temp sensor number 2 of the reference cavity shows much more (horrible) noise compared to the other ones (2K pkpk).

broken, oscillating?

Will check this tomorrow.

- personal notes -

things to do: change limit of both heaters to match new situation. much less power required and messing up things causes both cavities to heat up too much (50 C)...

added a single temp sensor on refcav2 (AD590) last week  in order to monitor the temp of refcav2. We tried to change the heater power on both cavities to match the frequencies. So far we have no tempctrl. Changes of the room temperature have too large (and different) influence on the individual cavities. So it"s almost impossible to predict the room temp changes and change the heater power to match both cavities. We need a real temp stabilization of the chambers...

the name of this sensor is C:PSL-FSS_RCTEMP

room temp next to both chambers is measured with C:PSL-FSS_RMTEMP

PT1000 value (chamber temp):

1083.9 Ohm

AD590 values:

RT:20.6
S1: 20.8
S2: 21.55
S3: 21.76
S4: 20.77

platinum temp sensor (PT1000) used for calibration of AD590 on PSL table:

Callendar-Van Dusen Equation:
R_T = R₀(1+AT+BT²-100CT³+CT⁴)
R_T = Resistance at temperature T (°C)
R₀ = Resistance at 0°C
T = Temperature in °C

*Both b =0 and C=0 for T>0°C

source:  http://content.honeywell.com/sensing/prodinfo/temperature/technical/c15_136.pdf
online calculator: http://www.isotech.co.uk/prtcalc-web.html

measured resistance: 1083.9Ohm = 21.55C

calibration offsets for chamber sensors:
S1: +0.75K
S2: 0
S3: -0.11
S4: +0.88

room temp sensors:

ACAV_AMB1 : +0.2
ACAV_AMB2:  -0.3

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.

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.

So the loop itself is functional now (in terms of all the hardware and software interfacing), but the metal block didn't actually heat up even at around the maximum voltage output. However, the output value of the PID loop changes as expected with the setpoint/error, as well as when I touched the temperature sensor to heat it by hand.

Here's a picture of the setup when testing the loop. We also tried it with a smaller (aluminum?) slab instead of the heatsink, but that also didn't heat up. It's pretty messy and very much a "before" making pretty packaging picture.

(Oh, also the measurement signal looks pretty noisy in this picture but it gets a lot better if you stick in one of the LP filter boxes)

So I'm not sure how much to chronicle software changes when they're works in progress, so I'll just list everything until someone tells me to shut up:

-Pulled out all temperature control related channels into a separate database file "TempCtrl.db"

-Modified the iocBoot file to include "TempCtrl.db"

-Made a configuration file "TempCtrlConfig.ini" in the scripts folder with parameters for the PID script

Wrap the heatsink in some socks or blankets or foam. In order to get the temperature to rise you need insulation, same as people in winter time.

Please gives us a few calculations on what the total heating power needs to be for the different temperature control servos. I guess the outer can needs more Watts than the inner shields.

the temperature in some of the cavity channels jumps about 5mK if you kick the table hard enough, so there is some problem with the d-sub connections on the table connecting the sensor readout box with the DAQ.

here a plot of the temperatute stability in peter's lab (upper graph) if nobody is working in the lab (christmas) and if someone opens the door to the lab (the large spikes in room temp). lower graph shows the temp of the refcav (in-loop, no out-of-loop sensor so far. Except the large spikes stability is about 4mK/pp. I will add a single ool-sensor today (will be C:PSL-FSS_RCTEMP). We used this channel so far for the second cavity but have a second temp box since christmas...

changed set point from 26.5degC to 26.6degC for a first test at 2:42PM local time on fb2. Tuned the VCO feedback to be ~zero. Tuning coeff same as mentioned in post 682

temperature time series for the last couple of days measured right above the table between both cavities.

thought a little bit about how to create the difference of 127MHz. Here are two ways how we can do this right now:

We only use one AOM (as before). We can't use the second one as we would have to use it way beyond it's operating range (which we already tried Friday) or in second order where we don't get enough light if double passed.
So using only one we have the following options:

1. we use the Isomet 19" 80MHz OM driver which can be tuned down to 63.5Mhz using an external voltage. The frequency tuning knob can't go that low.
   We know that the phase noise is very high and the output signal is not even close to a sine wave. So this is OK for alignment but i would not use that for an actual measurement without detailed noise characterization.
    
2. we get one of the Marconi's from the 40m and use our 2W RF amplifier which we already used Friday. The phase noise is is very very low and we don't need a lot of tuning range, so it's even better.
   This guarantees that we don't sit on the phase noise of the VCO even if we only have a low UGF for the second loop.

i'm currently adding new channels for RFAM monitoring and EOM temp control but before they work i used some other channels for now.

• The EOM temp setpoint is currently modulated using the SR785 as i cant add a large DC offset on a small modulation using the SR345.
• The current setpoint is 650mV which is equivalent to something around 30C.
• On top i have a modulation (sine, 1mHz) of 180mV (?, have to double check, might be 170mV) to modulate the EOM temperature.
• The setpoint value is recorded using channel C3:PSL-FSS_VCOMON
• The actual Temperature of the in-loop sensor is recorded using C3:PSL-FSS_FREQCOUNT
• The feedback to the laser is monitored using the fast actuator channel C3:PSL-FSS_FAST. The EOM feedback loop is disabled as well as the script for laser temp feedback.

i checked the temp modulation by using a PT1000 sensor as an OOL sensor on the EOM case using a multimeter.with min/max hold function.

• min value is 1120 Ohms = 30.9C
• max value is 1139 Ohms = 35.8C

Monitoring the the above channels using Striptool didn't show any result yet. Even the modulation of ~5K is not visible in the feedback to the laser. Will add all signals to their final channels tomorrow and do a more detailed analysis.

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?). 

Wow, nicely done!

For posterity, the design document is on the DCC: T1600071

The vendor report for the optics is here: Q1600019

I'm a little surprised at the finesse. The design value was 300. This should be set by the reflectivities of the flat mirrors, which are each supposed to be 99% ± 0.1% for p polarization.

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).

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

• 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

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)

I moved the circuits to the electronics lab to test it there, and I recreated the situation that was occuring in the lab. I found that when the current limit knob on the power supply was turned down, I consistently got the same response as I was getting in the lab, where the sensor reading would dip when the control voltage turned on, and then rise back up when the control voltage shut off, but the power supply did not display that it was current limited. I then turned the current limit knob all the way up and didn't get this dipping problem (I tried this for >10 toggles). So I think the problems from before are just a result of not having enough current, so I will  try to set it up again with the power supply from the electronics lab and see what happens. Some oscilloscope (of the temperature sensor signal) pictures below:

Current limited, toggle control voltage on:

Current limited, toggle control voltage off:

Current not limited, toggle control voltage on (the little bump I think actually corresponds to a rise in temperature from the heater being on):

Current not limited, toggle control voltage off (note decay in temperature reading ~5s in from left edge):

The current 1064nm, 100 mW laser in our setup, NPRO lightwave 126 is broken. We are looking for a new one to replace it.

The laser stopped working when I tried to lock the cavity and saw that RCTRANSPD fluctuated a bit even after I adjust the gain setup.

So I turned the HEPA filter above the table off to see if the signal would be more stable, it was not. When I turned it back on

the laser was off. I don't think the laser and the HEPA filter are associated, but that's what happened.

The power output, as indicated on the laser driver is 8 mW. When I turned the laser off for 5 mins and turned it on.

The temperature of  the crystal started from ~40 C and there was power out, then, in ~10 seconds, the temperature went up to  94 C, and the power dropped to 8 mW again.

The voltage supply for TEC went up to 4V which is the maximum V for cooling.

I switched to the 10W laser driver, the same symptom happened again, so the problem might be the head, not the driver.

Frank opened up the laser to find any burnt mark, but found nothing and put it back, and now the laser is working.

We don't know for sure yet, what's wrong with the laser. But I'll use this opportunity to work on modification of PMC servo.

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.

The cable tray will run along the wall, some of the cable rack will be lowered for a few inches.

took some images with the thermal imaging camera of the AOM installed in the PSL so far. The first three pictures show the AOM driven with 75MHz, 80MHz and 85MHz.

85MHz:

80MHz:

75MHz:

it is interesting that the hottest zone is at the end of the crystal, not where the pzt is mounted. It looks like the crystal is not proper mounted. Here a normal image for comparison...

so i took some pictures of a different AOM, but same model. Here are the pictures for different power levels @ 80MHz:

this looks normal...

*** Rana: I've replaced Frank's AOM picture with a zoomed in one. This circuit looks a little sloppy to me - why are the coils so loose?

when playing with the setup i've noticed that we are actually very sensitive to temperature fluctuations (more than i expected), so the absolute length (or CTE?) must be more different than we expected. So i've added the thermal insulation, drilled four new holes in the end caps, connected the temp sensors etc. Thermal control loop is running now. Made some changes to channels (mostly range limiuts, warning levels etc) and added four new software channels which i will use to generate absolute calibrated temp signals. The difference between the sensors is about 1K. I know it does not matter for the stabilization but i hate it as you never know if you have a gradient or degraded sensor or so...

I'ver started heating it up to 26.5C (you may ask why this number: laziness -  actually i've started the heater before finishing the insulation and that was the actual temperature at the time i finished it). The LO for the AOM already increased to 70.6MHz (from 64MHz). So my guess is that once we reach 30C we are back in range for the iLIGO VCO.

Will change the temperature over the weekend in a couple of steps to measure the temperature sensitivity  (differential length change with temp). This will tell us how good the thermal stability has to be to not be limited by thermal fluctuations. We then can make a decision on further things like second, external box, passive foam or metal box with active control etc..

we added two endcaps for the first refcav and additional insulation for the ion pump and valve.

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.

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.

Details about how to use the pump and the chamber can be found in CTNwiki.

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.

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.

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

It can be found here

• replace windows on acav chamber by ar-coated windows
• find source of reflection in cavity - my guess it is the dirty coating - the spot size is about 1cm, so it can't be a pure reflection from any of the other surfaces as the beam is too large. The concave mirror surface acts like a convex surface in reflection and intensity increases when cavity is locked.
• clean mirrors of current cavity, replace mirrors or replace entire cavity
• clean all optics on table - entirely and double check with real bright white-light source
• make acav RFPD resonant or wait for new RF photodiodes to be ready
• tune PMC RFPD to 21.5MHz (easy)
• add AOM in RCAV path (doesn't help right now to lower phase noise or increase range of PLL)
• lower beam height for beat setup in transmission
• replace RFPD for pll by larger size PD
• add notch for PMC
• replace current FSS stuff by adv LIGO version
• check max tuning range for wenzel vco
• add perl script for temp feedback to acav to keep beat constant (long term stability)
• replace LO for ACAV and RCAV by real sinewave (wenzel+amplifier)
• change gain in RFPDs for 1mW max light
• measure seismic on table
• float table to see what happens (75PSI are not enough, tested today)
• ...

• measure Marconi phase noise LOCKED to Rubidium clock for different input ranges and LO frequencies (make a list of possible frequencies)
• re-measure coupling from RIN into beat
• re-measure seismic coupling to beat - we have the stack TF and the coupling factor for RCAV so far
• measure RF-AM coupling - not limiting at the moment but we don't know where it's gona be

Today we connected the PT1000 thermostat on RCAV's stack to C3:PSL-GEN_DAQ16. This will be used for time constant between chamber to cavity measurement.

We want to learn the time constant between the chamber's outside surface to the cavity, so we need to know the temperature of these two points.

The temperature on the outside surface of the chamber can be measured by 4 sensors that we already have. For the inside,

Frank left one PT1000 on the top seismic stack, just under the cavity inside the chamber. It's wired to the D-sub connector. We made a circuit, as shown below, to measure the change of the resistance( which is proportional to T) of the sensor.

insert fig

R(T) at 30 C is about 1.2 k ohms.

SR560 is set to DC couple, gain x2. The output is connected to C3:PSL-GEN_DAQ16

This should give us the temperature of the stack inside the vacuum.

Now I'm waiting to see any change of the voltage.

The 10V supply is an AD587 with a 22 uF foil capacitor connecting its "noise reduction" pin (8) to ground which I had sitting around from the doubling phase noise experiment.

The signal goes like:

10V x (1 - 9.1k / {9.1k + R(T) } ) =10 V x (1 - 9.1k/{9.1k + 1.2k + dR} ) =10 x (1 - 9.1k / 10.3k x 1 / {1 + dR/10.3k } )

which by taylor expansion is:

10 x  (1 - 9.1 / 10.3 x (1 - dR / 10300 ) = 10 - 10 x 9.1 / 10.3 - 10 x 9.1 / 10.3 x dR / 10300 = 1.165 V - 8.6e-4 x dR/Ohms V

for a 1 kOhm RTD, dR/dT is about 4 Ohms / K so we get a total signal temperature sensitivity that goes like (after the 560 x2 gain):

V(deltaT) = 2 x (1.165V - 8.6e-4 x dR/dT x deltaT /Ohms V)

V(deltaT) = ( 2.33 - 6.8 x 10^-3 x deltaT / K ) Volts.

We plugged this into an epics channel and confirmed that it was being recorded.

Tara regularly makes ~ 0.1K adjustments to the temperature of the can in the course of the refcav beat experiement. This will show up as a 700 uV change on the 2.3 V DC signal. I'm not positive that it's this is large enough for us to see, but we get it for free. If we can't see this, we can try a 1K step for a 7 mV change, and if that doesn't work I'll put something less crappy in to readout the PT1000 sensor.

The DC signal read as 2.6 V on a scope and 2.43 V with the existing calibration of the frontend. The 1.2K number I plugged in is wrong by a bit. I trust the systematic calibration (as recorded by the frontend) to ~10%

WHY AM I DOING THIS?

Me and Frank disagree about what the radiative time constant is between the cavity and the can, in the case of both the cryo cavity, and the as built cavity in the PSL. (see this elog).

1. Envelope physics says it should be on the order of 41 hrs.
2. The reference cavity + can + heater at the 40m has a time constant of 4 hours
3. Frank has said "the radiative time constant from the can to the cavity is like 30 minutes," (though he may have since rescinded that statement)
4. I looked through the PSL elog for some info about what these time constants actually are. I found:
   1. This elog where they do a step in the heater temp, but I can't glean anything clearly useful from it
   2. Another elog claiming a 7.5 hour time constant just for the heater where Rana made a few criticisms on the measurement
5. The other salient point is that Frank has talked to some people (who exactly?) in both LISA and at UF, and seems to have been told "the only way to get long time constants is to have really good radiation shields with a bunch of layers." This doesn't seem to jive with the second point above, where envelope calculations give you a > 1 day time constant. It would be awesome if Frank could elaborate on exactly what is known here to help remove one person from the telephone chain.

My money is that the thermal time constant is totally dominated by the conductive pole through the stack, and that's what we see (in the PSL lab and at the 40m), so depending on what the time constant between the can heater and the stack thermometer is, we might be able to rule some things out.

Is there any model for the thermal conductivity of the stack + suspension?

 This morning, I decided to adjust RCAV's temperature setpoint (C3:PSL-RCAV_RCPID_SETPOINT) up by 1 Kelvin. As I saw not much change in the readout of the PT1000 sensor overnight, I'm afraid that the signal might be too small for a 0.1 k change.

We might be able compare the readout from C3:PSL-GEN_DAQ16 to C3:PSL-FSS_SLOWDC level. SLOWDC controls the temperature of the NPRO crystal for adjusting laser frequency.

The calibration for SLOWDC is 4700 MHz/V*. The resolution of SLOWDC is 0.0001 V => 0.47 MHz. 

Use df/f = dL/L to compute the equivalent cavity's length change, dL ~ 3 e-10 m.

Compare this dL to the effect of thermal expansion of the cavity.

dL = alpha x L x dT =>

3 e-10 [m]= 0.51 e-6 [1/K/m]  x 0.203 [M] * dT [ K]

dT [K] = 3 milliKelvin is the resolution of the temperature we can measure, so we should be able to measure 1 K change on the cavity easily, and we can compare with what we see on the sensor.

From above equations, the cavity's temperature is related to SLOWDC by

dT  = dV_slowdc x 32.91 [K/V].

 For 1 kelvin change on the cavity, SLOWDC should change by 0.0304 [V]

Cavity's parameters

cavity's length, L = 0.2032 [m]

SiO2 expansion coeff, alpha = 0.51 e-6 [1/K/T]

channel list:

C3:PSL-RCAV_RCPID_SETPOINT    set temperature for PID cavity's thermal control

C3:PSL-RCAV_TEMAVG                   average temperature on the outside surface of the can

C3:PSL-FSS_SLOWDC                     thermal control on laser's PZT for adjusting frequency

C3:PSL-GEN_DAQ16                         readout from the temperature sensor on the top seismic stack 

* About the calibration, I used sidebands (35.5MHz x2 =71 MHz) to calibrate SLOWDC to frequency. I also used the cavity's free spectral range (737 MHz) for comparison as well. The results agree well.

 Tara, please post what the channel names are, physically.

Result from a step function test.

          At ~1.5 h, I changed the temperature setpoint from 33.5 to 34.5 C, so the voltage supply to the heater rose up. It reached at 1.7 V and constant because the perl script limited the voltage at that value for preventing any mishap.  The can reached the equilibrium first, in ~ 5.5 hrs after the setpoint was changed. Then the cavity's temperature reached the equilibrium, ~ 3 hrs later. The stack temperature had not reached equilibrium yet.

         Heat transfer mechanism that heats up the cavity seems unlikely to be conduction from can-> stack -> suspension-> cavity. If that were the case, the stack would reach equilibrium before the cavity.

 I'm repeating the experiment, but this time I turn off the thermal PID feedback of the can temperature, and make a step down in the heater voltage supply, and keep the laser locked to the cavity so that we can read the continuous value from SLOWDC.

*I'm sure I made this entry last night. Where did it go?

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.

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], 

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) .

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.

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.

 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.
•  

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

 This setup has too much gain (i.e. not enough range). Please reduce the arm length asymmetry by a factor of 10 so that we can monitor over 24 hours.

Also the temperature channels ought to be calibrated (via the EPICS .db) so that the readout is in degC instead of ARB.

The temperature sensor reading is being railed to the negative power voltage whenever a new input command is entered:

The first ramp is at the output of the instrumentation amplifier AD620, the second ramp is at the output of the low-pass filter Andrew put in recently. Also, I looked at the signal going into the AD620 and it did not exhibit this drop, so the problem comes from the AD620 circuit. I tracked the current being drawn on the display of the power supply, and never saw it go above 250mA, but I'm thinking if it's a current problem (overdrawing), it could be happening fast enough to not register on the power supply.

I plan on using a resistor and the oscilloscope to track the power current more closely while I change the input command to see if it is the problem.

Note: if the leads on the temperature sensing circuit are flipped such that the signal is negative, the signal rails to the positive supply voltage whenever an input command is entered.

tuning range of the 80MHz VCO used for the frequency stabilization:

disconnected the turbo pump and enabled the ion pump  - initial current was 1mA  (value before venting was <0.1uA)

some old designs i've built some years ago using the cheaper DIP versions of the matched BJT pairs from Analog Devices.

Designs are DC-coupled, gain 1000 as they were designed for measuring the noise of photodetectors.
Given values are not 100% what i've used later, only for drawing the schmatics (e.g. gain setting resistors or compensation), but order of magnitude is right.

Eagle-files + Documentation: