnoise.png

I'll check their formula and GWINC to see where the differences are.

 

Quote:

[matt, tara] Got AlxGa1-xAs material parameters from Matt Abernathy. I plug the numbers (all in SI) in GWINC, but the result is still not quite similar to that in Cole etal paper.

 ioffe has materials parameters for TO noise calculation.

Specific heat: 0.33+0.12x J/gK
 Mass density rho = 5.3165-1.5875x g/cm^3
Thermal conductivity,kappa: 0.55-2.12x+2.48x^2 W/cmK  (There is also thermal diffusivity = kapp/(rho*specific heat) [m^2/s]. The results are the same)
Thermal Expansion: (5.73-0.53x)·10-6/K

dn/dT: 3.66-2.03x *10^-4/K
This is from a paper, "Thermal dependence of the refractive index of GaAs and AlAs measured using semiconductor multilayer optical cavities", by Talghader and Smith. Keep in mind that this paper has an important Erratum if you want use values from it.
Unfortunately, this paper measures dn/dT at a max wavelength of 1030nm, so it's not quite accurate, but probably good enough.

Note:

One of the variables in GWINC code is ThermalDiffusivity. But the numbers used in previous TO plot is thermal conductivity of materials. I'll check the TO calculation codes and see if it is just a naming error, or the calculation is actually wrong.

 

Attachment 3: RefCav_TOnoise.png
RefCav_TOnoise.png
Attachment 5: RefCav_AlGaAs_TOnoise.png
RefCav_AlGaAs_TOnoise.png
  1168   Thu May 2 03:03:48 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

[matt, tara] Got AlxGa1-xAs material parameters from Matt Abernathy. I plug the numbers (all in SI) in GWINC, but the result is still not quite similar to that in Cole etal paper.

 ioffe has materials parameters for TO noise calculation.

Specific heat: 0.33+0.12x J/gK
 Mass density rho = 5.3165-1.5875x g/cm^3
Thermal conductivity,kappa: 0.55-2.12x+2.48x^2 W/cmK  (There is also thermal diffusivity = kapp/(rho*specific heat) [m^2/s]. The results are the same)
Thermal Expansion: (5.73-0.53x)·10-6/K

dn/dT: 3.66-2.03x *10^-4/K
This is from a paper, "Thermal dependence of the refractive index of GaAs and AlAs measured using semiconductor multilayer optical cavities", by Talghader and Smith. Keep in mind that this paper has an important Erratum if you want use values from it.
Unfortunately, this paper measures dn/dT at a max wavelength of 1030nm, so it's not quite accurate, but probably good enough.

Note:

One of the variables in GWINC code is ThermalDiffusivity. But the numbers used in previous TO plot is thermal conductivity of materials. I'll check the TO calculation codes and see if it is just a naming error, or the calculation is actually wrong.

  1165   Wed May 1 01:45:55 2013 taraNotesPMCDebra matrix for PMC design

Considerations for PMC design is corrected and updated

  1163   Tue Apr 30 01:15:26 2013 taraNotesNoiseBudgetAlAs/GaAs layer structure optimized for TO

I'm computing  coating Brownian and thermo optic noise (TO)  in AlGaAs coatings using GWINC code to compare it with the result reported by G Cole etal. Brownian noise from my result is similar to theirs, but TO noise is still not correct.  I'm working on it.

==Background==

We have talked about what kind optimizations should we go for AlGaAs coatings in order to minimize TO noise. There are two choices for us to consider

  1. using the layer structure as proposed in T1200003, or
  2.  adding a half wavelength cap on top of the quarter wave stack coatings as suggested by the authors.

Since the second option is more desirable in terms of manufacturing because of its simplicity, I decided to check if it really can  bring TO noise below coating Brownian noise. If it is true, we can use it for our mirrors.

 ==calculation==

  • I use GWINC code for TO noise and brownian noise calculations to verify the result if they are agree or not.
  • Materials parameters used in the calculation are taken from the paper. But most of the coatings material properties of an individual layer of AlxGa1-xAs are not provided. There are only the average values of thermal expansion, heat capacity, thermal conductivity, dn/dT.  There are refractive indices (nh/nl = 3.48/2.977) and layer structure (81 layers, starts with nh, ends with nh). So, as a start, the values for high index material and low index material are the same as the averaged values.

==result==

  • My Coating thermal noise level is 8.4*10^-35 m^2/Hz while their result is 9.8 *10^-35 m^2/Hz, @ 1Hz. This is not very bad, since there are some differences in the formulas between GWINC and their calcualtion.
  • However, my TO is off the roof, almost 2 orders of magnitude above their result. I'm checking if it is because of the code is wrong(typo in the parameters) or the fact that I used all the averaged values.

(I'll add more details about the calculations later)

  1159   Thu Apr 25 10:57:52 2013 taraNotesPurchasesPomona BNC cable

I plan to order RG58 bnc cables from Pomona, here is a list of what I need

catalog

  1. Cables for Fast monitor (from TTFSS to electronic shelf) (~15ft , x2)
  2. For slow feedback (from the shelf to the laser controllers) (~20ft x2)
  3. For EOM temp control feedback (I'm not sure where the nim crate will be, this will be decided soon).
  1158   Tue Apr 23 22:28:53 2013 taraNotesTempCtrltemp sensor on heat shields

I checked both temp sensors on the heat shields. They are working. I can see the change in resistance when I the heater is on. It seems to be a wiring problem. I'm investigating it.

  1157   Tue Apr 23 18:43:39 2013 taraDailyProgressRefCavHOM for new sideband frequencies

35.5 MHz and 38MHz sideband frequencies are chosen for  1.45 " refcav. These frequencies will be suitable for cavities formed by 0.5/0.5m RoC mirrors and 1.0/1.0m RoC mirrors.

 

HOM35.5MHz0.5m.png                           HOM38MHz0.5m.png

  a) For0.5m/0.5m RoC mirrors 1.45" cavity, f1 = 35.5MHz.           b) For0.5m/0.5m RoC mirrors 1.45" cavity, f2 = 38MHz

HOM35.5MHz1m.png                                HOM38MHz1m.png

 c) For 1m/1m RoC mirrors 1.45" cavity, f1 = 35.5MHz                d) For 1m/1m RoC mirrors 1.45" cavity, f2 = 38MHz

 

Since we will use a crystal oscillator to drive the EOMs, I have to check how much power we need for the sideband.

If the crystal oscillator can provide us with enough power, we can use the crystal to drive a broadband EOM directly. Otherwise we will need an EOM driver, or a resonant EOM.

 

==shot noise level vs mod index(Beta)==

shot_vs_beta.png

To see how much should the mod index be, I plot shot noise level vs Beta, with Power intpu = 1mW and 2 mW, and Finesse = 1e5 (for  T=300 ppm mirrors)and 2e5 (For AlAs/GaAs coatings), with mode match = 80%. It seems that for the lowest shot noise level, we need beta = 0.8.

For resonant EOM, mod depth = 0.2 rad/V, for BB EOM, mode depth = 15mrad/V , see psl:745.  These correspond to 4V (25dBm) and 53 V (47dBm) for the resonant and BB EOMs, respectively.

 

  1155   Mon Apr 22 17:13:24 2013 taraNotesDAQslow feedback to laser via EPICS is on

The medm screen for the 1st laser is completed, the servo is on an stable. Refcav has been locked for a few hours as of now.

  • The output for slow feedback is on J9, slot 9/10 (VMIVME 4116 C2 S3). This is an unused channel previously assigned to PSL-ISS_ISET, I checked ISS.db file to look for the VMIVME address. For the slot number, I look up the channel name in D980535-C-C document.
  • I added low pass filters (~100mHz) to both input and outputs of EPICS.
  • All EPICS channels for slow feedback and perl scripts are in SLOW_LASER.db file.
  • The startup.cmd file is updated accordingly.
  • servo gain is optimized. ??? What does that mean??? How about some performance plots? (About the bode plot, I'm trying to get a transfer function of the NPRO slow input, with that I can estimate the bode plot of the loop. As of now I just adjust the PID gain so that the loop is stable)

KP.jpg

fig1: FAST feedback to the laser is shown in blue plot, vertical axis:1V/div, horizontal axis:  4 sec/ div. I adjusted proportional gain first, to get only a few overshoots with acceptable rise time.

KI.jpg

fig2: Then I adjusted integral gain to eliminate the offset, and Derivative gain to reduce overshoot. More about PID gain can be found here. Current Value KP = -0.0002, KI =-0.00015, KD = 0.

I set the output to be between -2 V to 9 V. Since we need to lock it to GYRO later, it has to be able to be tuned to match the gyro laser. Currently, Gyro laser is operated around 35 Deg C which is similar to 8V input to slow feedback.

 

I'm trying to draw a cartoon for DAQ wiring in CTN lab for future reference. This is what I have so far. I'll add it in WIKI page.

CTN_DAQ_wiring.png

  1151   Thu Apr 18 19:54:01 2013 taraNotesDAQEPICS channel for slow feedback to laser

I created a channel for feedback to slow DC to the laser head. The servo will be done digitally using a perl script similar to what we have for the vacuum can.

 

There are unused channels for temperature monitor, so I modify them for FAST MON instead.

In the database file "cavities.db" in the sun machine, I changed [C3:PSL-BOX_SENS1] to [C3:PSL-ACAV_FMON]. For input +/- 10 V.

The next thing is to create perl scripts for the servos. Then find an output channel for feeding back to the laser.

 I made an medm screen for controlling the slow feedback signals to both lasers.

C3PSL_LASER_SLOW.png

 Channels that will be created are:

input

  • C3:PSL-ACAV_FMON

soft channels

  • C3:PSL-ACAV_PID_KP
  • C3:PSL-ACAV_PID_KI
  • C3:PSL-ACAV_PID_KD

 output

  • C3:PSL-ACAV_SLOWOUT
  1149   Mon Apr 15 10:59:35 2013 taraNotesPMCDebra matrix for PMC design

Quote:

Considerations for PMC design:

  1. Stiffness(Acoustic susceptibility) & heavy material: With heavier material, the pmc motion on the support becomes smaller.(RXA: please quantify with a formula)
  2. Filtering factor (Finesse/FSR/Cavity pole), g-factor: Filter out intensity noise around 10 MHz (RXA: please quantify with a formula)
  3. Design for thermal expansion cancellation between the spacer and the end cap: So that the PMC is less sensitive to ambient temperature
  4.  3 or 4 mirrors?  3 is polarization selective. For general lab use with power less than 1 W,  3 mirror design should be good. (RXA: I don't follow this logic at all)

RXA: In general, all of these considerations need some sort of quantitative detail. Make a DeBra Matrix so that we can evaluate. 

 Some requirements for the PMC:

==Cavity pole==

 For intensity filtering. The modulation frequencies for the refcavs is ~ 15-25 MHz, we want the intensity fluctuation at this frequency to be shot noise limited.  We have to determine what should be the frequency pole. Intensity noise around 1MHz - 30MHz will be ~ 1/f^2, see the paper by Harb etal, eq1 and fig9, get the paper from psl:1156. Under the assumption that RIN remains constant, at 20MHz the laser will already by shot noise limited (@ 1mW input).  laser intensity noise / shot noise ~ 0.16. (laser intensity noise here means intensity noise from spontaneous emission/ pump-source intensity noise/ dipole fluctuation noise/ noise from intra cavity losses, any thing except shot noise)

laser_rin.jpg

  Thie pole can change with the cavity length and Finesse, [ Finesse = FSR/(2*cavity Pole)] , so our choices for mirror reflectivity, cavity length will affect this number as well. So for a fixed set of mirrors (fixed finesse), longer perimeter means lower cavity pole, but the cavity will be more susceptible to acoustic coupling.

==First longitudinal body mode==

  It should be at high frequency ( for high UGF servo). The shorter the length, the higher the frequency. See PSL:1134.

== g-factor==

 For a stable cavity, g factor has to be between 0 and 1.  Another reason: We should choose g-factor such that HOMs do not coincide with other cavity axial modes (FSR apart). For a ring cavity with 2 curve mirror R1,and R2, g = (1- p/R1) x (1 - p/R2) where p is the round trip length. (For 3-mirror cavity, g = (1 - p/(R))^2 . See HOM calculation.

==Stiffness==

 we want a solid, bulk shape PMC, not thin long one. This will make the PMC less susceptible to acoustic noise.

==Higher order mode suppression== 

Other transverse modes will be suppressed by a factor of (1-r)^2 / (1 +r^2 -2rcos(2*pi* dfmn/ FSR)  where dfmn is the gouy phase shift of m+n mode, r =r1*r2*r3.. (reflectivity of each mirror in the cavity) see evan's note. Transverse modes of the output of the NPRO can be found by scanning the PMC and measure the transmitted beam. Other modes beside TEM00, will be reflected back from the refcav and incident on the RFPD. This will cause the mode mismatch and increase shot noise level. Usually, higher r (higher Finesse), will suppress more HOMs.

==Build up power==:

= Pin x Finesse/ pi. CVI mirrorsfor high damage threshold power have maximum power for cw around 10MW/cm2. So I use this number as an upper limit for the power threshold. Assuming the power input is ~ 30 mW, average spotsize is 350 um. This gives ~ 8W/cm2. So Finesse can be up to ~ 3e6.  (10 MW/cm2 > (Finesse/pi) x 8 W/cm2) .

 

Some assumptions:

  • Losses(scatter/absorption) on each mirror is ~ 100 ppm. It seems that a super polished mirrors in vacuum has ~ 10 ppm loss. This comes from a Finesse measurement of the previous 8" refcavs, see psl:1046. The calculation shows that loss in one cavity is 25 ppm (for 2 mirrors), and 160ppm for another cavity. Since the PMC mirrors will be in air, and probably not as good as refcav mirrors, dust in air might accumulate over time and causes extra loss on the mirrors, 100 ppm loss assumption might be ok for this calculation.
  • PZT range is about 15um @1000V, as shown in the catalog, see PSL 1052 for the details, (we can drive it with ~0-300 V, so ~ 4um displacement),see PSL:1052

 

Let's see some of the designs that are available. Then we can decide which one we should modify to suit our requirement.

  1. Design1 iLIGO PMC: Isosceles triangular PMC, fused silica, perimeter = 0.42m, flat-curve (1m ROC)-flat mirrors. Round Trip = 0.42m See T-080195,here (it says the pole is 7 MHz).
  2. Design2 (Dmass' PMC): stainless steel PMC, perimeter =0.4m , same mirrors as those of design1, so its finesse is the same.
  3. Design3, AdvLIGO PMC style (4 mirrors, bow-tie): stainless steel (see PSL:)
  Cavity pole /FSR/ Finesse g-factor Stiffness  1st Longitudinal body mode  Approximate dimension(height x width  x length)  Note
Design1  cav pole = 7MHz / FSR=714MHz / Finesse =50 0.34    14 kHz  2" x 2.4" x 7.1"  The values are for p-pol, waist radius = 370um.
Design2  cav pole =  9MHz  /FSR = 925MHz / Finesse = 50  0.46    16.6kHz [PSL:1134]  2 x 2.6 x 6  assuming similar mirrors from design 1, w0 = 353 um.
Design3            
             
             
             

 

Attachment 1: laser_rin.jpg
laser_rin.jpg
  1146   Tue Apr 9 15:39:16 2013 taraNotesPMCAbout PMCs

Considerations for PMC design:

  1. Stiffness(Acoustic susceptibility) & heavy material: With heavier material, the pmc motion on the support becomes smaller.(RXA: please quantify with a formula)
  2. Filtering factor (Finesse/FSR/Cavity pole), g-factor: Filter out intensity noise around 10 MHz (RXA: please quantify with a formula)
  3. Design for thermal expansion cancellation between the spacer and the end cap: So that the PMC is less sensitive to ambient temperature
  4.  3 or 4 mirrors?  3 is polarization selective. For general lab use with power less than 1 W,  3 mirror design should be good. (RXA: I don't follow this logic at all)

RXA: In general, all of these considerations need some sort of quantitative detail. Make a DeBra Matrix so that we can evaluate. 

  1142   Fri Apr 5 13:56:01 2013 taraNotesRefCavestimated beat frequency

Note:A test to check which cavity needs to be heated up.

1) when heaters (on the shields) are off, C3:PSL-VAC_CHMBRTMP =31.2

  • CAV1 is locked @ (slow out Coarse/fine) = 774/500
  • CAV2                 @ 769/500

2 when heater on CAV2(4V), C3:PSL-VAC_CHMBRTMP = 31.2

  • CAV1             @ 775/500
  • CAV2              @ 756/500

So to bring both cavity to be resonant at the same time, the heater on cav1 should be on.

I'm not sure if the thermometers on the shields are working or not, I'll check them.

Quote:

At current temperature, the estimated beat frequency will be ~ 60-100MHz. This is not so bad, since we can use 1811 to measure the beat signal and use PLL to extract the beat noise.

 

We will need to use thermal expansion to tune the beat frequency. So, as a start, I try to figure out the beat frequency, and how much we have to heat up the cavity. The heaters on each cavity is off, only the heater around the vac chamber is on (but the servo is off). 

Right now we have one laser locked to one cavity, but the beam path to the first cavity has a beam splitter that we can borrow the beam and direct it to the 2nd cavity. I realigned the beam to have both beams into both cavities. By adjusting the temperature control on the NPRO (slow signal), I can bring the beam to resonant in each cavity.

1st cav is resonant @ (334/398) and (150/398). The numbers correspond to coarse and fine knobs of the slow feedback to the laser.

2nd cav is resonant @ (154/398). (I'll come up with a better name to call the cavities)

The FSR is 4.07 GHz (for 1.45" long cavity). This means 334-154 = 184 clicks on the coarse knob equals to 4.07 GHz, or 22MHz per coarse click. Both cavities resonant at ~3-5 clicks apart. So the beat frequency is ~ 60-100MHz. This is quite good, at least we are not close to half FSR apart. The power required to tune the cavity length should not be that high.

 

The next thing to do is try to see which cavity we need to heat up in order to bring both cavities resonant frequency closer together.

 

  1139   Thu Apr 4 18:45:38 2013 taraNotesDAQTemperature control updated

[peter, tara]The temperature servo for the chamber is back on, the current setup is at 31.2 C.

There was a problem with C3:PSL-VAC_CHAMBERTEMP channel, and I could not run the script for temperature control of the chamber. Peter helped me figure out what happened. It turned out that one of the parenthesis in the database file (cavities.db) was missing due to an accidental delete, and the name of the channel was too long (it was working before, I don't know why).

Anyway, the channel was renamed to C3:PSL-VAC_CHMBRTMP, in 1)cavities.db, 2)rcav_PID_2012_06_15.pl, and 3) medm screen for controlling the servo. The temperature servo is working again.

Quote:

We switched the temperature readout channels used for temp feedback control to improve the signal. The new signal is significantly smoother.

      The signals from 4 thermostats around the vacuum chamber were acquired through 4 channels, C3:PSL-RCAV_SENSE(1-4). These channels were then connected to DAQ. This made the signal noisy because the resolution of analog to digital converter was low. In order to fix that we use an analog circuit to sum and average the signals from 4 sensors then amplify it before sending to DAQ,C3:PSL-RCAV_TEMP, then calibrated it to C3:PSL-VAC_CHAMBERTEMP by comparing RCAV_TEMP [V] to RCAV_TEMPAVG[C] which is calibrated to deg C already. 

      CHAMBERTEMP = (RCAV_TEMPx-0.495) + 34.957

     

     We corrected the perl script (in SUN machine) used for thermal feedback on the heater jacket. Now the script is named rcav_PID_2012_06_15.pl, see wiki. The servo is now back on.

 

  1137   Mon Apr 1 20:59:19 2013 taraNotesPMCPMC Longitudinal Testing and Modal Analysis

I calculated some requirement for the beam jitter at the output of the PMC. A rough estimate shows that we need the angular stability at the PMC about half nano radian so that the frequency noise of the beam locked to the refcav is less than 10-2 Hz/rtHz.

==Background==

PMC also reduces beam jitters from the laser, so that the beam alignment to the cavity is kept centered. Since the laser is locked to the reference cavity, any misalignment of the input beam will cause the beam to sense the change of the cavity length.

So vibration that shakes the PMC will change the alignment of the output beam. With stiff material, the seismic induced deformation of the PMC will be reduced.

==Calculation==

  • calculate the ray tracing matrices from the PMC to the cavity. I assume that only the angle of the output beam changes due to PMC sagging, because of a long distance from the PMC to the refcav, with several mirrors in between. This gives me the position and the angle of the beam going to the cavity.
  • find out what is the change of the cavity length (dL), when the input beam is translated by dx, with angle theta.
  • convert displacement noise to frequency nosie (dL -> df), as a rough estimate I choose the requirement for df to be less than 10-2 Hz/rtHz (about the level of the estimated coating noise). This step is not really necessary, but I feel that it is easier to compare the noise in Hz/rtHz unit rather than m/rtHz.
  • The required angular stability at the PMC is ~ 0.5 nano rad. This number seems to be too strict. I will double check it.

==next==

Eavn is working on COMSOL to find out the angular tilt of the output beam due to PMC sagging. Optimum support points will be determined to minimize beam jitter due to seismic.

  1136   Sun Mar 31 20:06:16 2013 taraNotesPMCPMC Longitudinal Testing and Modal Analysis

Quote:

I think the analytical formula in terms of rho is going to be (1.57/2*pi) * sqrt(E / rho * L^2), since the Roark formula is (1.57/2*pi) * sqrt(A* E * g / w * L^2) and the weight per unit length is w = m * g / L = rho * A * g. With your values for L, A, E, and rho, this gives f1 = 16 kHz. Since A does not appear in the analytical formula, this also explains why changing the area in the Comsol model doesn't change the frequency.

 

 

 good catch! Thanks. Then both analytical and FEA results are the same. So our COMSOL results for PMC should be valid, the first body for a stainless steel PMC, see psl:1131,at 16 kHz is reasonable.

  1134   Sun Mar 31 03:03:28 2013 taraNotesPMCPMC Longitudinal Testing and Modal Analysis

[see PSL:1135]

I compared results between COMSOL and analytical solution. The first longitudinal mode from both results are comparable.

Peter sent me a note from Dennis about PMC longitudinal mode calculation. Dennis mentioned about a book by Young&Roark (here), so I looked it up and see how to estimate body mode frequencies of a simple block/beam.  I tried a simple geometry, a 0.1x0.1x0.175 (m) block. According to the book, cf situation 7b, table16.1 page 771, the first longitudinal mode is

f1 = (1.57/2*pi) * sqrt ( E/ rho*L^2), ), rho is the mass density of the material (2202 kg/m^3, for SiO2), E is the Young's modulus (72 GPa), L is the length of the block ( I use L = 0.175/2 because 7b situation is a uniform bar vibrates along its longitudinal axis, with upper end fixed, lower end free. This is similar to a whole beam resonate freely on both end because its center will be fix. Thus, to use the formula for our case, we have to use half length of the beam).

The analytical solution and COMSOL give f1 ~ 16 kHz.

psl_log2.png

 It is very strange that, according to COMSOL simulation, when the cross sectional area of the block is changed to 0.01x0.01 m^2 instead of 0.1x0.1 m^2, the frequency of the longitudinal mode does not change that much (still close to 16kHz. However, from the analytical solution, the frequency should drop by a factor of 10 ( around 165 Hz).

I'm going to think about this a bit more, but at this point, I think my COMSOL model is not correct. Might be some kind of bdy conditions that I'm missing.

psl_log.png

 

 

 

  1133   Sat Mar 30 01:42:16 2013 taraDailyProgressElectronics EquipmentFrequency readout noise of PDH loop

It is indeed really high. We expect something in the order of ten nV/rtHz level (like what we had before, see PSL:781.). We are investigating it.

One of the possible causes is the back reflected beam. When I reduced the power input from 6mW to 1mW (with common/fast gain = 999/720, boost on), the error noise was down to ~ 10nV/rtHz up to 5 kHz, with bumps and peaks around 50kHz up to 100kHz. Our mode matching this time is not very good.

 I replaced the PBS for PDH locking to a bigger one, since the beam spot at that position was quite large. This got rid off some high frequency peaks and bumps. I still have to align the beam to minimize back reflection.

 

Quote:

Is this really the input noise of the PDH servo (i.e., output noise / servo TF)? If so, it seems pretty high. With the right components, you should to be able to do ~100x better.

Quote:

Tara was able to tame the cavity servo loop. The third attachment is the result of several SR785 measurements of the error signal power spectrum. I converted this to a frequency noise power spectrum (fourth attachment) by extracting the voltage/frequency calibration factor from the error signal as follows.

 

 

  1131   Wed Mar 27 01:52:34 2013 taraNotesPMCPMC Longitudinal Testing and Modal Analysis

I use COMSOL to find the first longitudinal mode of a stainless steel PMC,  it is about 16 kHz. I'll find an analytical solution and compare them to make sure that the FEA result gives us a reasonable answer or not.

The FEA result in psl:1088 does not show the right body mode of the PMC. The frequency of 440 Hz is from some weird mode as seen from the figure in the entry. Evan checked the body mode of a simplified steel PMC, and I also check independently. Our results agree quite well that the first longitudinal mode is at ~16kHz.

pmc_eigen.png

However, this does not answer what we measured in PSL:1097, where the longitudinal motion is around 300Hz. I checked the body frequency of the base blocked and it is even higher than the PMC body modes' frequencies (this should be expected since the base is even bulkier).

Note: I just learned from Zach that the PMC in GYRO setup does not have 3-point support. It just sits on the base block. But this has not given me any clues about the possible modes yet.

 I'm writing some background and requirement for the PMC[coming soon]

 

  1124   Wed Mar 20 22:18:28 2013 taraNotesRefCavestimated beat frequency

At current temperature, the estimated beat frequency will be ~ 60-100MHz. This is not so bad, since we can use 1811 to measure the beat signal and use PLL to extract the beat noise.

 

We will need to use thermal expansion to tune the beat frequency. So, as a start, I try to figure out the beat frequency, and how much we have to heat up the cavity. The heaters on each cavity is off, only the heater around the vac chamber is on (but the servo is off). 

Right now we have one laser locked to one cavity, but the beam path to the first cavity has a beam splitter that we can borrow the beam and direct it to the 2nd cavity. I realigned the beam to have both beams into both cavities. By adjusting the temperature control on the NPRO (slow signal), I can bring the beam to resonant in each cavity.

1st cav is resonant @ (334/398) and (150/398). The numbers correspond to coarse and fine knobs of the slow feedback to the laser.

2nd cav is resonant @ (154/398). (I'll come up with a better name to call the cavities)

The FSR is 4.07 GHz (for 1.45" long cavity). This means 334-154 = 184 clicks on the coarse knob equals to 4.07 GHz, or 22MHz per coarse click. Both cavities resonant at ~3-5 clicks apart. So the beat frequency is ~ 60-100MHz. This is quite good, at least we are not close to half FSR apart. The power required to tune the cavity length should not be that high.

 

The next thing to do is try to see which cavity we need to heat up in order to bring both cavities resonant frequency closer together.

  1121   Fri Mar 15 11:23:45 2013 taraNotesopticAlAs/GaAs

I'm thinking about the spec for AlAs/GaAs coatings. Here is the list of what I have:

  • coating on concave side of the mirror for 0.5m x6 (I'm not sure if they can do the transfer on 0.5m mirror now) for 1.0m x6 for flat mirror x3 -
  • for circularly polarized light, normal incidence
  • Transmission @1064 = 100ppm +/- 10ppm. 10% error is still within the acceptable value for 10ppm loss (T ~ 67-73%), see T1200057v11 -
  • Absorption + scatter loss < 10ppm, this is what Garrett told us. -
  • coatings diameter = 8mm (The number is from Garrett), the loss around the edge for our beam with diameter=364 um is less than 10^-10 ppm. -
  • Max scratch surface and point defects are not determined yet. I can look up the specs from our current SiO2/Ta2O5 mirror since they are ok for us. -
  • I think we are aiming for the thermo-optic optimized coatings. The layer structure can be found in T1200003-v1.

==Coating diamter for 0.5m ROC mirror==

About the coatings diameter, Garrett said it depends on the aperture size/ coating diameter. So I made a plot to estimate the loss due to the finite size coating vs Coating diameter for our spot radius of 182 um. The loss is simply calculated by the ratio of the power not falling on the coating = Ploss/Pin = (exp(-2*r0.^2./w0.^2))*1e6*26000/pi   

where r0 = coating radius, w0 = spot radius, a factor of 1e6 for showing the result in ppm, 26000/pi is the total loss due to the light bouncing in the cavity.

edgeloss.png

fig1: Loss vs coating diameter (in meter)

 

It seems we can go to 2mm coating diameter, and the loss is still much less than 1ppm (the expected loss from absorption and scatter is ~ 10ppm). However, we have to consider about how well they can center the film, how well we can assemble the cavity. So larger coating diameter is always better. If we assume that 1mm error is limiting us, coating diameter of 4-5 mm should be ok for us.

 ==for mirror with 1m ROC==

If the ROC is 1.0m, the coating diameter can be 8mm. For the cavity with 1.45" long, the spot radius on the mirror will be 215um (182um with 0.5m mirror). This changes the noise budget of the setup a little bit. The total noise level is lower by a factor of ~ 1.2. (see below figure) at 100 Hz.

noise_compare.png

fig2: Noise budget comparison between setup with 0.5 m and 1.0m RoC mirrors, plotted on top of each other. Noises that change with spotsize are coating brownian, substrate brownian, thermoelastic in substrate, and thermo-optic.

 

==What do we choose? 0.5m or 1.0m==

For both 0.5 and 1m, the cavity will be stable (see T1200057-v11, fig11). So either choice is fine

if we use 1.0 m,

  • we loss the signal level a bit,
  • but we are more certain that the coating will work. 
  • The procurement should be faster (as promised by Garrett)
  • have large area coating up to 8mm diamter
  • need to check if we can mode match or not (I'm positive that we can, but I'll check or let Evan check)

So at this point, I'm thinking about going with 1.0 m mirror.

 

 

  1119   Thu Mar 14 18:20:04 2013 taraNotesopticfused silica pmc

Peter told me that  the fused silica pmc currently used in the lab is bonded by Vac-seal epoxy. So we don't need to polish any surfaces for optical contact.

IMG_0819.JPG

Traces of vac-seal can be seen between the mirror and the tip, the tip and the spacer bonded areas. Vac-Seal epoxy is chosen for its low out gasing, so that the mirrors won't be contaminated.

IMG_0820.JPG

  1117   Wed Mar 13 23:52:12 2013 taraDailyProgressVacuumCavities inserted; chamber pumping down

It's still improving, now the current is 0.3 mA (0.38mA yesterday). I'll wait until it stops improving and try to tighten the screws a bit. \

About the plastic pieces, they are peek. I think it is vacuum compatible, cf E960050-v11.

May be it is the temp sensor that I have to re-solder on the copper pieces. I did not bake them after soldering.

Quote:

The gap at the frange is OK as long as the gasket is evenly squeezed.

1. It the pressure still improving? => Your cavity is still out gassing.

2. Fasten the screws little more (not too much) for these two franges.
    => If there is any improvement there is some leakage.
    => If not, this is just a outgas from your cavity.

Do you think your black plastic pieces are vacuum compatible? Are they made of Delrin?

 

  1114   Tue Mar 12 18:20:21 2013 taraDailyProgressVacuumCavities inserted; chamber pumping down

After pumping the chamber down for two days, I disabled the turbo pump and turned on the ion pump. The initial current was 7mA. After a day now it is 0.38 mA. It was better before, see PSL842, (started the ionpump at 1mA, and operated at less than 0.1 uA). If this is true, this means the pressure will be about a factor of 0.38mA/0.1uA ~ 3800 higher than before. (the calculation is based on this equation, where the current is directly proportional to the pressure.

 

The flanges do not flatly touch each other. There's a tiny bit of something in the middle that causes a gap as shown below. This might be the reason why the ion pump current is high, or it might be some out-gasing problems of the in-vac materials. I think we can try to spray isopropanol around the flange to see if the current comes up, but I think it is a bad idea for the ion pump. I'll ask Steve or Koji for their opinions.

 IMG_0815.JPG

The pictures during the installation can be found onpicasapage

Quote:

 [Tara, Koji, Evan]

Tara and Koji spent the better part of yesterday afternoon inserting the new cavity assembly into the vacuum chamber. In the process of putting the window back on the chamber, the old copper gasket may or may not have hit the inner surface of the window, so Koji performed a drag wipe. Tara and Koji then inspected the window under a high-power bulb, and I think the consensus is that there's no visible damage.

The chamber is currently pumping down. Unfortunately, there appears to be a small speck of something trapped between the window and chamber flanges, so there's a small gap on one side of the joint. We'll see if we can achieve high vacuum.

 

  1111   Wed Mar 6 21:56:12 2013 taraDailyProgressVacuumnote on feedthrough

Feedthrough channel (as seen from the connector outside of the chamber)

1-6: heater on cavity#98 : 85.4 Ohm

3-7: Temp sensor on cavity #98

 

4-8: heater on cavity#99: 156 Ohtm

5-9: Temp sensor on cavity#99

 

  1109   Wed Mar 6 12:48:50 2013 taraDailyProgressVacuumthermal shield + peek supports are baked

I setup the small vacuum chamber to bake the shields and peek cavity supporting pieces. All pieces are baked and I'm assembling all the parts.

 photo(13).JPG

Details about how to use the pump and the chamber can be found in CTNwiki.

  1108   Sun Mar 3 22:08:26 2013 taraDailyProgressVacuumopening vac chamber

 I found all necessary parts for the heater (crimp connectors, heating wire). I'll bake all parts once all the wiring is ready

 

 IMG_2255.JPGIMG_2258.JPG

fig1: left Dsub connector for the cable, Right, heater(yellowish wire around the tube) is connected to wiring cable with crimp connectors.

download.jpg

fig2: crimp connector( vacuum compatible material). I need to borrow the suitable crimp tool from Down, see PSL:775

heater.png

fig3: above, previous wiring (one heater,3 sensors), below current wiring (two heaters, two sensors)

  1107   Thu Feb 28 09:12:22 2013 not taraDailyProgressTempCtrlHeater for refcav

 To make a low noise current driver, use the BUF634 wrapped around the loop of a low noise opamp (never, ever use the AD797) as illustrated in the BUF634 datasheet.

Don't use a commercial power supply, make a protoboard setup for a NIM box.

  1106   Wed Feb 27 10:16:09 2013 koji, taraDailyProgressVacuumopening vac chamber

We opened the vacuum chamber and brought the stack with the 8" cavities out to the clean bench. New 1.45" cavities are under preparation to go in the chamber.

The 8" cavities and the double seismic isolation stack were removed from the chamber. The connector had to be removed from the inside of the mini flange for the feedthrough [add pic]. We replaced the top seismic stack along with 8" cavities and their mounts, with the new stack/new mount for the shorter cavities. We reuse the bottom stack.

Next is to modify the wiring for heaters and temp sensors. Currently, the connector is wired for 3 sensors and 1 heaters (for 9-pin connector). Soldering at the 9-pin connector seems to be a tough job. Koji suggested that I remove one of the temp sensor at its legs, not at the connector end. Then connect the unoccupied cables to the heating wire.

  1105   Wed Feb 27 09:57:33 2013 taraDailyProgressTempCtrlHeater for refcav

I'm contemplating about how to drive the heater of the copper shields. So here is a list of what happening.

Objective: to drive the heater with ~ 1W power (see the previous entry). The estimated power is from my calculation. I have not taken heat conduction/uncertainties of surface emissivity into account, we might need more power to be on the safe side.

Problem: If we use a low noise driver (using AD797, cf PSL:765), the maximum delivered power will be only~0.5 W.

So, do we need the low noise driver? or do we need to have enough power? If we just need enough power, we can drive the heater with a commercial power supply.

At this point, I think it is more important to be able to tune the cavity length with larger range so that we can see the beat. Thus, sufficient heating power should be our top priority . Better heater/ control can be developed next, once we see the beat.

So the plan is:

  1. Wrap both shields up with heating wire. The length of the wire (total resistance) depends on the limiting current/voltage available from a power supply to ensure ~2W power.
  2. Put the cavity back and bring the laser to the cavities.
  1104   Mon Feb 25 01:02:58 2013 taraDailyProgressVacuumHeater for refcav

I did the calculation to estimate the required power for heating up the cavity by 20 K above room temperature. More details are coming.

 ==Motivation==

Since we plan to tune the beat frequency by tuning the cavity length with thermal expansion, we need to know how much power is needed to heat one cavity up by 20K above room temp (see LIGO-1200057 for more detail).

 ==Model==

To simplify the calculation, I use 2-D model for the cavity and the shield. Assuming the system is in equilibrium. In the calculation, I considered the effect from reflected radiation from both the shield, and the cavity itself.

photo(10).JPG

 ==result==

  •   At room temperature of 29 C, depending on the emissivity of the shield, the tube has to be from 20 C - 100 C above room temperature inorder to keep the cavity at 20 C above room temp. Since the emissivity of the copper can vary from, .03 (highly polished)-0.77 (oxide),  see ref. Our cavity is polished a bit, so the reasonable value should be ~ 0.1-0.2.  Thus, the shield temperature should be able to reach at least 50 [C]. 

heater.png

 

 

  •  Next is to estimate how long the wire should be for bringing the shield up to 50 degree C. If all the heating power from the wire goes to radiation, the radiated power from 50[C] shield, at room temp is sigma*epsilon*area*(T^4 - T0^4) which is ~ 0.6 - 1 Watt Note: I checked the calculation and Frank's note in PSL:765, the calculated number is reasonable.
  1103   Sun Feb 24 01:53:43 2013 taraNotesVacuumforgot to turn off the ion pump while venting

I vented the vacuum chamber and forgot to turn of the ion pump. It was on for ~ 5-10 minutes. I'm not sure if it is broken or not. I talked to Alastair and he said the metal plate might be contaminated and needed to be repair. I'll keep this in mind and see what happen when I pump down the chamber.

  1102   Sun Feb 24 01:16:26 2013 taraDailyProgressRefCavrefcav and mount assembly

Thermal shields and caps are ready. I cleaned them in ultra sonic bath with Isopropanol. They fit nicely to the mount.

The caps are teflon.

photo(9).JPG    photo(8).JPG

 

==Wire Heater==

I'm think about how to wire the heater around the cavity. I'm reading Frank's entries in PSL:765,768 ,776,786, . Seems like I might need to drill a few holes on the shields for the wires.  There are still more similar wire left. I'll calculate how long the wire has to be for heating the cavity up by 20 K.

 

  1101   Mon Feb 18 03:38:20 2013 taraNotesScheduleTask Sheet

Now that the layout is ok and the table is cleared, the next thing to do is open the chamber. The plan for this week will be:

==Cavities & Vac chamber==

  1. clear the clean bench
  2. Prepare all parts  for the new cavities that will go to the chamber. The assembly for the new cavities is almost done, I'm wating for the caps of the thermal shield.
  3. Open the chamber, remove the 8" cavities.
  4. Change the heaters/ thermometers. Currently, the feedthrough for the chamber is 9-pin. It is for 3 thermometer and 1 heater. The new setup will need 2 thermometers, 2 heaters. I don't know what kind of wire and where it is in the lab yet. This need to be checked.
  5. Once the sensors/heaters are installed, we can close the chamber and start the mode matching.

==1st laser==

  1.  The setup for the first laser should be easy. The setup from the laser to the PMC is not changed much except the BB eom for locking. I should be able to lock the first cavity soon

==2nd laser==

  1. Need to think about the PMC
  2. The EOM
  1100   Mon Feb 18 03:16:44 2013 taraNotesDrawingslayout for the new setup

The new optic layout is done. There are some changes from the previous version.

Some notes and concerns about the new layout:

  • I made sure that the beam size in all EOMs, EAOMs are in the right size (diameter ~ 250 - 500 um) to avoid any clipping/ diffraction.
  • Due to the small waist size of the new cavities and the long path between the cavity and the vacuum window. The beamsize on the input optic is quite large (almost 3mm). I'll have to check if the beam spiltter is large enough.
  • I assume that the beam size from the new PMC (the setup on the left) is 370um. It might change because the design for the PMC is not settled yet, but the spacer is enough for mode matching.
  • I add a beam splitter in the setup just before the vacuum window. This can be used for RFAM pickup or fiber distribution. It can be done in the future.
  • a set of flip mirrors are added, for scanning two cavities with one laser. The mode match should be good enough to get TEM00 in the 2nd cavity.

ctn_layout_2laser_v2.png

 

  1099   Wed Feb 13 19:54:32 2013 taraNotesLaserexternal cavity laser diode

Some notes about external cavity diode laser. I investigated in this about a year ago.  I think it might be a good time to work on it, since I'm modifying the  ctn layout and we can use a frequency stabilized laser (although locked to a short cavity) via fiber optic to test with a home made ecdl. 

Quote:

I just asked Aki a few more questions about the ecd laser. If we do not require the performance to be rival to that of the NPRO, making one is possible in a few weeks time scale.

Q1) The setup of the Littrow style laser you showed me had one mirror
behind the grating. Is the setup similar to this
<
http://rsi.aip.org/resource/1/rsinak/v72/i12/p4477_s1>? Where the
mirror is used so that the alignment does not change when the laser is
tuned.

A1) Actually, Yes, for the laser you took the photo. But, most of the lasers we use in the lab don't have a mirror, only diode and grating in the box. In our case, we only scan the frequency by ~1GHz and the pointing vector drift is negligible.

 

Q2) Did you remove the glass window of the diode laser when you assemble
 the laser? If so, how do you keep the diode clean, or it does not matter
for your requirement.

A2) We didn't remove the glass window. What we did is very simple. We mounted the bare diode on the thorlabs mount: http://www.thorlabs.com/thorProduct.cfm?partNumber=LT230220P-B

 

 Q3) You mentioned that the line width of the laser when locked to CS cell
is ~300kHz. Is it because of gain limited of the servo or the CS cell's
intrinsic noise?

A3) We haven't measure the linewidth with locking and without locking independently. It's possible that our laser linewidth (without frequency lock) might be ~300kHz. So I don't know what limits our linewidth.

One thing you may want to consider is that a diode laser is infamous for the broad background incoherent light, compared to the solid state lasers. We typically observe ~30nm-wide incoherent light around the carrier with 30-40dB suppression compared to the carrier. If your experiment is sensitive to the spectral purity, this might be an issue.

Aki

 

 So the question is do we want to try to build one similar to what they have? We know that with the time scale and experience we have it will not be as good as the performance of the ecd laser reported in Numata etal paper, but it might be a fun project for the SURF student.

Quote:

I'm looking into how to make external cavity diode laser (ecd).

Here is the list of what we need.

 

  • Laser diode 1064nm, diode (+AR coating)
  • Grating
  • Current driver for locking the laser (home made)

I ask Akihisa who works in Kimble's group about their home made 850 nm ecl. The performance is not as good as NPRO yet (300kHz line width when locked to CS cell), but it is certainly interesting.

  • They use Littrow configuration ( 1 grating to form a cavity). The mirror behind the grating is for adjusting the output beam.
  • The PD is not AR coated, a slight power (10 uW) that transmits inside the diode is enough for the feedback.
  • The linewidth is ~ 300kHz when locked to CS cell, the free running noise is not measured.
  • They use both PZT and current to actuate on the frequency stabilization, the bandwidth is 50kHz.
  • Temperature feedback is employed to keep the stability of the laser
  • Time for putting everything in the box (once all components are ready) is ~ 1wk

 (IMG_0887.jpg IMG_0888.jpg

 

 

  1098   Fri Feb 1 03:14:45 2013 taraNotesDrawingslayout for the new setup

Quote:

I'm making a new layout for the 2 laser setup. It is in progress. I just want to make a note about some features I want to have in the setup.

  • Move EAOM (for intensity stabilization,ISS) some where down stream, close to the chamber. Since the monitor is behind the cavity.
  • Pick up for optical fiber: This should be somewhere behind 1)EAOM, so that power can be stabilized, 2) BB EOM, so that the laser frequency is stabilized at higher frequency as well. 
  • Some beam splitter/ flip mirrors to bring laser to both cavities. so that I can scan two cavities with 1 laser at the same time. This should be handy when we need to find the beat, since the beat frequency is not well determined (by AOM) anymore.
  • Some space for an independent frequency stabilized laser. This is for the future, when we want to lock a laser to the 8" cavity and distribute it to other labs.  From the current setup, I think we might have to borrow the laser from the 2-laser setup, after the PMC, then lock it to the 8" cavity. Installing 3 sets of laser, PMC, refcav on the table is not easy. With this idea, we can lock the laser to a low noise cavity(8"), and just change the cable, when we want to switch to do the coating measurement.

 The tentative plan is shown below. I also move the BB eom for locking frequency(phase) next to the laser.

  • The pick up for fiber distribution can be at the PBS just after the PMC where we dump the power. However the laser is not intensity stabilized yet, but I'll worry about it later.
  • I add pick up for RFAM monitor. It is the unused beam after the EAOM. The power will fluctuate when ISS is active, but should be fine for RFAM monitoring purpose.

I'm doing mode matching calculation to see if the lenses can be placed in the available area. Otherwise I need to fold the beam a few more times.

I also started removing the acoustic box, some optics on the table so that I can try to position the real optics to see if they can fit on the table.

 

ctn_layout_2laser.pdf

  1096   Wed Jan 30 02:20:38 2013 taraNotesDrawingslayout for the new setup

I'm making a new layout for the 2 laser setup. It is in progress. I just want to make a note about some features I want to have in the setup.

  • Move EAOM (for intensity stabilization,ISS) some where down stream, close to the chamber. Since the monitor is behind the cavity.
  • Pick up for optical fiber: This should be somewhere behind 1)EAOM, so that power can be stabilized, 2) BB EOM, so that the laser frequency is stabilized at higher frequency as well. 
  • Some beam splitter/ flip mirrors to bring laser to both cavities. so that I can scan two cavities with 1 laser at the same time. This should be handy when we need to find the beat, since the beat frequency is not well determined (by AOM) anymore.
  • Some space for an independent frequency stabilized laser. This is for the future, when we want to lock a laser to the 8" cavity and distribute it to other labs.  From the current setup, I think we might have to borrow the laser from the 2-laser setup, after the PMC, then lock it to the 8" cavity. Installing 3 sets of laser, PMC, refcav on the table is not easy. With this idea, we can lock the laser to a low noise cavity(8"), and just change the cable, when we want to switch to do the coating measurement.
  1095   Tue Jan 29 13:49:02 2013 taraNotesEnvironmentleakage in the lab

There is a pool of water in the lab. It is localized only around the floor near the fume hood. I'm not sure where it comes from,but it drips down from the edge of the sink where there is a small pool of water on it. I'll keep an eye on it. If there is still more leakage, I'll contact pma to take a look.

I did not see anything yesterday, so I guess the leak just started last night.

 2012_01_29_leak.jpg

  1094   Thu Jan 24 16:21:01 2013 taraNotesScheduleTask Sheet

I updated the new task sheet for thermal noise probe experiment: 

The goal is to have the first beat measurement:

The plan for the upcoming week (Jan28) will be

  1. finish the table layout
  2. preparation for cavity installation (clear the table, opening the chamber, wire for heater). This means all the parts for in-vac use should be ready. I need to submit the thermal shield to the machine shop, and check some wiring for thermal sensing + heater.

I don't think the second PMC will be ready yet, I'll try to lock the second laser w/o the PMC first, and add PMC later.

For Feb4 week, I will

  1. modify the RFPD PMC servo of the first loop. The current resonant EOMs (14.75 and 21.5 MHz will be used for refcav locking).
  2. check the noise level of the current 21.5MHz RFPD as well (currently used for locking PMC) since it has not been done yet.   I'll try to modify the TTFSS box as well.
  3. clean the table and make sure it ready for the new setup.

 

Attachment 1: Thermal_Noise_Probe_Schedule_-_Sheet1-2.pdf
Thermal_Noise_Probe_Schedule_-_Sheet1-2.pdf Thermal_Noise_Probe_Schedule_-_Sheet1-2.pdf
  1093   Thu Jan 24 14:35:45 2013 taraNotesPurchaseslab supply for in vac preparation

I ordered some clean supplies which will be used when I open the vacuum chamber for installing the short cavities.

 Some of the items that might be needed

  • copper ring seal for vac seal
  •  .... (I'll write it down when something come to mind).
  1092   Wed Jan 23 19:29:26 2013 taraNotesEOMEOM driver

The EOM driver is working. For the same modulation depth, it can drive a broadband EOM using less power.

==measurement and result==

I used PMC setup to test this EOM driver because its frequency range is only ~21 - 24 MHz, and the sideband for locking PMC is 21.5 MHz. So what I did:

  • Checked the performance of resonant EOM as a reference: Scanned the laser and measured the error signal, with resonant EOM. The input power to the EOM is 9 dBm (5.7 V on the RF power scale with phase shift = 3.297 + 180 degree phase flip). The error signal was 3.13 Vpk-pk.
  • Replaced the resonant EOM with a BB EOM equipped with the driver. Re-aligned the beam to the PMC. (I turned down the input power to min before I connected the RF input to the driver).
  • Scanned the laser again and increased the power until I got the same error signal. With the driver on BB EOM, the input power is reduced to -9.5 dBm (on the slider: 3.9V with 0.6707 phase shift, no phase flip) for the same error signal. So the board adds ~18.5 dBm.
  • PMC cavity can be locked and stable.

   From the schematic, the board is supposed to have 30V output from 0.15 V output (x200). In dBm that will be 20log(200)~ 23dBm. So It is roughly ok.

IMG_2213.jpg

fig1: BB EOM with the driver. One of the unused output is for output mon.

IMG_2214.JPG

fig2: Error signal from scanning the laser, with BB eom and the driver, measured at mixer out.

==conclusion==

 The board is definitely working and will benefit us, definitely for locking PMC. If we use a marconi to drive a BB EOM, the max output is 13dBm. The power is halved (one to the EOM, one to the mixer). That means ~ 10dBm to the EOM (we will probably split more some where for RFAM pickup, but we can do that on the line that goes to the mixer), so assuming we have ~ 10dBm for the EOM. With the board it will be ~ 10+18.5 dBm = 28.5dBm (~6V) . It should give modulation depth of 0.09, see psl:745, This might not be enough for locking the refcav( see,psl:929. where we have beta of 0.18), but we can add another RF amplifer, or use the board for PMC servo . I'll check what are the appropriate modulation depth for locking PMC and refcav.

 

 

  1091   Tue Jan 22 21:01:38 2013 taraDailyProgressVacuumvented screws for vac use are cleaned

I clean the vented screws and peek pieces for cavity mount with ultra sonic bath, I'll check if I need to bake them or not.

IMG_2211.JPG

  1090   Tue Jan 22 19:12:24 2013 taraNotesEOMEOM driver

I got an EOM driver from Rich Abbott, I'm checking if this thing works well or not.

The EOM driver is just an amplified resonant circuit with +/- 18V input. With the driver connected to a broadband (BB) EOM, we can use the combination to add sideband to the laser. This is better than a resonant EOM because we can pick a range of frequencies, instead of having a fixed one from the manufacturer.

 I checked the TF between input and the RF mon, the resonant peak can be moved between 20.7 MHz and 24.5 MHz by adjusting an inductor on the board. Since the RFPD for the PMC is 21.5 MHz, I'll use it to check the modulation index of a BB EOM equipped with this board.

The plan is 

  1. Check the power vs modulation index from the current resonant EOM. To do this, scan the laser, measure the error signal.
  2. Remove the resonant EOM, and replace it with a BB one, adjust the power, check the modulation index vs power input again

Once I verify this I will check the frequencies for refcavs and pmcs, so that I can decide the value of L and C on the board.

 

 

        IMG_2209.jpg

Attachment 2: EOM.pdf
EOM.pdf EOM.pdf EOM.pdf
  1087   Tue Dec 18 19:07:24 2012 taraDailyProgressPMCANSYS result for PMC body mode

Kristen and Norna came to ATF for impact-hammering of the metal PMC in the gyro setup

LINK

  1086   Mon Dec 3 16:57:27 2012 taraDailyProgressPMCANSYS result for PMC body mode

Quote:

 

 

First_Mode_-_PMC.png

Above you can see the first mode shape of the PMC.  The colors represent the displacement - deep blue indicates no motion, while red indicates the gr

 

 This does not look like a longitudinal mode. Do you have the frequency for the first longitudinal mode(along cavity length)? the first longitudinal mode should look like this ( this model has no fixed boundary condition, just a block in space).

cavity_eigen.png

  1085   Wed Nov 28 20:37:57 2012 taraHowToRefCavassembling short ref cav

[Peter, Tara], we assembled 2 short reference cavities today. The bonding between the spacers and mirrors are strong and holding the mirrors nicely.

 I got the cavity fixtures (made from delrin) from the machine shop today, so I asked Peter to help me assembling the cavities. All picture can be found here

  • The mirrors are 1/4 stack SiO2, Ta2O5 T=300ppm, ROC =0.5m.
  • Two spacers' serial numbers are 98 and 99.

I tested the bond by lifting the whole cavity by handling at the mirror on top only, and wiggling it a little bit. The bonds weren't broken. The hardest part was cleaning all surfaces to make sure that there was no dust.

From hindsight, I don't really need to see the fringes to do the bond. If the surface is clean, the pieces will be bonded instantly after a light pressure. If there are particles on the surface that cause fringes, the bond will not form anyway. So for Si cavity, Dmass can try to do optical contact without a setup to see the fringes.

IMG_2029.jpg

fig1: the mirror is placed in position by the fixture. The mirror is not pressed on the spacer yet. Fringes can be seen on the polished ring on the mirror. See the video to see how the fringes vanish after applied pressure.

IMG_2034.jpg

  1083   Tue Nov 27 05:38:01 2012 taraNotesNoiseBudgetThermoelastic noise in spacer and substrate

 I calculated brownian noise in AlAs/GaAs coatings, brownian noise and thermoelastic noise in fused silica substrate for different beam sizes. From the plot, we can see that a smaller spotsize might be better for us.

     This is a quick study to see the how spotsize on a mirror affects Brownian noise and thermoelastic noise in coatings and substrate. The radii of the beam (where the beam intensity drops by 1/e^2) used in the calculation are 91, 182, 364 um. Loss in coatings is 10^-5, loss in substrate is 10^-7. Note for 1.45" cavity with 0.5m RoC mirrors, the beam radius is 182 um.

TE_Brownian_compare2.png

Reminders:

  • The plot is shown in displacement noise, not frequency noise from cavity.
  • The psd (m^2/Hz) of coating Brownian noise is proportional to 1/w^2 (w is the beam radius)
  • The psd of substrate Brownian is proportional to 1/w
  • The psd of substrate thermoelastic is proportional to 1/w^3 ,at high frequency where adiabatic assumption valids. But at low frequency, when heat diffusive flow rate is comparable to the beam radii, TE noise is reduced from that of adiabatic assumption.

The Brownian noise in the coatings is more comparable to TE noise in substrate with smaller beam size although the crossing between the two noises are at higher frequency. So it should be able to see the total noise from both effects. However, to get smaller beamsize, we probably have to use even shorter cavities, or smaller RoC mirrros. So it might not be practical for us.. Nevertheless, going to smaller beam size should  be a good idea.

Note:

  • for 1.45" long cavity, no choices of RoC give w = 92 um,
  • for mirror with RoC = 0.5m, cavity length of 0.1 inch(2.5 mm) gives w = 92 um
  • I think I made a mistake in the proposal since Brownian noise in substrate was higher than coatings' noise. I double checked it for this calculation and Brownian noise in substrate is always lower than coating brownian.

 

Attachment 2: TE_Brownian_compare2.fig
  1078   Thu Nov 15 18:59:17 2012 taraNotesPMCstainless steel pmc

The PMC round trip is ~ 0.32m. The end mirror has ROC = 1.0m. The spotsize is 384 micron.  The end mirror has radius ~2.5 mm. The clipping loss will be ~ 1*10^-43 on the curve mirror, and much smaller at the flat mirrors. The number seems very small but I think it is correct.

 This is just a simple integration for power of the beam P(a) where a = radius of the mirror (2.5mm). The total loss on all three mirrors per one trip is definitely way below 1ppm.

[add calculation]IMG_1982.jpg

  1077   Thu Nov 15 00:33:01 2012 taraNotesPMCstainless steel pmc

Kriten sent me solidwork part files for the steel pmc.  I'm checking all the parts and will decide what material we want to use.

She reported that a ss pmc will have the first body mode at 780 Hz, while an aluminum one will have the first body mode at 1kHz. But we have to take thermal expansion, stiffness into account. here are some material properties

 

  Stainless steel AL
Thermal expansion coeff x10^-6 16 22.2
Young modulus    [GPa] 180 69
density   x10^3 kg/m^3 7.8 - 8

2.7

I think the thermal expansion will be a problem, but their thermal expansion coefficients are not that different. I don't know about stiffness of the body. I'll ask someone about this. Otherwise Al might be a better material if we look for higher resonant frequency.

  1076   Wed Nov 14 19:46:47 2012 taraNotesNoiseBudgetThermoelastic noise in spacer and substrate

Found the problem. My noise budget code was wrong. So after I fixed it, the TE noise in substrate result from COMSOL agrees pretty well with the analytical result (within 20%).

substrateTE_compare.png

The result from COMSOL is plotted in dashed-black line. The result from Cerdonio is plotted in dashed pink.  Since my simulation uses the adiabatic assumption (used in BGV and Liu&Thorne paper), the results agree at high frequency. So I think the calculation is correct. I'll check some options (changing spot size, changing material) to see if TE noise can be made lower for AlAs/GaAs samples.

I attached my COMSOL file below. This is done in 3D model. It could have been done in 2-D axis symmetric setting, but I used 3-D for spacer sagging before, so I just used the same geometry I had.

 Quote:

I realized that the mesh size was too large, even with the finest mesh for default setting. So I reduced the mesh size around the beam area and the results got closer to the analytical prediction. It is still a factor of 2 below the prediction. I'll see if I can hunt down this problem . I think it will be a good idea to verify my model by using my model to calculate Brownian noise and comparing with the result reported by Braunschwig group. 

When I defined mesh size in COMSOL, I used the predefined value provided by COMSOL. The finest mesh has maximum element size ~500 um, and minimum ~5um. Since the beam size is ~ 180 um, the maximum element size should be ~10 um. So I changed the values around, defined new area for smaller mesh until the results did not change much. I ran the simulation a few time to make sure that the solution converges. Right now my substrate has 3 regions

  1. a cylinder at the center where the beam hits the mirror, Radius = 2x180um, depth=2x180um ,   Min/Max mesh size = 12.6/27 um
  2. an outer region, radius = 5x180um, depth = 5x180 um, min/max mesh size = 10/50 um
  3. The rest of the substrate, fine mesh

I tried to change the mesh size/boundary size a bit to get the result accurate enough without taking too much time. The TE estimation still a factor of 2 below the analytical estimate.

 

 

Attachment 2: TE_cavity_v2.mph
  1075   Wed Nov 14 00:25:48 2012 taraNotesNoiseBudget&#3642;Brownian noise in spacer and substrate

 I used COMSOL model to calculated Brownian noise in substrate. This was done for cross checking my model simulation. The result from model is within 2% compared to half infinite model calculation.

I followed Levin's Direct approach to calculate Brownian noise in substrate, basically, to calculate the elastic energy inside the substrate under the applied test force. This can be done using COMSOL and analytical calculation. The comparison between the two is shown below.

U is the stored energy in substrate.

 IMG_1975.jpg

 

Note: I used the same COMSOL model for TE noise calculation. I just asked it to produce the strain energy in the substrate (no spacer).

The simulation is very close to the analytical result. So I think my spacer-cavity model and all the factors in the calculation are correct. The TE calculation is a little more complicated, since I have to calculate the gradient of expansion in COMSOL and it might be wrong somewhere. I'll check that. 

  1074   Tue Nov 13 01:56:24 2012 taraNotesNoiseBudgetThermoelastic noise in spacer and substrate

I realized that the mesh size was too large, even with the finest mesh for default setting. So I reduced the mesh size around the beam area and the results got closer to the analytical prediction. It is still a factor of 2 below the prediction. I'll see if I can hunt down this problem . I think it will be a good idea to verify my model by using my model to calculate Brownian noise and comparing with the result reported by Braunschwig group. 

When I defined mesh size in COMSOL, I used the predefined value provided by COMSOL. The finest mesh has maximum element size ~500 um, and minimum ~5um. Since the beam size is ~ 180 um, the maximum element size should be ~10 um. So I changed the values around, defined new area for smaller mesh until the results did not change much. I ran the simulation a few time to make sure that the solution converges. Right now my substrate has 3 regions

  1. a cylinder at the center where the beam hits the mirror, Radius = 2x180um, depth=2x180um ,   Min/Max mesh size = 12.6/27 um
  2. an outer region, radius = 5x180um, depth = 5x180 um, min/max mesh size = 10/50 um
  3. The rest of the substrate, fine mesh

I tried to change the mesh size/boundary size a bit to get the result accurate enough without taking too much time. The TE estimation still a factor of 2 below the analytical estimate.

 

  1073   Mon Nov 12 22:56:10 2012 taraNotesNoiseBudgetThermoelastic noise in spacer and substrate

I'm checking the calculation for TE noise in substrate and spacer. I'm comparing the results from analytic calculation and simulation. The results still do not agree. Comsol gives a result ~ a factor of 4 lower than its analytical counterpart.

   Since the TE noise in substrate will be significant in AlAs/GaAs mirrors, the TE noise estimation should be correct. The TE calculation in substrate was done by (BGV, Liu and Thorne, and Cerdonio). The correction was noted in Numata 03 and Black 04 papers.  I think the calculation is well established because the calculations from all of the papers agree (with all corrections taken into account). So It will be nice if an FEA simulation predicts the similar result as well. 

   I followed the calculation done by Kessler etal paper where they calculated the Brownian noise in spacer. The mirror-spacer assembly is pushed by a static force with Gaussian profile, P = 2 F0 / (pi*w0^2) * exp(-2r^2/w0^2), where w0 is the spot radius = 182 um for 1.45" cavity with 0.5 mRoC mirrors, at r = w0, intensity drops by 1/e^2, F0 is the magnitude of the force (1N for my simulation) .

  1.    I simulated 1/8 of the cavity which is cut by xy,yz, and zx planes.
  2. Then I used COMSOL to calculated (gradient of expansion)^2,  (expansion = divergence of displacement in the body),
  3. integrated over the calculated body. (get 4.18x10^-12)
  4. Then multiplied by  8 to include all the sections of the cut cavity,
  5. multiplied by 2 for double cavities,
  6. divided by 2 for averaging the dissipated power over 1 period.
  7. then followed the calculation given by Liu and Thorne. The result is still lower than the analytical model.

cavity_TE_v2.png

Note about COMSOL: I used extra fine mesh in a small volume where the beam hits the mirror, fine mesh in the rest of the substrate, and normal mesh in the spacer. This reduced the memory used in the calculation a lot and should not introduce a lot of error, since all the deformation will concentrate near the beam spot.

  1072   Fri Nov 9 18:56:49 2012 taraHowToRefCavHowto optically contact mirrors

I have been practicing to optically contact two flat mirrors together. I think now I get it.

   Since I need to build my refcav by optically contacting mirrors to a spacer. I tried the procedure by contacting two flat blank mirrors together. I bought blank fused silica mirrors from Thorlabs, pf10-03. Here are the instructions:

  1. Clean your space.
  2. Wear lab gown, mask, gloves to prevent dust from your clothes
  3. Use a can duster to blow away any obvious dust/dirt from the mirrors. Clean the whole mirror if possible. Any dust on the mirror back or side might fall on the surface anytime.
  4. Wipe the mirror with acetone (I wiped the back and the edge first, before wiping the front face). Then switch to isopropanol. I tried drag wipe first, but it did not remove one of a particle right in front of the surface, so I had to wipe it with more pressure.
  5. Contact to surfaces together. I put one on top of another. You will see a fringe pattern changes as the surfaces become close together under gravity. Press it to make it contact properly. ( I pressed with my finger for ~ a minute with ~10-20 Newton). If the surfaces are clean, there should be no fringe pattern (see fig2).  There should be no change when you remove the applied pressure.

 

IMG_1955.jpg

fig1: On the right, fringe patter can be seen when two surfaces are put close together. On the left, after applying pressure the fringe should go away. The fringe pattern(purple-yellowing) on the left side indicates that these surface are not properly contacted.

IMG_1964.jpg

 fig2:  These mirrors are optically contacted somehow, except the center. I don't know what happen here that cause white area in the center. I might be that the mirrors are not flat enough. But the rim seems to have a nice optical contact. I tried to remove the mirrors by hands but they are well stuck. I'll ask peter for more about what causes the white area here.

 

As I tried to do this, I got an idea of a fixture for optical contacting the spacer to mirrors. It will be a cap with a center hole for the mirror position. Here is a solidwork drawing. The part can be aluminum. I have to think about the tolerance of the piece, but from the calculation it can be an order of a  cm (to keep the beam to go through the window). So the hole can be ~ .01 inch larger than the spacer and the mirror.

fixture_assem.PDF

  1071   Thu Nov 8 20:00:33 2012 taraNotesDrawings1.45" refcav mount

I finished the drawing for refcav mount and the top plate. Everything fits together, so I'll submit the drawing tomorrow.

New things I added:

  • Top plate for holding the thermal shields, and the cavities with screws.
  • wall between the cavities for radiation shield.
  • new top plate for the seismic stack, the dimension is similar to the one we have now, but the hole pattern is modified to fit the refcav mount
  • cap for thermal shield (not shown).

The refcav mount is a bit wider than the top seismic stack plate, but it fits inside the chamber, see the assembly. So I don't think it will be a problem.

 

 

vacuum_chamber_dualcav_new2.PDF

vacuum_chamber_dualcav_new2_cut.PDF

Attachment 3: dual_cavity_mount_base_v11.PDF
dual_cavity_mount_base_v11.PDF
Attachment 4: peek_v2.PDF
peek_v2.PDF
Attachment 5: stack_plate.PDF
stack_plate.PDF
Attachment 6: topplate.PDF
topplate.PDF
Attachment 7: wall.PDF
wall.PDF
  1069   Thu Nov 1 03:10:16 2012 taraNotesLaserLaser profile scan

I scanned the profile of the laser borrowed from 40m. The avg beam radius is 220um ~ 1 cm in front of the laser opening. This number will be used for a new table layout.

The laser was operated at full power (~700mW as expected). I used a mirror to attenuated the beam and use WINCAM to measure the beam profile (power incident on WINCAM was ~0.7mW). To measure the full power and avoid burning the power meter, I used a polarizing beam splitter with 1/2 wave plate to reduce the beam power by half then measured and summed the power from two sides of the PBS.

result.png

The beam shape is looking more like a blob than an oval. This might explain why the fitting does not match the measurement well.

[add fig]

 

Attachment 2: result.fig
  1068   Wed Oct 31 16:00:13 2012 taraNotesDrawings1.45" refcav mount

I finished the design for dual cavity mount. The assembly looks fine, all parts fit together. I'll make sure that the mount can be screwed down to the current seismic stack before I submit the drawing.

 

After finding the optimum support points using COMSOL, I redid my cavity support design. The picture below shows the assembly of a metal base, peek pieces for support points, and copper shield. The picture shows only half of the mount.

Assem2.PDF

Assem2_1.PDF

 

Personal note: 

The current design, the beam height is 1.32 inch above the top seismic stack, ~5.5 inch measured from the table.

With the new cavity mount design, the beam height will be 1.5 inch above the top seismic stack.

 

  1067   Fri Oct 26 04:05:18 2012 taraNotesRefCavcomsol simulation for cavity suspension

I looked into the model a bit more to make sure that I included all the effects and get the coupling right [more to come]

 

plot_line.png

fig1: displacement(beam line direction) per unit acceleration on the mirror surface. X-axis represents the position on the mirror along vertical line. Each plot represents result from different support positions. For optimum point (1.17"), the sensitivity to vertical seismic is around 2.1x10^-12 m/(m/s^2).

plot_angle.png

fig2: This plot shows the result as in fig1, with the means removed. Typically we want the tangent line at the center to be zero for minimum tilt.

 

  1. Now I use SI units in the simulation.
  2. The surface displacement along vertical direction is ~ 5 x10^-10 m. The spot radius is 100 um, so the effect from translation along vertical direction is negligible. We can assume that the beam still hits and senses the center of the mirror.
  3. Check the result between support length between 1.18" +/- 0.02" (+/- 0.5 mm). This tolerence is what I estimate to be our assembly tolerence.
  4. I calculated the tilt by using the tangent line at the displacement on the vertical line in the mirror center. Note that my mirror surface in the simulation is flat instead of curve. At 100 um away from the center, the surface height will change by ~ 10^-8 m due to the mirror's RoC. Meanwhile, the result from simulation (flat surface) shows that the displacement around 100 um away will change ~ 10^-12 m. However, I don't think it will effect our model much, since most of the deformation occurs around the support point, not the mirror. It means all the tilting comes from deformation on the spacer, not the mirror. So the simulation for flat mirror should be a good approximation.
Attachment 1: plot_line.png
plot_line.png
Attachment 2: plot_line.png
plot_line.png
  1066   Wed Oct 24 20:22:01 2012 taraDailyProgressNoiseBudgetThermoelastic noise in spacer

I used COMSOL to estimated thermoelastic noise in 1.45" spacer.  The noise is not significant for our coating Brownian measurement. I still need to verify the model with some analytical estimation.

 

dmasscavity_sagging_0_2012_10_04.png

 

 

  • The model is 1/8 of the spacer with symmetry boundary condition on each cut surface.
  • The applied force is a quarter of an annulus, 2 mm width. This annulus represents the contact area between the substrate and the spacer.
  • The deformation is simulated, and its gradient is calculated by COMSOL and integrated over the volume (1/8 of the cavity).
  • I followed the calculation on Liu and Thorne, 2000, and get thermoelastic noise from spacer. It is lower than coating's Brownian noise at least 1 order of magnitude and will not be a limiting source for us. The plot below show spacer' TE in black dashed line.

short_nb.png

Note:

  • I should compare the simulation result with an analytical result, but I'll do that later in parallel with other work. This is not an urgent one. I'll also try to reproduce the result reported by Kessler et al. about Brownian noise in spacer as well. Then calculate the Brownian noise in our cavity.
  • I'll check the spacer's TE noise for our previous 8" cavity to make sure that there is consistent with the result.
  1065   Wed Oct 24 11:16:52 2012 taraNotesRefCavcomsol simulation for cavity suspension

As Rana and Jan suggested, I thought about the effect of mirror's radius of curvature and DC tilt effect to cavity length noise. I ran a few simulation tests and the results were not changing much.

==cavity length noise==

The cavity length mainly changes from two effect

1) Actual position change:

2) Tilting of the mirror: If both mirrors tilt up or down together by theta, the cavity length will be longer by R*theta^2, see the attached picture. The calculation takes mirror's ROC and optical axis shifting into account.

IMG_1914.jpg

   So, to find the best place to support the cavity, the contribution from both effects should be minimize.

==COMSOL Simulation: effects from different boundary conditions==

We have been discussing about different boundary conditions for support points whether to use fixed in all direction, fixed in only vertical direction, point support, or finite area. So I decided to check the effect from the following condition

  •  Point like support, fixed in all direction
  •  Point like support, fixed only in vertical direction
  • Area support, fixed in all direction  (The size of the area does not change the result that much, see PSL:1061. This time I chose 1mm^2 which is 1.5x10^-3 [in^2].)

 ==result==

    There are no significant differences among the chosen boundary conditions. I varied the angle from 30, 45 and 60 degree, all the boundary conditions resulted in the same sagging behavior, so the best choice will be 30 degree as discussed in PSL:xx .The plot below shows the displacement per [m/s^2] of the mirror center along the beam line and tilt angle per [m/s^2] of the mirror, (the support angle is 30degree). With any boundary conditions, the optimum position will be quite the same (30 degree,  1.2" apart).

analyzed_plot.png

==where is the optimum spot? what is the coupling from seismic to cavity length?==

 From the simulation, with our restriction on support position (angle between 30 to 60 degree, 0.5 - 1.2" apart), the mirror positions always extend the cavity length. Since tilt will always increase the cavity length, we cannot cancel the effect between translation and tilt. The best way is to minimize translation and choose zero tilt as before.

About the coupling, at the optimum spot where tilting is zero, there will be no tilting motion. Displacement noise will only come from translation of the mirrors.

Attachment 2: analyzed_plot.fig
  1064   Thu Oct 18 01:20:49 2012 taraDailyProgressNoiseBudgetThermoelastic noise in spacer

I talked to Jan about how to calculate thermoelastic (TE) noise in a spacer. I will use comsol to estimate the thermoelastic noise in our cavity.

  Thermoelastic noise has not been estimated for our setup. I think it will not be that high. As the previous result with 8" cavity, the measured signal was very close to the estimated coating Brownian noise. However, our noise budget will be more comprehensive if we include TE noise in the spacer.

 Basically to do that, we have to apply force on the spacer , then calculate the gradient of the strain inside the spacer [Liu and Thorne, 2000]. I think this can be done by COMSOL. I am working on it. I'll add more details on CTN wiki page later.

 

  1063   Tue Oct 16 01:14:00 2012 taraNotesRefCavcomsol simulation for cavity suspension

I checked the cavity length sensitivity to horizontal acceleration ( normal to the beam line axis). Unlike the result from vertical acceleration, the cavity length did not change smoothly with the position.  For the optimum point(30 degree, 1.18 inch apart), the displacement sensitivity due to horizontal acceleration is about a factor of 2 larger than that of vertical acceleration.

 Since both horizontal(H) and vertical(V) seismic noise on the table are comparable [psl:xxx], I want to make sure that there will be no serious displacement noise due to acceleration on vertical direciton.

On the COMSOL model, I fixed only one side of the support to push against horizontal acceleration (see pic). As the support can only push against the cavity, not pull. It should make more sense to use this boundary condition.

For the effect from H acceleration, I varied the angle from 12 to 42 degree, and distance from 0.8 to 1.2 inch. The displacement did not change smoothly with the support positions. So I could not tell which way I should choose for the optimum support. However, the displacement seems to be around 2x10^-10 inch for most of the positions. [the unit on the y axis of the plot should be inch per acceleration]. disx_y.png

For the optimum support (1.18 Inch, 30 degree) the senstivity is about 1x10^-10 inch/ (m/s^2).

  1062   Sat Oct 13 23:38:08 2012 taraNotesRefCavcomsol simulation for cavity suspension

 After checking displacement and tilt of the mirrors from various support points, the tentative support positions will be 1.2"  and 30 degree, (see entry 1060 for their definitions). I'll check the sensitivity along other directions (horizontal), and see if the noise budget will be acceptable or not.

 

After running more simulations, I got the cavity's length sensitivity due to vertical acceleration. The angle varied between 12 to 45 degree. And the nice point seems to be 30 degree at 1.2".  I will check the length and tilt sensitivity on horizontal acceleration, and compute the noise budget to make sure that seismic noise will be acceptable. After that I'll finish the drawing for the cavity mount.

dis_tilt_12_45.png

 

Note: I just realized that I should have used strain/acceleration unit in the displacement plot. I'll fix that later.

  1061   Thu Oct 11 19:07:06 2012 taraNotesRefCavcomsol simulation for cavity suspension

I ran a few more simulations to see how support area would affect the displacement. It turned out that it was not significant, for area = 0.056 x 10-3, 0.9 x 10-3 and 5.6 x 10-3 [inch2]. This is good because we don't have to worry too much about the effective area of the contact points in the simulation. The errors will probably be dominated by other parameters (mostly, support positions). Judging from all the requirements, I think I'm close to making the decision for the support position.

     The plot below shows results from different support angles, (theta = [30,45,60]).

plot_degree_area.png

==a few comments about the plot==

  • The area of the support points are not significant in the simulation, see green, red, cyan plots.
  • For the support angle, it seems that the smaller theta, the less sensitivity to vertical acceleration (blue ,pink, green). However, smaller angle means the support area becomes more vertical. This will cause two issues that I can think of, (1) more force on the spacer which will increase the surface loss, (2) a bit harder to machine the mount. About the loss, I think it is still ok if the normal force from the support to the spacer is less than a factor of 5. This gives us the smallest angle of ~ 12degree, but at this angle, the optimum support point for zero tilt will be very close to the ends of the 1.45" spacer (based on the blue ,pink, green curve) and it will be unsafe to mount the cavities in case they slide out of the supports.

            I think possible choices (considering loss, machining, safety) for the support positions will be some where around 0.7-1.2 inch along the beam line, and the angle will be ~ 12 - 30 degree. I'll run more simulation to see if I can find it.

==Note==

  1. I reproduced the result for 8" cavity with wire support and got the same result as before (dL/L)/(acceleration) ~ 2 x 10^-10 (1.7x10^-10, to be more exact), but the tilt is 9x10^-9 rad which is a lot larger than the results from the short cavity. So I think my simulation for short cavity is fine as I can reproduce the same result.
  2. I tried to constrained the support area in vertical direction only, but the simulation failed. I think this is because of the bending of the spacer surface. If you let it slide horizontally, the surface will tilt a bit as well and cannot be kept fix in vertical direction. Then for the constrain in all directions, I don't think there will be a solution for zero coupling for the short cavity.

 

  1060   Wed Oct 10 21:31:57 2012 taraNotesRefCavcomsol simulation for cavity suspension

I used COMSOL with MATLAB to run the simulation. I tried to vary support position and checked the mirror displacement along the beam line axis and tilt angle.

 

With Aidan help, I am finally able to run matlab with comsol to get the results (displacement of the mirror surface and tilt).

 We are not planning to cut the cavities for support points, so we will choose the support positions on the spacer's surface, with parameter X and theta, see the figure below for their definitions.

cavity_sagging_0_2012_10_04.png

As a start, I chose theta = 30 and 60 degree. The displacement and tilt (due to cavity sagging under its weight at 1g)as a function of support position are plotted below.

plot.png

It is possible to minimize the tllt, but the displacement is still a bit bad. The result from 8" spacer, the sensitivity to acceleration is (dL/L) / (m/s^2) = 2x10^-10, while the current result will be about  1x10^-10. Since the cavity is shorten by ~ a factor 4, I expect a better sensitivity to vibration.

==next==

I'll try to change the area of the support points to check its effect on the displacment.

I have to check if I can constrain the support points in one direction or not.

Quote:

I'm still working on COMSOL, now my model has the following features:

  • support points are simulated by four rectangles on the spacer surface. The support areas can be changed, and their positions can be moved along the beamline and around the cross section directions. I'm still not sure what should be the effective area for the simulation. However, the paper from Milo's group [2009] showed that the areas of the supports points do not affect the optimum position that much (but the support must be constrained in the same direction as gravity only). I have not tried to constrained the points in only one direction yet.
  • I used a symmetry plane to model only half of the spacer. This will reduce some simulation time and avoid any asymmetry due to the mesh size.
  • I'm trying to use matlab with comsol and print out the result. The work is stil in progress.

 

Note:

Milo etal. 2009 Phys Rev A 79.053829.

 

  1059   Wed Oct 10 16:10:18 2012 taraNotesPurchaseslab supply

Faraday Isolator, for 700mW 1064 laser. This will be installed after the laser (and waveplates for polarization adjustment).

I looked at Thorlabs website and found one that meets our requirement.

IO-3-1064-HP, the max power is 15W, and the max intensity is 500 W/cm^2. If the 700mW beam has 300 um radius, the avg intensity is~ 250 W/m^2.

I'll ask around to check if this is a suitable one.

Quote:

Just a list for something we need to buy.

Electronic:

  • BNC connector(T,L,Y)
  • maybe some bnc cables for ATF
  • SMA to BNC connectors (all combination)
  • SMA 50 ohms terminator

Optics& opto-mech parts

  • lens, various f length (for 1064)
  • lens mount and post
  • some mirrors?
  • PBS
  • half wave plates (x2)
  • EOM (BB for spare), EAOM (for ISS)

Lab

  • sticky mat/ shoe cover
  • gloves
  • cable tie holder

lab supply

  • duster can

 

  1058   Wed Oct 10 01:38:49 2012 taraNotesRefCavcomsol simulation for cavity suspension

I'm still working on COMSOL, now my model has the following features:

  • support points are simulated by four rectangles on the spacer surface. The support areas can be changed, and their positions can be moved along the beamline and around the cross section directions. I'm still not sure what should be the effective area for the simulation. However, the paper from Milo's group [2009] showed that the areas of the supports points do not affect the optimum position that much (but the support must be constrained in the same direction as gravity only). I have not tried to constrained the points in only one direction yet.
  • I used a symmetry plane to model only half of the spacer. This will reduce some simulation time and avoid any asymmetry due to the mesh size.
  • I'm trying to use matlab with comsol and print out the result. The work is stil in progress.

 

Note:

Milo etal. 2009 Phys Rev A 79.053829.

  1057   Tue Oct 9 15:12:57 2012 taraNotesPurchasesvibration isolators (table legs)

The table is not floated. Either the legs are broken, or there is a leak in the tube system. I think it is likely that one (or more) of the legs is broken. Since it happened before with the older legs. Their rubber part in the leg gradually failed over time. We might need to reconsider buying brand new legs again.  The pump connected to the table could not keep up with the leaking rate, so I turned it off.

  1056   Thu Oct 4 23:28:06 2012 taraNotesRefCavcomsol simulation for cavity suspension

I'm using COMSOL to simulate the effect of cavity sagging to find the optimum suspension points. The answer is not yet ready, I'm still working around COMSOL.

 

  • Problem with point like contact: First I tried to simulated 4-fixed-point support, however the result was asymmetric along beam line. It might be the result from the point-like contacting areas between the support and the cavity. You can see the tilting along the beam line in the figure below. Note the constrained area  of the support effect the sagging slightly as well [add ref].

.cavity_sagging_0_2012_10_04.png

fig1: cavity sagging, on 4 point suspension. The cavity is not symmetric on left and right.

     So, as a start, I switched to half line support. As my cavity support will be rods placed perpendicular to the refcav, the simulation might not be off by much. Then I checked the displacement at the center of the mirrors. The result was, the further to the ends of the spacer, the less displacement of the mirrors. I think this is strange. I also remember a paper about this and their cavity dimension is similar to what we have, and their result is slightly away from the ends [ref]. I'll have to double check the result again.

Note: I think what is wrong is how I use the displacement of the mirrors along the beamline as differential length of the cavity, I have not taken into account tilting of the mirrors yet. Also, I'll try to position the venting hole downward to see if there is any differences in the result or not.

 cavity_sagging_2012_10_04.png

  1055   Wed Oct 3 17:48:26 2012 taraNotesTempCtrltemp control for the chamber is off

I checked the temperature control servo for the vacuum chamber and found out that it was off. I could not turn it back on yet since there was some problems with the channel. I'll ask Peter to help me on this.

About a week ago, I tried to add another channel for controlling the second PMC servo card. I did not write an elog since it was not done yet.

I created PMC2.db in /usr1/epics/psl/db. It had only one channel for C3:PSL-PMC2_GAIN. I used #C6 S2. This channel might already be used for temp control. I'll try to remove the channel and see if the problem can be solved.

 

  1054   Thu Sep 27 17:13:13 2012 taraNotesDrawings1.45" refcav mount

modification of the previous mount, work in progress.

cav_mount_assembly_3.PDF

peek_v2.PDF

  1053   Tue Sep 25 23:31:40 2012 taraNotesPurchaseslab supply

Just a list for something we need to buy.

Electronic:

  • BNC connector(T,L,Y)
  • maybe some bnc cables for ATF
  • SMA to BNC connectors (all combination)
  • SMA 50 ohms terminator

Optics& opto-mech parts

  • lens, various f length (for 1064)
  • lens mount and post
  • some mirrors?
  • PBS
  • half wave plates (x2)
  • EOM (BB for spare), EAOM (for ISS)

Lab

  • sticky mat/ shoe cover
  • gloves
  • cable tie holder

lab supply

  • duster can
  1052   Mon Sep 24 16:41:19 2012 taraNotesPMCstainless steel pmc

[with Zach and Dmass] We discussed about the stainless steel pmc design  and here are the list of what should be modified.

The drawing can be found, on svn.

pmc_endcap_v2,

  •  the hole for the beam exit can be larger (x1.3) so that the exit beam can pass without clipping. It should not be larger than the pzt
  •  [Important]the thickness of the PZT, thickness of the back mirror, end cap,  will determine the optical path of the PMC (with the beam centered at the input/output mirrors), see pmc_spacer_v2. Make sure to get the thickness right, so the beam is centered on all mirrors.

 pmc_clamp

  • the clamp for the input / output window can have a little larger opening to avoid clipping the beam. The through hole diameter can be 0.9 in stead of 0.8".

pmc_base

  • The height should be corrected for 3" beam height, measured from the table to the center of the mirror (the previous one was designed for 4" beam height. The size of the ball bearing has to be specified (Dmass said it was for 3 mm radius).

Materials

  • find the ruby/ sapphire bearings for the 3-point mount sapphire ball.
  • Other parts of the PMC (spacer, pmc clamp, end cap) will be made from stainless steel, the base will be made from brass.
  • press fit slot and press fit cone will be made from hardened steel dowel pin from McMaster

assembly

  • make sure the assembly has the right beam height (3" for CTN)
  • make sure to glue (with epoxy) the mirror, pzt, end cap carefully so all the parts are parallel, no tilt, no yaw.
  • order ruby or sapphire ball

PZT can be ordered from www.pi.ws.

The requirements for PZT from (LIGO-xx), are (A) pzt range = 2.7 FSR, for 0 - 375 V, (B) resonant frequency at 10kHz or above.

Zach is using model P-016.10H. The displacement is 15 um (with 1000V), OD = 16mm, ID = 8mm, L = 15mm, resonant frequency = 67kHz.  Assuming the pzt is linear, the displacement will be 5.6 um for 375V, this corresponds to 11 FSR for our cavity (FSR = 454 MHz). I don't know if this will cause some locking problem or not, or it might just give us an extra gain in the pmc loop.

If I follow the requirement, the displacment of 5um @ 1000V will be enough for us (model P-016.00H), but the length of the PZT will be 7 mm, and I think we have to fix the drawing accordingly.

pzt_snap.png

Above, an excerpt from the pzt catalog, the full one can be found HERE.

 

 

  1051   Sun Sep 23 18:28:04 2012 taraNotesDrawings1.45" refcav mount

I'm working on the new mount for 1.45" refcav. I 'll discuss the design with Eric G and some mech engineers (Mike Smith, Ken Mailand) later.

cav_mount_assembly2.PDF

Here is the assembly of the mount, with only one cavity shown.

copper thermal shield around the cavity is 1.75" OD, 1.686" ID, wll thickness = 0.064", 3" long. (I'll order the tube from McMaster-Carr.

bottom mount will be a single piece, holding both cavities together.

I'll add a top plate to hold the shield and cavity later.

 

Note: I'm thinking about using teflon to make all the mounting pieces (top and bottom) so the mount will act as heat insulation between the shield and the platform.

cav_mount_bottom.PDF

cavity_mount_top.PDF

Attachment 1: cav_mount_assembly2.PDF
cav_mount_assembly2.PDF
  1050   Tue Sep 18 02:35:56 2012 taraDailyProgressRefCavoptical contact practicing

I got the mirror blanks for optical contact practicing. I tried to contact them together, but I have not succeeded yet.

 The mirrors are not transparent on the back, but we can still see the fringe due to the gap between the two surfaces clearly with just room light, see the picture below. I might not clean it well enough. I'll try to do it again later.

IMG_1764.jpg

  1049   Mon Sep 17 20:59:15 2012 taraDailyProgressfiber opticsetup for fiber optic distribution.

 I adjusted the mode matching a bit ( changing lenses positions and rotate the lens on the collimator). The coupling efficiency was up to 66%. This should be enough for now.

The power input can go up to ~20mW, so the output is ~12mW which should be enough for gyro.  I also adjusted the polarization, so that the polarization of the input beam matched the fast axis of the cable. I tested the polarization of the output beam with a PBS and got the extinction ratio of ~ 670.

The fiber is polarization maintaining fiber, nufern pm980.IMG_1761.jpg

It has fast and slow axes, and we have to match the polarization of the input beam to the fast axis. To do that

  1. rotate the angle of the output state (B) to find the minimum and maximum transmitted beam behind the PBS. The extinction ratio is max/min.
  2. rotate the input stage (A) and repeat (1) until you find maximum extinct ratio

If the beam polarization matched the fiber axis, the output beam will have linear polarization which gives the maximum extinct ratio. Meanwhile if the beam polarization does not match the fiber axis, the output beam will have elliptic polarization and the extinction ratio will be lower, since certain amount of power will be transmitted and reflected no matter how you rotate the beam.

2012_09_18_fiber.jpg

==Note==

  • The output polarization is quite sensitive to the fiber motion/position. I could not really clamp it down because I had to rotate the fiber, and need to give it some slack. So the measured power changed a bit during the measurement. It could moved from 5 uW to ~ 30 uW for the minimum transmission. This increased the error by a factor of 6.
  • The reason for low extinction ratio may also come from the fact that the output beam angle changes with the rotation at B. So the beam did not always hit the PBS perpendicularly. I kept the PBS and PD close to output as much as possible to minimize any effects due to the beam position. This can be repeat by keeping the beam output stable, and rotate the PBS with a rotatable mount instead. I think Dmass mentioned that he had the stage. I'll ask him later.

 

  1048   Mon Sep 17 19:06:36 2012 taraNotesPMCstainless steel pmc

I got PMC drawing from Dmass, this will be similar to gyro's steel PMC. I'll submit the work to machine shop soon.

The drawing is on svn full PMC assemble can be found at ATF:1543.There are spare mirrors in PSL that can be used. I still have to look for a PZT.

The round trip length is 0.33 cm. this corresponds to FSR = 454.45 MHz. If I want to be able to scan through 2 FSR, the displacement range of the PZT will be dL = 2*FSR * L / f, where L = 0.33m, f = c/lambda.  dL ~ 1um. 

  1047   Sun Sep 16 16:42:16 2012 taraDailyProgressfiber opticsetup for fiber optic distribution.

I'm setting up fiber optic so that I can send frequency stabilized laser to ATF. Right now the power coming out is small (0.8 mW from 20mW input). I'm working on better mode match for better efficiency.

2012_09_16_fiber_setup.jpg

 

Note: due to the space limitation, I cannot pick the beam after the broadband EOM used for frequency stabilized the laser. The beam is taken after the PMC, where it was used to dumped the excess power.

Collimator: http://www.thorlabs.com/thorProduct.cfm?partNumber=CFC-2X-C

optical fiber: nufern pm980

modematching: The focal length of the collimator is 2.0 mm, MFD of the fiber is 8 um. The beam diameter at the lens is then ~360 um.

The coupled power is quite small. I'll check the mode matching again to get more power coming out.

 

  1046   Tue Sep 11 15:50:38 2012 taraNotesRefCavtranmisson and finesse of the cavity

I calculated loss in the cavity by using the cavity's pole and transmission. For our cavity with 38kHz pole, T=300ppm, loss on each mirror is ~13ppm. For AlGaAs coating with 10 ppm loss per mirror (absorption+scatter), T = 120 ppm for Finesse = 22,000 is a good choice for us.

   We want to choose a Transmission that is about 10 times larger than the mirror loss. If the transmission is comparable or smaller than the loss, most of the light will loss in the scatter/ absorption and there will be no light coming out.

I calculated to find the loss in the cavity following the instruction from  [Siegman, Laser page 436], 

IMG_1726.JPG

the cavities' poles were measured in PSL:425. RCAV = 38kHz, ACAV = 54kHz which corresponds to round trip loss of 24ppm and 160ppm. For now I assume that round trip loss = absorption x 2 + scattered light x2. I used 5 ppm for absorption on each mirror, since it seems to be a conventional number. This gives scatter loss on each mirror = 7 ppm (RCAV) and 75 ppm (ACAV). The number from ACAV is quite bad, and I think it might be due to the fume from the old package, see Frank's comment. I'll use internal loss  from RCAV (12 ppm for each mirrro) which gives the transmission of 120 ppm.

==Transmission vs coating layers (thickness) and finesse==

I calculated the transmission of mirror with various thickness [ half wave cap quarter,(N pairs of quarter/quarter) Subsrtate]  (where blue represents SiO2, and green represent Ta2O5) .

N total coating thickness [um] T(ppm) Finesse (assume 24 ppm round trip loss) note
 12    4.25 299 9,710  
13

4.56

148 18,300  
14 4.87 73 32,600  
15 5.18 36 52,800  
    120 22,000

 

table 1: coating structure vs T and finesse for SiO2/Ta2O5.

Note, the thickness for AlAs/GaAs might change, but the relation between T and Finesse will be  the same. G. Cole told us that scatter loss is ~ 3-4 ppm, and absorption is ~6 ppm. So the round trip loss is 20 ppm. Here the second table gives the values of finesse and T, with 20ppm loss.

T(ppm) Finesse  
100 26,000  
120 22,400  
150 18,500  
200 14,300  
300 9,800  

table2: T vs finesse for 20ppm loss roundtrip (AlGaAs).

 For SiO2,Ta2O5 coating, I understand that the actual value of transmission can be adjust by changing the thickness of the last few layers, and it should not change the total coating thickness that much. So the above table is still a good quick reference for thickness vs tranmission. If we want T = 120 ppm, the thickness should be around 4.56 micron. This increases the Brownian thermal noise by a factor of sqrt(4.56/4.25) = 1.03, not a big advantage here. For T = 120ppm, the finesse is 22,000.

    Photothermal noise will become larger when the cavity's finesse is increased, but it will not be a problem. Photothermal noise is proportional to sqrt of finesse, PSL:1014. With the finesse of 22,000, it will be comparable to the coating thermal noise at DC upto 25 Hz, PSL:1037. For AlAs/GaAs, the coating will be optimized for photo thermal noise, so intensity noise will not be a problem. So I think with T = 120 ppm for finesse = 22000 will be an appropriate choice for us.

  

  1045   Mon Sep 10 22:39:32 2012 taraNotesRefCavtranmisson and finesse of the cavity

Here is a note about choosing finesse of the new cavity. I'll try to incorporate all the change in the system due to the change of the finesse.

1) Coating thickness(thermal noise level):

     For high reflectivity, the finesse is ~ pi*sqrt(1-T)/T, where T is the transmission (300ppm). A further simplification give         Finesse ~ pi/T. So, with thicker coating, lower transmission, higher Finesse. ( add plot of finesse vs coating thickness). With thicker coating, we also have higher Brownian noise in the coating.

2) Gain in frequency discriminator for PDH lock (shot noise and electronic noise)

    With higher finesse the gain in the PDH signal is increased. We can better suppress laser noise, and noise in RFPD electronics and shot noise. With the servo we have, we can suppress laser noise down to thermal noise already. However, we can improve shot noise and electronic noise if we have higher finesse.

 

  1044   Sun Sep 9 01:18:23 2012 taraNotesScheduleTask Sheet

I make a task sheet for Thermal Noise Probe experiment for the next few months. This will be reviewed every week. The xlsx file is on svn in documents section

tasksheet.png

Attachment 2: task_sheet_2.xlsx
  1043   Mon Sep 3 01:02:46 2012 taraDailyProgressBEATcode schmidtt trigger for beat

I used IQ readout method to measure the frequency noise of Marconi. The sensitivity was good, it was lower than 10^-3 Hz/rtHz up to a few hundred Hz.

 

From previous entry, I finished up the measurement and analyzed the data. I did two things:

  • Compared IQ read out with PLL technique, (green and red against black and dark green): The measurements agree well so I think IQ method is reliable.
  •  Measured Marconi noise( carrier@160MHz, modulation off) when the marconis were locked and not locked with one Rb clock( pink and neon green). When the modulation is off, we cannot use PLL to measure marconi's frequency noise. So I used IQ to measure the noise.  

IQ_compare.png

 ==comments==

From the measurements, I can conclude that IQ method has enough sensitivity at least down to 10^-3 Hz/rtHz (might be up to 1kHz) (neon green).  One thing we should keep in mind is that the frequency noise from marconi is still quite high at 1kHz, even with modulation frequency is off. So we still have problem with measure noise up to 1kHz .

Anyway, I have not determined the noise floor of IQ method yet. I will think about it. If we can keep the beat frequency drift small enough, IQ methods should be ok for beat measurement.

Attachment 2: IQ_compare.fig
  1042   Fri Aug 31 00:48:00 2012 taraDailyProgressBEATcode schmidtt trigger for beat

 I compared results from three different readout techniques, it seems that my Schmidtt algorithm does not work at frequency above 10Hz for our required sensitivity, but IQ readout is very promising.

First, to check if I can produce the same results from the same data analyzed by two different technique  (Schmidtt, and IQ). I used 2 data set (chI and chQ) for IQ read out technique, then for Schmidtt technique, I used only chI data. Then I compared the results with what I got from PLL (I used the old data because I have not measured the new one yet, but it should give a rough idea).

compare_IQ_schmidtt.png

fig1: comparison between IQ readout and Schmidtt technique. Black line is the old measurement for Marconi noise, 10khz input range.

==comments==

The signal was demodulated down to ~1kHz and obtained by 32kHz DAQ. The data was taken with 150s time stretch. It turned out that the Schmidtt method is not sensitive enough.  IQ readout seems to be senstive down to 2 x10^-2 Hz/rtHz. The mismatch between the IQ and PLL was probably the setup between the two are different. (I have not measured the noise level of the current setup with PLL yet).

==next==

So I moved on to use IQ readout with other data. I chose marconi input frequency modulation range to be 1kHz and 100Hz. IQ method can measure down to 3x10^-3 Hz/rtHz. (I'll still have to verify this with PLL, but from a quick look it seems that the results are very reliable (I don't use any delay line in IQ read out at all and I can get sensitivity better than 10^-2 Hz/rtHz). It seems to be very promising for our beat readout.

I will try to compare this will results from PLL, if they agree. I'll move on to use this method to measure the noise level of marconi with frequency modulation function off (we can not do that by PLL technique). The result will give us an upper limit of the Marconi noise plus the readout technique noise which can be used in the noise budget.

IQ_compare.png

fig2: Marconi noise at different settings, measured by IQ methods. The demodulated frequency is 1kHz. This plot is intended to show the sensitivity of IQ method which can be used to measure coating noise upto 1kHz (if the noise from an oscillator used for demodulation is not too noisy).  I have not plot the results from PLL on the same plot yet, but I have attached Marconi noise measured by PLL in fig3 below (data from 2010).

vco-frequency-noise_2010-03-12.png

fig3: Marconi noise at different setting (Carrier @160MHz). The noise level for 1kHz and 100 Hz input range are almost the same and agree with what I got from IQ readout.

 

==note==

  •  I will check how sensitive IQ read out be (by compared it with PLL). However, I also notice one possible problem with this technique. The peaks around 1kHz, from demodulated signal, and its harmonics can be seen clearly in the plot . (there is also this mysterious peak at90 Hz as well, I'm not sure yet where it comes from). It means that if our beat frequency drifts around, the psd of the beat will have weird bumps all over those frequencies.
  • Think about digital delay line, I have not succeeded yet. I'm still confused with the calibration of this technique, but if IQ readout is working ok, I can use this method for beat measurement.

 

Attachment 2: compare_IQ_schmidtt.fig
Attachment 4: IQ_compare.fig
  1041   Thu Aug 30 02:52:01 2012 taraDailyProgressBEATcode schmidtt trigger for beat

I'm trying to check if the schmidtt trigger algorithm will work as our beat readout or not (observe beat from dc to 1kHz), I also revisit IQ readout technique that we tried before as well. I'm analyzing the data and found that some data was not taken carefully (from chosen the wrong time). Here I'll just explain my plan and setup:

Objective: Check if readout methods (Schmidtt PSL:xxx, IQ PSL:xxx readout) are suitable for beat measurement or not. Do they provide a valid result?,

Method:  Compare the result obtained from the mentioned method with a reliable result. My plan is to use PLL to measure the frequency noise of a Marconi at various setup and use it as our reliable result. The setup is similar to what I did in PSL:Xxx, but I did not use 10MHz standard frequency input for Marconi.

   Setup 1: simple demodulated signal.  [add fig] . Both marconis' carrier frequencies were ~ 160MHz. Then the two signals were demodulated down to 1kHz and sent to DAQ (@32kHz). Three levels of frequency deviation (10kHz, 1kHz, 100Hz) were set on Marconi. The data will be used for testing Schmidtt trigger technique

 

IQ_2012_08_20.png

fig2: setup for measurement 2 and 3.

   Setup2:  IQ technique.  With this setup, I measured I-Q signals with three different setting The signals were also demodulated to 1kHz and the frequency deviation were chosen to three levels (10kHz, 1kHz, 100Hz)as well.

  Setup3:  This setup was similar to that of setup 2, but the setting was different. I kept the frequency deviation at 100kHz, and varied the demodulated frequency instead. I chose 1 kHz, 3kHz, and 9 kHz. This data will be used for checking if our beat frequency drift around 9 kHz, will the read out still ok or not.

  1040   Mon Aug 27 14:24:06 2012 taraNotesScheduleplan for upgraded thermal noise probe experiment

 

 The 1.45" cavities have arrived. So I think it is a good time I layout the plan for the next phase of the experiment:

IMG_1579.JPG

 

To Do:

  • Design optic layout for 2-laser setup
  • Try assemble the cavities with our SiO2/Ta2O5 mirrors
  • modify seismic stack / design for holding the cavities
  • getting AlAs/GaAs coating on the substrates
  • find substrate with AdvLIGO coating
  • Choose resonant frequency for 2nd PMC, and 2nd cavity

 

2_laser_setup.png

fig2: proposed new setup with 2 laser sources

Since we want to upgrade the current setup to 2-laser setup, we need to find:

  • another NPRO
  • another PMC
  • broad band EOM, and two resonant EOMs (one for locking PMC, one for locking to the cavity)
  • RFPD, servo, oscillator for locking 2nd PMC to NPRO
  • RFPD for 2nd cavity (with the correct resonant frequency)
  • second oscillator for locking NPRO to the cavity (we can reuse the marconi for AOM)
  • another servo for PMC (I think there is one spare that we used for driving a PZT, I'll check)
  1039   Fri Aug 10 19:50:37 2012 taraDailyProgressBEATcode schmidtt trigger for beat

I used Schmidtt trigger process to track frequency of beat measurement. This is a first step for digital PLL.

==Intro==:

      we are trying to do offline PLL digitally, so we can avoid extra frequency noise from the LO used in PLL. The first step is to track down frequency of the beat measured by the PD.

==the code==:

       I use Schmidtt trigger algorithm to covert analog signal to digital (the plot below show (-1,1) instead of (0,1) for easier comparison with the analog signal). The data below is taken from beat measurement  with +/- 5000 count. The level is set to +/- 0.2 from amplitude of +/-1. Then I record how long the digital signal stay at 1, or 0 before the signal flip, then use that time to calculate the frequency of half cycle and plot it in the below figure.

 

schmidtt.png

Plot: Above, measured signal from daq (blue) and digitized signal via Schmidtt trigger (Green). Below, frequency of beat as obtained by the calculation from the digitized signal. Note the different time span between the two plots.

      

 ==next==

     I have not FFT the frequency drift in time series yet because I just realize that the way I collect the frequency drift vs time might be a problem. The time step for frequency drift would be varied from point to point depending on the current drift frequency. For example,sat at 1Hz, the signal crosses zero twice per second, and twenty times per second at 10Hz. This means the data density (point per time) between the two frequencies are different, see the below zoomed picture. And it might cause a problem when I do FFT with varied dt size.   To fix this, I 'll try to assign constant frequency to fill in the space. Once the problem is fixed, I can just FFT the signal. I'll think about using PLL code as well and compare the two methods.

 zoom.png

plot2: zoom in of the first figure.

Quote:

Quote:

 This is overdoing it. Please just post the existing beat data somewhere and I can show you how to do it easily with a few lines of matlab code. Then you can go back to your usual noise hunting.

 Here is the demodulated beat signal, with 32kHz sampling rate, 120 second time strecth. I used SR560 to amplify the demod signal so that pk-pk value is ~10 000 counts. The data is store in demod.data with a signal column .

 

Attachment 3: schmidtt.zip
  1038   Thu Aug 9 16:52:09 2012 taraNotesSafetywooden cabinet mounted to the wall

The wooden cabinet is mounted to the wall as required by PMA.

  1032   Fri Aug 3 02:46:15 2012 taraDailyProgressNoiseBudgetTransfer Functions (RIN to Frequency noise via photothermal)

I fixed the calculation for photothermal noise and compare it with the measured results. This time, they agree quite well.

  I include Thermo expansion and Thermo refractive effects in coatings in to the calculation, and make sure that they are treated coherently. However I still use Farsi's calculation, not GWINC code (see below for why).  Now the calculated TF shape agrees well with the measurement. However, the magnitude is still off by a factor of ~5. This can come from the calibration from power to Volt on the PD_trans_dc (from PSL:xx), the measurement data is taken from PSL:1021.  The absorption I use for calculation is 5ppm, finesse is 7500. These numbers contribute to the magnitude of the calculated TF. I'll try to plot it with confidence interval to see if it will be close to the measured result or not.

RIN_coupling_2012_08_03.png

 

 

==why not GWINC?==

      Since GWINC code has a section(s) for thermo-optic noise (TO) calculation (following Evans 2008 paper), we should be able to modify it to produce the result for RIN induced noise in our case. However, there is one complication for this option. GWINC calculates (TO) by using the [ Power spectral of temperature fluctuation on surface] x [dphi/dT]. [dphi/dT] is derived from the coating structure and materials' properties. However, the temperature fluctuation on the surface is derived by fluctuation-dissipation theorem then averaged over a guassian beam profile(see Levin,2002).  Hence, to use GWINC code to calculate our result, we need to calculate the pwr spectral density of the surface temperature which is heated up by gaussian beam profile, and averaged over gaussian beam as well. I'll try to do this later when I have more free time.

     I reviewed Farsi calculation carefully They follow the sign for TE and TR in the coating(practically following Evans' paper),so I think it is valid as well. Even though Farsi calculates the effect from the first few layers which, as they claim, contribute most to TR, their argument agrees with the result provided by Evans' paper where they compare TR effect between ITM(8doublets), and ETM(19doublets) which are almost the same.

Plus, the effect from coating becomes comparable to that of substrate around 100 Hz and higher frequency. This means the contribution from substrate and coating must be calculated together, which is nicely done by Farsi as well. (This means you cannot just calculate the effect from a bare substrate, ie. from cerdonio, and add the effect from the coating on top of one another) Since we are going to use the calculation from substrate with coating from them, why not the coating calculation as well?

Attachment 2: RIN_coupling.fig
  1028   Mon Jul 23 19:32:50 2012 taraNotesRefCavHeater power

 Just a reminder for myself what I have to do for short cavities setup:

1) Heater power for cavity. We have not checked how much power we can add to the spacer. This can be done by sending a constant current to the wire (all pin can be found at  PSL:841) Then measure the frequency change between the two cavities. This can be converted to differential length between the two cavities (dL = df/f *L). Then we can estimate how much the cavity is heat up (dL = alpha L dT). With this number and an assumption that heat flow in and out for short and long cavities are about the same, we can roughly see if the current heater is good enough for the short spacer or not, or we need more current/ longer coil to be able to heat up mote.

2) Optics layout for shorter cavities: We also have to decide if we want to go for two laser setup, or two AOM setup as well.

  1027   Fri Jul 20 01:32:45 2012 taraDailyProgressNoiseBudgetTransfer Functions (power fluctuation to Frequency noise via photothermal effect)

1) The coating term is account for thermal expansion only(TE), no thermo refractive has not been included yet.

Farsi treats the effect from TR by assuming that all the TR contribution comes from the first few layers with 180 degree phase different from TE. From their result, TR contribution is much smaller in our frequency band, so I haven't included it in the calculation yet.

2) I agree that our fixed spacer setup will have different result. I'll think about it. Since the coupling is high at low frequency, and the thermal diffusion length is large  ~sqrt(k/C/2pif), which is ~ 1.7 cm [sqrt(1Hz/f)],  (SiO2 material properties) This is certainly comparable to the radius of the mirrors, and any effect on the edge may not be negligible.

Mistake!!, the heat diffusion length is sqrt[ k/ (rho*C*2pif) ] ,  where k is heat conductivity (W/mK), rho is mass density, C is specific heat per kg. This gives 367 um x sqrt(1Hz/f) in fused silica, which is ~ 1.2 mm at 0.1 Hz, and it is still small compared to the mirror's size. Then the boundary condition of our optics might not be as bad as I thought.  I'm thinking about using COMSOL to estimate the effect due to heat escaping from the substrate to the spacer. However, I expect that the boundary effect will help us a bit. The contact area at the spacer will provide additional heat bath for the substrate and reduce the heat at the spot area, thus reducing the thermal expansion effect. Plus, the expansion of the spacer/the substrate at the contact area will be in the opposite direction of that on the spot area. The expansion at the spot area will make the cavity shorter, while the expansion of the spacer/contact area will make the cavity longer. So I think the calculation will still give us the upper bound level, the shape at low frequency ( around 0.5 Hz and below), may change due to the longer heat diffusion length and the boundary effect is not negligible.

Quote:

 

 Does the coating term include the thermo-optic cancellation between thermo-refractive and thermo-elastic? I think you have to use the Evans/Ballmer paper for that (i.e. the GWINC code that I sent you before).

I am surprised if we can use these other papers without modification since the boundary conditions of our optic are fixed. The edge of the mirror doesn't move since its contacted to the spacer. How do you account for that?

 

  1026   Fri Jul 20 00:47:10 2012 taraNotesEnvironmentthe wall is plastered

The wall between PSL and ATF was drilled, chiseled and plastered. This hole is for laser distribution via optical fiber. I'll contact the carpenter shop to have cable tray installed in PSL and ATF soon.

IMG_1478.jpg

IMG_1479.JPG

IMG_1480.JPG

The cable tray will run along the wall, some of the cable rack will be lowered for a few inches.

  1024   Thu Jul 19 23:55:48 2012 taraNotesAOMellipticity of the beam through AOM

I'm optimizing the AOM to make sure that we have max power, and to minimize pointing  due to frequency change, and minimize ellipticity of the transmitted beam.  With the current status, the ellipticity of the beam is 0.59. I'll check what happen to the visibility of ACAV with this beam shape.

  Today I decided to optimize the AOM since I removed it for inspection (I thought it was broken, but it's not). Before I put the AOM back to its place, I made sure that the double-passed beam path overlaps with itself nicely, and parallel to the table plane. Then I inserted the AOM in the beam path, and adjusted the position(tilt/yaw) until the first order beam's power was maximized and got the beam with ellipticity = 0.59. 

   With the non gaussian beam profile we will have less power in ACAV, more noise on RFPD. In addition,  the visibility of the cavity will be reduce and we have to measure it so that we can estimate the power input to the cavity accurately. This number will effect the calculation of our photothermal noise.

  1023   Thu Jul 19 03:01:26 2012 taraDailyProgressNoiseBudgetTransfer Functions (power fluctuation to Frequency noise via photothermal effect)

I fixed the code for photothermal effect (following Farsi), the calculation for substrate effect agrees with Cerdonio at low frequency. And both calculations (Cerdonio/ Farsi) agree if the coating thickness is zero.

 

 I found a mistake in my code for photothermal noise calculation and fixed it. Now the result agrees well with that of Cerdonio's. However, the effect from thermal expansion of coating may be wrong. I try using the parameters in their setup and have not come up with the similar plot they have. So, I'll have to check my calculation again.

Farsi_compare.png

 fig1: comparison between Cerdonio's and Farsi's calculation. The effect from coating(red) may be still incorrect.

Note: I use Riemann sum for the integral in both calculation. The step size has to be small enough for both calculations, otherwise the results won't match up nicely.

Attachment 2: Farsi_compare.fig
  1022   Thu Jul 19 00:32:41 2012 taraNotesElectronics EquipmentRF amplifier

Datasheet for RF amplifier ZFL-500LN. This will be connected after RFPD, when we reduce the power (for RIN TF measurement).

 

  1021   Tue Jul 17 19:05:32 2012 taraDailyProgressNoiseBudgetTransfer Functions (RIN to Frequency noise via photothermal)

Tara and I re-measured the transfer function coupling the RIN to frequency noise at five different power levels.

We were having problems with the ACAV gain at low powers, so we chose appropriate attenuation through an OD filter to compensate. 

The following plot shows the TF's measured, with their corresponding ACAV power levels:

TF_compare.png

The transfer functions measured at 2.2mW, and 1.9mW were measured with a .15 OD filter. Those measured at 1.77mW, 1.6mW, and 1.42mW were measured with a .10 OD filter. 

As shown in the plot, the magnitudes of the transfer functions measured at different power levels match up quite nicely, confirming the validity of the transfer function. The phase plots are pretty consistent, with slight variation up to 20 degrees at higher frequencies.

I converted the above transfer function magnitudes to units of [Hz/Watt] and plotted the results with the calculation using Cerdonio's result:

tf_compare_cerdonio.png

The main source of error between the calculated and measured transfer function is from the voltage to power calibration. We will re-measure this and see if a result closer to the calculation is achieved. Since the measured transfer functions are all fairly consistent with each other in magnitude, I used the transfer function measured at 2.2mW and calculated the residuals for the calculation w/ Cerdonio's results:

residuals.png

I attached the raw data, where the first column of each .mat variable is frequency and the second is dB for magnitude variables and degrees for phase variables.

Attachment 4: residuals.fig
Attachment 5: tf_compare_cerdonio.fig
Attachment 6: TF_compare.fig
Attachment 7: TF.mat
  1020   Mon Jul 16 19:08:34 2012 taraDailyProgressNoiseBudgetTransfer Functions (RIN to Frequency noise via photothermal)

Sarah and I are trying to remeasure the coupling from RIN to frequency noise. Here we list the problems that might cause the error in the measurement.

 Before measuring anything, we aligned the beam to ACAV and RCAV , adjusted the power so the power to ACAV is 2mW and RCAV has 0.2mW.

We found out that:

  1. EAOM alignment does not change the measured TF. We misaligned the EAOM and measured the TF. There was no significant change in the measured TF.
  2. The measured TF does not change with excitation level (from SR785). With output level of 1-5Vpkpk, the measured TF remains the same.
  3. However, the TF changes if we change the power to ACAV. The measure TFs with different power level differ at high frequency and they converge at low frequency depend on the power level. For example, if we choose RFpower on Marconi to 12 dBm, the measured TF will be higher than the TF with 13dBm power ~ 2dB at 1kHz, and converge around 300Hz.

Since the TF remains the same at lower frequency, we thought that the gain of ACAV loop cannot keep up with the frequency noise and cause the deviation in the measure TF, so we checked the UGF of ACAV with different power level (*this is done roughly for a quick check, we will come up with more careful analysis later).

RFpower[dBm] [P_acav mW] UGF
10  [0.4] 16kHz
12.2 [1.3] 39kHz
13 [2.0] 50kHz

The measurements above were done with an extra ND filter that reduce the power on RFPD by a factor of 2. The shape of the TF is roughly 1/f.  The UGF is significantly higher than the interested frequency (DC-1kHz), so we think it is not the insufficient gain in ACAV loop.

So we change the gain in RCAV loop, by increasing the power to 0.4 mW. With this level to RCAV, the TF does not change with power to ACAV that much, unless the power goes down to ~0.4mW.  On the other hands, the TF(RIN coupling) also changes if we reduce RCAV gain to certain level. We will investigate more about this.

Quote:

I tried to remeasure the coupling from RIN to frequency noise (same setup as in psl:1005).  However I have not been able to reproduce the result yet.

 

The result we got in psl:1005 does not have the same TF shape as we expected from the calculation. The slope is off by 1/f^0.25. So There might be something wrong with the setup, for example bad alignment in EAOM that adds PM to the beam and effect the PDH signal. So in order to verify the problem I redo the measurement with the similar setup. This time, however, when I change the power into ACAV, the measured TF changes as well (this did not happen before), so I'm checking what's wrong with the setup.

Checking plan:Once I can remeasure the TF (ACAV AOM feedback/ ACAV TRANSPD). I want to check

  1. effect of beam EAOM alignment to the TF
  2. comparison between TF from (ACAV AOM feedback/ACAV TRANSPD) and (beat signal/ACAV TRANSPD). I tried this already and they are quite similar except some phase shift.
  3. To verify that the measurement is valid, I'll make sure that the TF is independent of excitation level and power input to ACAV.

 

  1019   Sun Jul 15 23:20:32 2012 taraDailyProgressNoiseBudgetTransfer Functions (power fluctuation to Frequency noise via photothermal effect)

I calculated the photothermal noise following the method from Farsi etal 2012 paper. The results is plotted below. I think I did something wrong, by looking at the low frequency part, the results from Cerdonio and Farsi do not agree. I'm still checking on it.

==Cerdonio  vs Farsi==

Cerdonio calculated the displacement noise due to thermal expansion of substrate induced by shot noise. This can be modified to estimate the noise due to absorption of RIN (see PSL:). Farsi extended the calculation to take thermal expansion and thermo refractive effect from coating into account. Since our measurement does not match up with Cerdonio's calculation that well.  I check if the mismatch comes from coating expansion or not. (I did not expect much, since the spotsize is 291 um, and the coating thickness is ~4um. Heat diffusion length ( ~ sqrt(k/2piCf ) )tells us that the effect from coating will be ~ 1000 Hz and above)

==result==

Farsi_compare.png

There are a few things that make me uncertain about my calculation code:

  1. The effect from substrate is going up with wrong slope while it should match the calculation from Cerdonio at low frequency because the thermal diffusion length is much longer than the coating thickness and most of the effect comes from substrate only.
  2. With our parameters the effect from coating is not significant until ~ a few kHz while it is dominating around 100 Hz for their setup.Their spot size on the mirror is~ 100um, so I expect that for our setup, the effect from the coating should kick in before 1kHz.

I'll double check my calculation again.

 

Attachment 2: Farsi_compare.fig
  1016   Tue Jul 10 02:55:37 2012 taraDailyProgressLaserTransfer Functions

I tried to remeasure the coupling from RIN to frequency noise (same setup as in psl:1005).  However I have not been able to reproduce the result yet.

 

The result we got in psl:1005 does not have the same TF shape as we expected from the calculation. The slope is off by 1/f^0.25. So There might be something wrong with the setup, for example bad alignment in EAOM that adds PM to the beam and effect the PDH signal. So in order to verify the problem I redo the measurement with the similar setup. This time, however, when I change the power into ACAV, the measured TF changes as well (this did not happen before), so I'm checking what's wrong with the setup.

Checking plan:Once I can remeasure the TF (ACAV AOM feedback/ ACAV TRANSPD). I want to check

  1. effect of beam EAOM alignment to the TF
  2. comparison between TF from (ACAV AOM feedback/ACAV TRANSPD) and (beat signal/ACAV TRANSPD). I tried this already and they are quite similar except some phase shift.
  3. To verify that the measurement is valid, I'll make sure that the TF is independent of excitation level and power input to ACAV.
  1015   Sun Jul 8 22:59:50 2012 taraNotesEnvironmentDrilling between ATF and PSL for fiber

Eddie told me that the wall in ATF lab is plaster, so he could not anchor the machine thing on the wall. It will be drilled by a hand drill from ATF to PSL.

We tried to move the clean bench in PSL with 4 people, but it wouldn't budge.  So instead of moving the bench, I'll leave it as it is. Since I can cover the wall with Al foil/ plastic / wet sponge around the drilling area so that the dust is confined.

 

IMG_1444.jpg

fig1: ATF lab, The hole will be drilled ~3 inches below the pipe.

IMG_1445.jpg

fig2: PSL lab, there is ~7" space between the wall and the clean bench. I cannot move the clean bench, but I can  cover the wall with something, so I think it should be fine drilling with that.

 

Attachment 3: IMG_1446.jpg
IMG_1446.jpg
  1014   Fri Jul 6 18:59:49 2012 taraDailyProgressLaserTransfer Functions

I have been thinking that the TF measurement (coupling from RIN to frequency noise) we plotted before has unusable unit [Frequency noise /RIN]. We have to correct it by converting it to [frequency noise/ watt]. With that we can compare the result to other experiment/ calculation or use it in the noise budget.  As an example, I plot the result calculated by Cerdonio etal 2003 here as well.

 

==we use wrong unit==

  I am editing my noise budget code to incorporate the effect from RIN induced noise and find out that, the unit we have in Fnoise/RIN is not making sense. In order to get the noise due to RIN I have to multiply measured RIN to the measured TF, that is    Freq noise due to RIN = [measured TF] x [measured RIN behind CAV] = [Hz/RIN] x [RIN] = Hz. As we can see, it does not change with the input power level. In reality, it should depend on the power level as well.  That is why I think Hz/RIN unit is unusable.

 

==result from Cerdonio==

 Braginsky etal calculated the noise from thermal expansion due to heat absorbed from shot noise. The result was later corrected for all frequency span by Cerdoniot etal in 2003. The result tells us the displacement noise due to shot noise. With some modification, we can apply the result to get the displacement noise due to RIN. This will be compared with the measurement later. [add calculation]

IMG_1447.jpg

fig1: calculation for TF using Cerdonio etal's result.

RIN_Fnoise.png

fig2: calculated TF in unit of [Hz/Watt] (I convert the result from m/watt to Hz/watt). Frequency noise can be calculated by multipling the TF with RIN x Pin. [fig file, code]

==some thing about the measurement setup==

The nice thing from the measurement is that we can see the Cerdonio effect when the thermal diffusion length is comparable to the spot size (around 3Hz) nicely. However, the asymptotic behavior of our measurement does not agree well with the prediction. It has a slope of 1/f0.75 , while we expect 1/f.  I'll find out what's wrong with the setup (bad alignment on EAOM, PDH signal, etc) or other mechanism that might cause this effect.

Attachment 2: RIN_Fnoise.png
RIN_Fnoise.png
  1012   Mon Jul 2 23:44:59 2012 taraNotesPDphotodiodes behind cavities

After installing the second pd behind ACVA, I tried to re-measure RIN behind ACAV and RCAV again. The measured RIN from ACAV and RCAV are about the same as we measured before.

 

    I was having problem with the measured RIN after we installed the second PD for ACAV. It turned out that the beam was not dumped properly. The measurement was done after I realigned the beam to ACAV/RCAV and centered the beam on the PD. I'll use the coupling we measured in PSL:1008 to add the contribution from RIN in ACAV to beat noise (assuming no common mode rejection).

 RIN.png

 

 

  1010   Mon Jul 2 00:46:05 2012 taraNotesNoiseBudgetmechanical loss in spacer

I check how much loss in the spacer would have to be to have the contribution comparable to the current beat signal. It turns out to be 10-4 (the current number we are using in the nb calculation is 10-7. I'll check in the literature to see if the spacer loss angle can go up that much due to the non polished surface and back-action loss or not.

       Since the spacers in our setup is not polished on the side and the supports are on two o-rings which are not optimized on the nodal supports, the mechanical loss in SiO2 substrate might be higher than its usual number. So it is a good idea to check what would be the loss of the spacer to be able to give the thermal noise at the same level as what we measure. By adjusting the loss angle to 10^-4 (in the spacers only, not substrates), the calculated noise is about the same as the measured result.

spacer_loss.png

 fig1: The total noise estimation with loss in spacer = 10^-4 plotted with the latest beat measurement.

  1009   Sat Jun 30 05:01:04 2012 taraDailyProgressBEATbeat measurement with 2mW input power

I changed the power input from 1mW to 2mW for both cavities and measure the beat signal. From the measured results, there is no difference between the beat signal at different power input levels. This means that the current signal is unlikely to be sitting on shot noise or electronic noise that depend on power level.

 

     With the current laser (500mW), we can increase the power to both cavities up to 2mW. With higher input power, shot noise and electronics noise can be brought down. Even though the noise budget suggests that we are not limited by shot noise and electronic noise, I think it is a good idea to verify it . If we are limited by the shot noise due to low input power or electronic noise due to low frequency discriminator gain, we can see the change if we increase the power.

==setup==

  1. The power into both cavities are increase to 2mW.
  2. For RCAV, common/fast gain are 630/750.
  3. For ACAV I used an ND filter to reduce the power incident on RFPD. I chose the one that reduce the power roughly in half.

==results and comments==

     beat_2012_06_29_3.png

I plot the beat measurement with  1mW (black) and 2mW(pink) power together to show that there is no apparent improvement. Note that the noise budget is corrected for 2mW input power for both cavities. (I have not include the noise due to power fluctuation yet, I'll add that soon). 

 The slope around 50 Hz to 200 Hz is strongly go with 1/f^0.5 which is the slope of Brownian noise. However, it is ~ a factor of 1.2 above the upper limit of our estimation. I will check with Peter if he still has some information about the coating or not, and also the parameters in the noise budget calculation (for example, the loss in SiO2 for spacer, substrate to see if the values I use are sensible or not.

  1004   Wed Jun 27 17:47:29 2012 taraNotesPurchasesvoltage variable attenuator

voltage variable attenuator

This will be used for ISS in ACAV loop. It will be connected between the oscillator and the amplifier (see the schematic below).

The feedback signal will be sent to the attenuator to change the amplitude, thus changing the power of the diffracted beam in order to stabilize the power.

 

  997   Fri Jun 22 22:34:28 2012 taraDailyProgressNoiseBudgetRIN coupling to Frequency noise

I tried to  measure the coupling from power fluctuation to frequency noise in RCAV, but I could not. The RIN induced frequency noise is probably much lower than we expected in RCAV.

 ==Intro==

    In the previous entry, we measure dP to df coupling in ACAV, by locking RCAV with small power, and measure the power fluctuation behind ACAV and the feedback signal to AOM. This will tell only the coupling in ACAV (assuming the measured coupling is correct). However, the coupling might be different between ACAV and RCAV, so I'm trying to measure the coupling for RCAV.

==how to measure pwer fluctuation to frequency noise==

 Use swept sine measurement, source out is sent to EAOM for power modulation. The reference signal is the signal from PD behind RCAV, the response signal will is the feedback to pzt on NPRO (FAST). The calibration for FAST can be measured by scanning the cavity (3MHz/V), see PSL:182 .

==estimation of SNR==

  1.     The noise for this measurement will be the free running noise of the NPRO (~ 10^4 /f  [Hz/rtHz]), so ~ 100 [Hz/sqrtHz] @100 Hz.
  2. Signal: The signal will be RIN ( with excitation) x coupling(RIN to df). By modulating the power with EAOM, the RIN of the laser after being modulated is ~ 10^-4 [1/rtHz] flat. Since the exact coupling{RIN to fnoise}  is not known, I assume that it is about the same as the measured coupling from ACAV (300Hz/RIN @100Hz). seePSL:989:.   Combining the coupling and the excited RIN, the signal is ~ 3*10^-2 Hz/rtHz @100Hz.
  3.  SNR @ 100Hz is then ~ 3*10^-4. This is bad, to have SNR of one we need ~10^7 integration cycles. (I feel that the number is a bit too large, I'll double check it, maybe the RIN number is wrong).

I tried to measure the coupling anyway (from 50Hz to 100 Hz), with 5000 integration cycles, 7 mW input power to RCAV (I don't want to increase more power because the loop becomes unstable). The result is not good, the SNR is still too small. So we probably cannot determine the coupling for RCAV with this method. [add TF]

  995   Fri Jun 22 03:35:54 2012 taraDailyProgressBEATfrequency noise of demodulated beat

I added the result from the I-Q measurement method in PSL:993.

  994   Thu Jun 21 16:04:39 2012 taraDailyProgressBEATlow frequency beat with 32kHz ADC

Quote:

 This is overdoing it. Please just post the existing beat data somewhere and I can show you how to do it easily with a few lines of matlab code. Then you can go back to your usual noise hunting.

 Here is the demodulated beat signal, with 32kHz sampling rate, 120 second time strecth. I used SR560 to amplify the demod signal so that pk-pk value is ~10 000 counts. The data is store in demod.data with a signal column .

Attachment 1: demod_2012_06_21.mat
  991   Wed Jun 20 16:06:50 2012 taraDailyProgressBEATlow frequency beat with 32kHz ADC

Koji suggested I try to measure the frequency noise of the demodulated beat this way. By using I and Q signals of the demodulated beat signal, I can plot the signal of I and Q, which should be a circle. From that I can get dphi/dt which is the instantaeneous frequency, then FFT to get the beat noise. 

To do that I need:

  • Power splitter with 90 degree phase shift - ZMSCQ-2-180, datasheet.
  • another cable for DAQ (one for I, one for Q)
  990   Tue Jun 19 00:08:19 2012 taraDailyProgressBEATlow frequency beat with 32kHz ADC

I'm using simulink to do software PLL. 

As Zach suggested that using simulink to do the PLL might be easier than using his codes, I looked up this  example and trying to build a model for PLL. It's not working yet, I'm still trying to get FFT from the feedback signal to the vco.

Quote:

Quote:

==comments==

     The FFT results from the fix demodulated signal are not very stable. Thus, the line width is quite large and depends on the number of average and time of the measurement we choose. The plot shows the FFT results from two different average, 200 and 100, (and one more 100 from different time).  We might need a code to track the center frequency of the time series data and then FFT it to get a valid result for this technique.

 Yes, absolutely. If the frequency has some low frequency wander, there's certainly no way to measure the frequency noise with a straight FFT. Instead, you make sure that the beat frequency is up at ~5-10 kHz and then record a long time series (100 - 1000 s). You then use a software PLL to estimate the frequency noise.

Make sure the beat shows up as at least 10000 counts peak.

 

  988   Fri Jun 15 21:41:16 2012 frank, taraNotesDAQTemperature control updated

We switched the temperature readout channels used for temp feedback control to improve the signal. The new signal is significantly smoother.

      The signals from 4 thermostats around the vacuum chamber were acquired through 4 channels, C3:PSL-RCAV_SENSE(1-4). These channels were then connected to DAQ. This made the signal noisy because the resolution of analog to digital converter was low. In order to fix that we use an analog circuit to sum and average the signals from 4 sensors then amplify it before sending to DAQ,C3:PSL-RCAV_TEMP, then calibrated it to C3:PSL-VAC_CHAMBERTEMP by comparing RCAV_TEMP [V] to RCAV_TEMPAVG[C] which is calibrated to deg C already. 

      CHAMBERTEMP = (RCAV_TEMPx-0.495) + 34.957

     

     We corrected the perl script (in SUN machine) used for thermal feedback on the heater jacket. Now the script is named rcav_PID_2012_06_15.pl, see wiki. The servo is now back on.

  985   Wed Jun 13 11:12:55 2012 taraNotesopticcavity mirrors

Frank showed me where we keep the spare cavity mirrors. They are in a cardboard box labeled REO in the left cabinet. There are 7 substrates with the coatings similar to what we use in the current setup. They are specified as polished annulus, and wedge (details are added in the proposal). So, if we have short spacers, we can assemble the cavities asap. The coatings profile is not written anywhere(# of layers, transmissivity), I'll ask peter if he has the information about this.

IMG_1357.jpgIMG_1358.jpg

IMG_1359.jpg

 

  982   Sat Jun 9 01:49:52 2012 taraNotesPMCPMC servo was acting strange

 Today when Sarah and I tried to align the beam to PMC,  we lost PMC lock and could not bring it back. So we investigated it. The cause of the lost lock is not yet exactly concluded, but we can lock pmc back at ~ 40mW with usual stability.

 What we did before we lost lock:

  • We realigned EAOM in front of PMC. Since we replaced the laser head, the beam misaligned a bit and clipped/ scattered at EAOM. This turns out to be irrelevant with the problem (see details below).

What we did to check what was wrong with the PMC servo:

  • Made sure that the power input to PMC did not exceed  80 mW, so the RFPD for PMC was not destroyed (It was not saturated either).
  • Check the PZT response, by scanning the DC offset. By doing this we still see HOMs showing up on ccd behind the PMC, so the PZT is working fine.
  • Measured error signal from mixer out while scanning the cavity. The error signal had sine wave around 30 Hz added into it, so the signal was error signal + sine wave (30Hz) with amplitude larger than of pk-pk level of error signal (when the alignment was still bad).
  • So I disconnected RF signal, terminated the input with 50ohm, the fluctuation was still there. It was gone after I disconnected LO singal.
  • I checked the signal from LO, nothing was wrong with it.
  • Then I tried to align the beam as best I could without locking the PMC. Once the alignment was good enough, the 30 Hz fluctuation amplitude was smaller than pk-pk signal of the error signal. However when I locked the PMC, it was not very stable. A light tap on the table could kick the PMC out of lock.
  • I tried to reverse all the setup back to original and measured the error signal. With 20mW power input, the error signal was totally fine.IMG_1343.JPG
  • Then I increased more power to PMC, by adjusting the half wave plate in front of the first Faraday Isolator. The shape of the error signal changed as the power increase, but I could still optimize it by adjusting the phase shift (I don't know why would the phase change with the power it should be independent from each other). However, at 60mW input power to PMC(this corresponds to ~110+ mW at the 21.5 MHz EOM) the error signal turned bad. It was distorted, and the peaks from sidebands were gone. It could not be adjusted by changing RF power to EOM or phase shift.     So it has to do with the power. I'm not sure if the alignment of the EOM will be the cause of this behavior, but I aligned it quite well. There was no obvious beam clipped or scattered light on the EOM openings.

IMG_1345.JPG

Current status:

  • PMC is locked with 40mW input, gain 15 dB.
  • The origin of 30 Hz modulation at mixer out is not yet verified, but it disappeared after I investigated the problem.
  • PMC lock is stable, tapping on the table won't kick it out of lock
  978   Mon Jun 4 22:38:44 2012 taraDailyProgressBEATlow frequency beat with 32kHz ADC

I tried to measure beat signal without using PLL, the FFT of the demodulated signal does not look good. We probably need to do a code for software PLL to improve the signal for this method.

==Why not PLL?== ,

     as we can see in the previous beat measurement with PLL readout technique, frquency noise of the oscillator dominates the signal at high frequency (500Hz and above. Its frequency noise is dependent on the setup (carrier frequency and f modulation range). By turning off the modulation range, and demodulating the beat signal with a fixed frequency signal, we hope to reduce the frequency noise of the oscillator a by little bit.

==what I did==   

     We have a cable from PSL to ATF that goes to DAQ. The channel name is C2:ATF-PSL2_DAQ_OUT. 

  • Check DAQ: By sending in a sinewave signal, I can check that DAQ is working properly. [fig1, FFT of sinewave 15 Hz]. 
  • To calibrate the signal, I used DAQ to measure the signal at V feedback to VCO, then get the plot from diaggui in fb0, and scale it so that it is comparable with the calibrated beat measurement.The calibration is ~0.45  times the result from FFT, see the figure below.

beat_2012_06_04_b.png

  • Turn off the frequency modulation on the vco, then measure the demodulated signal at mixer out, use the calibration from (2) to convert to absolute frequency

demod.png

==comments==

     The FFT results from the fix demodulated signal are not very stable. Thus, the line width is quite large and depends on the number of average and time of the measurement we choose. The plot shows the FFT results from two different average, 200 and 100, (and one more 100 from different time).  We might need a code to track the center frequency of the time series data and then FFT it to get a valid result for this technique.

   

 

 

  975   Fri Jun 1 01:00:30 2012 frank, taraDailyProgressAcousticsacoustic enclosure for input beam

The beat signal is measured with an adhoc lid covering the enclosure on the beat part. There are no significant improvement in the acoustic region of the beat signal yet.

     The closing lid is the only missing piece for our acoustic box, so I just use smaller pieces of acrylic planes I can find to cover the top, and measure the beat signal. There is not much improvement in the acoustic region, see fig2 below for a closer look.

     There might  be dust on the optics somewhere, because the signal at low frequency is not as stable as it used to be. I could see scattered light bump coming up when I averaged for too long (this did not happen last time). I'll check all optics and make sure that they are clean.

     NOTE: The temperature servo is back on. We had to disconnect it in order to install the acoustic box. I set C3:PSL-RCAV_TEMPAVG to 32.5 ( the previous value was 35.03). I changed it because I noticed that as the temperature reached up to the set point, the beat frequency went up as well.  I'll check if this set point will reduce beat frequency down to ~ 160 MHz or not.

    

 

beat_2012_05_31.png

beat_2012_05_31_b.png

  974   Thu May 31 15:10:57 2012 taraNotesPurchasesquote from Coastline

I called Coastline about the lead time for the substrates and asked if we could speed up the process by changing the specs to be less demanding (coarser roughness/ sphericity). The answer is no, all the main features of the substrate (curve surface with polished annulus, wedge surface) are the main driving factors, not the roughness or how flat it is. Additionally, they told me that, from their experience, the width of the annulus of 2-3 mm is enough for optical contact. 5mm width would be too large and it causes the surface near the annulus to deform, and it takes longer time (he could not tell exactly how longer). I put the quotes on CTN wiki.

  973   Wed May 30 23:39:41 2012 frank, taraDailyProgressAcousticsacoustic enclosure for input beam

We started installing the new acoustic enclosure box for the beat path. It covers the whole beat setup and the part for power detection. The walls are installed. The lid will be made later.

 

IMG_1300.jpg

The mirror mount in front of new focus 1811 PD has long knobs which extend into the wall, so I replace it with another mount that has no knobs, and the panel can fit without obstruction.

The current box has only three holes, 2 for input beams, one for output beam at the beam splitter. Windows might be installed in these holes for better acoustic enclosure.

  972   Wed May 30 23:39:15 2012 frank, taraDailyProgressAcousticsacoustic enclosure for input beam

We started installing the new acoustic enclosure box for the beat path. It covers the whole beat setup and the part for power detection. The walls are installed. The lid will be made later.

 

IMG_1300.jpg

  970   Mon May 21 21:11:16 2012 taraNotesNoiseBudgetthermo optic noise added to noise budget

Frank corrected me that the acoustic coupling was not improved with the legs changing around 500 Hz. The measurement in comparison was not equipped with acoustic box yet. So I compared the measurement with the result from psl:955 , and there is no improvement around acoustic frequency. This means that these peaks are really from acoustic coupling through air, not legs.

beat_2012_05_18_compare_lid.png

  969   Mon May 21 16:53:38 2012 taraNotesopticwindows for vacuum chamber

 I edited the drawing for 10" flange. The wedge surfaces for 2" windows are tilted by 2 degrees sideway.

 

I tried to assemble the pieces with 2" OD window, 0.25" thickness (without Oring). I think the clearance for the window might be too tight. I'll fix it.

 

window_assembly.PDF

window_assembly2.PDF

 

Attachment 1: edit_blank_10_wedge.PDF
edit_blank_10_wedge.PDF
  968   Mon May 21 02:38:48 2012 taraNotesNoiseBudgetthermo optic noise added to noise budget

Thermo optic noise (coating) calculation has been added to the noise budget code.

      The previous result (for example, see psl:901) was not done properly. What I did was calculated the TO noise using GWINC code independently then plotted the result on top of the noise budget. Now the calculation is included in the main noise budget file (by calling function from GWINC). The noise budget for shorter cavity can be done easily by changing the length of the cavity only. For 1.45" long cavity, the TO noise in coating will not be the dominant noise source.

 

nb_short_cav.png

  967   Fri May 18 18:44:38 2012 taraDailyProgressBEATbeat measurement update

I'm trying to fix the mechanical peaks in the beat signal. The work is still in progress.

There are several mechanical peaks around our most sensitive band (60 Hz - 600 Hz). It is important that we damp these to improve the beat signal. 

One of unidentified peaks are around 240 Hz , which might be originated inside the vacuum chamber. I tried tapping the chamber, and saw the peaks went up, but I could not pinpoint to where(stack/heat shield/ cavity motion) exactly.

There are peaks around 500Hz that come from QWPs behind the cavities. I checked their components individually to pinpoint where was the part that produced 500 Hz peak.  Each QWP was mounted on a rotatable square mount that are screwed down to a solid aluminum block.

  1. I inserted a sheet of rubber between the two mounts to damp any possible motion from the structure, but there was no improvement.
  2. I replaced the Al block with a steel 1" post.-> The peaks still there. So,
  3. I replaced the rotatable square mounts to the circular desing, on a steel post, and made sure that the QWPs were secured. -> The peaks still there
  4. I tilted the wave plates a little bit to prevent any possible back reflection, still no improvements.

It might be the motion of the plate itself. If this is the case, we have to have a better acoustic insulation box. The current one for the beat path has to many holes.

  966   Fri May 18 03:56:22 2012 taraDailyProgressBEATbeat measurement update

The beat signal is getting better at lower frequency, there is no obvious scattering bump around 80 Hz anymore. It is possible that the previous legs + leveling valves were bad and caused extra vibration.

After legs and leveling valves replacement,  the beat signal has better stability at low frequency. There is no bump due to scattered light around 80 Hz anymore. Plus, acoustic coupling (400-600Hz)f is significantly reduce without any improvement on the acoustic shield. Thus, it is very likely that acoustic coupling is related to legs' isolation performance.,Wrong statement, see PSL:970.(Mon May 21 21:13:11 2012 )

 Mechanical peaks around 25-50 Hz are probably from horizontal, or tip/tilt motion of the seismic stacks.

beat_2012_05_18.png

fig1: beat measurement. floated table, floated air spring. 1mW input power for each cavity, 1kHz input range on PLL.

 The result around 80-200 Hz has slope  f^(-0.5). Alas the span is only ~100 Hz bandwidth, with hideous unidentified peaks around 240 Hz. Otherwise we can be more certain that we reach thermal noise of something even though it is ~ a factor of 1.5 above the upper limit.

zoom.png

fig2: close up plot of the above figure, around 80-300 Hz. The measurement result is about a factor of 1.5 above the upper estimated level.

compare.png

fig3: comparison with f^-0.5 line (cyan).

 

 

  965   Thu May 17 17:35:17 2012 taraNotesPurchasesquote from ATF for substrate and spacer

I got the quote for substrates from ATF (AR coated on flat-wedge, blank on concave, R=0.5m side) quantity of 10 and 20. They also offer the cavity assembly as well, so I sent them the drawing of our cavity to ask for the quote.

For the spacer, I asked two other companies,cidra, and tecoptics, for the quote, but they have not replied back yet. So, I just asked another company,aiceramics.com, in hope of getting it done soon.

  964   Wed May 16 22:27:13 2012 taraNotesopticwindows for vacuum chamber

I finished the drawing for new vacuum windows. The o-ring for the windows will be #223 (1/8" thickness). I'll consult with Steve one more time before I submit the drawings.

A few comments about this desing:

  •  The design is intend for 2" OD, 0.25" thickness window. The blank has 2 degree wedge surface for the window.
  •   The grooves for the o rings are based on the instruction on previous entry.
  •  The material for the small window, and the 10" window will be stainless steel.
  •   I feel that the drawing is a bit unclear, I'll try to draw it properly.

2_small_window.PDF

blank_10_wedge.PDF

Quote:

Nice reference for O-ring + groove design. I'll put it on CTN wiki as well.

 

The o-ring I plan to use for 2" OD window is #223, 0.139" thickness, ID = 1.609", OD = 1.887". McMasterCarr.

 

  963   Wed May 16 22:25:06 2012 frank, taraDailyProgressSeismicreplacing table legs

We check the leak. There are no obvious leak on the hose or connectors. It appears that the current leveling valves are more sensitive than the previous sets. That explains why the compressor is activated every 30 minutes while the beam alignment is still quite good.

  962   Tue May 15 21:57:49 2012 frank, taraDailyProgressSeismicreplacing table legs

The leveling valve for table legs are installed. With the new legs, the beam can maintain its alignment better than before (from 30 minutes observation period).

      Three leveling valves are installed, all of them have T connectors for linking two legs together, so we use a short tube, filled with epoxy to blind the unused side of the T for two legs. The compressor works every 30 mins. Although the period is shorter than before (~1 hr), but  it takes only ~ 2 minutes to compress the air, and the beam alignment is stable before and during the air compression, see fig 2. 

IMG_0930.JPG

fig1: new legs with new leveling valve.

IMG_0931.jpg

fig2: RCAV_RFPD_DC, over 100 seconds while the compressor is working. Generally, the reflected power on the RFPD increases if the alignment changes. The stable DC level indicates good stability. For this setup, the DC level is 1.7V. when the beam is off resonance. The previous setup had bad stability, it drifted from ~ 100mV to ~400mV.

We will use soap water to check for any possible leak tomorrow to make sure that there is no obvious leak in the setup.

  960   Fri May 11 17:52:17 2012 taraNotesopticwindows for vacuum chamber

Nice reference for O-ring + groove design. I'll put it on CTN wiki as well.

 

The o-ring I plan to use for 2" OD window is #223, 0.139" thickness, ID = 1.609", OD = 1.887". McMasterCarr.

Attachment 1: O-Rings.pdf.zip
  959   Fri May 11 01:00:42 2012 taraNotesPurchasesreplacing table legs

Frank told me to get the leveling pressure from Cryo table. Now I have enough for the table (a set of three), so I don't need to buy the new set. All CTN table legs are replaced. I'll connect the pressure tube to the legs tomorrow.

Quote:

There are no spare leveling valve at 40m, I'll buy a set of new one from Newport. It costs ~ $ 800. I asked Eric and he said to go ahead and buy it.

leveling valve set of 3 

It should be able to mount on the leg with an adapter piece easily.

This is how it looks like when it is mounted on the leg

http://search.newport.com/?q=*&x2=sku&q2=S-2000A-819.5

 

 

 

  958   Thu May 10 16:04:24 2012 taraNotesPurchasesreplacing table legs

There are no spare leveling valve at 40m, I'll buy a set of new one from Newport. It costs ~ $ 800. I asked Eric and he said to go ahead and buy it.

leveling valve set of 3 

It should be able to mount on the leg with an adapter piece easily.

This is how it looks like when it is mounted on the leg

http://search.newport.com/?q=*&x2=sku&q2=S-2000A-819.5

Quote:

I'm replacing our table legs with the ones from Jan's lab.  Those should be a little newer than the one we are using. However, the regulators we have are not compatible with those new legs.

 The pressure regulator for the old legs (which we are using) need a plate with a slot for holding it lever (in red square). However, the legs I took from Jan's lab do not have such the structure because it has different regulator style (in green square) and there is only one.  Basically, we don't have enough regulators for the new legs (need 3, have 1). I'll look around, check the 40m if they have some unused regulator.

IMG_0893.jpgIMG_0894_1.jpg

Above, left) the structure for the regulator for the current legs. Right) the regulator for the new leg.

 

 

 

 

 

 

  957   Wed May 9 22:26:26 2012 taraNotesLaserexternal cavity laser diode

I just asked Aki a few more questions about the ecd laser. If we do not require the performance to be rival to that of the NPRO, making one is possible in a few weeks time scale.

Q1) The setup of the Littrow style laser you showed me had one mirror
behind the grating. Is the setup similar to this
<
http://rsi.aip.org/resource/1/rsinak/v72/i12/p4477_s1>? Where the
mirror is used so that the alignment does not change when the laser is
tuned.

A1) Actually, Yes, for the laser you took the photo. But, most of the lasers we use in the lab don't have a mirror, only diode and grating in the box. In our case, we only scan the frequency by ~1GHz and the pointing vector drift is negligible.

 

Q2) Did you remove the glass window of the diode laser when you assemble
 the laser? If so, how do you keep the diode clean, or it does not matter
for your requirement.

A2) We didn't remove the glass window. What we did is very simple. We mounted the bare diode on the thorlabs mount: http://www.thorlabs.com/thorProduct.cfm?partNumber=LT230220P-B

 

 Q3) You mentioned that the line width of the laser when locked to CS cell
is ~300kHz. Is it because of gain limited of the servo or the CS cell's
intrinsic noise?

A3) We haven't measure the linewidth with locking and without locking independently. It's possible that our laser linewidth (without frequency lock) might be ~300kHz. So I don't know what limits our linewidth.

One thing you may want to consider is that a diode laser is infamous for the broad background incoherent light, compared to the solid state lasers. We typically observe ~30nm-wide incoherent light around the carrier with 30-40dB suppression compared to the carrier. If your experiment is sensitive to the spectral purity, this might be an issue.

Aki

 

 So the question is do we want to try to build one similar to what they have? We know that with the time scale and experience we have it will not be as good as the performance of the ecd laser reported in Numata etal paper, but it might be a fun project for the SURF student.

Quote:

I'm looking into how to make external cavity diode laser (ecd).

Here is the list of what we need.

 

  • Laser diode 1064nm, diode (+AR coating)
  • Grating
  • Current driver for locking the laser (home made)

I ask Akihisa who works in Kimble's group about their home made 850 nm ecl. The performance is not as good as NPRO yet (300kHz line width when locked to CS cell), but it is certainly interesting.

  • They use Littrow configuration ( 1 grating to form a cavity). The mirror behind the grating is for adjusting the output beam.
  • The PD is not AR coated, a slight power (10 uW) that transmits inside the diode is enough for the feedback.
  • The linewidth is ~ 300kHz when locked to CS cell, the free running noise is not measured.
  • They use both PZT and current to actuate on the frequency stabilization, the bandwidth is 50kHz.
  • Temperature feedback is employed to keep the stability of the laser
  • Time for putting everything in the box (once all components are ready) is ~ 1wk

 (IMG_0887.jpg IMG_0888.jpg

 

  956   Wed May 9 19:59:44 2012 taraNotesSeismicreplacing table legs

I'm replacing our table legs with the ones from Jan's lab.  Those should be a little newer than the one we are using. However, the regulators we have are not compatible with those new legs.

 The pressure regulator for the old legs (which we are using) need a plate with a slot for holding it lever (in red square). However, the legs I took from Jan's lab do not have such the structure because it has different regulator style (in green square) and there is only one.  Basically, we don't have enough regulators for the new legs (need 3, have 1). I'll look around, check the 40m if they have some unused regulator.

IMG_0893.jpgIMG_0894_1.jpg

Above, left) the structure for the regulator for the current legs. Right) the regulator for the new leg.

 

 

 

 

 

  955   Wed May 9 03:11:33 2012 frank, taraDailyProgressAcousticsacoustic enclosure for input beam

I check the performance of the enclosure box for input optics. It neither improves the beat signal that much.

   ==Is the acoustic box good?  No==

   To check how good the acoustic shield can be, I measured the beat signal and feedback signal to ACAV AOM when the lid were on and off. There were no much improvement in both signals, see fig1 below.

IO_lid_compare.png

fig1, beat signal and ACAV feedback, converted to frequency noise. Beat signals between the lid close and open (red, purple) are very similar. Feedback signal to AOM are also the same (blue, cyan). I plot the 4 traces together to see if there are any coincided peak, so I can know where it happens (beat path or input optics). Note the peak at 280 Hz in Cyan trace is not real, it pops up after ~50Avg. I could not find its origin yet.

    ==Is it really acoustic coupling?  Yeah, kind of==

    The results were so similar between the lid open and close, so I wondered if those were really acoustic. To test this, I turned off the two computer (PC and fb2) and remeasured the beat. Those computers' fans are quite loud when they are on. For fb2, the fans still work even when it is shut down, but definitely much quieter. The beat signal was improved a bit, see figure 2. The results were real, I repeated them twice. Note that the room are still not totally quiet with the two computers off, sounds from sun machine and electronic rack are still there, and they are as loud as the two computer and closer to the beat setup as well. 

computer_compare.png

fig2: beat signals when the computer are on (blue) and off (red), several peaks are obviously reduced when the computers are off.

  ==discussion and plan==

    Since the computer are sitting on the floor, it is not certain if the peaks due to the computers are from acoustic transferred through air or vibration transferred through the ground. But the peaks in question are at high frequency (almost 1kHz), and we have 3 stage seismic isolation on (except floating table). It is very likely that these peaks are caused by acoustic. To make sure that they are really acoustic, I'll float the table and repeat the measurement again.

  954   Tue May 8 16:28:30 2012 taraNotesLaserexternal cavity laser diode

I'm looking into how to make external cavity diode laser (ecd).

Here is the list of what we need.

 

  • Laser diode 1064nm, diode (+AR coating)
  • Grating
  • Current driver for locking the laser (home made)

I ask Akihisa who works in Kimble's group about their home made 850 nm ecl. The performance is not as good as NPRO yet (300kHz line width when locked to CS cell), but it is certainly interesting.

  • They use Littrow configuration ( 1 grating to form a cavity). The mirror behind the grating is for adjusting the output beam.
  • The PD is not AR coated, a slight power (10 uW) that transmits inside the diode is enough for the feedback.
  • The linewidth is ~ 300kHz when locked to CS cell, the free running noise is not measured.
  • They use both PZT and current to actuate on the frequency stabilization, the bandwidth is 50kHz.
  • Temperature feedback is employed to keep the stability of the laser
  • Time for putting everything in the box (once all components are ready) is ~ 1wk

 (IMG_0887.jpg IMG_0888.jpg

  953   Fri May 4 20:19:58 2012 frank, taraDailyProgressAcousticsacoustic enclosure for input beam

The enclosure box for input optics are done. We still need to order more of the nuts for one panel, but the box should provide certain acoustic shielding for now.

We will measure the beat signal once the temperature stable.

IMG_0863.JPG

IMG_0864.jpg

    The box has four 1-inch diameter holes, 2 for periscope, one for input beam, another one is for the beam to RCAV curve mirror which we cannot fit in the box.

    We had to rearrange the cable for ACAV AOM to have fewer cables going in and out the box. The cable for driving the AOM was remade so that it did not block the panel.

  952   Thu May 3 22:26:14 2012 frank, taraDailyProgressAcousticsacoustic enclosure for input beam

We are still working on the acoustic shielding panels. The work should be done by tomorrow.

IMG_0855.JPGIMG_0856.jpg

Fig1: Left, one panel with damping foam + black pad on top( to prevent scattered light). Right,a panel with two layers of good damping material with a damping pad under it. This type can damp acoustic noise pretty well.

We prepared all sides of the acoustic box. However, we don't have enough damping materials, so all the panels are not similar, but they all have some soft foam to provide acoustic damping. All the holes for cables/ beam are marked and will be drilled tomorrow. 

  951   Wed May 2 19:48:32 2012 frank, taraDailyProgressAcousticsacoustic enclosure for input beam

      We measured the frequency response(microphone out/signal to speaker) to see how well we can shield the outside acoustic.  The test panel did help reducing the acoustic coupling, but there is still room for improvement.

     Den lend me a blue bird microphone from 40m.  So we setup a measurement to compare two panels we have. The first one was what we made yesterday (plastic with damping pad), the second one was the aluminum panel(1/8" thick) with soft foam on the inside and foam strip on the edge where the panel met the frame.

     ==setup==

     We measured the frequency response between the microphone signal and the speaker driving signal. The source was white noise (band limit) 100Hz - 6.5kHz, 1.4V. The output has a T so that one was sent to the speaker, another one was for chA.  The SR785 chB input for microphone signal was floated  since the mic gave differential output. This should prevent the pre-amp output to see "ground" at the output and break the opamp.The measurement was average over 5000 samples.

      We measured with the speaker on and off (but the white noise ref to chA was still connected) to check we have a good SNR for every setup. Three setups were:

  • 1) wihout panel (see fig1),
  • 2) with plastic panel + damping pad (see fig2,left),
  • 3) with aluminum panel + soft foam (fig3, right).

.IMG_0853.jpg

fig1: setup, with the panel remove.

IMG_0854.JPG

fig2: two panels for testing. Left, a plastic piece with damping pad attached on (from yesterday). Right, an aluminum panel with soft foam

IMG_0852.jpg

fig3: panel under test.

 

result.png

fig4: result.

  ==conclusion + plan==

    From the plot, it is not very clear if the aluminum panel (panel2) is better than the plastic one (panel1). It might be that noise coming from other panels(which we have not changed) is the dominating signal. We will put the mic in a smaller container surrounded by acoustic damping with an opening for the material/structure to be tested. Then we can test a sample easily without removing/installing the panel all the time.

     For now, we are planing to use another kind of foam to put inside the box. We check by ears and found that it is better than the current foam we use with the aluminum panel.

  950   Tue May 1 23:58:21 2012 frank, taraDailyProgressAcousticsacoustic enclosure for input beam

We added the lid on top of the enclosure. More work is needed to complete the box.

    We made the closing lids by cutting a 1/8" acrylic panel. A strip of soft foam was added between the frame and the lids to form a seal.

    We did a qualitative test by placing a white noise source inside the box and listening. The aluminum bubble wrap we used did not provide good noise reduction. So we replaced one side by a plastic piece (~1/8" thick) with a damping pad on. It could damp the noise pretty well. I'll borrow a blue bird microphone from Den tomorrow, so we can measure the TF or just the relative noise signal to check how much attenuation we get from our structure.

 

IMG_0851.jpg

  949   Tue May 1 20:37:16 2012 taraNotesopticwindows for vacuum chamber

I got the reply from Thorlab the flange can't accept the thicker optical windows. So I think we have to make our own custom small flanges. I'll check TCS small windows design and make a drawing and consult with Steve again.

 

------------------------

Hi Tara,

Thank you very much for your response.  It looks like our flanges can only fit
windows 0.1 mm thicker, with a tolerance of +0.0/-0.2 mm, so these flanges would not
be cross-compatible with existing windows.  I apologize for any inconvenience this
may cause.  Please let me know if you have additional inquiries, as I am very happy
to help.
-------------
  948   Tue May 1 03:12:58 2012 frank, taraDailyProgressAcousticsacoustic enclosure for input beam

We started making the acoustic enclosure around the input beam area (second half of the table, before the chamber). The frame is done. We haven't received all items for the panels yet, so we just tried to use the aluminum bubble wrap as test panels. And we just used a piece of plastic planes to cover the top. There is no improvement in acoustic coupling yet.

 

IMG_0849.jpg

  947   Mon Apr 30 01:41:18 2012 taraDailyProgressBEATnoise due to air leaking from legs

I suspected that the air leaking from the legs might cause  noise in acoustic bandwidth, so I measured the beat signal with unfloated /floated table. The beat signals from the two look similar. Air leaking from legs does not cause any extra acoustic coupling from 100Hz - 3kHz in the beat signal. 

 

 beat_2012_04_29_compare.png

fig1: Beat signal between floated and unfloated table. There is no significant difference between the two.

       On Friday evening, I turned off the air compressor. When I came back on Sunday night, the table was not floated. I measured the beat signal, then turned on the compressor, realigned the beam, then measured the beat again. The results were similar in 100Hz- 6 kHz band. Floating the table with these leaking legs will not add any extra acoustic noise to our signal (at least, at this level).

      Since the result looked nice with fewer mechanical peaks at night, I took a chance to check if it was limited by frequency noise of marconi in PLL loop or not. I changed the input range from 1kHz to 200Hz (their frequency noise level should be different and observable at 100-1kHz bandwidth, see here), but there was not different between the two input range, see fig2 below.

beat_inputrange.png

fig2: beat with different input range. There is no significant change in the results.

    It is likely that the flat level we are sitting on are detection noise + shot noise. This will be carefully checked next.

  946   Fri Apr 27 22:37:54 2012 taraDailyProgressBEATreplacing one mirror mount in beat path

One of the mirror mount in beat path was not properly mounted on the board because of the limited space. I changed that mirror mount with a block mount (similar to the one we use for the beam recombining beam splitter). The acoustic coupling is getting better.

 

IMG_0842.jpg

fig1: the mirror with the mount similar to that of the BS, see psl:818.

beat_2012_04_27.png

fig2: beat signal, comparison between before and after the mount replacement.

I'm not sure why the beat signal at 2kHz and above does not match. It might be that I did not align the beat well enough, or the alignment in front of the cavities changed. However, there is a significant improvement in beat signal, except the new mechanical peak around 1.2kHz, it might be from the new mirror mount.

 

Note: I turned off the air compressor switch after I measured the beat before the mount replacement to make sure that the seismic isolation for both measurements will be similar.

 

 

Attachment 1: IMG_0841.jpg
IMG_0841.jpg
  939   Wed Apr 25 15:15:03 2012 taraNotesLaserborrow NPRO from TNI

Duly noted, I'll write the sop for this one and put it on svn and wiki.

Quote:

Don't forget to write an SOP for that laser before using it. A few things changed recently if you want to use the old SOP as an template. The hazard zone and emergency laser shutdown procedures, table layout etc. have slightly changed since the 10W laser has been moved. The map outside the lab is also not up-to-date anymore. The main kill switch at the door is still active but the second one which was attached to the 10W laser is gone now, but also not required for the NPRO. The required OD for safety glasses has to be recalculated for the document, but the ones we have are all good. The rest of the old SOP should be the same.

You also have to redo the mode matching to the PMC as the beam waist changes if you switch to the higher power laser.

For a two-laser setup we don't have all the parts at the moment to do so. There is no EOM for the PMC nor the refcav sidebands, no servo for the PMC, and no PMC. We don't have all parts to build a new PMC, especially no PZTs. Mirrors exist, spacer parts too but have to be assembled and we have to ask P.K. if we can have them, but shouldn't be a problem.

 

 

 

  935   Tue Apr 24 22:10:45 2012 taraNotesElectronics Equipmentmodified UPDH

[Frank, Tara]

We checked the input referred noise of the UPDH and found it to be 31nV/rtHz, so we bypassing U6, and the input noise is reduced to 3.3nV/rtHz. Most of the excess noise was introduced at the adjustable gain, U6.

        Before the modification, the noise level at TP13 was 170 nV/rtHz. The gain at U1 was 26.4dB, and -6dB at U6 (0 on the knob). Thus the total amplification was 14.8 (a factor of 5.5). The input referred noise was then 170/5.5 ~ 31 nV/rtHz.   After we scraped out the trace from U6 Vout to TP13, and short TP3 and TP13, the noise at TP13 level was 37nV/rtHz. This level is reasonable (cf OP27 datasheet). The amplification was changed to 20.8 dB (a factor of 11). The input referred noise now is then 37/11 ~ 3.3 nV/rtHz. This input referred noise can be convert to frequency noise by the frequency discriminator gain (latest value was 31kHz/V). It is only 0.1 mHz/rtHz which is  low and will not be the limiting source. This input referred noise will be added to the noise budget. The cavities can be locked properly after the modification. 

 

zoom.png

fig1: part of the schematic of UPDH after modification on R14 and R12, and U6 is bypassed. The whole updated schematic can be found on svn

 

Right now the gain is not adjustable, we will solve this problem later.

  934   Tue Apr 24 22:07:10 2012 taraNotesLaserborrow NPRO from TNI

I borrow an NPRO from TNI. The model is similar to what we have, but I think the maximum power is 500mW (what we have is 100mW). I'll check the beam waist position tomorrow.

     Since we might switch to 2-laser setup, it is a good idea to have the second one ready. I checked a few companies for 1064 laser diode, but I could not find the information about how to control the frequency and its performance on the webpages. So I asked Eric Black and got the laser from TNI. Plus, the TTFSS is proved to be sufficient to suppress NPRO free running noise below coating noise level, the NPRO laser should be good for our setup.  We might need to think about preparing the rest of the equipments (resonant EOM for 2nd PMC, resonant and Broadband for refcav, PMC, and the servo circuits). For now, if the borrowed laser works fine and has more power, we might just switch it with the current one to gain more power.

 

IMG_0818.jpg

  933   Mon Apr 23 22:06:14 2012 taraNotesPurchasesvibration isolators (table legs)

I'm looking for the new table legs. The height will be 13.5". The plate that connects to the table will have 3 holes, 4" inches away from the center. I need to check what is the weight of the table and if the new legs can be mounted with the table  or not. 

For New port legs, (set of 4), the price are around $2500 (can be cheaper for less weight requirement). I'll check what is the weight of our table.

      They also offer Pneumatic Vibration Isolators with Automatic Re-leveling legs (I don't know what are the differences), but the height are wrong for our setup, the shortest one starts at 16" height. I looked into this category because our current legs also say Pneumatic

IMG_0809.jpg 

 

IMG_0805.jpg

  932   Mon Apr 23 16:52:16 2012 taraNotesopticwindows for vacuum chamber

I asked Steve about the choices, he thought the Thorlabs window should be ok for us.

      What Steve suggested are:

  • The seal between the 10" blank and the windows should be copper seal, (the window already comes with knife edge),so
  • the blank will have knife edge seals for two small windows as well.
  • Thorlabs window does not have an o-ring between the window frame and the optic, we should add the o-ring between them to avoid a direct metal-glass contact.

About the blank with two openings for beam access, he said a vacuum company could do it for us. I'll make a drawing and get a quote from Nor-Cal and MDC. I have to specify that the blank will be for ultra high vacuum system (UHV).

Quote:

For small window option, I can either have it made from scratch ( based on LIGO's drawing) or buy the commercial windows from Thorlabs. Here I listed down all pros and cons for each choice as I discussed it with Frank. I 'll ask Steve tomorrow for his opinions.

 == Using Thorlabs 2.75" OD windows:==

       Pros

  • easy to replace for damaged optics,
  • ready in short time (parts are in stock)
  • minimum time on machine shop
  • The thickness of the window is only 0.63", It should be able to fit in the set up which has ~ 1" clearance.

      cons:

  • Have more rubber seals in the system due to the design, I'll check Cryo:194 to see what will be the minimum pressure we need.

==Making custom parts (like LIGO, see quote window)==

     Pros:

  • There are only one o-ring used for each window (better vacuum pressure)

     cons:

  • Spend more time on designing/machining, probably more money for making the parts as well.
  • Take longer time if we need more spare pieces.
  • The available space for the window is quite limited (1"). If we follow what Zach did for Gyro, it is already to thick for our setup

Quote:

 

I'm searching DCC for window/viewport examples. The following drawings give me some ideas how to make a window for our setup.

 TCS viewports details

double viewports

septum window flange

 

 

  931   Sun Apr 22 23:09:38 2012 taraNotesopticwindows for vacuum chamber

For small window option, I can either have it made from scratch ( based on LIGO's drawing) or buy the commercial windows from Thorlabs. Here I listed down all pros and cons for each choice as I discussed it with Frank. I 'll ask Steve tomorrow for his opinions.

 == Using Thorlabs 2.75" OD windows:==

       Pros

  • easy to replace for damaged optics,
  • ready in short time (parts are in stock)
  • minimum time on machine shop
  • The thickness of the window is only 0.63", It should be able to fit in the set up which has ~ 1" clearance.

      cons:

  • Have more rubber seals in the system due to the design, I'll check Cryo:194 to see what will be the minimum pressure we need.

==Making custom parts (like LIGO, see quote window)==

     Pros:

  • There are only one o-ring used for each window (better vacuum pressure)

     cons:

  • Spend more time on designing/machining, probably more money for making the parts as well.
  • Take longer time if we need more spare pieces.
  • The available space for the window is quite limited (1"). If we follow what Zach did for Gyro, it is already to thick for our setup

Quote:

 

I'm searching DCC for window/viewport examples. The following drawings give me some ideas how to make a window for our setup.

 TCS viewports details

double viewports

septum window flange

 

  930   Sun Apr 22 22:10:17 2012 taraNotesDocumentationProposal is on SVN

The problem is fixed, I just deleted my local folder, check outed from the svn. Now when I commit the subversion, it shows up in docs/CTN_docs/proposal folder. I added Frank's drawing of the spacer in v-3. I'll add more materials later.

Quote:

The proposal for CTN lab is on SVN. The original folder was /CTNlab/docs/Proposal.

 

There is a little problem. I copied all the files from our original folder, and saved them in the /trunk/docs/CTN_docs. After I edited the proposal_v3, and commited, the new file showed up in the original folder. I think the copied files still carry their "svn address" with them, so when I commit them, they show up in the old folder. I'll fix this later.

 

 

  929   Sat Apr 21 00:55:00 2012 frank, taraDailyProgressBEATBeat measurement update

We measured the beat after a few changes in the setup (modulation index, air spring, small enclosure around the beat). We update the noise budget with suppressed laser frequency noise and Marconi's frequency noise in ACAV loop as well. There is not much improvement in the overall beat noise.

A few changes in the setup:

  •  Air Springs: We activated the air springs (it was off because we though it causes the vacuum chamber to move too much during the work), so there is one more seismic isolation stage. The peak at 58 Hz disappears.  However, there are peaks around 6-12 Hz showing up this time. They are probably from the table and the legs that are not set to float the table properly*.
  • Modulation index: now the modulation index is 0.18. I measured the power in the carrier and sideband to calculate the mod index, see PSL:855 . This is the maximum we can have with the 13dBm from Marconi with 4-way splitter (-6dBm),-0.5 dBm loss from the cable, so the power to EOM is 6.5dBm.  With the new mod index, the frequency discriminator gain (inverse) for RCAV/ACAV become 22 and 31 kHz/V.
  • A prototype enclosure box around beat setup: PSL:925.

beat_2012_04_20.png

Updated Traces:

  •   Suppressed Laser frequency noise: Since we changed the OLG of TTFSS by changing the modulation index, we remeasured the OLG of TTFSS, applied to the laser frequency noise (estimated to be 1e4/f Hz/rtHz). Good news, we have enough gain in RCAV loop to suppress the laser noise to coating level. The gain setup for common/fast is 750/750
  •  Frequency noise of Marconi for ACAV: We had not done this so far, so we wanted to make sure that it would not be the limiting source. This is done by dividing the frequency noise of marconi by ACAV OLG TF. We assume that the frequency noise is the same as we measured from the marconi for PLL plotted in the noise budget.
  • Shot noise is updated with the current modulation index (0.18) and cavities visibilities (~80%). It is not shown in the nb because I included it in the PDH detection noise.

* The table legs have small leaks, so we have air compressor to keep the pressure high, PSL:880. It works once every hour when the pressure is low.  The change of pressure can screw up the beam alignment a lot (for example, DC readout from RCAV RFPD can be varied from 100mV - 1.2V, with 1.7V maximum value (off resonance). I think it affects the alignment to AOM and change the diffraction efficiency/ beam shape as well.  It also changes the seismic isolation property. The peaks around 10 Hz show up when the pressure is low.  I'll find new legs from New Port and ask for the quote.

 

==Plan for the next few days==

1) Try using DAQ to measure beat signal, see if we can get rid off the frequency noise from PLL

2) Find out about the new legs(prices/availability)

3) Make the acoustic enclosure box around the whole setup

4) Work on the design for new vacuum windows.

 

  927   Thu Apr 19 19:58:19 2012 taraNotesDocumentationProposal is on SVN

The proposal for CTN lab is on SVN. The original folder was /CTNlab/docs/Proposal.

 

There is a little problem. I copied all the files from our original folder, and saved them in the /trunk/docs/CTN_docs. After I edited the proposal_v3, and commited, the new file showed up in the original folder. I think the copied files still carry their "svn address" with them, so when I commit them, they show up in the old folder. I'll fix this later.

 

  926   Thu Apr 19 03:12:23 2012 taraNotesopticwindows for vacuum chamber

 

I'm searching DCC for window/viewport examples. The following drawings give me some ideas how to make a window for our setup.

 TCS viewports details

double viewports

septum window flange

  925   Wed Apr 18 23:53:01 2012 Frank, TaraDailyProgressFoamsimple insulating box

to test how much insulation we need to reduce our acoustic noise (or air currents) problem we've added a simple aluminum-foil plated styrofoam box around the beat breadboard.
There are plenty of holes for all the beams and cables :

  • two for incoming beams
  • two for the power stabilization
  • aux beam from combining beam splitter
  • reflected beam from beat PD
  • video, power, PD cables,...

Even with all those holes we can already see a reduction with a simple lid on top of it. Due to power outage yesterday we still have no stable system yet, so waiting until tomorrow for measurements.

 IMG098.jpg

  924   Wed Apr 18 02:38:59 2012 taraNotesEOMChange modulation depth for 14.75MHz EOM

I changed the attenuator to -1dBm. From a quick measurement, there's no improvement in the beat signal at high frequency. Any improvement in the band we might be able to see is dominated by acoustic and PLL. [more details later]

Quote:

I increased the power to the 14.75MHz EOM from 0.5 dBm to 3.5 dBm. This was done by changing the attenuator from -6dBm to -3dBm.

As a quick test to see improvement in electronic noise level, I increased the detection gain. The modulation depth was changed from 0.14 to 0.2 rad (following the EOM's paramters in psl:855). I could not see any improvement yet. So I'll try to damp the mechanical peaks in the signal so that flat noise level can be measured better.

 

The slope of error signal for RCAV and ACAV are 44.25/57.62 kHz/V

 

  923   Wed Apr 18 00:49:07 2012 taraDailyProgressopticvibration damping on optics

I planned to measure the beat at night with the air springs activated, but the power went out around 11:45 pm. I think the temperature servo got a kick and it is drifting very fast. So I cannot keep the cavities locked long enough for the low frequency measurement. I'm just turning the systems back on for now.

The laser, 3 Marconis for 14.75MHz EOM, for ACAV AOM, for beat are set back to the original setup, PMC medm screen are back on, the air springs are up and working.

The linux machine is on but I forgot the password, will ask Frank tomorrow.

  922   Tue Apr 17 19:51:54 2012 taraNotesopticwindows for vacuum chamber

If we go with plan2,

1)window and flange

  • I think the window size of 2.75" diameter is the largest size for us ( with 3" clearance between the 2 beams). Thorlabs has a 2.75" window with 1.5" optic,with 1.18" opening, so it might not be compatible with its wedged window:http://www.thorlabs.us/NewGroupPage9.cfm?ObjectGroup_ID=5546 which is only 1" diamter.
  • Or we can order 2.75" flange with 1" bored, from N-C. to use with Thorlabs' window. Though I'm not sure how to assemble the two together.

2) Two Half-Nipple will be welded to the blank on the 10" flange. They will be 3" apart, as the input beams are. We might need something smaller than 2.75" diameter for accessing all the screws.

3)  blank 10" flange: I think Frank said that we have one in the lab. For another one, we can order it from N-C, blank. It is ~$ 300.

 

I'm not sure how to mount the window and the flange together. If we buy the window set from Thorlab, I think it can be directly assemble them similarly to the current 10" flange, see figure below. Or we might need to mount the windows like Zach does for Gyro, see ATF:1601.

 IMG_0757.jpg

 

Quote:

I'm checking the properties/prices/availability of window for the vacuum chamber.

 

Plan1: 10" diameter window (6" window opening)

  •  A&N: ($775), no info on optical quality. This is probably just a regular viewport similar to the one we use.
  • Pfeiffer also offers viewports for visually monitoring, so I think they are not good enough.
  •  MDC fused silica window, for 10" flange ($ 3,297), no optics properties. Only lens grade for ultraviolet are specified, but they claim that for IF also available (viewport)
  • Nor-Cal also has flange for 10" and 2.75" with glass,fused quartz, fused silica material. no optics properties shown.

 Plan2: 10" diameter blank with 2 smaller windows (1.5"/2" diameter)

  •  Thorlabs: 2.75" OD window, 1.18" window (windows are replacable, I think we can switch to CVI windows ) ($244 x2) + machining .
  •   MDC offer 2.73" with 1.5" window, no info on optics properties 

 

 Most of the manufacturers do not have good window for laser with 10" flanges. Finding two smaller windows with good optics properties is probably easier.

 

  921   Mon Apr 16 22:02:08 2012 taraDailyProgressopticvibration damping on optics

I forgot to change the code to disable the air springs, now the seismic coupling makes more sense.

beat_2012_04_15_fixed.png

Quote:

I measured beat signal, after damping most of the optics, realigning the beams to the cavities, measured the slope of error signals and applied it to the measured detection noise.  Acoustics peaks around 200Hz to 1kHz still present.

beat_2012_04_15.png

Fig1: beat measurement, I added shot noise and electronic noise from both cavities to a single trace called detection noise (from measurement).

 I turned off the HEPA fans on the table and on the clean bench before measured the beat signal (after I finished, I turned on the fans as usual).

     The peak at 58 Hz shows up this time. I think this is the peak from beam line motion of the stacks, see PSL:716.  (I think that was before we switched to the softer springs, I'll double check). Note that the air springs were not activated during the measurement, we can try using it and see if there is any improvement.

     There is a good improvement on minimizing the acoustic peaks, although still not enough. Also, increasing the modulation depth seems to help with the flat noise part at high frequency, we may really sit on detection noise.

 

  920   Mon Apr 16 18:17:05 2012 Frank, TaraSummaryNoiseBudgetmechanical resonances

I was told at JILA that anything less than 3/4" thick is lame.

This place has thick stuff:

http://www.estreetplastics.com/Clear_Acrylic_Plexiglass_Sheets_3_4_Thick_s/35.htm

Don't worry about the cost of the plastic - if it helps the noise, we have the money.

  919   Mon Apr 16 16:21:01 2012 taraNotesopticwindows for vacuum chamber

I'm checking the properties/prices/availability of window for the vacuum chamber.

 

Plan1: 10" diameter window (6" window opening)

  •  A&N: ($775), no info on optical quality. This is probably just a regular viewport similar to the one we use.
  • Pfeiffer also offers viewports for visually monitoring, so I think they are not good enough.
  •  MDC fused silica window, for 10" flange ($ 3,297), no optics properties. Only lens grade for ultraviolet are specified, but they claim that for IF also available (viewport)
  • Nor-Cal also has flange for 10" and 2.75" with glass,fused quartz, fused silica material. no optics properties shown.

 Plan2: 10" diameter blank with 2 smaller windows (1.5"/2" diameter)

  •  Thorlabs: 2.75" OD window, 1.18" window (windows are replacable, I think we can switch to CVI windows ) ($244 x2) + machining .
  •   MDC offer 2.73" with 1.5" window, no info on optics properties 

 

 Most of the manufacturers do not have good window for laser with 10" flanges. Finding two smaller windows with good optics properties is probably easier.

  918   Mon Apr 16 15:35:15 2012 Frank, TaraSummaryNoiseBudgetmechanical resonances

we mapped the mechanical resonances in our system using two techniques :

  1. tapping each optical component and check spectrum for excited modes. This technique is difficult as we usually excite a lot of other mechanical modes nearby as well.
  2. exciting everything with white noise (loudspeaker) and then add damping on individual components and checking spectrum which modes we reduce. Very precise but you have to start with the dominant wants and damp them first to see the smaller ones nearby.

resonance frequency is attached to each component. Green background behind numbers means we could verify the frequency using both techniques. Yellow means measured using tapping but could not see using the damping technique as the contribution to the beat signal is to weak. Input optics needs to be checked and finalized. Picture is temporary only.

beatboard_resonances.jpg PDH.jpg

  916   Mon Apr 16 02:20:58 2012 taraDailyProgressopticvibration damping on optics

I measured beat signal, after damping most of the optics, realigning the beams to the cavities, measured the slope of error signals and applied it to the measured detection noise.  Acoustics peaks around 200Hz to 1kHz still present.

beat_2012_04_15.png

Fig1: beat measurement, I added shot noise and electronic noise from both cavities to a single trace called detection noise (from measurement).

 I turned off the HEPA fans on the table and on the clean bench before measured the beat signal (after I finished, I turned on the fans as usual).

     The peak at 58 Hz shows up this time. I think this is the peak from beam line motion of the stacks, see PSL:716.  (I think that was before we switched to the softer springs, I'll double check). Note that the air springs were not activated during the measurement, we can try using it and see if there is any improvement.

     There is a good improvement on minimizing the acoustic peaks, although still not enough. Also, increasing the modulation depth seems to help with the flat noise part at high frequency, we may really sit on detection noise.

Attachment 1: beat_2012_04_15.png
beat_2012_04_15.png
  915   Sat Apr 14 06:03:23 2012 frank,taraDailyProgressopticvibration damping on optics

 We are damping most of the optics with rubber cones. There are a few peaks that we still could not find their origins.  We are thinking to build an acoustic insulation box to cover the setup.

   [details will be added soon]

IMG_0744.jpgIMG_0742.jpg

  914   Sat Apr 14 01:10:48 2012 taraNotesEOMChange modulation depth for 14.75MHz EOM

I increased the power to the 14.75MHz EOM from 0.5 dBm to 3.5 dBm. This was done by changing the attenuator from -6dBm to -3dBm.

As a quick test to see improvement in electronic noise level, I increased the detection gain. The modulation depth was changed from 0.14 to 0.2 rad (following the EOM's paramters in psl:855). I could not see any improvement yet. So I'll try to damp the mechanical peaks in the signal so that flat noise level can be measured better.

 

The slope of error signal for RCAV and ACAV are 44.25/57.62 kHz/V

  913   Fri Apr 13 13:35:37 2012 Tara, FrankSummaryNoiseBudgetPDH readout noise level verification

we re-checked the electronic noise level for each PDH loop and also checked how the noise level rises with a shot noise limited light source (halogen bulb) using the exact same setup as we use daily.
Before we had only checked the individual parts, e.g. the shot noise limit of our RF-PD, not including the rest of our setup (mixer, LO or filter stages). This was just to confirm that nothing is broken and our PDs are still OK.

Measurements were taken right after the mixer / LP-filter, but still connected to PDH servo. This is OUT1 of the common path of the TTFSS-box  and the input on the front of our ACAV PDH servo. We blocked the laser and adjusted the power of the bulb and measured the electronic noise floor using the SR785. The dark noise spectrum is the one already shown in the last noise budget. Below the plots of noise level, measured around 500Hz (100 avgs, wide marker) vs different photocurrents of the PDs. DC impedance is 2kOhms, regular DC level for RCAV is 1.7V and 1.47V for ACAV. RF-impedance is the same as measured some time ago for those diodes. Data in folder 4/11/2012 on svn.

SN_RCAV.png  SN_ACAV.png

Conclusion: New measurements agree very well with the old measurements. So our PDs still work fine and there is no excess noise in our readout we missed.

  912   Fri Apr 13 03:15:12 2012 koji,frank,taraDailyProgressopticvibration damping on optics

We tried to damp mechanical peaks from each optics. For now, by putting a rubber piece on a mirror mount can suppress mechanical peaks effectively. We are still thinking about more robust way to damp the peaks.

    Beat signal has a lot of acoustic peaks from 100Hz up to 1kHz, and they may mask any improvement we work on flat noise. Damping them is necessary before we can work on the flat noise hidden underneath.

    By tapping each optic, we can see peaks raising up in beat signal or feedback signal to ACAV AOM. We used the feedback to ACAV AOM to identify peaks in ACAV path first. The curve mirror behind AOM has a strong peak which can be damped by a rubber cone placed on top of the mount, see fig1 below.

IMG_0731.jpg

fig1:  Mirror mount1, with a damping rubber on top.

     We also tried using different mounts to see if the peak would be reduced. The original mount was an anodized aluminium mount. We switched to different two stainless steel mounts, mount1 and mount2. The spectrum of the feedback signal to AOM (not calibrated) between two mounts with and without damping rubber are shown below. From the spectrum, there are not much different between the current anodized Al mount (not shown) and the steel mount in fig1.

damped.png

 

Note: We also tried to damp the mirror mount with small rubber pieces placed between the frame and the body of the mount, but it did not help at all. The springs of the mount are stronger than the rubber, so this method is not effective.

    To sum up,

  • we need to damp most of our optics. The current plan is to use a rubber cone and just place on top of the mirror mounts. We are also thinking about better damping schemes. 
  • There are not much different between a stainless steel mount [add model#], and an aluminium mount[add model#]. It is probably unnecessary to change mirror mounts.
  • We will order more of the rubber cones for damping.
  911   Thu Apr 12 22:22:12 2012 frank, taraDailyProgressNoiseBudgetElectronic noise + shot noise verification

We measured shot noise level with white light source. The noise level is slightly off from shot noise calculation in the noise budget.

  After identifying electronic noise in the detection part, we characterized shot noise in RCAV and ACAV systems.  We shined a white light source on the RFPD, and adjusted its power so that V_DC level was the same when the cavity was locked.  Then measured the noise at mixer out (out1 for RCAV system). This voltage noise has contribution from shot noise and electronic noise. It can be converted to frequency noise by multiplying with the slope of error signal.

  ACAV RCAV
Vdc [V] 0.4 0.1
Vnoise(flat) [nV/rtHz] 49.4 40.7
Slope [kHz/V] 81.7 40.3
 f Noise [mHz/rtHz]  4.0

 1.64

 

  The frequency noise calculated in the last row has contribution from shot noise and electronic noise (from previous entry). By subtracting(quadraturely) the electronic noise, we should obtain the contribution from shot noise only. Electronic noise (from previous entry) are 1.8 and 1 mHz/rtHz for ACAV and RCAV, respectively. This gives shot noise level = 3.6/1.3 mHz/rtHz for ACAV/RCAV. The calculation in the noise budget gives  1.6/0.9 mHz/rtHz.  The measured and calculated results do not match. I think because the error in modulation index, finesse, cavity mode matching are too large. The results are summarized in the table below. I will use the measured shot noise/ electronic noise in the noise budget for now.

  ACAV RCAV
Electronic noise(measured)[mHz/rtHz] 1.8 1
Shot+electronic noise [mHz/rtHz] 4 1.64
Shot noise [mHz/rtHz] 3.6 1.3
Calculated shot noise* [mHz/rtHz] 1.6 0.9

 

* The calculated shot noise use 1mW power into each cavity. Modulation index = 0.14. The following numbers are in [ACAV/RCAV] format.  Finesse = (7500/9700), Visibility = (90%/80%)

  909   Wed Apr 11 20:30:50 2012 frank, taraDailyProgressNoiseBudgetElectronic noise verification

  We are checking the noise contribution from electronics readout of our system to get better noise budget estimation. We added detection noise from RCAV and ACAV systems in the noise budget. They are comparable to the estimated shot noise level.

    We measured the noise level of the PDH detection for RCAV and ACAV system, the corresponding error signal slope.The results are summarized in the table below.

Setup: 1mW input to each cavity, TTFSS: Common/Fast = 900/800,

  RCAV ACAV
Pwr input [mW] 1 1
V_DC (RFL RFPD) [V] 1.7 1.47
Err signal Slope [kHz/V] 40.3 81.7
Detection Noise [nV/rtHz] 35 23

 

Detection Noise x Err signal slope gives us the electronic noise contribution in absolute frequency unit. From the above table, we obtain

1 mHz/rtHz and 1.8 mHz/rtHz from RCAV and ACAV systems, respectively.

 The electronic noise from RCAV and ACAV systems are added to the noise budget. In the plot below, I removed the traces for Spacer noise and thermo optic noise to make the plot less crowded, but their contribution are included in the total noise.

beat_2012_04_11.png

 

More to come:

  • Shot noise measurement for ACAV/RCAV/beat
  • TTFSS OLG TF, suppressed laser frequency noise

 

  906   Wed Apr 11 02:44:29 2012 frank, taraDailyProgressopticbad PBS

Update, beat measurement after several optics replacement. Peaks around 10 Hz, 35 Hz show up this time.

    Optics that we replaced are:

  •  Beam splitter that divides the beam to ACAV and RCAV path. Now the new BS is 1" cube for large beam
  • Fixed the orientation of mirrors on the periscope and the turning mirror for ACAV RFPD. A few of them were flipped back causing stray light in the beam path.
  • Two of the mirrors on periscope had transmission of 2% or so, we replaced with a high reflective ones (0.1% transmission).

The problem with the curve mirror from last entry has not been fixed yet. It turns out that the mirror we borrow from 40m is worse than the one we have (surface is more milky), so we leave the original mirror as it is.

Note: The beat measurement was done when the air springs were inactive. Noise at high frequency goes down a bit.

The power input to each cavity is 1mW, setup on PLL is 1kHz input range, with gain = 200.

beat_2012_04_10.png

  905   Tue Apr 10 00:50:56 2012 frank, taraDailyProgressopticbad PBS

We noticed wide angle scattered light behind the PBS in front of RCAV. The scattering source is probably the curved mirror behind RCAV AOM. We borrowed the similar mirror from 40m and will try to compare them.

      The wide angle scattered light behind the PBS in front of RCAV might contribute to the noise in beat signal. The picture shows the scattered light with area larger than the half inch PBS cube. This picture was taken when the beam's polarization was changed to P-polarization so that most of the light was reflected from the PBS. With small transmitted light through the PBS, the scattered light can be seen clearly behind the PBS, see here

     After the inspection, it is very likely that the curve mirror behind RCAV AOM is the source. So we borrowed another R=0.3 mirror from 40m to see if it will be better or not, this will be done soon.

     Note: during the inpsection, we also identified another bad PBS,pic. This is the one in front of RCAV AOM. Its center surface looks dirty, so we replaced it with a better one.

  904   Mon Apr 9 23:05:44 2012 taraDailyProgressSeismicSeismic coupling from curtain frame to beat

Ting's code for new coating thermal noise is attached. This is the code used for the calculation in their paper (not published yet). They use loss in shear and bulk mode which are assumed to be equal to the loss measured from ring down. The result is ~5% different from the calculation from Harry et al's result.

    We talked to Huan about how their coating Brownian noise calculation differs from that of Harry etal. The main points are:

  • different notation for loss angles
  • light penetration in the coating is considered
  • correlation between substrate surface and coating noise

Since phi_bulk and phi_shear are not well studied in the literature, the exact values are undetermined. By setting them to be equal to the regular phi obtained from ring down measurement, the result of the coating noise is not significantly different from the model we have been using.

Attachment 1: brownian_mult.m.zip
  903   Sun Apr 8 23:35:05 2012 taraDailyProgressSeismicSeismic coupling from curtain frame to beat

  I used ciplot command to plot the noise budget with confidence region. 

beat_ciplot.png

Quote:

http://lmgtfy.com/?q=matlab+ciplot

 

Attachment 1: beat_ciplot.png
beat_ciplot.png
  902   Fri Apr 6 01:36:39 2012 Tara, FrankSummaryNoiseBudgetACAV loop changes

summary of all changes made and more detailed plots will be posted soon, so check back later

writing in progress

  • replaced New Focus power supply for RFPD with Agilent power supply (+/-15V)
  • added isolation transformer between mixer LO and main splitter of 14.75MHz LO
  • changed gain of input stage from PDH-box from 10 to 20
  • replaced thickfilm resistors in input stage with thin film ones
  • added offset potentiometer to input opamp
  • added 25MHz HP-filter to AOM driver/amplfier

ACAV loop is now completely free from line harmonics and the in-loop level dropped to ~10nV/rtHz (includes 7nV/rtHz from analyzer). Mixer dark noise is ~25nV/rtHz.

  901   Fri Apr 6 01:07:47 2012 taraDailyProgressNoiseBudgetThermoelastic noise from substrate

I corrected the code for substrate thermoelastic noise (TE) in my noise budget. The substrate thermoelastic of 8 inch cavity will not be a problem, but it will be comparable to coating thermal noise around 10 Hz for 1.45" cavity.

 

     After reviewing the papers about measurement of thermoelastic noise in substrate, I realized that I have not fixed my calculation for substrate TE as noted in both publications by Numata et al (2003) and Black et al (2004). Apparently, the calculation from Cerdonio et al(2003) uses different notion of beam radius, and misses one factor of pi.  The new result is plotted below. It is about a factor of 1.5 higher than the old one at adiabatic limit (high f), but it is still smaller than the estimated coating thermal noise, see figure 1.

 corrected.png

fig1: TE noise is corrected and plotted in comparison with the previous result.

     However, for a planned 1.45" long cavity, due to the smaller spot size on the mirror, TE noise is very close to the coating thermal noise around 3 Hz up to50 Hz, see figure 2 below. So this will be another issue to keep in mind.  Note: shot noise seems to be very high, I'll double check the result again if it is correct or not. I just changed the cavity length parameter and ran the code to get the result.

beat_145in.png

fig2: Noise budget for 1.45" long cavity.

  900   Fri Apr 6 01:03:01 2012 frank, taraDailyProgressBEATground loop problem

We fixed the ground loop problem in ACAV setup and got rid of most of the harmonic lines in beat signal.

    The ground loop problem in our current setup comes from the common ground of the two Marconis. The first one is for 14.75 MHz EOM. Its signal is split, and sent to demodulate the signal from ACAV RFPD. The second Marconi is for driving the AOM around 80 MHz for frequency lock. By using a transformer between the mixer (for PDH demodulation) and the LO signal from the first Marconi, we broke the ground loop, hence got rid of the harmonic lines in the signal.

beat_2012_04_05.png

  898   Wed Apr 4 23:58:43 2012 taraNotesNoiseBudgetthermal noise from spacer + support

Following the calculation from Kessler, I estimated brownian noise from the spacer and the o ring support from our setup. From the cursory check, it seems that we don't have to worry about both sources too much.

      Kessler et al. refined the calculation for brownian noise of a spacer from Numata's et al and included  Brownian noise of viton supports. For spacer's noise, they corrected the result for a cavity with a hole along the cavity's axis, and also did the FEA simulation with COMSOL for the mirrors that are smaller than the diameter of the spacer. For their chosen geomtry, the noise from the spacer, as obtained from FEA, can be 30% larger than the analytical result.

    So, as a quick check for our cavity, I used their analytical result and increased the result by 30% to get an estimate. The new spacer noise is about an order of magnitude below the coating noise, so it should not be a problem.

    For viton noise calculation, they assumed a 4-point supported cavity, and calculated the energy stored in shear deformation, then used fluctuation-dissipation theorem(FDT) to compute the displacement fluctuation. However, our cavity is supported by two o-rings wrapping around the cavity's groove and sitting on two  U-shape teflon pieces. I don't know the exact thickness of our o-rings, so I use (1/8)" = r_o . Assuming that the surface area is  pi* cavity's radius * r_o *sqrt(2), and thickness of the o ring under pressure = 1.5 r_o. [add pic], I can get the noise contribution from the support. Parameters for O-rings' are assumed to be the same as those of viton in the paper. The result is about the same as substrate's Brownian noise, which is not very threatening for our experiment. I believe that the actual number will be even lower, since my estimation exaggerates, to get the upper bound, the surface area of the o ring to be half of the spacer's perimeter times the ring's thickness. The actual shear deformation should be localized in smaller area.

 Parameters for our cavities can be found here.

  

 

beat_nb.png

  896   Tue Apr 3 20:56:10 2012 frank, taraDailyProgressSeismicSeismic coupling from curtain frame to beat

We measured the beat signal after we fixed the wiring problem. We are close to what we had before, but we still have a scattering light problem.

      Just after we fixed the cable wiring between the table and the frame on 3/30, we measured the beat signal (beat 03/30, blue). There was a hughe scattering noise bump from 100-200 Hz, and it was not stable. We did not see this before (beat 03/12, pink).  We found that there were plenty of dust on the periscope mirrors, lenses in front of RFPD. These optics are in the sensing path, and are more sensitive to scattering light. After cleaning them, the beat signal is better now (beat 04/03). The bump at low frequency is still higher than before, but now it is stable. 

Note that the acoustic peaks around 1kHz are not reduced from fixing the cable, but the bump at low frequency is slightly lower. This is reasonable, the cables should be very soft at high frequency, and do not transfer any vibration from the frame to the table at acoustic bandwidth.

beat_2012_04_03.png

     Other changes we did:

  • replaced the PBS before ACAV AOM with a new one. The old one has defect coating, which we can see by eys, and causes extra back reflection.
  • replaced the PBS in front of RCAV. This one also has high back reflection.
  • re-adjusted the AOM angle. It was tilted down a bit, and this causes extra beam distortion. The beam ellipticity changes from ~0.7 to 0.88 when the AOM is almost parallel to the table surface.
  • replaced translational stage for the curve mirror behind ACAV AOM. Now it should be more stable ( but I don't see any obvious suppression in acoustic peaks yet).
  • sealed the gap between the curtain frame and the plastic covers on top of the table where dust might fall off.
  • change the air spring setup ( I'm not sure about the details, need to ask Frank)

      It is very likely that the when we removed all the cables on the table, dust was stirred up and stuck on the optics, because they are dirtier than usual. We might need to clean more optics on the table. However, from today measurement, it seems that the bump at low frequency might be the effect from the seismic as well (from the feature of the peaks @35 and 55 Hz). Noise budget from seismic will be revisited, also the flat noise at high frequency will be investigated next.

  895   Mon Apr 2 22:33:21 2012 taraNotesNoiseBudgetparameters for coating brownian noise calculation

Coating Brownian noise with uncertainty (worst & best case scenarios)

 nb_with_uncertainty.png

I use the parameters found in the literature for coating Brownian noise calculation. 

The upper limit ( high noise level)  has

  • loss (silica) =1.2 e-4,
  • loss (tantala)= 4.6e-4,
  • Young's modulus (Tantala)= 186 GPa

The lower limit (low noise level) has

  • loss (silica) = 0.1 e-4;
  • loss(tantala) =3.6 e-4;
  • Young's modulus (Tantala) = 102 GPa

The rest of the parameters are their nominal values. The max/min values are ~ 18% from the average level. @ 100 Hz, the average noise level is 4.097 mHz/rtHz. The upper limit is 4.815mHz/rtHz, the lower limit is 3.324 mHz/rtHz.

Quote:

I make a list of parameters found in literature. This will be used for estimate the coating Brownian noise level and its error.

 SiO2 loss:

  • 1 +/- 0.2 x10-4  (Crooks et. al. 2006)
  •  0.4 +/- 0.3 x10-4 (Crooks et. al 2004)
  • 0.5 +/- 0.3 x 10-4 ( Penn et al. 2003)

Ta2O5 loss:

  • 3.8 +/- 0.2 x10-4  (Crooks et. al. 2006)
  • 4.2 +/- 0.4 x10-4  (Crooks et. al 2004)
  • 4.4 +/- 0.2 x 10-4  ( Penn et al. 2003)

 * the values reported by Crooks etal in 2006, are supposed to be more accurate than the results in 2004, because of the better estimation of energy stored in the coatings and the corrected thermoelastic contribution. They mention that the Poisson's ratio has small effect on the level of the estimated thermal noise.

The numbers from Penn et al are extracted from the multilayer coating ringdown measurement. Since they measure the ring down of the coating which has both materials. The values depend on Young's moduli of the materials as well. They use Ysio2= 72 GPa/ YTa2O5 = 140GPa. Thermoelastic loss is not taken into account.

 

The values for Young moduli are usually measured directly with nanoindentation technique.

 SiO2, Young modulus (Thin film)

  • 72 GPa (used in Penn et al 2003)

Ta2O5, Young modulus (Thin film)

  • 140+/-14 GPa  (Alcala et al, 2002, nano indentation,stacks.iop.org/Nano/13/451)
  • 140 GPa (Martin PJ et. al.,1993 Mechanical and optical properties of thin films of tantalum oxide deposited
    by ion-assisted deposition Thin Films: Stresses and Mechanical Properties IV, Mater. Res. Soc. Symp. Proc.). This source is not available by caltech connect. This source is cited in the paper by Penn et al, 2003.
  • 144+/- 42 GPa (Crooks et al 2006, this assumes that Young's modulus of SiO2 is 72 GPa)

                      

SiO2, Poisson's ratio:

  • 0.17: (Bamber, et al,2001), but they use SiO2 for calibration and assume that its Young's modulus and Poisson's ratio in thin film form are known.

 Ta2O5, Poisson's ratio

  • 0.23 (assume that it is the same as in bulk, Crooks, 2006)

The uncertainties in Poisson's ratios of the materials have small effect on the coating noise level. For examples, the 10% increase of SiO2's, and Ta2O5 Poisson's ratios, causes the thermal noise to increase by 0.09%, and 0.06%, respectively.  

list of all materials' properties,here.

 

  894   Fri Mar 30 00:50:43 2012 frank, taraDailyProgressSeismicSeismic coupling from curtain frame to beat

The coupling from frame motion to beat signal is now significantly reduced after we fixed the cable wiring .

 

      The plot below shows the acceleration measured on the frame and the corresponding beat signals. I did not calibrate the signal since this is just a qualitative check.  I tried to excite the frame so that the signals from the frame motion were comparable (blue and cyan). After we fixed the cables wiring, the beat signal (pink) are less sensitive to the frame vibration than before (red).

      Note that the set up were similar in both measurements. The input range for PLL was 5kHz, with gain 200.

compare.png

 

I have not measured the beat signal after we fixed the cable yet. It will be interesting to see if the beat has been improved or not.

  892   Thu Mar 29 18:50:51 2012 taraNotesNoiseBudgetparameters for coating brownian noise calculation

I make a list of parameters found in literature. This will be used for estimate the coating Brownian noise level and its error.

 SiO2 loss:

  • 1 +/- 0.2 x10-4  (Crooks et. al. 2006)
  •  0.4 +/- 0.3 x10-4 (Crooks et. al 2004)
  • 0.5 +/- 0.3 x 10-4 ( Penn et al. 2003)

Ta2O5 loss:

  • 3.8 +/- 0.2 x10-4  (Crooks et. al. 2006)
  • 4.2 +/- 0.4 x10-4  (Crooks et. al 2004)
  • 4.4 +/- 0.2 x 10-4  ( Penn et al. 2003)

 * the values reported by Crooks etal in 2006, are supposed to be more accurate than the results in 2004, because of the better estimation of energy stored in the coatings and the corrected thermoelastic contribution. They mention that the Poisson's ratio has small effect on the level of the estimated thermal noise.

The numbers from Penn et al are extracted from the multilayer coating ringdown measurement. Since they measure the ring down of the coating which has both materials. The values depend on Young's moduli of the materials as well. They use Ysio2= 72 GPa/ YTa2O5 = 140GPa. Thermoelastic loss is not taken into account.

 

The values for Young moduli are usually measured directly with nanoindentation technique.

 SiO2, Young modulus (Thin film)

  • 72 GPa (used in Penn et al 2003)

Ta2O5, Young modulus (Thin film)

  • 140+/-14 GPa  (Alcala et al, 2002, nano indentation,stacks.iop.org/Nano/13/451)
  • 140 GPa (Martin PJ et. al.,1993 Mechanical and optical properties of thin films of tantalum oxide deposited
    by ion-assisted deposition Thin Films: Stresses and Mechanical Properties IV, Mater. Res. Soc. Symp. Proc.). This source is not available by caltech connect. This source is cited in the paper by Penn et al, 2003.
  • 144+/- 42 GPa (Crooks et al 2006, this assumes that Young's modulus of SiO2 is 72 GPa)

                      

SiO2, Poisson's ratio:

  • 0.17: (Bamber, et al,2001), but they use SiO2 for calibration and assume that its Young's modulus and Poisson's ratio in thin film form are known.

 Ta2O5, Poisson's ratio

  • 0.23 (assume that it is the same as in bulk, Crooks, 2006)

The uncertainties in Poisson's ratios of the materials have small effect on the coating noise level. For examples, the 10% increase of SiO2's, and Ta2O5 Poisson's ratios, causes the thermal noise to increase by 0.09%, and 0.06%, respectively.  

list of all materials' properties,here.

  891   Thu Mar 29 00:46:33 2012 frank, taraDailyProgressSeismicSeismic coupling from curtain frame to beat

We rearranged the cables and the setup a little bit to isolate the table from the frame.The work is still in progress.

 

We removed some unused cables that ran from the frame to the table, and rearranged some equipments. They are:

  • PMC RFPD DC out, PMC TRANS,
  • 4 CCD-camera cables, which are now on the table only.
  • Laser cables (Fast feedback, Slow control, laser control, laser power). The laser controller is now on the table.
  • Marconi for 14.75 MHz sideband is now on the curtain frame.
  • 35 W laser and the unused vacuum chamber are removed from the table and rest on the floor.
  • The hose for air spring will be replaced. It is also connects between the air springs and the frame directly.

IMG_0687.JPG          IMG_0688.JPG

  890   Tue Mar 27 23:32:22 2012 frank, taraDailyProgressSeismicSeismic coupling from curtain frame to beat

We are trying to decouple motion from the curtain frame to beat signal. Currently, just by tapping the frame, we can see the effect on beat signal clearly.

The cables running between the table and the frame are very likely to transfer the motion from the frame to the table causing extra acoustic noise in the beat. So we want to see the improvement by removing some of the unused cables. First, we need to check the current coupling from the frame motion to the beat. An accelerometer is installed on the frame (see picture). We tried to drive the table with a PZT, but it failed. The PZT is not strong enough to shake the frame at 5 Hz. As an alternative, we will just tap the table and see the motion on the frame, on the table, and on the beat. Then we can estimate the coupling (at least for the resonant frequency).

 IMG_0681.jpg

  889   Tue Mar 27 17:01:23 2012 taraNotesNoiseBudgetCalibration verification for PLL

I check the calibration factor used in PLL measurement to make sure that we have used the right calibration factor. The result is fine. The calibration we have been using is correct.

     From the beat measurement, the total noise budget is very similar to the beat signal at high frequency with some offset. So we suspect that the calibration for LO frequency noise / beat we have might be wrong. 

     I set the tuning range to be 10kHz(marconi), with gain 200 because this range has frequency noise large enough to be seen on beat signal, and I have the data for this setup from PSL:874.

     The calibration for beat signal with 10kHz input range is 7e3 Hz/V. For 1kHz input range, we have been using 700 Hz/V (rescaling with the tuning range). The direct measurement using a voltage calibrator + frequency counter gives 712 Hz/V, see figure below. The calibration remains the same with 160MHz or 100MHz carrier frequency.

cal_1k_pll.png

    Electronic noise from PLL (straight blue plot with ~f dependent) is also added for completeness. The level measured after SR560 is 40uV flat, the OLG TF of PLL is 4.44e4/ f, the electronic noise from PD can be calculated following the instruction in PSL:816.

it seems that we have a good agreement between the measured beat and the sum of 2 limiting sources at high f.

freq_noise_cal.png

 

Note that the frequency noise from Marconi is measured with 160 MHz carrier, while our beat is currently ~ 180 MHz. However, the carrier does not change the frequency noise that much (cf PSL:874).

To sum up, the calibration we have been using is correct. The mismatch between the beat signal and the total noise budget is probably be the other noise sources we have not taken into account yet.

 

  888   Mon Mar 26 19:51:41 2012 taraNotesNoiseBudgetThermal noise vs spotsize and cavity length

I calculated thermal noise from different mirrors to find out what factor we might gain compared to the current thermal noise level. For the possible design with 0.5m-0.5m mirrors with 1.45" cavity length, we gain a factor of 8.6.

factor_ROC_length.png

 Fig1: The factor gain for Coating noise with different mirrors ROC combination and cavity length. note: the title of the plot should not contain "R=0.5, concave-flat".

It turns out that using flat-concave style will not be the winning solution, we win more with 0.5m-0.5m mirrors with the same cavity length.

 

The code for this calculation can be found on svn

  887   Wed Mar 14 17:31:51 2012 taraSummaryNoiseBudgetThermal Noise calculation using gwincDev

The discrepancies between in GWINC code and our code are  different values of phi_perp/ phi_perpendicular, and the simplification of the formula.

 

The coating thermal noise from Rana's plot is about a factor of sqrt(2) lower from the usual code we have been using. The reasons are that

  1. We use different values of phi_perp and phi_para. Values from GWINC phi_perp/phi_para are 1.3e-4/1.7e-4,  ours are 1.7e-4/3.1e-4. This is the main reason why the results do not agree
  2.  Our code uses a simplified Thermal noise formula, where all the terms with 1- sigma are ~ 1 see [fig.1], but another simplified version[fig.2] that keeps one more term of 1-sigma gives a result that agrees with that from GWINC.  With the same parameters, the better simplified version (with term 1-sigma) gives the result smaller than that of the exact formula by only ~ 1%.
  3.  Also, the temperature we use in the code is 35 C, while the temperature in GWINC is 20 C. This contributes to 5% reduction from our result.

 

Harry_etal_2002.png

fig1: excerpt from Harry etal. Class Quantum Grav 19 (2002) 897-917

Harry_etal_2005_p060072.png

 

 fig2. excerpt from Harry etal. www.ligo.caltech.edu/docs/P/P060072-00.pdf

  884   Tue Mar 13 22:06:46 2012 taraNotesNoiseBudgetcalibration verification

The flat noise in beat signal  around 100 Hz now is very likely to be the sum from electronics and in loop noise. Better measurement will be done again to confirm the result.

==motivation==

We are investigating what would be the sources of the flat noise we see in the bet signal. Since it is flat, we suspect that it should be shot noise or electronic noise in our setup.

==setup&procedure==

Before we can say anything about contribution from electronic noise to beat frequency, we need to be sure that the calibration from V noise to Frequency noise is correct. The calibration we need is the slope of error signal which can be measured directly, see PSL: .

Once we know the calibration, we check that it is correct. This is done by locking both cavities, measuring the beat signal. Then reduce the gain in RCAV loop until the feature of the inloop noise shows up in the beat. Then we can compare inloop noise to beat. The result looks fine within 10%.

[add fig1] fig1: Inloop noise convert to absolute frequency using slope of error signal and frquency noise from beat.

 

Now we can use this to convert inloop noise and electronic noise to absolute frequency.

 We also want to see the contribution from shot noise. Lower the power is an option to bring up shot noise. We try:

  1. reduce the power only in RCAV, until the beat signal is flat, check inloop noise & electronic noise & shot noise
  2. reduce the power only in ACAV, until the beat flat, check inloop noise & eelctronic noise & shot noise

Error signal slope from

  • RCAV : 76 kHz/V for 1mW input, and 2.5MHz/V for 30uW input*.
  • ACAV: 157kHz/V for 1mW input, and 4.1 Mhz/V for 38 uW input*.

*I did not measure the slope with lower power, I just scaled down them with power.

beat_rcav_30uW.png

fig2: Beat noise in comparison with shot noise, in loop noise, and electronic noise. RCAV power is reduced to 30uW. Shot noise is corrected for the corresponding input powers.

beat_acav_38uW.png

fig3: Beat noise in comparison with shot noise, in loop noise, and electronic noise. ACAV power is reduced to 38 uW. The peaks in ACAV in loop noise does not show up in the beat. I think it might be harmonic lines in the measurement, and not real.

 

So if I plot the electronic noise and in loop noise from regular setup the result:

 beat_regular.png

 fig4: Beat with regular setting (1mW input to each cavity, modulation depth = 0.14, power to EOM = 0.2dBm). The in loop noise and electronic noise are very likely to be the sources of the limiting flat noise.

 

The electronic noise and the in loop noise are very close to the beat level, by increasing the gain in the detection point, we should be able to suppress this to level. We can try using larger modulation index(current value is only 0.14). Another measurement with better resolution will be done in order to add them in the noise budget.

  882   Mon Mar 12 23:31:38 2012 frank, taraDailyProgressBEATbeat with floated table, air spring.

We measured beat signal with the upgraded seismic isolation system. It seems that we are sitting on some scattering noise source at low frequency.

 

      Most of the mirrors mount in beat path, are damped with rubber cones. A lead block is put on the board for beat setup to damp down any resonant peaks. With air springs and floated table, seismic noise is significantly reduced. From the shape of the bump around 70 Hz, we suspect that it is the scattering noise. That will be investigated further.

beat_2012_03_12.png

Fig1:  In grey: beat signal after softer RTV springs were used. In pink: beat signal with air spring, unfloated table. In blue: beat signal with air spring, floated table.

  881   Mon Mar 12 15:28:37 2012 taraHowToNoiseBudgetNoise estimation for delay line technique

I finished the calculation for delay line. The detail is in psl:868. Plus, I edit my plot so that it looks nice ( see PSL:878).

  878   Sat Mar 10 18:42:42 2012 taraDailyProgressBEATbeat- with air spring isolation

 Beat measurement with new air springs is measured with unfloated table (the N2 supply is empty). Good improvement can be seen at frequency above 20Hz.

beat_2012_03_10.png

The data is compared with beat signal before the air springs, unfloated table on 2012_03_03.  Once we float the table we should see no seismic/mechanical peaks around 100Hz. Then effects from RFAM/RIN couplings might reveal themselves.

Note about the setup:

  • Power into both cavities are ~ 1mW (I also adjust the power for beams to RFAM measurement PDs to be ~0.8 mW).
  • I replace the plastic legs for RCAV RFPD, so that the height is 3 inch, the reflected beam is dumped properly.
  • The measured Marconi frequency noise from Marconi in this nb is not updated yet. I'll update that soon.
Attachment 1: beat_2012_03_10.png
beat_2012_03_10.png
  876   Thu Mar 8 14:56:21 2012 taraHowToNoiseBudgetNote on Noise Hunting

Seem that people have been asking what have we done to reduce the noise in the beat measurement, so I think it is a good idea to summarize it down here as a future reference.

I'm listing the topics for now, the corresponding details will be added in the future.

  •      RFPD for PDH : we had problem with the PDH loop(psl:706). This might be the combination of bad RFPD, small modulation depth, problem with TTFSS.
  •      Modified RFPD (PSL:792, PSL:795), with that the inloop noise is below coating noise level and the loop is not gain limited. (PSL:806)
  •      14.75 MHz resonance EOM: We added a resonance EOM for adding the sideband frequency. The broadband EOM that we used before could not give us enough modulation depth (PSL:745).  Plus, we got rid of the RF summing box which might contribute to weird behaviors of the loop (PSL:711,720). Now the broadband one is only for feedback control.
  •      Lower side band frequency: We lowered the sideband frequency from 35.5MHz to 14.75 MHz, because have better Q at lower frequency.
  •      TTFSS position on the table (PSL:778) : Beofre: lower UGF(33kHz, psl:654)
  •      Polarization of the beam (and RFAM effect)
  •      Scattering light: (dump beam properly, use super polished mirrors, psl:647)
  •      Seismic and Mechanical peaks (seismic, PSL:690), seismic stack(psl:668,837), softer spring(PSL:762,814), float table(psl:696), mechanical peaks (PSL:818,820,827)
  •      Noise in PLL readout:
  •      Put two cavities in the same vacuum chamber: this reduce the thermal drift b/w the two cavities, and allow us to use lower tuning range on PLL, (psl:667), better thermal sheild (PSL:776)
  •      Reduce RFAM: Thermal control EOM(PSL:744), setup(PSL:854)

 

 plan:

bring up coating noise level, (psl:657), using shorter cavity, smaller spotsize.

 

note:

some useful numbers: psl:733

  874   Wed Mar 7 22:54:06 2012 taraNotesNoiseBudgetIFR2023B frequency noise lock w/ Rb clock

Frequency noise of IFR2023b (locked to Rb clock) at different settings are measured. This data will be used in beat's noise budget. With reference signal from Rb clock, Marconi's noise is lower than coating noise level below 300Hz.

        Noise contribution from IFR2023b/Marconi used in PLL for beat read out has to be updated. The current data in the nb is  from Marconi with no reference signal.  With Rb clock 10MHz reference signal, the frequency noise will be lower at frequency below 1kHz,see ATF:822. Since we are going to reduce the LO frequency noise by reference it with Rb clock,  we need to measure noise level for the nb.

     The settings are:

  • carrier @160MHz, with 3 different input ranges, 10kHz, 1kHz, 100 Hz.
  •  carrier @100MHz,  with 10kHz, 1kHz, 100Hz input range.
  • carrier @ 10MHz, with 10kHz, 1kHz, 100Hz input range.

     The carriers are chosen at different 3 frequencies because we want to check if the noise will be lower or not. As we aim to lower beat frequency down, 100MHz, and 10MHz are our choices.

 ==setup==

IMG_0399.jpg

Both Marconis are locked to the same Rb clock(not shown in the diagram). I use a transformer between the clock and each marconi to prevent ground loop.

From the block diagram, to convert the voltage noise to absolute frequency noise, we simply need to know the calibration factor of the Marconi. This can be obtained by measuring the marconi output frequency vs input tuning voltage.

==result==

freq_noise_ifr2023b_rbclock.png

Data sets from 160MHz carrier are plotted in red tone, 100MHz sets are in green tone, 10Mhz sets are in blue tone.

==Comments==

  • We might gain some reduction in frequency noise for going to lower beat frequency 10MHz, but with 100Hz tuning range.
  • Reference signal from Rb clock does not reduce Marconi noise at 10 kHz input range. It helps only with smaller tuning range (1kHz, 100Hz).
  • For data with 10kHz input range, the shape of 100MHz is different from 160MHz and 10MHz sets. I double check it and it seems real. The UGF is definitely above 20kHz.
  • Peaks in the plot come up when I lock both Maronis to the same Rb clock. They are gone when I don't lock, so the peaks might not be real.
  • Data are available on SVN.
  873   Wed Mar 7 11:39:37 2012 Tara, FrankNotesTempCtrlacav temp tuning - 2nd try

previous measurement is invalid - due to changing in the alignment from floated table to unfloated table over night we don't know what we really measured.

What we can say is that the beat frequency changed from 181.265MHz to 185.9MHz, so by 4.66MHz for a 0.2K increase of the shield using 1.53V heater voltage.

aligned everything properly, locked both cavities, changed tuning input range to 100kHz and turned heater off. We might stop the new measurement at any time as we got the basic information we need

  872   Wed Mar 7 01:39:39 2012 Tara, FrankNotesTempCtrlThermo modulation for ACAV

I turn on the temperature modulation on ACAV. For ACAV loop, I attenuate the signal to marconi with -30dB, reduce the power to AOM to 9dBm (original value is 13dBm) and increase the tuning frequency range to 500 kHz (max @800kHz).

weird shit going on when modulating ACAV temperature with the rad heater. Looks like we have some sort of parasitic cavity. Will redo measurement with smaller VCO input range

On the left: two temp sensors on ACAV shield in degC
bottom right: heater voltage monitor in Volts (temporary stolen channel :-) )
top right: feedback to AOM VCO input in Volts

stepresponse.jpg

  870   Tue Mar 6 20:48:47 2012 frank, taraDailyProgressBEATbeat is back

After debugging the RCAV loop, we measured the beat signal again.  We still cannot reach coating thermal noise, the noise floor now seems to be mostly seismic related.

 

beat_2012_02_22.png

 Fig1: Beat measurement, grey trace shows the beat measurement from 2012-02-22, floated table. After that we changed the springs on seismic stack. Blue is the beat signal, unfloated table, and Pink is the beat signal with floated table.

 

     The new beat signal has more mechanical peaks around 300 Hz - 1kHz than the old one does. This is probably because I do not properly damp a few mirrors. The input range used in today's measurement is 2kHz. It does not change between 1 or 2kHz input range, so we probably sits on other technical noise sources.

     In conclusion, with the new springs, we can reduce seismic noise in 5Hz- 100Hz bandwidth (see grey and pink traces in the plot). Next step, we will work on damping mechanical peaks in the signal properly, and adding air spring for the vacuum chamber.

    Note: The setup was optimized for unfloated table. The blue trace was measured after we optimized everything, from polarization to beam alignment, both before and after the cavities. When I floated the table, the beam was barely aligned and could not be locked. I realigned the beam with periscopes, and adjusted the beam alignment on beat RFPD before measuring the pink trace.

  869   Tue Mar 6 17:30:27 2012 Tara, FrankDailyProgressNoiseBudgetTTFSS in-loop noise floor

the last couple of days we had observed that our TTFSS in-loop noise floor we can reach is much higher (40nV/rtHz) than it was before (8nV/rtHz), but we couldn't figure out what it was. The general shape was the same. So we kept backtracking what we did the last week and finally ended up with the proper alignment and temp stab of the EOM we use for our PDH sidebands.

It turns out the in-loop noise floor is sensitive of how we align the broadband EOM (PC) we use as a frequency actuator. We can't properly measure RFAM when applying a modulation on it (SNR too low) and are already good aligned. Instead we aligned the EOM properly to the beam and aligned the polarization removing a L/2 waveplate after it. We are using the first PBS of the faraday isolator as our polarization reference (there is almost no space between the parts to put anything else in the beam path). It turns out that there is a slight mismatch in polarization if we do so. So we glued a little cube PBS on a post and used that as a reference instead of the PBS of the isolator.

With the old technique we have about 21uW out of 20mW in transmission of our new reference PBS - if we align the half waveplate with the micrometer screw right in front of our PC we can get as low as 8uW. This tiny difference we can't see with our RFAM detection technique at the moment. Now the noise floor is back to 8nV. My guess is that we create high frequency AM instead of phase modulation which screws up the sensitivity somehow. However we didn't see this on the spectrum analyzer (LF and RF). UGF is back to ~1MHz.

Next we will try stronger modulation using higher voltage and minimize it using RFAM detection.

  868   Tue Mar 6 00:53:02 2012 taraNotesNoiseBudgetNoise estimation for delay line technique

I made a diagram for estimating the noise of the beat signal(or marconi) as measured by delay line technique. This will tell us if this method will be able to reach the coating noise sensitivity or not. The calculation gives the estimated frequency noise of Marconi to be ~ 10 mHz/rtHz, but the measurement has unidentified noise level of 20 mHz/rtHz. We still have not figured out what is the source of the noise.

delay_nb.png

  1. Start with a source, we use a Marconi with signal = 10 dBm,noise = -150 dBm.
  2. An amplifier with Gain = 16dB, NF = 8dB. S becomes 10+16 = 26dBm, N becomes -150 +16+8 = -126 dBm.
  3.  We use 4 ch splitter, thus the power on each channel is smaller than the input by a factor of 4 (-6dBm): S/R =+20/-132 dBm 
  4.  For the signal that goes to LO side of the mixer(13 level), it is attenuated to 13 dB. From here, we will assume that the signal from this side is perfect and does not contribute to the output.
  5. For the delay line side(-9dB), the S/R are +11/-141 dBm. It should be ~10dBm, so 11 dBm is fine.
  6. The mixer has conversion loss ~ 6dB, the signal is attenuated by -6dB, S=11-6 = 5dBm. Noise is also attenuated by -6dB, but the double sided frequency will be folded to one sided f due to the mixer. So the noise power is increase by a factor of 2(+3dB)*. The final S/R are +5 dBm and -144 dBm respectively.

* This is under the assumption that noise from both sides are incoherent. However, both sides are from the same source with 200 ns delay. The coherent time and line width (FWHM) of the signal is related by FWHM = 1/(pi*Td), see, coherence time. For upper limit approximation, let the Maroni's FWHM to be 10kHz (it should be smaller than the tuning range as well, even down to mHz linewidth), then the coherence time will be 10 micro seconds. So with 200 ns delay time, both sides should still correlate. We have to be careful about how to calculate noise at this point.

Note: I use ZX05-1MHW-S+ mixer for this setup.

 For calibration:

I check the slope of the free running signal. I use two Marconis. One for LO side, it is set at 160MHz,13 dBm. Another one for RF side, @160MHz 10dBm. The slope is 1V/ rad. The peak to peak is saturated at 1.2 V pk-pk.   To change to frequency noise, multiply by 2pi*Td*f, where Td is the delay time in the cable (206nS for our setup), so the calibration is  1.26 * f [V/MHz]. 

From step 6, noise power is -144dBm =  14nV/rtHz. with the calibration factor this gives  14e-9 / (1.26 V/MHz) = 11mHz/rtHz.

 ==comparison with measurement==

The setup is similar to the above figure. The amplifier at the end is SR560.

delay.png

  1. Determine the noise floor of the setup: Noise level of the system is measured by terminating the LO side of the mixer with 50 Ohm. It is around 5 mHz/rtHz (black line). This is lower than the expected signal (purple line).
  2. Measure the signal(frequency noise of Marconi). The oscillator is set at 160MHz, with modulation off. The result is ~20mHz/rtHz (in green). This is higher than the expected level (10mHz).
  3. Verify that the unidentified noise is real: Marconi frequency noise with modulation on, 10kHz, is measured by delay line to check if we can see the frequency noise. This setup is chosen because the frequency noise of Marconi at 10kHz input range is well identified from beat measurement and from direct measurement (plotted in red), and it will serve as a reference for us.  The signal measured by delay line is plotted in blue. It turns out that the feature of 10kHz input range noise is barely seen. It seems that the sensitivity of delay line technique is not good enough because of some unexpected noise source.

 

    The sensitivity of delay line technique is too high to measure the frequency noise of Marconi with 10kHz input range. Unless we figure out what causes this flat noise, PLL is probably a better choice for our readout technique.

 

Attachment 2: delay.png
delay.png
  864   Mon Mar 5 16:29:50 2012 frank, taraDailyProgressBEATbeat is back

As mentioned in the previous entry that the in loop noise is higher than before. We investigate what might be the causes. We suspect that mode hopping or relaxation oscillation might be the case, but they are not.

The loop gain for TTFSS cannot be increased up to the usual setup, one thing we notice is that the signal from error point is quite large ( the input range selected on SR785 cannot go down below -30 dBVpk, it used to be -48 or -50 dBVpk)and oscillating at 200kHz. So we are also looking for what causes the oscillation at this specific frequency as well.

1)Checking for mode hopping:      Mode hopping might produce unexpected behavior of the laser. We want to make sure that the   usual SLOW_DC value is not close to where mode hop occurs.

 method: scan PMC and look at the signal from reflected power. The dip from transmitted TEM00 should become shallower when  mode hop occurs because some of the power is lost to other TEM00 mode.

 result: we are no where near mode hopping. The SLOW_DC coarse knob can be adjusted between 200 - 700 easily without seeing mode hopping. The usual set value is around 500 +/-20.

 

2) Relaxation oscillation: (cf siegman, laser, ch 25)

   Since we are looking for what cause the oscillation at 200kHz, relaxation oscillation is a possible candidate. We use a PDA10CS to measure the spectrum of the laser power. We use the beam reflected from the first Faraday isolator just after the NPRO. A broad peak at 462 kHz is observed, so this is unlikely to cause the oscillation.

relax.JPG

fig1: relaxation oscillation is measued (green) with the frequncy at 462kHz. The yellow line is the noise floor of the PD.

    To sum up, we are looking into what degrade the performance on RCAV loop. We ruled out two possibilities which are laser mode hopping and relaxation oscillation.

  863   Sun Mar 4 00:45:26 2012 frank, taraDailyProgressBEATbeat is back

Beat is back.

The beat signal( with the new softer springs) is measured and compared with the previous result (unfloated table). 

several notes about this measurement:

  • The resonant peaks from the stack show up as expected at 10, and 35Hz.
  • I have not added the contribution from seismic in the nb yet since TF between beat signal and seismic is not measured. We will measure that once the temperature settles.
  • Beat noise at high frequency  and  PLL readout noise do not match. This is probably because the gain setup on PLL changes and the readout noise changes with the gain, see psl:816 .
  • The table is not floated. If we float the table,seismic noise at frequency above 3-4Hz along with acoustic noise around100- 1kHz should decrease, and we might have a good chance to measure thermal noise around 100 Hz.
  • In loop noise for RCAV is around 50nV/rtHz instead of 10 (we could go down to this level before, see psl:855). The gain on TTFSS loop cannot be increased to what it was before (Common/Fast=920/850)We are checking what are the problems.
  • A quick measurement for ACAV in loop noise gives flat noise floor of ~40nV/rtHz. The slope from ACAV's error signal is 52kHz/V. Thus, the frequency noise floor from ACAV is ~ 2mHz/rtHz. Although it is higher than the designed value  (smaller than shot noise level), it is certainly not the current limiting noise source. 

 

beat_2012_02_03.png

  861   Sat Mar 3 17:50:45 2012 taraDailyProgressBEATbeat is back

NO!! no dance party for you because you didn't return our label maker.

Quote:

Quote:

We expect it to be stable by tomorrow afternoon.

 What about my dance party?! 

disco.png

 

  858   Fri Mar 2 23:37:36 2012 taraDailyProgressBEATbeat is back

 Beat signal is back now, but we have not measured the spectrum yet since the temperature is still drifting fast (10kHz/ min). We plan to measure it tomorrow afternoon.

    The optics for RCAV are aligned. The visibility is ~95% without adjusting the lenses for mode-matching. I do not redo the mode match yet because we probably have to move the chamber for installing the air springs very soon. For ACAV, the beam is aligned, with visibility only ~ 80%. I also adjusted the cable length for PDH locking so that the error signal looks symmetric (Frank added the 4-ch splitter for demodulating RFAM, so the phase shift changed a bit.  All reflected beams  are properly blocked with razor blade blocks.

    The beat frequency is ~ 188MHz instead of the nominal value of 160 MHz because the temperature is not settled at the set point yet. It takes longer than before because of the copper shields around the cavities.  We expect it to be stable by tomorrow afternoon.

  855   Thu Mar 1 10:18:13 2012 Tara, FrankDailyProgressNoiseBudgetRCAV loop characterization

We locked RCAV, optimized beam alignment, and measured the noise from error point. The measured noise@error point is ~ 10nV/rtHz (equivalent to 0.4 mHz/rtHz ~a factor of 3 below shot noise limit).  This noise will be add into the noise budget.

     1) in loop noise( I make some change as Frank suggests in PSL:856)

    ==setup==

     The power is adjusted so that the DC output from the RFPD is 2V (about 1mW) when all the laser power is reflected on the RFPD. Common/Fast gain on TTFSS = 920/850. error signal slope is 35.5kHz/V.

   ==comparison with calculation== 

   This calculation serves as a sanity check for the signal we measure. The measured in loop noise at error point will contain several noise sources (electronic noise floor, shot noise, free running noise of the laser which is not completely suppressed), but, with the current known information, I can estimated only the electronic noise. The result will at least give us some lower limit of the noise floor.

   If the loop gain is high, it can suppress laser free running noise until it reaches flat electronic noise. The electronic noise can be estimated if we know impedance(Z), gain(G) of the RFPD. The mixer(6dB conversion loss) + 50Ohm terminate reduce the level by a factor of 4 (-12dB).  Thus the noise level is ~ sqrt(4kTZ) * G/4.  For RCAV RFPD, Z = 2000,G = 10, so the noise level is ~ 15nV/rtHz. (The measured noise is lower than the approximation, I'll double check this before posting the noise budget).   

    Since the real part of the impedance of the whole circuit will take sometime to calculate, I'll just look at the measure noise output and use that number. Frank measured the voltage noise of the RFPD (in psl:795.) to be 20nV/rt(Hz). The noise is attenuated through the mixer (-6dB), so the expected thermal noise at the output is 20/4 = 5 nV/rtHz . This is not too bad, the noise floor of SR785 is ~7nV, so the incoherent sum is sqrt(5^2+7^2) = 8.6 nV.

   

 

  2) modulation index

    I measured the transmitted power from carrier and sideband which are 1.3mW and 6.45 uW. Power ratio from sideband/carrier is = 0.05. This corresponds to modulation depth(beta) = 0.14. [from (J1(beta)/J0(beta))^2 = 0.05 where J0 and J1 are Bessels fn of the first kind.

     We use 4ch splitter instead of 1 (see setup here) so the power to EOM is reduced to .2 dBm (measured, and it agrees with the setup with 6dBm attenuator). The EOM has 50 ohm load as specified in the manual, with .2dBm the input voltage is 0.23V. The datasheet says that the modulation efficiency is more than 0.2 rad/V. This gives beta = 0.046, ( from the measured value, the modulation efficiency can be calculated to be ~ 0.6 rad/V), so I think it is ok.

    The current mod index is ok, since the inloop noise is below shot noise, and we don't loose much power to the sideband.

     

 

  853   Wed Feb 29 21:43:19 2012 Tara, FrankDailyProgressRFAMPD's for both cavities installed

We've installed two pd's, one for each cavity. As the demodulated signal was very small when we optimize the EOM the usual way so we decided to replace one PD with a 14.75MHz resonant PD we've stolen from the TNI to get some more signal. The other one is a 10CS from Thorlabs (which i wanted to use for both of them). It actually turns out that we don't gain very much using the resonant PD, so we might switch back to the other one.

The DC level of the demodulated signal is less than 1mV, the noise level is limited by the readout electronics when optimized (nV level). We currently have one SR560 (DC-coupled) each with a gain of 1000 and sitting on the white noise floor. We will check with a better pre-amp tomorrow.

We temporarily sample the signal taken in front of RCAV with EPICS to monitor it over time. The EOM is not stabilized, only passive isolation is installed. When we misalign the EOM and start heating the EOM we can see the noise spectrum going up and down periodically, as well as the (tiny) dc signal, so everything is as expected, except that we can't measure anything when optimized.

schematic, pictures,diagram, etc  later when we have a final setup.

  846   Mon Feb 27 19:17:57 2012 taraNotesElectronics EquipmentNoise in delay line read out technique

I estimate the noise sensitivity in the delay line technique. The calculation should tell us how good this method can be used to measure frequency noise in beat signal.

 The calculation follows the setup in PSL:828. All relevant datasheets can be found here.

      ==Methods==

      I try to compute how each component in the setup generates noise and shows up at the end of the stream. With the calibration factor, I can convert the noise back to its equivalent frequency noise at the input.

      The only component that introduces noise in the setup is the ZHL-1A mixer. Its noise figure is ~ 8dB. Assuming that the only noise from the input side is thermal noise in 50 ohm, then the noise level after the amplifier is ~ 50nV, see details in the note below.

     For each signal trace, the signal goes through 4-ch splitter (-6dB), cable, then the mixer (~3dB conversion loss). These components give a factor of (1/4) x (1/2) to the signal. I'm not sure how noise in both delay line will sum up at the mixer. So for now I just assume noise coupling from one side of the mixer. The noise level after the low pass should be 50nV/8 = 6.3 nV. The calibration from Voltage to frequency noise at the mixer output is 2.5 MHz/V (from measurement on 2012_02_22,160MHz, svn)  Thus the absolute frequency noise  is  6.2nV x 2.5 MHz/V = 15.5 mHz.  This level is ~ a factor of 1.5 lower than the frequency noise of Marconi (10kHz tuning range) , see psl:834, psl:833. If this is the only limiting source, we should be able to measure coating noise upto ~500Hz (limited bandwidth due to the chosen delay time is not taken into account yet).

     It would be nice to be able to measure the noise and compare it with the calculation. To measure the noise, we need a low noise input source (Marconi with 1kHz tuning range should be ok) with power as specified in the setup. However, the measured noise level is higher than the expected noise and we don't know what the cause is. So we can not verify the calculated noise level yet.

==note==

Noise figure = 10log10( Noise Factor). From the datasheet, noise figure of the amplifier ~ 8dB which corresponds to Noise Factor = 6.3.

Noise Factor = SNR from input / SNR from output. For our setup, the signal from input is 5dBm (0.4 V), with noise ~ 1nV flat (50 Ohm thermal noise). the output signal is 22.8dBm (3.1V). Thus the expected noise at the output = Noise Factor x (signal_out/signal_in) x noise_in = 6.3x (3.1/0.4) x 1nV ~ 50nV.

  845   Mon Feb 27 10:56:43 2012 taraDailyProgressopticbeam realignment after new seismic stack installation

I realigned ACAV and found TEM00, but now the transmitted beam is completely missed the opening on the insulation, it is off from the center by ~ 1 cm.

  844   Sun Feb 26 17:40:43 2012 taraDailyProgressopticbeam realignment after new seismic stack installation

I'm trying to re-align the beams to the cavities. Due to the new RTV springs for the seismic stack, the cavities' natural axes shift by ~1/4 " with respect to the previous position.

     I had to adjusted the height of the top mirror of the periscope before I could align and lock RCAV (visibility ~ 95%) again. The pictures below show the position of the current beam. With the previous setup, the beam position was almost at the center of the holes. Now, for RCAV, the axis shifts closer to the edge. RCAV might yaw with respect to the previous position. Left picture shows the incoming beam position, Right picture shows the outgoing beam position.

IMG_0374.jpgIMG_0373.JPG

      For ACAV, however, it seems that the position changes a lot and the beam clips on the outer edge of the top mirror before I can even find TEM00. I think I'll have to add a spacer between the mirror mount and the vertical plate in order to re align the beam.

     I think we can keep the stack position as it is for now, if I can lock both cavities and the transmitted beams can be adjusted on the breadboard for beat path. We might also have to increase the hole size on the insulation cap as well depending on where the beam position of ACAV will be.

 

  840   Fri Feb 24 21:59:44 2012 Tara, FrankDailyProgressVacuumupgrades installed - pumping started

we've installed:

  • the new stack -see earlier post with measured TF
  • both copper radiation shields
    - one shield has a NiChrome heater wrapped around (the other one not as we first want to see if we need more/less power, but calculation says this one should be right)
    - three temp sensors (AD590), two on the shield with heater, one on the one without
    - those sensors won't be used for temp control, only for temp and gradient monitoring and/or safety shutdown (if required)
  • cable from D-SUB feedthrough to rack breakout terminal

window is back on and pumping down over the weekend. Thermal insulation for vacuum chamber is back in place, so we should be ready to go for a new measurement on Monday afternoon after installation of the beat breadboard

P1820701.JPG P1820756.JPG

  838   Fri Feb 24 13:01:46 2012 taraNotesNoiseBudgetNoise budget for beat measurement by PLL

I added another noise budget,"beat_nb.m", on SVN. This noise budget is for beat measurement via PLL. It calculates both cavities' noise sources which will add up incoherently in the beat measurement. The code is still incomplete. More noise source will be added later. This noise budget is supposed to be the correct nb for our beat measurement.

 

This code is based on the previous one which is the noise budget of a single cavity.  I remove a few traces e.g. noises from ambient temperature fluctuation coupling through thermal expansion. Since they are way smaller than the other. I still keep Brownian noise in substrates and spacers. These might be removed later so that the plot is not too crowded.  

beat_nb.png

The current code includes noise from:

  1. Spacer Brownian (x2 from both cavities)
  2. Substrate Brownian (x2 from both cavities, x2 from two substrates on each cavity)
  3. Coating Brownian   ( x2 cavities, x2 substrates)
  4. Substrate Thermoelastic (x2 caviteis, x2 substrates)
  5. shot noise from PDH locking(x2 from acav and rcav loop, they can be independently adjusted)
  6.  LO phase noise from PLL
  7. PLL read out noise

The other noise sources that will be added later are:

  1. Seismic coupled noise
  2. RIN induced noise

      For noise from seismic, especially from vertical direction, we can measure the TF between seismic -> beat. This will tell us the real coupling. Koji and Frank measured the TF between the table and the stack in the previous entry, so there should be no problem with the setup.  We can also try other directions (horizontal), but from vertical direction, we should be able to get the idea how seismic couples into beat signal.

     For RIN induced noise, I'm not quite sure yet if the SNR will be large enough to be able to measure, but we can approximate.

 

  832   Thu Feb 16 23:27:50 2012 Frank, TaraNotesBEAToptimized cable-delay setup sensitivity

Started characterizing the cable-delay setup with the right length of cable (134ft of RG58 for 160MHZ). After checking the change in sensitivity with load impedance i've changed the load to 500 Ohms (instead of the usual 50 Ohms). I think an additional low-impedance path for the 2f has to be put in parallel later (to have proper 50Ohms @ 2f) to not get it reflected at the input of the low-pass filter back into the IF port of the mixer. (see first schematic).

mixer_termination-01.png

 

However, the following simple setup has been used for the measurements:

cable-delay_v5.png

 

I've measured the output signal vs different LO power levels while keeping the RF signal strength constant (8.29dBm) to find out the optimum signal strength in terms of size (not noise at this point!).
The following plots show the result:

  • The peak mixer output signal (for pi phase shift) increases with LO power
  • at the same time the slope around the zero-crossing shows a maximum around the specified LO power value (13dBm) - update: had a typo in vertical units, now correct - plot itself was correct

LO_power.png LO_power2.png

[discussion]

  831   Thu Feb 16 19:44:23 2012 Frank, TaraNotesBEATmixer output scaling with load impedance

 

==Some intros about this measurement:==

We are using a mixer as a phase detector. Usually, mixers are not optimized to be used as phase detectors (that provide DC output), they do have 50 ohms impedance output. However, when we use a mixer as a phase detector, the output is DC, and the output behaves like a current source. We want to know how load impedance will affect the readout system, so we measured the output and varied the load impedance.

 

==Setup and Result==

The setup is shown here. We checked how much we can gain in the cable-delay readout scheme by using a different (higher) load impedance. Used the standard setup with a Marconi as the source, modulated the frequency with a triangular signal and looked after the LP filter with the scope for different termination impedances after the mixer. Plot is in arbitrary units as only the relative change is relevant. Signals are triggered at slightly different times.

mixer_slope.png mixer_slope2.png

==comments==

 The result shows that with higher load impedance, we have better sensitivity (steeper slope). This is as expected from a current source with load impedance ( V = IR). At this point we are using 500 Ohm load impedance in our regular setup because we are not sure if the higher load impedance will introduce any extra noise.

  830   Thu Feb 16 02:40:58 2012 frank, taraDailyProgressBEATBreadboard setup for beat path

We still working on the breadboard setup. There will be several things we have to modify for the setup.

Now we can mount the mirrors on the three screw blocks, with beam on the hole pattern.

[add details about what to modify]

photo(8).JPG

 

mech_peak_2012_02_15.png

 

Note: I have been looking at frequency only around 100 - 3 kHz. Here is the beat for broadband with other noise. The seismic noise in the plot is for floated table, I'll edit that, but it is pretty similar for what we had before. So the breadboard setup has not introduced any noise at low frequency that shows up in the beat yet.

nb_2012_02_15.png

  827   Tue Feb 14 22:47:45 2012 frank, taraDailyProgressBEATBreadboard setup for beat path

I checked the mechanical peaks in breadboard setup. We get rid of the peak at 800 Hz from the periscope and a big peak around 200 Hz. However, there are some new peaks popping up which are not identified yet.

 

   As the beams are not on the whole pattern, Frank suggested that I move the whole board and clamp it down instead. In order to do that, I had to remove the QWPs. I clamped it down with steel clamps around the legs. After that I inserted some damping posts (see below pictures). It turned out that the rubber damping does not help much at this point the noise spectrum does not change at all between with or without the rubber. I think it is because I cannot insert enough rubber between the board and the post, as I slid them in after I clamped the board. I should have placed the damping posts in their places then clamped down the board.

[add clamping fig]

IMG_0329.JPG

 

mech_peak_2012_02_14.png

fig: beat noise in different setup: Blue: beat path with periscope, Magenta: beat with breadboard setup, Green: with damping on mirror mounts. The data were taken with 2kHz tuning range, gain 200.

==result&comments==

Since only the beat path that has been changed, all the peaks that popping out are contribution from the breadboard setup.

So things that are changed are:

  •      Mirror mounts: all mirror mounts on the bread board for beat path (except ones for picking up light for CCD/PD) are all the same, (the mount with whole frame around the mirror). This causes several peaks cluster around 1-1.2 kHz. By placing rubber cones on those mounts and the peaks were damped down by a factor of 3 or so. So we might need to insert some foam for better damping later.IMG_0332.JPG
  •       Heavy lead blocks are placed on the board to  help reducing noise at lower frequency around 100-200 Hz. Better damping/clamping for the board have to be done.IMG_0328.jpg
  •      The broad band noise was gone after all the stray beams were properly blocked with razor blade beam block.

There are a few new peaks due to the breadboard setup which have not been identified yet. It is very hard to check, since tapping with slight force already excite the peaks of the mirror mounts around 1 kHz. Once the mounts are damped, other peaks might be easier to be found.

  824   Tue Feb 14 02:39:53 2012 frank, taraDailyProgressBEATBreadboard setup for beat path

We removed the periscopes in beat path and use breadboard setup instead. There are higher broadband noise in the beat around 100 - 3kHz. At least, the peak around 800 Hz is slightly small, since the contribution from the periscope in the beat path was removed.

 

     We are trying to get rid of individual mechanical peaks in the beat signal. One of the major peaks comes from the periscopes and the associated mirror mounts. So instead of using periscopes to bring the beam height down, we use breadboard setup to bring the whole beat path up. This new setup gets rid out the periscopes, and 4 mirror mounts on it.

    Frank has a solid work assembly file showing how the setup should look like. However, the beam coming out of the cavities are not exactly on the whole pattern, and without the periscope we have no way to steer the beam to the designed path. As a temporary solution, I use a post mounts which are clamped on the board, not screwed.

IMG_0325.jpg

 Fig1: The beam path is not on the whole pattern on the table, so the block mirror mount cannot be used.

IMG_0326.jpg

fig 2: current breadboard setup. The mirrors behind quarter waveplates are mounted on a regular post and clamped down on the board. If I use the block mount, the beam won't hit the mirror.  Note: I use the previous beam splitter post (with 1.4kHz resonant peak) for now, because we don't have anything that fits right now.

 

==result==

beat.png

fig3: Comparison between original setup (in blue,with periscope) and breadboard setup (green). Both signals were taken with 2kHz tuning range, gain 200.

=comment=

I think the broadband noise might come from the scattering on the PD. THe spotsize on the PD is not much smaller than the PD. I haven't found the appropriate lens for modematching yet.  The peaks around 1kHz-2 kHz also seem to be more than the regular setup. This will be investigated.

  819   Fri Feb 10 23:47:22 2012 Tara, FrankDailyProgressBEATbeam splitter mount replaced

we replaced the mount for the combining beam splitter in the beat setup as it caused a large, broadband peak in the spectrum around 1.4kHz. The new mount is one of the old, fixed turning mirror blocks they used in initial LIGO at LLO as far as i know. After replacing the mount the peak is entirely gone. I've used two springs instead of one to increase the pressure.  We could not determine the resonance frequency of the new mount. Tapping the mount excites only known mechanical resonances from the surrounding mirror mounts. Tara posted a plot for comparison before and after replacing that mount (see here). He also has prepared a nice plot combined with a drawing which mount corresponds to which resonance we see in the spectrum. We will use this to start reducing (or even eliminating) those resonances starting with the most dominant ones close to 1kHz

Attached a copy of the drawing.

SCAN1623_000.pdf

 

  818   Fri Feb 10 23:29:39 2012 frank, taraDailyProgressNoiseBudgetidentifying mechanical peaks in the beat noise

We try to identify the origins of each mechanical peaks in the beat signal from frequency between 100 up to a few kHz, so that we can damp it down below coating thermal noise. For today, we get rid of the peaks from the beam splitter in the beat path.

 mech_peaks.png

fig1: beat signal on linear x scale. Blue and red was taken with 10kHz and 5 kHz input range respectively. The broad band noise on Blue is from LO phase noise.

IMG_0307.jpg

The beam splitter with a steel post and a clamp is replaced by ->

IMG_0308.jpg

this block which holds the beam splitter inside. The whole pattern does not line up with the table, so we have to use clamps for now.

 

  816   Thu Feb 9 22:31:03 2012 frank, taraDailyProgressBEATnoise budget and beat

Noise calculation from PD in PLL: ( I actually asked Koji once and did this already, see psl:730 . The results are similar)

1) determining which setup gives the best performance:

  • Gain on SR560 = 200. This gives UGF of 33kHz with 60 degree phase margin. Gain 500 has phase margin of 18 degree, which is too low, see fig 1.
  • Tuning range on Marconi = 1kHz. Currently, we cannot go with lower range. Usually, this noise couple directly to the readout and cannot be suppressed, so the lower noise (smaller range) the better.

TF_gain.png

fig1: OLG TF of PLL with different gain setup.

 

 

2) Measure electronic noise from readout system with the chosen setup. This noise will show up (after some correction) in the beat and determine what is the limitation of PLL readout technique.

     The PD was blocked, the feedback signal (Vfb) to the actuator (LO) was removed and measured.

3) Block diagram

[add block diagram and calculation]

4) After Koji explained on how to calculated noise budget from electronic noise in PLL to us, here the nb with PLL noise. (note: the LO phase noise has updated to 1kHz input range)

nb_2012_02_09.png

With the electronic noise from PLL, the sensitivity of this technique will prevent us from observing coating noise above 1kHz.

I'll calculate the noise from cable delay technique later and compare which one will give us better sensitivity.

 

Attachment 2: nb_2012_02_09.png
nb_2012_02_09.png
  815   Wed Feb 8 22:10:56 2012 frank, taraDailyProgressBEATnoise budget and beat

The noise budget is updated and plotted with today's beat measurement.

(*The electronic noise plotted in the graph is not correctly calibrated, see psl:816 for the complete calculation)

     After we replaced the table's broken leg, we floated the table and measured the beat signal as a reference before modifying the seismic stack. The calculation agree with the measured data quite well.

   We also measured the electronic noise from PLL. This was the signal which was fed back to the LO (with SR560 gain = 20). Apparently, we are sitting on it at 1kHz and above. We definitely need to work on PLL readout system to measure at lower sensitivity.

beat_2012_02_08.png

New traces in the noise budget:

  1. Beat noise (floated table)
  2. PLL electronic noise

I removed noises from room temperature fluctuation/ heater noise and spacer thermal noise because they are way smaller than coating noise and crowd the plot.

==Details about some traces in the noise budget==

Seismic noise:

     The vertical seismic noise coupling is calculated by applying the seismic measurement times stack transfer function times cavity bending coupling

     [ Frequency noise from seismic ] = [measured data] x [stacks TF] x [bending factor].

  • For seismic noise, The data was taken by a seismometer on a floated table.
  • Stack TF is computed based on springs and the weight of the blocks, see PSL: .. 
  • The cavity bending coupling is obtained from COMSOL simulation. However, the value used in the noise budget is about 1 order of magnitude larger than the value COMSOL provides. This is probably due to the fact that the cavity's support positions do not match exactly at the optimum point.

The peak around 6 Hz might be coupled from horizontal direction. This will be added soon.

RIN induced length noise is still an estimated. We have not been able to measured the real coupling yet, as the SNR is so low.

LO phase noise:  This is from measurement. I'm not quite sure if I miss some calibration factors. The phase noise does not show up in the beat yet even though it is very close together right now.


  814   Tue Feb 7 21:55:05 2012 Tara, FrankDailyProgressSeismicRTV springs

We measured Q and k of RTV springs. Currently, there are 9 of them. The results are ok

An aluminum block with accelerometer (0.398kg) was placed on a spring. The block was tapped, and the ringdown response was measured.

 

piece freq [Hz] spring constant k (N/m) Q
1 38.1 22,808 13.2
2 35 19,248 13.1
3 35.2 19,468 11
4 38.5 23,290 14.5
5 36.5 20,933 13.8
6 37.0 21,510 13.9
7 37.2 21,743 13.5
8* 32 16,090 9.55
9** 31.5 15,591 9.8

* the piece is broken, so it is softer and more lossy.

** the piece is made of other material.

 

Mean Q (piece 1 to 7) = 13.29 +/- 1.11

Mean k (piece 1 to 7) = 21,286 +/- 1,500

  813   Tue Feb 7 20:10:34 2012 Tara, FrankDailyProgressScheduleupdate

list of finished items

  • in-vacuum cables and connectors, screws, washers, teflon pieces, sensors etc cleaned - baking over night
  • cut & cleaned RTV springs - baking over night
  • polished and cleaned second shield
  • measured spring constant for all RTV springs to compare later with stack TF - see next elog entry for details
  • got lots of parts from the machine shop (periscope, pd mount, parts for beat setup - all parts cleaned and ready for assembly

all parts needed for upgrading the stack and adding radiative shields/heaters are tested/ready or currently beeing baked. Installation can start Wed/Thu

unfinished things left for tomorrow:

  • replace leg
  • replace opamp in PDHbox

P1820549.JPG P1820550.JPG

  811   Mon Feb 6 22:35:49 2012 frank, taraDailyProgressElectronics Equipmentacav characterization

TF from all components except the sevo is measured (collectively). This will help us to determine what kind of servo(UPDH) we want for ACAV.

 

TF from other components in ACAV loop except the servo (collectively) (frequency discriminator, marconi, amplifier) and fit]

TF.png

TF from ACAV loop without servo. The red curve is fitted with 1pole at 50kHz with 70kHz delay (exp(-i*f/70kHz)) . Since our bandwidth of interest extends upto only a few hundred kHz, this fit is good enough for a model. It starts to diverge from the data at 300kHz.

Note: The TF looks ok, it is flat as expected from most of the parts (frequency disc/Marconi/amplifier). The pole and time delay is from the AOM. We can see the phase changes as we change the AOM position so that the beam is closer to the PZT side. We gain ~5 degrees from adjusting the position.

[current updh schematic]

The TF has a pole around 50kHz. C18 with 3300pF gives a zero at that frequency and cancels the pole. We are designing the TF of the servo that is suitable for our need (UGF ~100kHz, with 1/f roll off at UGF, and ~45 degree phase margin, and, 1/f^2 at low frequency)

Problem with UPDH, on stage 2, the resistors' values might be wrong. We will check and fix it tomorrow.

 

 

  810   Sat Feb 4 03:18:08 2012 taraDailyProgressElectronics Equipmentacav characterization

I checked the TF of the amplifier used in the ACAV loop because I did not measure that yesterday.  The amplifier's TF is flat at least up to 200 kHz. The Bode plots between the loop with and without the amplifier are pretty much the same. Thus, it is ok and won't cause a problem in the loop.

==setup==

The setup is similar to what I did in the previous entry, except the output of the marconi is connected with the amplifier. See blue arrow.

 2012_02_03_pll.png

 

==result==

PLL_TF.png

The TFs between the two cases have similar shape. So the amplifier will not be the limiting component. The magnitudes (in arbitrary unit) are slightly different because I did not attenuate the power by the same factor it was amplified.

 

The UPDH box will be modified next to see if we can increase the loop bandwidth.

  809   Thu Feb 2 23:51:32 2012 frank, taraDailyProgressElectronics Equipmentacav characterization

We measured the TF of marconi using PLL loop. Marconi has flat response up to around 200kHz. This is quite good and we can certainly use it in ACAV loop.

 ==block diagram of ACAV loop==

  • D: frequency disc
  • G: servo
  • A: Marconi
  • H: Amplifier

2012_02_02_block.png

The whole OLG TF was measured in PSL:...  This time we looked into the marconi to see if its TF has bandwidth high enough for ACAV loop or not.  We know that Marconi has lower phase noise than LIGO homemade VCO (Megan's elog), but we have not learned about its bandwidth yet.

==PLL setup for Marconi TF==

The actual magnitude TF at DC can be determined by using a voltage calibrator to inject DC signal and measure how much the frequency of the output changes. This depends on the tuning range setup on the Marconi. However, we don't know the bandwidth of the TF, so we use PLL to find out. The setup is shown below.  The gain from SR560 was set to be low, so that the signal at high frequency will be the TF of the Marconi + frequency discriminator.

2012_02_02_pll.png

==results==

Since the mixer output gives 1/f response (flat in [rad/rtHz] unit), we corrected the TF by multiplying back with f to get the TF of the marconi. The magnitude on the plot below has arbitrary unit. We are only interested in its shape. We tried 100Hz, 1kHz(not shown), 10kHz tuning range. The magnitude varies with the tuning range as expected. The phase does not change that much.

 

marconi_TF.png

If we want to have phase margin of 45 degree, assuming other components in the loop have no phase lag. The best UGF we can do is upto 200kHz, according to the phase response of the Marconi (the phase drops by 135 degree around 200 kHz). Therefore, using a Marconi as an oscillator for driving the AOM is also possible because its bandwidth is high enough for measurement up to 1kHz.

Note: we will check what is the TF of the amplifier (H) used in our setup to make sure that it is not the limiting component.

We can definitely use Marconi as a VCO in our ACAV loop.

NExt: The next step is checking the UPDH box. At a glance, we found that the TF shape of the current UPDH is not suitable for our requirement.

  808   Wed Feb 1 15:46:51 2012 taraDailyProgressElectronics Equipmentacav characterization

I'm working on characterizing ACAV loop to check if the loop is good enough for the coating noise measurement. The results show that there we need better improvement on this part.

 

I realigned the cavity, and the visibility is only 85%. I have not figured out yet why the visibility decreases from 90%. Then I measured dark noise @ error point, inloop noise with different gain setting, and error signal's slope ( all with 50 ohm system).

 

 Error slope (with 2V DC on RFPD) =122.7 kHz. The visibility is @ 85%

1) Dark noise and inloop noise:  The in loop noise changes with gain setup on UPDH box. I changed the gain from 3 to 7 and made sure that there was no oscillation (oscillating @ gain7.5).

inloop.png

 With the error signal slope, I can calibrate in loop noise to absolute frequency noise from ACAV loop and plot them on noise budget. On the figure below, we can see that at high frequency noise from ACAV loop is dominating.

nb_2012_02_01.png
 

Notice the bump around 3kHz. It comes from the phase noise of the marconi. If I change the tuning range from 10khz to 1kHz the in loop noise also change (blue to green in the below figure). It is a trade off between lower frequency noise, less gain.   The peaks around 80 Hz go up because we have less gain to suppress the noise.

inputrange.png

 

2) OLG TF at different gain setup

ACAVOLGTF.png

 

Back to the 10kHz input range. Since the in loop noise is getting worse with more gain, I look into the OLG TF of the loop. The UGF is around 3-10kHz, depending on the gain. TF looks ok at gain 3-5, the magnitude increases along with the gain. At gain 7, the TF starts to deviate at high frequency (it might be oscillating at high frequency).

The in loop noise should be suppressed with 1/(1+G) factor, but the results above do not tell the same story. The gain increases but the in loop noise gets worse. So the next step is to look deeper into this problem. I'll use SR650 instead of UPDH and check if the same problem occur or not. This should verify if the UPDH box is bad or not.

Note: I'll calculate what we need for ACAV loop, and check if LIGO homemade VCO will be good enough for ACAV loop or not.

  807   Tue Jan 31 01:26:03 2012 taraNotesDrawingslegs and base for RFPD

These drawings are for RFPD's bases. The square plate will be made of Aluminum. It is for mounting RFPD on the table. The block is for RFPD legs which will be made from Delrin.

Attachment 1: RFPD_base.PDF
RFPD_base.PDF
Attachment 2: RFPD_leg.PDF
RFPD_leg.PDF
  806   Mon Jan 30 22:53:30 2012 frank, taraDailyProgressNoiseBudgetLoop characterization

swap the RFPDs. The SN002 for RCAV was not working (see psl:803). So we use SN01 for RCAV and characterize it again.

1) dark noise in Vrms/  rt Hz

noise_volt.png

Dark noise is not particularly high. ~40 nV/rt Hz flat. In loop noise is only around 10 nV/rt Hz. It is about a factor of 2 above SR785 noise for the input range setup. When I measured the dark noise, I made sure that monitor screens in the lab were off, no cables connected to a scope to create any ground loop. So the measured signal this time does not have harmonic lines as much as the previous measurement does.

2) error signal slope :  measured to be 34.4 kHz/V ( with 20dB attenuator on EOM. if I use 16dB attenuator, the calibration can be lower to 22 kHz/V). We will decide what should be the modulation index once we calculate the acceptable level of shot noise and compare with the gain we need.

Note that the numbers in psl:802 missed a factor of 2 (they should be twice as much).

3) absolute frequency noise from dark noise at error point and inloop noise: The gain setup for common/fast are 1000/750. It becomes unstable if I increase more fast gain. 

nb_2012_01_30.png

 The inloop noise lies below the estimated coating noise up to 100kHz. So if we can decrease any other technical noise sources, we should be able to reach coating noise level.

 

Next will be the complete OLG TF measurement.

  802   Thu Jan 26 21:05:21 2012 frank, taraDailyProgressNoiseBudgetLoop characterization

We need to characterize RCAV and ACAV loop, here is a list to do

  1. Power adjustment
  2. RCAV loop characterization
  3. ACAV loop characterization

Power adjustment: We now choose some appropriate and easier numbers of the setup for future reference.

The DC out from RCAV and ACAV RFPD are 2 V which correspond to ~ 1mW input.

The DC out from RFAM PD is 0.2 V which corresponds to 1mW input as well (this one has different R, hence different gain)

[add pic], and power to EOM is 0dBm. This will be our current standard setup.

RCAV loop characterization:

We use different RFPD, different TTFSS set, so this has to be done again.

The power to EOM is 0 dBm, 1mW input to RCAV:

The error signal slope:

ACAV 0.0149 MHz/V (I'm not sure why the error signal from ACAV is clipped,see below picture)

RCAV 0.0224 MHz/V

[add fig]

Dark noise level in Vrms/ rtHz

electronic_noise.png

-> Dark noise level in absolute frequency:

nb_2012_01_26.png

 It looks like the noise is low enough almost up to 1kHz. Yay!

The OLG transfer function as well as in loop noise, will be measured later once we optimize the gain.

  801   Thu Jan 26 12:40:21 2012 TaraNotesopticDual periscope base

 I fixed the drawing for periscope base. Will submit to the machine shop soon.

Attachment 1: dual_periscope_base.PDF
dual_periscope_base.PDF
  800   Thu Jan 26 00:34:23 2012 frank, taraDailyProgressBEATBeat improvement

 We investigated the beat noise, and found out a few issues we have to fix to improve the sensitivity.

  •  Periscopes

Resonant peaks from periscopes around 800 Hz are high. We use an aluminum bar to tighten the periscopes together. This bring down the peaks. However, the peaks are still high. We certainly have to work on the design for the periscope to get rid of the mechanical peaks.

2012_01_25_2.jpg

upper beam for dual periscope. This help reducing resonant peak around 800 Hz a lot.

 

  • Marconi's phase noise for ACAV loop.

The local oscillator we use for driving the AOM in ACAV loop is Marconi. We are sitting on its phase noise at 10kHz tuning range .When the tuning range is set to 1kHz, noise at frequency above 1kHz drops. However, with 1kHz tuning range setup, we don't have enough gain to suppress noise at lower frequency and noise around 100Hz goes up.

We need to characterize ACAV loop to project LO's phase noise on the noise budget (this has not been done yet). We might have to add another EOM for feedback on ACAV loop.

  •  Acoustic coupling

 We also noticed that talking noise can easily couple to the beat signal, but we are not certain where it happens. All mounts and posts will be checked if they are loosen or can be improved.

 

beat_20120125.png

  799   Wed Jan 25 23:41:38 2012 frank, taraDailyProgressopticnot much improvement on mode matching

We tried to redo the mode matching to RCAV by adjusting the lense position using translational stages. However the result does not improve that much. The visibility is still roughly the same at 96.5%. 

2012_01_25_3.jpg

 

We will do the mode match for ACAV. Right now the visibility for ACAV is ~90%.

We also monitored the beam reflected from ACAV. TEM02 shows up (see below figure), but we could not get rid of it by beam alignment. It is probably the distortion from the AOM.

2012_01_25_1.jpg

  798   Wed Jan 25 03:05:57 2012 frank, taraDailyProgressBEATBeat with 14.75 Mhz sideband

We added a ccd camera to monitor the reflected beam from the locked cavity. The shape of the beam(turns out to be LG10) tells us that we have to do a better mode matching.

==Motivation==

We found that bad alignment can cause extra noise in the beat noise, see  (psl:796).So we monitored the reflected beam from the locked cavity to check what kind of misalignment we have to fix. Currently, the shape of LG10 is the dominant one which tells us that we do not have a good spotsize/spot position match which can be fixed by moving the lenses in front of the cavity.

 2012_01_24_2.jpg

above, top right panel, the shape of the beam reflected from the locked RefCav.

2012_01_24.jpg

==To Do==

  So we will move the lenses in front of the cavity to optimize the mode matching.   The current value is ~ 96.5%, we want to improve it without spending too much time, not more than a day. We will mount the lenses on translational stages for better position adjustment.

  797   Wed Jan 25 02:51:44 2012 frank, taraDailyProgressopticFaraday Isolator is back

With the new EOM bases, we can place 2 EOMs and the Faraday Isolator back in to the setup.

The half wave plate(HWP) between the two EOMs is temporarily mounted with two posts mounted together on a cross holder, because there is not enough space. We will make a special post, so that it can be mounted between the EOMs.

 

2012_01_24_1.jpg

  796   Tue Jan 24 02:47:41 2012 frank, taraDailyProgressBEATBeat with 14.75 Mhz sideband

After switching to 14.75 MHz sideband setup, aside from the RFPDs, we did not change anything yet. We use TTFSS setup on the table. The seismic stack is still the same. The table is not floated. So we measure the beat  noise as a reference.

beat_2012_01_23.png

==Setup==

we use 14.75 resonant EOM for adding sideband, and the broadband EOM for feedback.  The resonant EOM is driven by an LO at 14.75 MHz @ 20dBm. The broadband EOM is supplied with 25V power supply, it is operated at low voltage because of the limited current source. 

RCAV is locked by TTFSS, ACAV is locked by UPDH, the phase shift for both loop is done by cable length adjustment.

 Note that we removed the Faraday isolator from the setup due to the limited space. It will be installed back later once we change the EOM base.

The power input is 0.2 mW on both cavities. The visibility is more than 80%.

==Problem and Plan==

We saw that beam alignment caused the bump around 1kHz to change significantly, so we will re-align everything to make sure that we have good mode matching.

The power input was chosen to be around 0.2 mW because at 1mW the error signal is irregular.  It might be the RFPD problem, we will look into it.

  789   Thu Jan 19 23:43:43 2012 taraDailyProgressopticRCAV is locked

RCAV is locked using TTFSS.

It took awhile before I could lock the cavity because the 14.75 MHz EOM tilts the beam path, and I had to realign the beam. We don't know why the beam path was changed that much. We checked the EOM with impedance kit. It has 14.75 MHz peak and the crystal looks nice, so we use it anyway.

2012_01_19.jpg

 The error signal looks nice after cable length adjustment.

I locked the cavity with fast feedback only to measure the transmitted power through the cavity. P_side band is 0.16mW, P_carrier is 0.57mW. So Psideband/Pcarrier ~0.3, this corresponds to modulation depth ~ 0.95. This is close to the calculation Frank did.

I have not tried to measure noise at the error point yet, since I have to flip the phase by 180 for feedback to EOM (TTFSS has a phase flip switch for FAST feedback only). I used a long BNC cable to change the phase by 180 degree, I think making an adaptor for EOM connector to flip the signal might be a better idea to try.

2012_01_19b.jpg

Next step:

  • We will use a Marconi with some amplifiers for LO to drive the EOM and demodulate the signal later. The current function generator is noisy, but it's a good start to see how much better we can gain from resonant EOM.
  • Use Jenne laser to characterize the 14.75 MHz RFPD. Then we can calculate how much gain we get, and how much improvement we need.

 

  787   Thu Jan 19 01:19:31 2012 taraDailyProgressoptic14.75MHz EOM

The 14.75 MHz EOM we have is for visible light, so we went to TNI and borrowed a 14.75 MHz EOM for IR and an 14.75 Mhz resonant RFPD.  I will re-aligned the beam and measure the error signal tomorrow.

 The current function generator can provide power up to 23dBm. So the EOM can be driven around ~ 19 dBm(~2V@ 50Ohm) (-3 dBm for a splitter, -1 dBm for loss in the cable). So we can expect the modulation index to be 0.2*2 = 0.4 rad.

  785   Tue Jan 17 21:59:47 2012 taraDailyProgressoptic14.75MHz EOM

As we decided to use lower sideband frequency (14.75MHz, instead of 35.5MHz), I replaced the Broadband EOM with 14.75MHz EOM.

==Motivation==

   The current broadband EOM give only small modulation depth (~0.06 rad with maximum power from the LO, seepsl:745) With a resonant EOM, we can get higher modulation depth with the same amount of power.

   Plus, in general, the RFPD's Q will be also higher at lower frequency, so we should get higher gain to suppress more frequency noise (the exact number of Q has not been measured yet).

==To Do/ Problems==

  We no longer use LIGO's old LO cards. All of the spares in the lab are also broken. We will use a function generator and adjust the cable length to change phase between LO and PD. 

 After I added the resonant EOM to the setup the beam path changed quite a lot, I need to re-aligned the beam before I can see the error signal and lock the cavity.

  784   Fri Jan 13 16:51:29 2012 TaraNotesopticIsolator base, EOM base's height

 Note: new EOM base

This one will have total height of 1.31" . THe height of the EOM (base to aperture) is 0.56". The height of the 4-axis stage (new focus 9071) is 1.06 - 1.18 " (min to max), I use 1.13" as operating height. So the total height is 1.31 + 0.56+ 1.13 = 3" .

 

The drawings of EOM and 4-axis stage can be found here:

EOM

4-axis stage

 

 

Mon Jan 16 17:10:25 2012

Base for Dual periscope is added. This will allow us to mount the plate to the table with screws. Clamps can be used to provide additional support as well.

dual_periscope_base.PDF

Attachment 1: eom_base_2011_01_13.PDF
eom_base_2011_01_13.PDF
  783   Thu Jan 12 16:04:05 2012 TaraSummaryPMCIsolator base, EOM base's height

As mention in the previous corresponding entry, the height of the base for the faraday isolator is not correct. I removed the thing from the table and measure its height again in order to have it fixed. It has to be cut by 3/128 inch from the base (see figure below). The groove for the isolator is well leveled. I got the same height for both ends. I'll bring it back to machine shop tomorrow.

faraday_base.jpg

There are other mechanical parts I need to fix:

  • Broadband EOM's base: Frank noticed that it is a bit too high, and the 4- axis stage height has to be adjusted close to minimum which is not good for mechanical stability.
  • New design for EOM base: If we decide to go for two EOMs for 1)adding side band, and 2) feedback, we need some space for installing the second extra EOM. With the current EOM base design, it takes up to much space along the beam path, so I might need to make a new EOM base.
  • RFPDs' base: Now they are only a plastic post. We will need a sturdier design.
  • Base for dual periscope

I'll try to have these parts before we open the chamber to change the seismic stage.

  782   Wed Jan 11 20:50:08 2012 taraDailyProgressNoiseBudgetcurrent TTFSS noise budget

 As Frank proposed to use side band frequency at 21.5 or 14.75 MHz, I checked the HOM of both frequencies.

See more details about HOM from psl:106

21_5MHz.png

Fig 1: HOM from 21.5 MHz sideband.  At 17th HOM the frequency difference between the sideband and the HOM is ~2MHz, which is much bigger than the cavity's linewidth.

 

14_75MHz.png

Fig2: HOM from 14.75 MHz sideband. The difference between 10th HOM and the resonant frequency is 2.2 MHz, which is much bigger than the cavity linewidth (~75 kHz).

 

It is unlikely that one of the HOMs will resonate inside the cavity if the frequency difference is much larger than the cavity linewidth. So, there is no apparent overlap  we have to worry about for both frequencies.

Thu Jan 12 10:53:29 2012: The code for HOM is on svn.

 

  780   Fri Jan 6 12:18:08 2012 taraDailyProgressNoiseBudgetThermoelastic noise from substrate

The correction for thermoelastic noise (by Cerdonio) has been added in the noise budget code. The substrate TE noise estimation from previous code uses Braginsky's result which does not take the spot size of the beam into account.  The code is now on svn.

The TE noise correction given by Cerdonio has to be integrated twice at each frequency point.

cerdonio.png.

The integration limits are 0 to infinity. I use Riemann sum to do the integration. Luckily, the integral converges quickly, so with integration limit from 0 to 10, stepsize = 0.1, the error between Cerdonio's and Braginsky's result is only 1.5% (at high frequency) and the calculation is not exhaustive. This can be changed later if we need better accuracy.

ctn_nb.png

For our experiment the thermoelastic noise is below coating Brownian noise at all frequency.

Attachment 1: TE_noise_compare.png
TE_noise_compare.png
  759   Fri Dec 9 06:32:24 2011 TaraSummaryPMCrealignment

The V-block's height is a bit too high. The beam height is very close to 3".

Quote:

thanks for the info.

I only did the PMC so far as i had to fix the daq and add the new channels. What's the problem with beam height of the isolator? Is the beam too low or the mount too high? Do you know?

 

 

 

  754   Thu Dec 8 20:28:06 2011 taraSummaryPMCrealignment

A reminder for Frank about the setup, 

  • The AOM's case on RCAV beam path is not screwed down to the body, so it will be blocking the beam.
  • The height for Faraday isolator behind the PMC is not correct, i think only ~80% of the power is transmitted. I haven't had it fixed yet
  • When I measured the visibility of RCAV, I scanned the cavity and minimize the dip as seen on the REFL RFPD. I got sth ~90% transmission, but when I measured the actual tranmitted power, the visibility is lower than that, maybe only~ 75%.
  • There is some weird reflection on ACAV RFPD, I'm not sure if it comes from the window or not.
  746   Sat Dec 3 01:12:00 2011 taraNotesopticV-block for faraday isolator

     I replaced the isolator mount with the V block I drew. The height is a bit to high. I'll send it back to the machine shop to reduce the height.

 

  745   Fri Dec 2 20:03:50 2011 taraNotesFSSmodulation index for TTFSS

I got the wrong model on the EOM. It is actually model 4004, not 4003. Based on the specification, the modulation index is closer to what we expected (measured ~ 0.15, estimated ~0.09).

I checked the modulation index of the sideband.  It is unusual that we had measured the index of ~0.06.

      I checked the power going to EOM, to make sure that there was nothing wrong, and found out that the power dropped by ~8dBm when it passed through the RF summing box. I replaced the box with a spare one, and the power to the EOM remains almost the same, see figure below.

2011_12_02_setup.png

After replacing the box, I rechecked the modulation index again using two methods.

  •  Check the error signal, by scanning the laser and observe the error signal from mixer out. The pk-pk values of the carrier and the sideband's error signals are 16mV and 8mV respectively.

This give the ratio of Psiedband/Pcarrier to be 1/2 which means beta is ~ 1.

  •  Measure the transmitted power through the cavity between the carrier and the sideband. I measured both power by a power meter and V out from TRANS_PD. The ratio between Psideband/Pcarrier is still ~ 1.8 x10-3. This corresponds to beta ~ 0.1 (see the plot in the attachment). The result is higher than the previous result with the old summing box (0.06),  but still very small.

(The manual says that the modulation depth is more than 0.2 rad/Volt, see figure below, so if we have 160 mW coming in, with 50 ohm impedance => V = sqrt(RP) = sqrt(0.16*50) ~ 2.4V. So Beta should be ~ 0.2 * 2.4 =  0.48 rad. not 1.8 x10-3.)  This part is wrong, the modulation depth is  15 mrad/V see specification for 4004 below, the calculation is in the correction below. 

IMG_1933_copy.JPG

Snapshot_2011-12-05_20-00-53.png

I suspected that the RF summing box might have a problem, may be the impedance match is not 50 ohm. We have not really checked every components in the spare box. 

     Next step, I'll remove the RF summing box, and connect the signal from LO to the EOM directly, and lock the cavity with FAST feedback only. If this gives us higher modulation index, it proofs that RF summing box does not have 50 impedance output to EOM and should be fixed.

==Correction as of Mon Dec 05 17:39:04 2011==

    I connect the power from LO directly to the EOM, and lock the beam using FAST feedback only. The ratio between sideband power and carrier power (Ps/Pc) is = 19.5uW/ 3.45 mW ~ 0.006 which corresponds to beta ~ 0.15

 

    The EOM we use is New Focus 4004 which has modulation depth = 15 mrad/V. The impedance is 22 pF (measured), and specified as 20-30 pF in the data sheet. For 35.5 MHz signal, the impedance is abs(1/ 2 pi 22 pF * 35.5MHz) ~ 200 Ohm. 

    Then for 22  dBm power (170mW), assuming all the power is transferred to the EOM, V =sqrt(PR) = sqrt( 0.17 Watt * 200 Ohm) ~ 6V -> Beta = 6*15mrad = 90 mRad. This is ~ a factor of 2 smaller than the measured modulation depth (0.15).

   Then let's calculate the input power we need for Beta = 0.6.  The voltage across the EOM must be 0.6/15mrad per volt = 40V. power = V2/R = 402/200 = 8 W -> 39 dBm .

 So to sum up,

  1. the modulation index increases from 0.1 to ~ 0.15when I remove the RF summing box and lock the laser with FAST feedback only.
  2. the modulation index is close to what we expect from the datasheet. To increase more modulation depth, we need more power from the LO.
Attachment 2: Snapshot_2011-12-03_01-17-33.png
Snapshot_2011-12-03_01-17-33.png
Attachment 3: beta.png
beta.png
Attachment 4: Snapshot_2011-12-05_19-21-10.png
Snapshot_2011-12-05_19-21-10.png
  743   Thu Dec 1 19:35:04 2011 taraNotesEnvironmentSprinkler Installation

The sprinklers are installed in the lab, and I cleaned the lab afterward.

 

  742   Wed Nov 30 23:15:35 2011 taraDailyProgressNoiseBudgetISS

 ISS is on, I use RCAV_TRANS_PD to close the loop. Currently, we have only one way to actuate through the EAOM. We will use AOM for ISS soon, so that both intensity noise on each cavities can be controlled individually.

 ==setup==

I use SR560 as a gain control for feedback to the EAOM, which is installed before the PMC.

The setup on SR560 is AC coupling, DC pass, with gain of 100.

==Result==

ISS.png

fig1: RIN from RCAV is reduced by ~ a factor of 8 once the loop is closed (cyan -> blue) . However, there is not much improvement on ACAV's RIN, as the coherence between the two cavities are quite small.

 ==Comments==

The Noise suppression level is as expected. The plot below show the transfer function of the plant times the control gain in comparison with  RCAV_RIN(open loop) divided by RCAV_RIN(close loop). The pole around 33kHz in the plant TF is from the cavity pole.

TF_ISS.png

fig2: Comparison between the expected OLG TF of the whole system( measured TF from plant times gain setup on SR650) and the result from the measurement (free running noise divided by suppressed noise). They agree well.

  741   Wed Nov 30 19:00:42 2011 taraNotesNoiseBudgetvertical seismic coupling to beat noise

  From 40m page, COMSOL FEA gives us the coupling (from vertical seismic to frequency noise ) to be 53 [kHz / (m/s2)].

Seismic_resp.png

Fig1: TF measurement( from PSL:735). This time the estimation from FEA is plotted as well.

    ==comments==

  •      From ACAV/seismic (Green), the measurement is ~ 1order of magnitude higher than the FEA result. This might come from the fact that the support is not exactly at the right place
  •      From RCAV/seismic (red), the TF tells us nothing. The coherence is close to zero, and the SNR is very low (~10-6 - 10-5). This requires 1010 average to bring SNR to 1. Below is the plot for Signal (frequency noise due to seismic) and noise (NPRO free running noise. And SNR (Signal/Noise)

SNR.png

     fig 2: Signal and Noise in TF measurement between RCAV noise and seismic.

        The NPRO free running noise is from psl:617. Seismic noise is measured by an accelerometer (PSL:716)and converted to frequency noise using coupling from FEA result (53kHz / (m/s^2))

  ==what's next==

  1.  Since the SNR for RCAV noise/seismic is very small, we might have to shake the table and remeasure it.
  2.  Use a better seismometer. The current accelerometer might not be sensitive enough.
  3. Check the coupling from FEA, check what is the coupling as a function of supporting position.
Attachment 2: Seismic_resp.fig
  740   Tue Nov 29 18:40:09 2011 taraDailyProgressNoiseBudgetISS

 I measured RIN from RCAV_TRANS_PD, ACAV_TRANS_PD, and PMC_TRANS_PD, and coherence between the three signals. It looks like that RIN from ACAV and RCAV are mostly from pointing (from DC upto a few hundred hertz), rather than intensity fluctuation.

 

==details&setup==

There are 5PDs in total, 2 for ACAV_TRANS, 2 for RCAV_TRANS (see PSL:738), and 1 for PMC.  The gain on ACAV and RCAV PDs are set to 20dB so that the outputs were not saturated. Dark noise from every PD(not shown) is approximately 50 nV/rtHz flat, which is well below the signal at all frequency.

The plot below shows:

  • RIN from RCAV_TRANS (behind RCAV) (BLUE) as measured from RCAV_PD1 and RCAV_PD2. They are similar (as it should be).
  • Likewise, RIN from ACAV_TRANS (RED)from ACAV_PD1 and ACAV_PD2 are the same. This is just to check that the PDs work fine.
  • RIN behind PMC in GREEN
  • Coherence between RCAV_TRANS_PD and ACAV_TRANS_PD (magenta)
  • Coherence between ACAV_TRANS_PD and PMC_TRANS_PD  (Yellow)
  • Coherence between RCAV_TRANS_PD and PMC_TRANS_PD  (Cyan)

 RIN.png

==comments==

      From the shape of the measured RIN, Frank suggested that we might be suffering from pointing effect. Since we pick the beam after the cavities  any pointing instability will cause the transmitted power to fluctuate, and this will show up on RIN measurement.

      Coherence between PMC and RCAV (Cyan) at ~ 200Hz and above is almost 1, this should be caused by real intensity noise. However at low frequency the coherence is close to zero. Any power fluctuation observed at the transmitted beam from RCAV is not really caused by the fluctuation at PMC. Pointing effect can be one explanation.

       I have to mention that the AOM on RCAV path is not being used. So the beam just travel through the AOM. But we still use AOM on ACAV path to frequency shift the beam. The first order beam which is frequency shift can be suffered from pointing effect if the alignment of the reflected beam does not trace back the incoming beam. Pointing from AOM might explain the lower coherence between ACAV and PMC (Yellow).

==To do next==

    To confirm that it is mostly pointing effect, we'll use the pickup beams for RFAM for RIN measurement. Since these beams are picked up before entering the cavities, the intensity noise due to pointing effect should be reduced. We expect to see better coherence between PMC and RIN infront of the cavities.   Unless we fix the pointing, ISS won't improve any RIN measured from TRANS_PD.

Attachment 2: RIN.fig
  739   Tue Nov 29 01:50:40 2011 taraDailyProgressopticoptics for RFAM

I made some minor adjustment to the optics layout so that the reflected beam at the PBS before the cavity can be used to measure RFAM. Now RCAV's beam can be picked up for RFAM measurement.

    The PBS just before RCAV was moved Eastward a bit so that the reflected beams from both PBSs are not blocked. I removed mirrors with soft mounts and use only rigid 1" posts only. 

    I used a spare 35.5MHz RFPD for the pickup beam from RCAV path (in red). The power cable for RFPD was made and checked. It works properly. There is a spare new focus 1811 RFPD, but the connector is broken, the pins are bent. I'll try to fix this and use it for ACAV's RFAM pickup.

RFPD_setup.jpg

     The AC signal from RFPD will be demodulated with 35.5 MHz signal which is split from the LO signal for ACAV PDH's lock. I have not adjusted the phase by trying different cable lengths yet. This will be done later.

     There is one thing I'm a bit concerned with. The RF signal from the RFPD has DC level ~ 120 mV, I'm not sure if it's unusual or not. I'll check with another RFPD.


  738   Tue Nov 29 00:40:01 2011 taraDailyProgressNoiseBudgetISS

Photodiodes for ISS are all setup and are ready to be used.

 ISS_setup.jpg

All four PDs are Thorlabs PDA10CS InGaAs amplified Detectors. Since InGaAs is not very sensitive to room light, I do not put room light filters over the PDs.

For each beam path (RCAV/ACAV) two PDs are required, one for feedback signal, one for out of loop measurement. The one for feedback signal can't be used for taking the noise level because it is in loop and the signal is suppressed by the control gain, thus does not reflect the real intensity noise of the laser.

Next step: verify the noise level of the RFPD, measure RIN from the PDs.

  737   Wed Nov 23 01:22:48 2011 taraDailyProgressNoiseBudgetISS

The previous entry was wrong. I did not use another PD to pick up the suppressed noise. So I fixed the setup and add another PD for picking up the signal.

 

 ==designing the control for the loop==

photo(6).JPG

The above diagram is a block diagram for a simple ISS. We can measure dP (free running noise). The suppressed noise (dPs) is picked up by another PD when the loop is closed. To design the TF of the controller, we have to know what are the TF of each compnents, so the whole TF can be decided.

I measured the TF of actuator and PD ([A] and [D]) in the diagram. The setup was hooked up similar to the drawing, but SR560 was set to AC coupling, A-B, flat response for injecting the excitation. The PD's gain can be adjusted, the measurement below was taken when the gain was 20dB.

TF.png

The gain on the PD can still be adjusted, I have to check this again.

 

The next step is to decide how much gain do we want. We know what is the RIN (measured from last entry) for the current setup, but we have to estimate how much it couples into frequency noise, then the gain level can be decide.

  736   Mon Nov 21 21:38:36 2011 taraDailyProgressNoiseBudgetISS

 ISS was re-installed to the setup, with the EAOM as an actuator. I used SR560 as a temporary servo. A better servo card will be design and installed in the electronic crate later.

==motivation & background==

       Relative intensity noise (RIN) of the laser, and other mechanisms that cause power fluctuation, can couple into the frequency noise of the laser locked to a Fabry-Perot cavity via random absorption. Thus, we want to take precaution and reduce it by applying an intensity servo to the setup. Our current intensity actuator set is consisted of an EAOM followed by a polarizing beam splitter. An EAOM changes the polarization of the beam, then the power transmitted through the PBS is changed accordingly. 

==current setup==

I used a Thorlab PD to dectect the transmitted beam from RCAV and measured RIN. Then I used the same signal to feedback to SR560 and to the EAOM. The setting on SR560 is AC coupled with bandpass with cutoff frequency at 10Hz and 3kHz, gain 1k.

==result==

RIN.png

 

==comments & future plan==

I'll check what other groups have with the intensity noise and project it in our noise budget. For now the only group that report the effect of intensity noise I known is Alnis' group. I'll also add the result from our measurement as well.

Attachment 1: RIN.png
RIN.png
  735   Thu Nov 17 23:30:19 2011 taraNotesNoiseBudgetvertical seismic coupling to beat noise

TFs between 1)vertical seismic and beat, 2) v seismic and ACAV feedback, and 3) v_seismic and TTFSS feedback are measured. The contribution from vertical seismic to beat signal is mostly from ACAV.

 ==Setup==

I used frequency response measurement (FFT1/FFT2) to measure frequency response [dB], phase [dB], and coherence between.

  • vertical seismic and beat, -> D/A
  • Actuator point for ACAV loop -> C/A
  • Fast Actuator point, RCAV loop ->B/A  (I used a T connector to pull the signal from feedback to NPRO)

The calibration for beat signal[D] is 7kHz/V, for ACAV actuator point [C] is 7kHz/V, for RCAV fast actuator [B] is 3.07 MHz/V, for accelerometer [A] is 1023mV/g (psl:716)

SETUP_2011_11_17.png

==result==

 1)calibrated signal, in the unit of Frequency noise in beat/ acceleration  [Hz/ (m/s^2) ]

freq_resp.png

fig1: calibrated frequency response, with 250 vector avg.

==comments==

  The plots show that the seismic contribution in the beat is mostly coupled through ACAV. RCAV is not sensitive to seismic at all, since the coherence is almost zero. We have to open the chamber and fix the ACAV's support.

Attachment 3: freq_resp.fig
  734   Wed Nov 16 19:11:38 2011 taraNotesNoiseBudgetseimic coupling to beat noise

Seismic noise is currently a limiting source for our setup, here is a list for what we have to do for figuring out the coupling from seismic to beat. Once we know exactly what's going on we can fix it correctly.

  1. TF measurement between seismic and feedback to NPRO (RCAV)
  2. TF measurement between seismic and feedback to VCO    (ACAV)
  3. TF measurement between seismic and beat             (psl:)

[more details soon]

  733   Tue Nov 15 18:53:25 2011 taraNotesNoiseBudgetupdated number for everything

A list of useful numbers for noise budget calculation.

 

==Reference Cavity==

  • Cavity Finesse:  10^4    PSL:424
  • Free Spectral Range:  737 MHz <=> cavity length = 8" = 0.2 m
  • Cavity Pole = 38.7 kHz.
  • FWHM = 2xcavity Pole = FSR/Finesse = 73.7 kHz
  • Spotsize on mirror = 291 um

 

==PMC==  PSL:370

  •  Finesse: 188
  • FSR     : 714MHz
  • FWHM: 3.8MHz
  • P-polariztion

 

==35.5MHz RFPD==

  • RFPD noise level:

==NPRO==

  • Fast actuator calibration factor: 3.07 MHz/V psl:182
  • Slow actuator

==LIGO VCO==

  • Tuning range vs frequency psl:22

==Beat==

  • Marconi(IFR2023b) calibration factor = 70e3  Hz/V for 160MHz carrier/ 100kHz input range (tuning range)
  •  Marconi(IFR2023b) calibration factor = 7e3 Hz/V   for 160MHz carrier/  10kHz input range

==Seismic Stacks==pslL668

  • translation beamline: f0 = 6.96 Hz  Q = 21.53
  • Vertical motion          f0 =  Q= 
  • Horizontal transverse, f0 = 6.35Hz, Q = 25.85

==RCAV TTFSS==

==ACAV servo==

  732   Tue Nov 15 18:38:20 2011 taraNotesFSSmodulation index for TTFSS

 

By measuring the power of the transmitted carrier and sideband, we can compute the modulation index of the laser.

P_carrier = P0 x J0(Beta)^2, and

P_sideband = P0 x J1(Beta)^2 . (P0 is the input power, P0 ~ P_carrier + 2 P_sideband), J0,J1 are Bessel functions of the first kind.

By plotting P_sideband/P_carrier vs Beta ,we can see where our modulation index is.

==setup & measurement==

  1. The RF_adj = 9.5V
  2. Phase adj = 0V
  3. Use old RF summing box

 The Transmitted power of carrier, and sideband are 8.3 mW, and 5.7 uW respectively. This gives Ps/Pc ~ 1/1000, or Beta ~ 0.06. This is surprisingly low. I'll double check it again.

  731   Mon Nov 14 21:51:07 2011 taraNotesFSSFSS debugging, OLG TF measurement

I remeasured OLG TF of TTFSS, the bandwidth can be increased upto ~300 kHz for 12 mW power input. It seems that the servo gain is certainly will be the limiting source for the current setup. We will need a higher gain to suppress laser frequency noise down to Brownian coating thermal level.

 ==Motivation==

 

 ==setup==

    I locked the laser to refcav, measured noise at error point, and converted it to absolute frequency by the slope of error signal. Then I compared it with the beat signal. Then I checked the OLG TF of the TTFSS and its  UGF. I decided to check it again, since the beat signal now became better than before.

==result==

 1) beat and noise at error point

beat.png

 

fig1: Beat, with comparison from noise @ error point. Note that harmonic lines appear again. I think because I use external power supply on the TTFSS. I'll use the power supply from the electronic shelf later.

 

The bump around 1kHz can be reduced further by increasing more gain, but not much. However, it was almost unstable.

    2) OLG TF: Details about how to measure TTFSS OLG TF can be found here(PSL:)

OLGTF.png

fig2: OLG TF of TTFSS, at three different input power, the plots from lower input power (1.6 mW and 6.6 mW) are offset to compare with the highest input power (12mW). The UGFs for 1.6mW, 6.6mW, and 12mW input power are ~1kHz, 2kHz, and 3kHz respectively.

From previous measurements  ( psl:594, psl:654), the UGF was quite low (less than 200kHz), and the TF looks terribly noisy.  Now with much better TF, I'll use this result and compare it with LISO model.

==comment==

The UGF of 300 kHz is quite high compared to what we had before (I have not yet measured the actual value of UGF for power input of 12 mW, I just estimated by eye. The real value might be a bit less.) However, from the current setup, it is might be too hard to increase more power to RCAV, because ACAV will not have enough power.  An RFPD with higher gain might be needed, since we will need at least a gain 2 order of magnitude more than the current one so that the suppressed noise can be down to coating noise at 1kHz.

Attachment 2: beat.fig
Attachment 4: OLGTF.fig
  730   Thu Nov 10 16:37:19 2011 taraDailyProgressNoiseBudgetRFPD noise in PLL

 I measured noise level from PD for beat measurement. It will not be the limiting source at frequency below 1kHz.

 ==back ground and motivation==

see psl:728

 ==setup==

 (i) dark noise :from the diagram in psl:728, I just disconnected the feedback signal from actuator to Marconi, blocked the beam and measured from Vfeedback to actuator. The signal is similar to the signal from Verror (after the mixer) multiplied by gain from SR560.

(ii) shot noise: A white light source was setup on the PD such that the DC level from the PD was similar to what the regular beam would produce (0.47V). Then repeat the measurement as in (i)

==result==

beat_2011_11_10.png

     The signal at Vfeedback was converted to frequency noise by dividing by SR560 TF, and frequency discriminator TF (gain 20, Low pass @ 10kHz and 0.45/f V/Hz respectively) so that the Voltage noise from RFPD is converted to absolute frequency noise.

      The frequency noise and shot noise from PD go up with frequency and crosses the estimated coating noise around 1kHz. The current PD should be sufficient for the setup for  measuring coating thermal noise at frequency below 1kHz.

 

  729   Thu Nov 10 00:05:45 2011 taraNotesSeismicPressure in N2 bottle

 

 One of the leg has a leak a gain. It's the one on SouthWest corner of the table. When I tried to floated the table, I can hear the air leaking from the leg. That leg is a slave one, which has no pressure control, so I'm not sure if this happened because I tried to adjust the pressure on NorthWest leg to balance the table or not. The table on NW leg was too high and the leg height was pushed up against the pin and not "floated".

  728   Wed Nov 9 20:31:40 2011 taraDailyProgressNoiseBudgetRFPD noise in PLL

I measured PLL's open loop gain(OLG) transfer function. This will be used to determined the noise contribution from beat RFPD.  For the current setup, PLL has UGF ~20kHz.

==Motivation==

I have not really checked how beat PD's noise will show up in the beat signal. I want to make sure that it will not be the limiting source. After discussing with Koji, I did some calculation and planned how to measure the noise level.

IMG_2188.JPG

 fig1: some calculation for PLL's block diagram. [H] -> frequency discriminator, [F]-> Servo (SR650), [A]->Actuator(Marconi)

 

==setup== [add fig]

PLL_OLG.png

 During the measurement, I reduced the gain of SR560 to 5, so the signal at low frequency was not suppressed too much. And the UGF was reduced to ~ 9kHz. (For regular setup, Input range = 10kHz, SR560 gain = 5e2, LowPass at 10kHz).

During close loop operation, I measured:

  1. Open Loop Gain of the TF [H]x[A]x[F] as shown in the setup, and
  2. Actuator x Frequency Discriminator[H]x[A], the response to SR785 was taken after the mixer (and the low pass filter)

 

==results==

TF.png

The OLG TF can be fitted with a pole at zero, and a pole at 10kHz( from SR560).

The pole at zero comes from the combination of Marconi and Frequency discriminator [H]x[A] (green plot). I think Marconi's TF [A] has flat response, at least up to a few kHz, since all the calibration we have used so far is flat, and it matched up with LO noise up to a few kHz. For examples, psl:506, psl:459. 

(I'm not sure if Marconi really has flat response or not, but let's assume it's true for now, and the pole comes from the frequency discriminator [H] only. I think [H] will be flat response if we convert it to V/rad by multiplying by f)

Green plot is [H]x[A] which is 20*log10 (3.16e3/f) or the actual transfer function is ->[H]x[A] = 3.16e3/f [V/V]

Since [A] = 7e3 [Hz/V] and flat, [H] is then-> 3.16e3/f / 7e3 = 0.45/f [V/Hz]

Once we get [H] we use it to calculate frequency noise contribution from Verr or Vfb when the feedback to Actuator is not connected (loop is not closed)(cf fig 1)

Attachment 3: TF.fig
  727   Wed Nov 9 02:10:58 2011 taraNotesFSSFSS debugging, harmonic line problem fixed

 From PSL:725, it seems that beat signal at high frequency is limited by  FSS servo gain. So I took a quick check by switching from old FSS to TTFSS. There is some improvement at high frequency. I'll try to float the table to see if I can reduce the bumps at frequency above 100 Hz or not.

beat_2011_11_08.png

 Note:

  • The LO phase noise from PLL (10kHz input range)does not match the red plot (TTFSS, 10kHz input range) I'll have to check the noise budget again.
  • When I used 1kHz input range (green plot), I used calibration = 0.7 kHz/V. This was not measured, I just scaled it from other measured calibrations form other input ranges ( 100kHz -> 70kHz/V, 10kHz -> 7kHz/V). The result seems ok, the features between blue and green curves line up nicely.
  726   Tue Nov 8 18:51:38 2011 taraDailyProgressopticoptics for RFAM

I added mirrors to pick up stray beams just before the cavities. These beams will be used for monitoring RFAM.

    I arranged the optics so that stray beams at the beam splitters (just in front of the cavities) could be used. The power of the beam is ~ 9 uW, but it can be increased by changing the polarization of the input beam later.

     Two photodiodes are needed, I haven't checked yet if I still have some spare PDs left.

     Then the signal from PD will be demodulated with 35.5 MHz signal (modulation frequency). The cable length + PD position will be adjusted so that the phase is the same as the PDH signal.

IMG_2186_ACAV.jpg

IMG_2187_RCAV.jpg

 

  725   Tue Nov 8 03:15:10 2011 taraDailyProgressFSSFSS debugging, harmonic line problem fixed

 

 

From psl:722, the beat noise at high frequency was getting a bit worse from before. I brought it back to previous level. It turned out that misalignment at AOM, was the problem.

    I had to re align the beam in ACAV path a lot, some mirrors might be touched when I adjusted the cables around that area. After that the beat level came back close to what it was.

 

beat_2011_11_07.png

Fig1: Beat, compared to error point from RCAV and ACAV loop.

==comments==

  • Noise from RCAV's error point (at high f) is very close to the current beat level and LO phase noise, but it seems that seismic noise also present around high frequency. We may have to identify what causes those peaks around 200 Hz and above first.
  • Noise from ACAV's error point is just slightly above the noise floor of SR785(not shown), so this might give only the upper limit of ACAV's error noise.
Attachment 2: beat_2011_11_07.fig
  724   Sun Nov 6 22:54:57 2011 taraNotesNoiseBudgetRFAM/ ISS

A list of things I plan + have to do this week

 1) RFAM 

  • Pick up a stray beam somewhere for RFAM measurement when the cavity is locked. Demodulate the signal to get the time series of RFAM level.
  • Add RFAM to noise budge.
  • Design a case to cover the EOM for temperature control.(Take a few pictures of the EOM)

2) ISS

  • Add intensity servo. Use AOM to actuate on the power of both beams to ACAV and RCAV (might start with the exist EAOM first).
  • Use a simple servo system for a start, the final design will be a servo card installed on the electronic shelf.
  • Add the effect of RIN induced noise to the noise budget
  723   Thu Nov 3 20:36:42 2011 taraNotesSeismicPressure in N2 bottle

I checked the pressure of N2 bottle used for floating the table. I floated the table again today for measuring beat. The pressure of the bottle does not decrease much. It is still ~ 2000 psi. I closed the valve on the bottle after I finished the measurement.

  722   Thu Nov 3 19:51:07 2011 taraDailyProgressFSSFSS debugging, harmonic line problem fixed

  I identified that harmonic lines in ACAV loop come from UPDH box. When I used SR560 to lock ACAV loop, there was no harmonic. Now the lines in beat signal are suppressed considerably. 

      From previous entry psl:720: I eliminated the harmonic lines in RCAV loop (FSS). I tried to bring the harmonic lines back in ofrder to see what caused them, but I could not. So I'll just have to remember to check the RF phase adjustment if the lines come back again. 

     After the lines in RCAV loop were gone, the beat signal still contained harmonic lines, so I check ACAV loop. The procedure is similar to what I've done for checking FSS loop. The lines were gone when I replaced the UPDH servo with SR560. Then I measured the beat from that setup.

=result=

 beat_2011_11_03.pngzoom.png

Left: Current beat signal (in blue) as ACAV loop was locked by SR560 instead of UPDH. Right: Close up at high frequency, line harmonics are greatly suppressed.

 

==setup==

[add fig

==comments==

The beat signal becomes slightly worse at high frequency. This might be a result from:

  • Not enough gain in ACAV loop (since I changed the servo), or
  • Scattering noise/back reflection. ( The visibility of ACAV reduced to~50% from 90%)

    I think  because I plugged and unplugged the cable connected to the EOM all the time, the EOM might be mis-aligned, I might also touch the mirror for double passing AOM on ACAV path as well. Right now the visibilty for ACAV become ~ 50% (from 93%), the visibility for RCAV decreased as well, but I brought it back to ~95% again.  I could not bring the visibility for ACAV back to the same level. I have to check what's wrong with the setup. This misalignment might be the reason for worse beat signal.

Attachment 3: beat_2011_11_03.fig
  720   Wed Nov 2 20:05:15 2011 taraDailyProgressFSSFSS debugging, harmonic line problem fixed

 I got rid off the harmonic lines in FSS loop.  I'm still trying to understand what actually caused the harmonic lines.

 errorpoint.png

 Fig1: PSD from error point of FSS loop. Blue, with harminic lines, Green, when it is fixed.

 

==Details==

       I started the setup similar to the setup in the latest test. psl:711 . However, I used the spare RFsumming box ( the pomona box in the picture)

IMG_2177.JPG

Fig2: RF summing boxes on the table, the spare one is sitting on the stage above the table for easy access.

Then I continued to investigate by adding the following components to the loop:

  1. cable for EOM feedback, from the electronic rack to RF summing box
  2. 35.5MHz oscillator card from the rack instead of a Marconi to drive EOM and mix signal for the mixer.
  3. Original cables in the setup, (a) from RFPD to mixer, (b)from oscillator to EOM,
  4. FSS without then with PC feedback

     After all the above tests I still found no harmonic lines, so I switched back to the original RF summing box, and found all the lines.  I though it might be coupling from the cables close to the box, so I move the summing box up where I placed the spare box, but the harmonics still presented.

    

      Then I adjusted the phase for the LO signal to the mixer so that the error signal was optimized. After I optimized it, the harmonics line were gone. I think the phase adjustment is what I have to look into. I'll try to understand that more later.

     Next step will be trying to lock ACAV with SR560, make sure there is no harmonic line, then check beat measurement to see any improvement at high frequency.

  719   Tue Nov 1 16:11:48 2011 taraNotesopticV-block for faraday isolator

I made a drawing for faraday isolator's base. I'll submit the drawing tomorrow.

Attachment 1: isolator_vblock.PDF
isolator_vblock.PDF
  718   Fri Oct 28 14:04:22 2011 frank, taraNotesNoiseBudgetNoise budget for unfloated table

 We think it might be useful to see the seismic contribution to the beat signal when the table is unfloated (so that the seismic contribution can show up clearly). Here is the beat and noise budget when the table is not floated.

 beat_27.png

For vertical seismic noise, I used measured seismic noise on the unfloated table, converted to acceleration noise, then applied the calculated TF (psl:697)of the stack and the result from comsol (acceleration noise to length noise). That is

[Measure Seismic converted to Acceleration] x [TF of the stack] x [COMSOL]  and converted to absolute frequency noise using df/f = dL/L.

[m/s^2] x [x/x] x [ m/(m/s^2)] x [Hz/m],

Note: This is the contribution from a single cavity, no common mode rejection is taken into account.

 

 

For East West seismic (beamline), I obtained that by using the measured TF (psl:716)between beat and signal from accelerometer, applied calibration for acceleration, multiplied by measure seismic on the unfloated table (converted to acceleration) x calibration for beat.

[Measured TF beat/acceleration] x [seismic converted to acceleration] x [calibration for acceleration] x [calibration for beat (7kHz/V)]

[V/V] x [ m/s^2] x [V/(m/s^2)] x [Hz/V]

Note: The mismatch between E-W seismic and the measured beat might come from the measured TF. We drove the table with the PZT and this may cause some non-linear response at the resonant frequency which shows up in the plot.

Attachment 2: beat_27.fig
  717   Thu Oct 27 20:45:58 2011 frank, taraNotesSeismicring down measurement for rubber spring

We did ring down measurement of the rubber cone used for supporting the seismic isolation stacks. The resonant frequency and Q of the spring will be used for better TF of the stack later. The measured values are f =46.7 Hz, Q = 14.67. (mass will be added later)

[details about the measurement will be added later]

These are the results from the data Frank gave me.

There is a problem in the result. The frequency of the ring down response can vary from 42 Hz to 47 Hz. So when I fit it I cannot really get the "best" frequency fit from the whole train of data.

From figure (1)

  • At point A) The data and fit seem to be well matched.
  • At B) The phase of the fit curve advances the measurement ( the fit frequency is too large)
  • At C) The phase of the fit curve lags the measurement (The fit frequency is too small)
  • At D) The frequency totally changes causing the fit and the measurement to be almost out of phase

 fit4.png

fig1: showing fit with the data. Non-linearity behavior of the spring can be seen clearly.

 

There are 5 sets of measurement. I notice that the nonlinear behavior was very small in the first data set which has its initial amplitude smaller than 0.4 V

fit1.png

fig2 Fit from data TEK00000. The initial amplitude is less than 0.4 V.

 

For other data sets, where their initial amplitude exceed 0.4, it does not matter where I analyze the data, fitted frequency still varies a lot. I take the data set #4, and  fit the data at high amplitude and low amplitude (when it is less than .15V). The frequency still varies. See fig3 below.

comparefit.png

fig3: fit from data TEK00003. Even when I use data at smaller amplitude, frequency still varies from the fit, similar to what happens in fig(1).

 

The problem is, the fitted frequency from fig(2) is ~ 46 Hz, while the fitted frequency (at low amplitude) from fig(3) is ~ 43 Hz. The difference is too large. So we decided to sample the frequency by averaging the frequency over 3 adjacent cycles of the ring down data, ignoring the first few peaks. Then histogram the samples.

hist.png

Most of the counts are around 46.5 Hz, which is contributed mostly from the first data set where the variance is very small. I think it is ok to pick the value from that data set

f =46.7Hz, Q = 14.67.

  716   Wed Oct 26 18:05:49 2011 frank, taraNotesSeismicModeling of the vertical isolation of the stack

Today we floated the table, measured beat, measured TF between beat and horizontal, and added horizontal seismic to the noise budget.

  • Replaced 2 legs, floated table: monitor pressure, 2000psi @ Wed Oct 26 18:06:41 2011
  • Measure TF between beat and accelerometer signal
  • measure beat signal, when table is floated, optimized optics.
  • add Horizontal motion to noise budget
  • ring down measurements from rubber spring, see psl:717

details coming soon.

 

IMG_2170.JPG

fig1: accelerometer was mounted on the table to measure East-West motion (along beam line)

IMG_2171.JPG

fig2: details of the accelerometer

TF.png

fig3: TF between Beat signal and signal from acceleromter (East-West motion). The data is not calibrated yet. Sensitivity for beat = 7 kHz/V.

beat.png

fig4: Current beat noise: floated table. Seismic from horizontal motion is calculated by applying the measured TF to the measured acceleration on the floated table.

beat2.png

fig5: Current beat noise: floated table. Seismic from horizontal motion is calculated by applying the measured TF to the measured seismic noise (converted to acceleration) on the floated table.

Attachment 4: beat.png
beat.png
  715   Tue Oct 25 23:21:55 2011 Frank, TaraDailyProgressBEATTF from hor. acceleration to beat signal

as we still don't have a good model for the horizontal isolation of the stack we will try to measure the TF from horiz. accel. of the table to the beat using the PZT actuator between steel frame and table. With the table resting on the legs we only get a nice TF around the resonance of the stack at ~6.5Hz, the rest is too noisy. We will fix the broken legs to float the table tomorrow and then try again.

  714   Tue Oct 25 23:13:57 2011 Tara, FrankDailyProgressSeismicre-measured the spring constant for the RTV springs

as we only did a quick-and-dirty measurement of the spring constant last time we re-measured it a couple of times. The resonance frequency varies by a few Hz from measurement to measurement and it also depends on over how many periods (and where, beginning or more to the end) of the ring down we measure. So we will analyze some ring down measurements using the same setup (Al-block with accelerometer resting on one RTV spring sitting on the optical table) and take the average for a new calculation.

  713   Tue Oct 25 22:03:32 2011 frank, taraNotesSeismicModeling of the vertical isolation of the stack

From Koji's code. I added vertical seismic noise from a single cavity, no common mode rejection. There are some modification from the code. The spring constant is 45 e3. M1 = 12.3, m2 =5.44.

The seismic noise in this projection is the data taken last week, see psl:696.

 nb.png

The coupling between acceleration noise to length noise is1e-9 * L  [m/(m/s^2)]. It is different from the value from FEA because our supporting positions are not exactly the same as the optimum point. It might be off by a few mm. The peak around 6.7 Hz is from Horizontal motion which has not been incorporated in the plot yet.

The noise budget code and auxillary files & functions will be committed on svn.

 

  712   Mon Oct 24 21:20:26 2011 frank, taraDailyProgressFSSrf summing box

 We check modulation depth of the PDH signal to see if we have enough modulation depth or not. The result will be analyzed later.

==motivation==

Based on the design of the RF summing box we got from Peter ,

IMG_2176.JPG

the TF from 35.5 MHz input to EOM does not have a resonant frequency at 35.5 MHz. Rather, the resonance is ~ 10MHz. We are not sure if we need to have the resonance at 35.5 Mhz or not. So we decide to check if the modulation index is already large enough for the current design, and compare it with that of the modified one. If the modulation depth is already large enough, we can keep it as it is. Basically we just need a high pass for 35.5MHz input to EOM.

By changing the inductance L from 1uH to 80 nH, the peak can be moved to ~ 35.5 MHz.

==Setup & Measurement==

 We want to check the slope of the error signal when the RF power changes, so there's a small change in the setup, see fig1.

setup.png

The input power is 2mW.

Then the slope of the error signal was record, the pk-pk of the sideband and carriers were also record between the input power of 10dBm and 13dBm to the EOM.

Then the RF summing box was modified (changing L), and the measurement was repeated.

==results==

coil       |  input pwr (dBm) |  Carrier slope ( MHz/V) |   pk-pk value (mV) (carrier/side band)          

 80nH   |     10                   |     1.43                          ||   28/18

            |             13           |      1.11                          ||  38.8/22.6

1 uH    |       10                 |      0.91                          ||   49.2/27.6

            |             13          |       0.74                         ||    68.4/37.2

 

From the result, the best setup is 1uH coil with maximum side band power which provides the best slope ( maximum gain at frequency discriminator). Note that 1uH coil is the original value for the schematic, so modification to 80nH does not help.

-> Modulation depth

  711   Fri Oct 21 01:36:52 2011 frank, taraDailyProgressFSSFSS debugging/ checking

We locked RCAV with a very simple FSS servo to debug what components in the FSS are not working correctly. For now, we found out that RF summing box does not work as expected and reduce the detection gain.

 

==Motivation==

We decided to check the RCAV fss loop seriously in order to understand why the performance is not as expected. So we used a very simple setup an SR560, a marconi, with FAST feedback only to lock the laser and measured the in loop noise of the laser. Then add more components in the setup and see how it changes the performance.

 ==setup=

            set1.png

                 set 1: The laser is locked with SR560, VCO: Marconi 2023A.  All cables used were extra cables not from the original setup. The in loop noise was measured with gain setup = 1e3 and 2e3. 2e3 gain level was barely stable, but we added it for comparison.

We checked the slope of error signal, and optimized it by changing the length of cable A and B, to change the phase between incoming signals to the mixer. Note that the signal sent to EOM were not 50 Ohm terminated. It turned out that with 50 Ohm, we got smaller gain.  The calibration for this setup at error point is 1.2 MHz/V. [show result]

               -set 2: RF summing box is added to the setup

 

                 setup2.png

comment:

  1. The setup for SR560 was changed.
  2. The gain (slope of error signal) was reduced, once we added the RF summing box to the system. The slope was ... ,
  3. I could increase more gain (up to 5000 without losing lock)which further reduce the in loop noise at low frequency, but a peak at high frequency ( ~70kHz) appear. At this point the time series of in loop noise was oscillating. And 180 Hz harmonic also showed up. This is a good example of non linearity in the system.
  4. The length of A and B cables were adjusted again to obtain the best error signal. 

 ==Results==

inloop_volt.png

Plot1: uncalibrated in loop noise from the above setups. Notice the absence of higher harmonics line when the system was not oscillate (red, blue and cyan). I did not save the data up to 100kHz to see the peak over there since our bandwidth of interest is only around 1 Hz - 1kHz.

calibrate_inloopnoise.png

Plot2: calibrated signal to absolute frequency. I omitted the result from Seup2 gain 5000 (magenta), because the system oscillates. Results from other setups are kept in the same color. As we can see, adding the RF summing box setup 2(red) causes the overall frequency noise to be higher.

It's true that the gain is different by a factor of 2, which might be the reason for lower gain. But we have differnt slope from error signal, and the system oscillates at higher gain, so the RF summing box definitely changes the loop performance.

 

==RF summing box==

   Our goal is to fix the RF summing box so that the gain is not reduced and the in loop noise level is maintained. This is the schematic of RF summing box we got from Peter King.

[add fig]

We also checked the spare one we have in the lab.  [add fig]. Components on 35.5 Mhz input path are similar to the schematic. We have not checked the tank circuit on the feed back path yet, but that's not necessary for the current test.

Attachment 4: inloop_volt.fig
Attachment 6: calibrate_inloopnoise.fig
  710   Thu Oct 20 11:43:36 2011 taraNotesNoiseBudgetcalculation from kenji's paper added to the nb

 Result from Kenji's paper (in brown) is added to the noise budget.

 

Snapshot_2011-10-20_11-41-49.png

[Numata etal., PRL 93, 250602 (2004)]

nb_2011_10_20.png

Attachment 3: nb_2011_10_20.fig
  709   Wed Oct 19 19:32:20 2011 Tara, FrankDailyProgressSeismicleg of optical table replaced - table now floating- beat

 Today we measured frequency response between beat signal and seismic noise, and coherence when the table was not floated. The data are calibrated based on the information from 40m:5196 (with gain 1 for out setup).(The plot is not calibrated yet)

 ==Motivation==

     We wanted to learn more about how seismic couples to beat measurement for the current setup. So measuring the frequency response between beat signal and seismic should tell us about the TF between the table to the cavities. For example, we can put a noise budget due to seismic when the table is floated by measuring the seismic on the floated table, then multiply by the TF we measure, plot it on the noise budget.

 

==setup & calibration==

    Details about the seismometer we used can be found here psl:696.  The table was not floated. The direction and positions of the seismometer (pink circles) are shown below in the map. All the results in this entry come from position 1 only. We checked that the signals of seismometer did not change much among each position.

Signal from seismometer was sent to SR785 chA. Signal from beat was sent to chB. We used Frequency response measurement (B/A) with vector average. The TF between beat and seismic noise from 3 directions were recorded along with coherence between them.

The beat signal has calibration = 70 kHz/Volt. Seismometer has calibration of 800 [Volt/velocity] or [Volt *sec]/ [meter].

The result (recorded in dB [V/V]) can be converted to [Hz / (m/s) ] by 10 dB/20 x 70 k [Hz/Volt] x800 [Volt / (m/s)]

CTN_map.png

 

==Results==

 TF_beat_seismic.png

The top panel shows the magnitude of the transfer function. Phase is shown in the middle panel. The bottom panel presents the coherence between seismic and beat for each direction. We are quite surprised that there is no strong peak around 6-7 Hz which is the resonant peak of the stack motion. We certainly see the peak around 16Hz which probably comes from bounce mode of the stack.

Attachment 2: TF_beat_seismic.fig
  708   Mon Oct 17 20:26:57 2011 taraDailyProgressFSSGround Loop, RFPD

Today I verified that fss is not gain limited. I also tried to fix the ground loop problem in fss by replacing the power supply, but it did not work.

 

 ==FSS gain limit==

     From psl:706 I mentioned that I did not check the in loop noise level vs gain. So I checked it again.The in-loop noise (MIXER OUT signal when the RCAV is locked) does not change at all when the gain reaches 25 or more. There's a change as the gain was increased from 20 to 25.  This means the noise is probably from the detection point. Modifying/changing RFPD will be the next thing on the list.

 ==Ground Loop==

   From psl:700, I noted that the ground loop might come from the power supply which had ground output same to protective ground. I found a power supply which has float ground and replaced it with the current one. Then I measured MIXER OUT(in loop noise) when the cavity was locked. 

 The results:

  1.  In loop noise look similar from either power supplies I use.  Odd harmonics (3f, 5f, ...) are still present.
  2. There are no even harmonics(2f,4f,...) in rcav loop. I found it to be from acav loop.

 

==beat PD noise level==

 I just realized that I have not checked and thoroughly understood the noise level of the beat PD and how it shows up in the beat noise. This will be the next thing to do as well.

  707   Mon Oct 17 12:01:26 2011 taraNotesSeismicN2 bottle ordered

I called central warehouse to order another N2 bottle.  It should be here this afternoon. Then we can find which leg has a leak.

  706   Fri Oct 14 14:40:51 2011 Tara, FrankDailyProgressSeismicleakage in the legs, RFPD noise test

I think another leg, or both of the old legs have leakage. The nitrogen tank is now empty.  So the latest beat signal (in blue) here was measured when the table was not floated, but the optics are optimized.

I also tried to lock the cavity with both servos (old fss and TTFSS), The beat signals look similar (the result was not saved, only through inspection by eyes), so either noise from detection point (RCAV RFPD) or noise from ACAV loop might be the limiting source at high frequency.

 

 == beat signal after optic optimization==

 beat_2011_10_13.png

           Fig1: Current beat signal (blue), the table was not floated.

 

== RFPD==

As a quick test, we checked the RFPD, by measuring the in loop noise at error point (MIXER OUT) with the current RCAV's RFPD. Then compare the results with the RFPD Raphael modified (see here). Although we don't know the performance of the current RFPD, we expected them to be somewhat different.

I used old FSS for this test, input power is 2mW, Common gain = 30 (max), Fast gain =15.6.  Higher Fast gain causes the system to be unstable.

 

errnoise_volt.png

Result 1: The noise level at error point: The noise level is ~ 100 nV/rtHz. at high frequency. Not particularly good. Both RFPDs give very same noise level.

 errnoise.png

Result2: Noise at error point calibrated to absolute frequency noise, compared with beat signal.

As both RFPDs give the same noise level at the error point, this should mean that the gain of both RFPDs are the same, and the calibration factor to absolute frequency will be similar in both cases. I measured the error signal slopes to be 3.6 e5 [Hz/V] and 2.9 e5 [Hz/V] for current and modified RFPD respectively. I decided to use measured calibrations from each case in the plot so that the level of noise due to possible error bar from the measurement can be seen. The level of error point noise is quite close to the beat level (a factor of ~1.4 - 1.6)

This means that we might be limited by RFPD noise at high frequency.  It might be something else. More investigation will be done. (I did not see if the noise level change with the gain or not at the current setup, I just cranked the gain up to what ever it was before it started to oscillate. If the noise still goes up/down with the gain, it's not RFPD noise)

 

Attachment 1: beat_2011_10_13.fig
Attachment 5: errnoise.fig
  705   Thu Oct 13 02:36:58 2011 Tara, FrankSummarySeismicsecond leg replaced

After optimized everything, I repeated the measurement that was done in this entry (noise at detection point). There is some improvement, the result is shown below.

 

==What have been done==

By "optimize everything", I meant:

  1. The Faraday Isolator was installed back in the setup, and optimized for maximum isolation. [add pic]
  2. Beam splitter and quarter wave plate sets (for double passed AOM and cavities) were optimized for minimum back reflection.
  3. EOM with half wave plate were adjusted for minimum RFAM (reduced by ~ 20 dB)
  4. Beam alignment to the cavities: visibility are up to 93% for both cavities now.

 Measurement Recap:  We want to check the noise from scattering noise or RFAM at the detection point, so we measure the noise at error point when the beam reflected off the cavity, or a mirror in front of the chamber (cf entry:700). We also want to see the dependent on power input, so we chose 10mW, 5mW and 1mW input power.

    Below are measurements from error point (Mixer out) which are calibrated to absolute frequency noise through the slope of error signal from each setup (power input of 10mW, 5mW and 1mW)

plot_2011_10_12.png

==Comments for the results==

  1.    The noise level when the beam reflected off the cavity goes down when we float the table and optimize everything ( red, green, blue are lower than pale pink (result from 2011_10_07).
  2.    The noise level when the beam reflected off the cavity do not change much with input power ( red, green, blue are about the same)
  3.    At 1mW the noise at high frequency (above 50Hz)  raises up for both cases (reflected off cavity / mirror) I'm not sure what happens. The calibration seems to be ok since the noise level at low frequency matches the results from 5mW and 10mW setup.
  4.    Seismic stack's resonant frequency at 6.7 Hz shows up more clearly after RFAM/ back reflection are minimized.

 

To Do: We will check the beat signal tomorrow. There should be improvement, since optics are optimized and the table is floated.

Attachment 2: plot_2011_10_12.fig
  703   Wed Oct 12 00:53:40 2011 Tara, FrankSummarySeismicsecond leg replaced

As the table was floated, we measured the noise from error point again.

 

     We tried to determine if the noise bump we saw were from the window, so we place an extra window in front of a mirror [add fig] and compare to the noise when there was no window. The results are not different that much.

     From a quick look, by adjusting the input power, from 1mW to 10mW. The shape of the noise from error point changes substantially. This could be come from RFAM or scattering.  I'll measure the noise vs input power after I optimize everything first. RFAM, beam splitter, back reflection have to be optimized.

 

 

  702   Mon Oct 10 21:30:45 2011 Tara, FrankSummarySeismicsecond leg replaced

we have replaced the second leg which was leaking. It could be that the legs are simply to old and the rubber got brittle. As far as i know the table has never been operated floating as it had the suspended reference cavity on it since the beginning. We operate at around 85 PSI, maximum operational pressure is specified at 100PSI so that should be OK. The second leg started leaking after one day of floating operation. We disassembled one today to have a close look but we can't really tell where the leak is. We will check with some pressure to see what's broken within the next few days. Let's see what the other legs do in the near future. We still have plenty of "spares", as Aidan bought two new sets (taller ones) for the old tables in the TCS lab. So we have 8 short ones which are currently not used (and seem to be newer than the ones we currently have). And they still work as passive legs.

  700   Sat Oct 8 02:09:56 2011 frank, taraDailyProgressFSSNoise Hunting: Ground Loop, RFPD

Today we did noise hunting stuff. We focused on ground loop and noise from RFPD. The noise from ground loop is probably come from the power supply of the RFPDs, and noise from RFPD looks very suspiciously similar to beat. More investigation is needed to be done.

  ==Ground Loop Problem==

     We saw a lot of odd harmonics from 60 Hz peak (180Hz, 300Hz, and so on). This indicates that we have a ground loop somewhere. We find that the table has the same ground from the cables (which come from the electronics shelf). We tried unplugging all the cables that connected to items on the table ( PDs, RFPDs, EOM), but it did not fix the ground loop. We have not tried unplugging the laser's cable yet. However, we think we know where the ground loop comes from. The power supply for the RFPD are three pins +/- 15 V with ground. Its ground is the same as power supply's ground which directly connects to the RFPD.  To fix this, we have to use a power supply from the electronics shelf, and we haven't tried to fix this yet.  This will be tested later.

 

==Noise at High Frequency==

We switched back to old FSS to compare the noise level. [add fig, details]

 

==Noise from RFPD==

   We checked the signal from error point of RCAV FSS loop. It did not changed with power input of the laser (from 1mW to 10mW), so we think it might come from the detection point, RFPD. We check the noise at error point of the FSS loop (old FSS) when

  1.   the beam incident on RFPD was blocked, so only electronics noise is observed, and
  2. the beam reflected from the cavity (off resonance) and fall on the RFPD, so we know what is the effect from the incident beam on RFPD (RFAM, scattering etc) and
  3.  a mirror is used to reflect the beam back to the RFPD in front of the cavity, so we know what is the effect from the incident beam without contribution from the cavity.

2011_10_07_lay.png

 Fig1: setup to check noise from detection point.

 

We measured the slope of the error signal (1.1e5 Hz/V) which is used for converting the noise at error point to absolute frequency noise.

The result is shown in comparison with beat measurements (red and magenta) below:

noise_hunt.png

Note: I have only coating noise in the noise plot so that the graph does not look to busy. The whole plot can be found in the matlab fig flie attached below.

Comment about the result:

  1.      The result from case (2) (beam reflected off the cavity) has a shape very similar to that of beat measurement. It is only about a factor of 10 away from the beat. If we assume that the calibration we got from error point could be off by a factor of 3, and the noise in beat measurement includes the similar effect from ACAV's path which might have larger calibration. It is very likely that light falling on the RFPD is the origin of the bump at 100 Hz. 
  2.      When we used a mirror to reflected the beam of in front of the chamber (case 3), the noise went down considerably. This means that the scattering bump come from the window of the chamber or the cavity itself. From inspection, we think it may be the window, because it is not very clean. A new AR window might be needed. 
  3.     The electric noise at the detection point (case 1) is already higher than coating thermal noise level. RFPD has to be modified later.

 

 

To do: fine adjustment on beam splitter and waveplates, to reduce an

Attachment 3: noise_hunt.fig
  698   Thu Oct 6 15:30:14 2011 taraNotesPurchasesNitrogen bottle

We can buy a tank of Nitrogen/ pressurized air  from Caltech warehouse, the extension is x4891. They have delivery service as well.

  696   Thu Oct 6 00:47:28 2011 Tara, FrankDailyProgressSeismicleg of optical table replaced - table now floating- beat

We measured seismic noise (on three directions) on the table when the table was floated and not floated. Seismic is substantially reduced at frequency 10 Hz and above.

Note: the data is calibrated to velocity/rtHz. ( I checked the 40m entry, I used this calibration here. The data was corrected for 50Hz pole as well.

(after calibrating, the result look similar to what Jan did last year, see here psl:435)

 

==guralp cmg-40T setup==

We used Guralp CMG-40T 3-axis seismometer (borrowed from 40m with Jenne's help) to measure the noise on the table. The setup on the seismometer is "Broadband velocity". Signals from "Low velocity" channels are used to acquire the data.

Voltage supply is 12V, the output ground of the box is fake ground and it should not be connected to the ground of the oscilloscope/ spectrum analyzer. I used float ground on the spectrum analyzer, so it should be ok.

The voltage output for each channel is +/- 10V.

The position was tuned by adjusting the legs of the seismometer, so that the position readout for each channel is close to zero (we got less than +/-0.3V), the manual says it should be below 3.5V, so we are ok.

==seismic improvement==

3 directions and the corresponding resonant frequencies are

  1. Vertical    (2.5 Hz)
  2. North-South: horizontal transverse motion of the cavity (normal to cavity's beam line)  (1.8 Hz)
  3.  East-West:  beam line direction       (1.7 Hz)

calibrated_seismic.png

The seismometer has flat response up to 50Hz, the data has been corrected for the transfer function of the seismometer.

***The noise is getting worse at low frequency below 3 Hz is typical [link to Newport], and the signal above 50Hz are mostly noise, so both signals from floated and unfloated table are similar. (2011/10/06)

Snapshot_2011-10-06_18-34-00.png

Transfer function from the floor to floated table. [Newport]

 

===beat improvement===

Then we checked the beat signal. We just made a quick check to see if the peaks between 10-100Hz would change or not, so we did not try to optimize the loop or anything. The result (red) improves, those peaks are decreased significantly, but it is bad that noise at frequency above 100 Hz goes up a little bit. This might be from the un optimized servo. So, the next step will be checking the servo, noise at high frequency.

beat_2011_10_05.png

 

Attachment 1: seismic.png
seismic.png
Attachment 2: seismic.fig
Attachment 4: beat_2011_10_05.fig
Attachment 7: calibrated_seismic.fig
  693   Tue Oct 4 23:20:57 2011 frank, taraDailyProgress re mode matching

 

  I readjusted the lenses for mode matching a bit more and the visibility for both cavities are now ~93%. We will check beat measurement with floated table tomorrow.

  692   Tue Oct 4 22:13:23 2011 Tara, FrankDailyProgressSeismicleg of optical table replaced - table now floating

we moved a nitrogen bottle from the tiltmeter lab to the CTNlab to float the table. Last time i tried this we realized that the pressure of the compressed air supply in the lab is not high enough (~35PSI max). As we now know that we need more seismic isolation we wanted to give it another try to see how much gain from floating the table and to determine how much we have to change the actual cavity support.

After connecting the table to the bottle and trying to adjust the regulators on the legs we saw that three legs seem to work fine but the forth one didn't move at all and one could hear that some air was leaking somewhere from inside the top of the leg, which we couldn't further investigate (no space when attached to the table). However the regulator seem to work just fine. So we decided to replace that leg. We took the pallet jack from the 40m to lift the table. After replacing the leg and adjusting the valves the table is now floating.

The maximum pressure for the legs is specified as 100PSI. In order to float the table we need about 90PSI, at 80PSI only part of the table floats. So we have to stick to the nitrogen bottle (or compressed air). We have a bottle rack so it's not a problem. We just have to run a hose to there which we have to buy. The hose has an O.D. of 1/4" and less than .17" I.D. We got the Guralp from the 40m to measure the isolation we gain from floating the table which we will do tomorrow.

.P1050673.JPG

 

  691   Mon Oct 3 23:54:19 2011 frank, taraDailyProgress re mode matching

As the cavities' height changed, I adjusted the lenses to fine tune the mode matching for RCAV, the visibility is ~85%

We might want to use a bigger beam splitter (the current one is 10mm cube) where the beam split to ACAV and RCAV paths, the spot radius is ~ 3mm. It might cause some diffraction problem.

The next step is to check the beat signal with the previous FSS. We suspect that something might be wrong with the TTFSS. If the old FSS can give us with better signal,

we will use the old one.

 

  690   Wed Sep 21 01:39:11 2011 taraDailyProgressBEATseismic noise coupling

The beat signal I measured today(orange) has noise level close to what we had before (blue,purple), so I'll try to check it again tomorrow. Meanwhile, this is the plot with beat signal when we modified the suspension (removed the spring), and the cavities were in two separate chambers (blue). The noise level around 20-100 Hz is very similar to what we have right now. Since the spring filters mostly vertical seismic, these peaks should be mostly vertical seismic coupling into the system.

beat_2011_09_20_m.png

Quote:

Tara showed me a quick plot which showed the spectrum taken with the new (current) setup and the one taken when we removed the springs from the wire suspension. They look pretty identical between 10Hz and 100Hz or so. So it is likely that we see a lot of vertical seismic. I've measured a little bit at low frequencies to see where we are and we are better than before now, i would guess an order of magnitude or so, without any optimized stabilization, alignment and loops. Tara will take a nice set of measurements tomorrow and make a nice plot. The peak at 6.7Hz or so is actually the horizontal motion of the two top stack plates (not only the top plate). I used our pzt-shaker to shake the table and even with a small signal i could increase it until i got scatter noise bumps around 100Hz. So we have to damp this somehow. Is there an easy way to get some more vertical isolation? what about putting the top plate on a few springs instead of rubber? How much do you typically get when floating the table? I don't have realistic numbers for that...

We also don't have a seismometer. Jenne took her's back and Jan shipped the others back to where they came from i think. i think we should get one which we can share between labs in bridge which we can keep for longer. We needed one quite often in the past couple of months and i guess once we start with the cryo cavity we will even more frequently. Any idea where to get a cheap one? We don't need an STS-2 or so... Seismic sucks anyway in bridge...

The problem with moving supports is that the spacer has pretty wide groves. But we have to think about a clever support anyways. Currently it's sitting on viton in the groves of the spacer, which, according to comsol, is very close to the optimum position but who knows in reality.

Quote:

By taking the passive transfer function between a vertical seismometer on the table and the individual cavity signals, we should be able to see which direction to move the cavity supports so as to minimize the seismic coupling.

Our first iteration probably will have a sign error, but by making a few iterations we ought to be able to home in on a better support. Also, we should compare the theoretical estimate with the measured coupling in units of strain/(m/s^2).

 

 

Attachment 2: beat_2011_09_20_m.fig
  678   Wed Sep 14 19:17:19 2011 frank, paul, taraDailyProgressBEATbeat layout

 Beat signal from double cavities in the same chamber was measured. At DC to 100Hz, it seems to be dominated by seismic. Above 100 Hz, the frequency's laser noise is the limiting source. The setup has yet to be optimized. This is for a quick check to see how beat signal changes with the new seismic isolation setup.

      I measured beat signal after Paul and Frank locked ACAV. At frequency above 100 Hz, beat signal changes with RCAV's gain setup. Error signal from RCAV's mixer out matches the shape of the beat signal ( I did not do the  calibration with error signal slope, just observed the displays on SR785). However, at low frequency (DC - 100 Hz), the beat signal does not change with anything, RCAV gain, ACAV gain, PLL gain, so I'm quite certain that it's the real signal we see here.

beat_2011_09_14.png

Tuning range on PLL loop was 10kHz, the calibration is 7kHz/V. A peak close to 7 Hz might come from the beam line transverse mode of the stack.

At DC - 10 Hz, the noise is lower than before. This might be the result from common mode suppression. However, the two spacers are not identical. They have significantly different holes' sizes. This may explain why the seismic cancellation is not that good at higher frequency.

Snapshot_2011-09-14_19-39-37.png

We might have to suspend the whole vacuum chamber to win against seismic.

  677   Tue Sep 13 20:54:11 2011 taraDailyProgressBEATbeat layout

 

 Beat path was setup. Beat RFPD sees the signal around 125 MHz. Beat measurement will be done tomorrow after ACAV is locked.

  674   Mon Sep 12 01:21:14 2011 taraDailyProgressBEATbeat layout

New beat layout, both paths travel with the same distance before combining at the beam splitter. I'll try if this layout work tomorrow.

I'm a bit worried that the CCD cameras might not fit on the table. 

Attachment 1: beat_2011_09_12.png
beat_2011_09_12.png
  673   Fri Sep 9 22:53:32 2011 frank, taraDailyProgressRefCavfrequency difference

 I made a mistake in the last entry. When I measured the frequency difference, I did not notice that two beams path were shifted at different orders from AOM.

One was +160MHz, another was -160 MHz. So I re measured the frequency difference again, with both beams shifted by 160MHz, the frequency difference between

the two cavities is 127 MHz. Without thermal adjust on the cavities,This falls into a hard-to-work-with frequency region with the current VCOs we have.

 

To lock both cavities at the same time with df = 127 MHz, for now,  we use 0th order beam on RCAV's AOM, so that the beam's frequency is not shifted, and use a VCO [model's number] to drive ACAV's AOM at 63.6 MHz, so the frequency different between the two paths is 127 MHz, from this both cavities should be able to lock simultaeneously.

We should be able to see beat signal by this Tuesday.

  669   Thu Sep 8 20:53:17 2011 taraDailyProgressRefCavfrequency difference

 I aligned the beam to ACAV and RCAV and measured the frequency difference between the two cavities to be 289 MHz.

 

I used SLOWDC for calibration, the beam resonates in ACAV and RCAV at

 

 DClevel                                  RCAV                 ACAV

     1st resonance       -0.0520 [V]                0.0088  [V]

      2nd resonance       0.1046                      0.1661

     dV                           0.1566                        0.1573

 

 For a quick calculation, I used nominal FSR = 737 MHz. The calibration is then FSR/dV = 4700 MHz/V.

The frequency different between the two cavities give (0.1661 - 0.1048)*4700MHz/V = 289 MHz.

(I think the frequency might be too high for the beat PD, we may need to have thermal control before we can measure the beat)

 use df/f = dL/L, the differential length between the two cavities is   dL =  df * L * lambda / c ~ 0.2 micron.

So common mode suppression for thermo-elastic or cavity sagging due to seismic can be approximated to be ~ dL/L = 0.2 micron / 0.2 m =  1e-6.

  668   Thu Sep 8 02:58:18 2011 frank, taraDailyProgressSeismicRing down measurement for new seismic stack

We did ring down measurement of the new seismic stack with double cavities on top. Q and f0 are 25.85, 6.35Hz for horizontal transverse mode, and 21.53, 6.96 Hz for beam line mode.

 

P1040902.jpg

   

longtitudinal.png transv.png

 Left) beam line, translational motion, f = 6.96 Hz, Q = 21.5                                   Right, horizontal tranverse motion, f = 6.35Hz, Q = 25.9.

  

     We used shadow sensing technique to measure the ring down of the top stack with respect to the table. The cavities were placed on the stack, we will add copper tubes and heaters later, so the resonant frequencies might change from these measurements a bit).  We noticed that the motion of the top stack was almost similar to the motion of the middle stack. They moved together. We have to think how the transfer function of the stage looks like before calculating the new noise budget.

  667   Wed Sep 7 22:41:44 2011 frank,taraDailyProgress Two cavities are in the same chamber

   We put two cavities in the same chamber. Next, we will measure the frequency difference between the two cavities when they have the same temperature to determine which frequency we need for the second AOM.

 

P1040900.JPG

Before we put the stack back in to the chamber, we measured Q from two modes of the stack(with cavities on it).

The modes are longtitudinal motion (translation along the beam line) and horizontal transverse motion [see this entry]. Other modes i.e. pitch, yaw die away too fast to be measured with ring down measurement.

The cavities' mirrors were checked and cleaned. ACAV's mirrors had a trace of smudge which is now cleaned.

Since the pendulum suspensions were removed, the cavity's height is reduced from 7" to 6" above the table. The periscope's height were adjusted accordingly.

The beam is aligned to RCAV, the visibility is ~85%, it decreases because the position of the cavity's height changes.

Now I'm trying to align the beam to ACAV. Once the beam aligned, we can lock the beam and measure what is the differential length of the two cavities, hence the frequency.

  665   Fri Sep 2 20:19:31 2011 Frank, TaraDailyProgressRefCavnew stack parts

checked if the cavities fit on the new support, they do . We've put the assembled new stack top plate in vacuum over the weekend to clean it a bit more. Also turned on the heaters which won't do much but won't hurt. Will move everything into the other, final chamber and start aligning the cavities on Tuesday.

  659   Wed Aug 17 20:41:13 2011 taraDailyProgressopticACAV path is up

I put most optics on ACAV path. I have not tried to lock the cavity yet. I'll install ACAV RFPD next.

layout_2011_08_17.jpg

 

 

  658   Mon Aug 15 23:13:53 2011 taraDailyProgressopticRCAV modematching optimized

 I recalculated the mode matching so that the spot radius in AOMs is 100 um. Now the visibility of RCAV is 90%.

     From the previous mode matching calculation, the spot radius in AOM is 220 um. This was too large for ISOMET AOM and caused beam distortion. The AOM was designed for much smaller spot radius (50 - 110 um). So I recalculated to make the spot radius inside the AOM to be 100 um. This spotsize is small enough for ISOMET and not too small for Crystal Tech AOM.

Rise time is 35 ns (28.5MHz) for 100