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ID Date Author Type Category Subject
  1507   Mon Sep 8 04:20:41 2014 ranaDailyProgressFSSTTFSS OLTFs, residual frequency noise

 

The AD829 cannot drive 100 Ohms rail to rail, so you can only go into the 50 Ohm input devices if the signals are small (less than 1-2 V I guess).

  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
  1504   Sun Sep 7 11:54:21 2014 EvanDailyProgressFSSTTFSS OLTFs, residual frequency noise

Acutally it does look like it's a 50 Ω loading issue. I find that when 50 Ω inline terminators are added to OUT1 and OUT2, the measured OLTF is reduced by a factor of 1.6. This explains the discrepancy between the SR785 and HP4395 measurements. I've attached the corrected OLTF plots, along with plots of a vector fit, and the expected residual frequency noise [assuming a free-running NPRO noise of 104 Hz/Hz1/2 × (1 Hz / f)].

South UGF is at 200 kHz with almost no phase margin. We need to fix this.

Attachment 1: pdhTFs.pdf
pdhTFs.pdf
Attachment 2: oltfNorth.pdf
oltfNorth.pdf
Attachment 3: oltfSouth.pdf
oltfSouth.pdf
Attachment 4: frnoise.pdf
frnoise.pdf
Attachment 5: noiseBudget.pdf
noiseBudget.pdf
  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
  1501   Thu Sep 4 11:47:42 2014 EvanDailyProgressNoiseBudgetAlGaAs python noise budget: photothermal TFs added

The first attachment shows the photothermal TFs which take absorbed power (in watts) to the mirror displacement (in meters) as sensed by our 215-µm beam. Since last night, I've fixed the coating TE part and committed the updated ipynb to the SVN.

The second attachment shows the noise budget, with the photothermal shot noise contribution.

Attachment 1: photothermalTF.pdf
photothermalTF.pdf
Attachment 2: noiseBudget.pdf
noiseBudget.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
  1498   Tue Sep 2 17:11:27 2014 EvanDailyProgressBEATNo beat

Quote:

Quote:

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

On Koji's suggestion, I set up a second 1811 to monitor the beat on the input side of the cavities, so that we can see what the lasers are doing independent of the cavity resonances. For each path, I am using the s-polarized light that is rejected from a PBS, so that we don't need to add extra optics to the beam paths.

For example, for south slow at 1.206 V and north slow at 5.565 V, I get a 69 MHz beat (with both cavities unlocked).

We should use this in conjunction with the cavity locking optics to figure out what the correct axial cavity modes are, and whether the cavities need any temperature adjustment.

With this setup, I find that a sub-100-MHz beat occurs for north slow at 5.015 V and south slow at 0.725 V. This south slow voltage corresponds to a south cavity TEM00 mode, but the nearest north slow voltage is at 5.298 V.

  1497   Tue Sep 2 15:04:05 2014 EvanDailyProgressBEATNo beat

Quote:

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

On Koji's suggestion, I set up a second 1811 to monitor the beat on the input side of the cavities, so that we can see what the lasers are doing independent of the cavity resonances. For each path, I am using the s-polarized light that is rejected from a PBS, so that we don't need to add extra optics to the beam paths.

For example, for south slow at 1.206 V and north slow at 5.565 V, I get a 69 MHz beat (with both cavities unlocked).

We should use this in conjunction with the cavity locking optics to figure out what the correct axial cavity modes are, and whether the cavities need any temperature adjustment.

  1496   Tue Sep 2 11:06:54 2014 EvanDailyProgressBEATNo beat

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

  1495   Sat Aug 30 19:10:11 2014 EvanDailyProgressNoiseBudgetAlGaAs python noise budget: seismic coupling added

Same data and same isolation model as for the silica/tantala noise budget. Since we have new table legs, we should retake this data (and make a spectral histogram).

The resonance frequencies of the stack are given as 10 Hz and 35 Hz in the noise budget. Are these for the old stack? I recall that with the new stack we measured resonances at 3, 7, and 10 Hz.

Also I want to double check the sequence of interpolation steps we've used on the silica/tantala noise budget. There are some seismic peaks and silica/tantala beat peaks that almost (but don't quite) match up in frequency, and I wonder whether this is an artifact of the interpolation.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1494   Fri Aug 29 19:56:06 2014 EvanDailyProgressBEATBeat breadboard in place

Quote:

Beat breadboard is slid back into place. North transmission appears on north camera. Still need to do south transmission.

Tara has found south transmission on camera. I steered the transmitted beams onto the beat PD and then made the k-vectors as parallel as I could as seen on an IR card.

The DC voltage on the PD is okay (ca. 50 mV from each beam), but I cannot see a beat note on the AC path using the HP4395. Tara will give a temperature kick which hopefully will bring the beat note within the range of the 1811.

  1493   Fri Aug 29 15:35:55 2014 EvanDailyProgressBEATMode-matching for beat

I predict (via alm) that the spot size on the diode (z = 991 mm) is 79 µm in the current configuration.

Attachment 1: ctnbeat_algaas.pdf
ctnbeat_algaas.pdf
Attachment 2: ctnbeat_algaas_alm.zip
  1492   Fri Aug 29 12:35:54 2014 EvanDailyProgressBEATBeat breadboard in place

Beat breadboard is slid back into place. North transmission appears on north camera. Still need to do south transmission.

  1491   Fri Aug 29 09:21:26 2014 EvanDailyProgressFSSPDH shot noise estimate

Ignoring for the time being the issue of offsets in the PDH error signal, here's my prediction for the new PDH shot noise level, assuming a visibility of 0.92 × 0.7 = 0.64 and an incident power of 2 mW.

So our beat will be slightly worse around 1 kHz, but we aren't completely hosed by the shot noise. I'd think the true solution here is to find (or buy) two large-aperture Faraday isolators to replace the PBS+QWP setup (according to alm, the spot size in this region is about 1.1 mm). E.g., we might consider a large-aperture ThorLabs isolator.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1490   Thu Aug 28 20:47:24 2014 EvanDailyProgressFSSPDH error signal on north cavity

Quote:

North PZT sweep: 10 Vpp triangle wave, 3 Hz

North slow control voltage: 3.6805 V

Actuation on north broadband EOM removed

Phase tuning needed, mode-matching needed

Find TNC-SMA converters

 Here's what I expect to happen given

  • perfect mode-matching,
  • critical coupling with 150 ppm transmissivity for each mirror,
  • p/s mode splitting of 2.0 MHz, and
  • perfect demod phase.

It seems to match up qualitatively with the measurement. In particular, it does not appear possible to exceed 70% visibility for each mode.

Attachment 1: npdh_sim.pdf
npdh_sim.pdf
Attachment 2: npdh_sim_narrow.pdf
npdh_sim_narrow.pdf
Attachment 3: fpbirefringence.pdf
fpbirefringence.pdf
  1489   Thu Aug 28 19:10:40 2014 EvanDailyProgressopticMode-matching solution for north cavity

Quote:

Current configuration:

  • Target waist: 180 µm, z = 0 mm
  • Lens 1: 140 mm focal length, z = −711 mm (24″ from center of vacuum chamber + 4″ through periscope)
  • Lens 2: 84 mm focal length, z = −991 mm (11″ further behind lens 1)
  • Seed waist = ??

Since we know we were mode-matched fairly well into the 180 µm waist of the silica/tantala cavity (>93% visibility), I asked alm to propagate this waist backward through the lenses in order to find a seed waist. It reports a waist of 161 µm at z = −1373 mm.

I asked alm for a new configuration using the same two lenses. The best configuration (mode overlap = 1) is as follows:

  • Seed waist: 161 µm at z = −1373 mm
  • Lens 1: 140 mm focal length, z = −743 mm
  • Lens 2: 84 mm focal length, z = −1023 mm
  • Target waist: 215 µm, z = 0 mm

So we should move lens 1 back by 32 mm (=1.3″), and move lens 2 back by the same amount.

I moved both lens mounts back by 1″, then adjusted the Vernier knobs and periscope mirrors to try to maximize the visibility as seen on north REFL DC.

The best I am able to do so far is a visibility of v = 1 − 0.57(1) V / 1.74(1) V = 0.672(6).

  1488   Thu Aug 28 17:36:03 2014 EvanDailyProgressopticMode-matching solution for north cavity

Current configuration:

  • Target waist: 180 µm, z = 0 mm
  • Lens 1: 140 mm focal length, z = −711 mm (24″ from center of vacuum chamber + 4″ through periscope)
  • Lens 2: 84 mm focal length, z = −991 mm (11″ further behind lens 1)
  • Seed waist = ??

Since we know we were mode-matched fairly well into the 180 µm waist of the silica/tantala cavity (>93% visibility), I asked alm to propagate this waist backward through the lenses in order to find a seed waist. It reports a waist of 161 µm at z = −1373 mm.

I asked alm for a new configuration using the same two lenses. The best configuration (mode overlap = 1) is as follows:

  • Seed waist: 161 µm at z = −1373 mm
  • Lens 1: 140 mm focal length, z = −743 mm
  • Lens 2: 84 mm focal length, z = −1023 mm
  • Target waist: 215 µm, z = 0 mm

So we should move lens 1 back by 32 mm (=1.3″), and move lens 2 back by the same amount.

Attachment 1: ctn_algaas_alm.pdf
ctn_algaas_alm.pdf
Attachment 2: ctn_algaas.zip
  1487   Thu Aug 28 13:40:05 2014 EvanDailyProgressFSSPDH error signal on north cavity

North PZT sweep: 10 Vpp triangle wave, 3 Hz

North slow control voltage: 3.6805 V

Actuation on north broadband EOM removed

Phase tuning needed, mode-matching needed

Find TNC-SMA converters

Attachment 1: npdh_broad.pdf
npdh_broad.pdf
Attachment 2: npdh_fine.pdf
npdh_fine.pdf
Attachment 3: npdh_sweep.zip
  1486   Wed Aug 27 03:21:53 2014 ranaSummaryopticoptimization for ETM with a-Si/SiO2 coatings

 I filled in more values for a-Si at 120 K into the wiki that Matt Abernathy set up. Then I ran the optimization code for Brownian noise only:

 aSi_120_Layers_60000.pdf

The above plot shows the comparison between the optimized aLIGO coating (silica:tantala at 300K) v. the a-Si coating at 120 K.

 aSi_R_60000.pdf

Then, finally, I compared the TO and Brownian noise of the two designs using the plotTO120.m script:

 aSi_120_TOnoise_60000.pdf

The dashed curves are silica:tantala and the solid lines are a-Si:silica. The Brownian noise improvement is a factor of ~6. A factor of ~1.6 comes from the temperature and the remaining factor of ~3.9 comes from the low loss and the lower number of layers.

I think this is not yet the global optimum, but just what I got with a couple hours of fmincon. On the next iteration, we should make sure that we minimize the sensitvity to coating thickness variations. As it turns out, there was no need to do the thermo optic cancellation since the thermo-elastic is so low and the thermo-refractive is below the Brownian almost at all frequencies.

  1485   Mon Aug 25 20:54:38 2014 EvanNotesNoiseBudgetAlGaAs python noise budget: TO implemented

Quote:

Quote:

I have started a python implementation of the AlGaAs noise budget. All parameters, functions, etc. are defined in a single notebook, and this same notebook generates the plot. The python uncertainties package facilitates estimation of uncertainties in material parameters, optical parameters, etc.

Currently, the coating thermo-optic trace is not an actual calculation; it is just a flat line culled from figure 5.9 of Tara's thesis.

The PDH shot noise trace is shown assuming an incident power of 1 mW on each cavity, a PDH modulation index of 0.2 rad, and a cavity visibility of 0.92.

To do:

  • Finish implementing true TO calculation
  • Add photothermal (requires RIN data)
  • Add seismic (requires seismic data, seismic stack TF data)

I've implemented the TO calculation following Evans et al. (2008), along with the so-called Yamamoto correction for the CTE.

These changes are on the SVN.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  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.

  1482   Wed Aug 20 19:00:03 2014 EvanNotesNoiseBudgetAlGaAs python noise budget

Quote:

I have started a python implementation of the AlGaAs noise budget. All parameters, functions, etc. are defined in a single notebook, and this same notebook generates the plot. The python uncertainties package facilitates estimation of uncertainties in material parameters, optical parameters, etc.

Currently, the coating thermo-optic trace is not an actual calculation; it is just a flat line culled from figure 5.9 of Tara's thesis.

The PDH shot noise trace is shown assuming an incident power of 1 mW on each cavity, a PDH modulation index of 0.2 rad, and a cavity visibility of 0.92.

To do:

  • Finish implementing true TO calculation
  • Add photothermal (requires RIN data)
  • Add seismic (requires seismic data, seismic stack TF data)
Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  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
  1480   Tue Aug 19 10:35:07 2014 EvanDailyProgressRefCavSouth cavity finesse

I swept the south NPRO PZT with a 2 Vpp, 5 Hz triangle wave and watched the transmission of the south cavity using a PDA100A. I saved three such transmission sweeps from the oscilloscope, and then performed Lorentzian fits on each of them in order to get the cavity pole.

From the Lorentzian fits along with the 4.4(2) MHz/V calibration found earlier [and FSR = 4070(30) MHz], I find the following:

  • Lower resonance: cavity pole of 135(3) kHz, corresponding to a finesse of 15100(340). This gives the total round-trip loss as 417(10) ppm.
  • Upper resonance: cavity pole of 139(9) kHz, corresponding to a finesse of 14600(1000). This gives the total round-trip loss as 429(28) ppm.
Attachment 1: southsweep.pdf
southsweep.pdf
Attachment 2: southsweep.zip
  1479   Tue Aug 19 09:25:19 2014 EvanDailyProgressRefCavPolarization selectivity of south AlGaAs cavity

I temporarily removed the QWP immediately before the periscope. Then I added a HWP directly in front of the vacuum chamber window.

While sweeping the laser across the south TEM00 resonances, I monitored the peak voltage of each resonance.

For this particular HWP mount, rotating the mount to 23(1) degrees produces s polarization, in the sense that placing this HWP between two PBSs causes the second PBS to reflect 100% of the beam.

Attachment 1: pol.pdf
pol.pdf
Attachment 2: southpol.zip
  1478   Tue Aug 19 08:55:24 2014 EvanDailyProgressLaserSouth NPRO PZT acutation coefficient

I used a function generator to drive the south NPRO PZT with a triangle wave. Then with the 14.75 MHz sidebands on, I used a PDA100A to watch the south cavity transmission.

Looking by eye at the carrier and sideband transmission peaks, I find an actuation coefficient of 4.4(2) MHz/V, which is higher than what Tara measured in 2010 (maybe the coefficient depends on which axial mode the NPRO is operating on?)

From the attached plot, we can also see that the mode splitting for the south cavity is 2.0(4) MHz.

Attachment 1: south_npro.pdf
south_npro.pdf
Attachment 2: southnpro.zip
  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.

  1475   Sat Aug 16 13:06:35 2014 EvanNotesRefCavExpected AlGaAs cavity parameters

Just so we have a concise table that we can refer to:

  North South Note
Length 1.45(1)" = 3.68(3) cm  
FSR 4070(28) MHz c/2L
Mirror ROCs 1.000(3) m Uncertainty is a guess
g factor 0.9632(3) 1 − L/R
TMS 353(3) MHz νFSR×arccos(g)/π
Transmission loss 297(6) ppm 317(3) ppm ctn:1468
Scatter loss 30 ppm 30 ppm Crude guess
Absorption loss 30(30) ppm 30(30) ppm Extremely crude guess
Finesse 16700(1400) 15100(340), 14600(1000) 2π/(losses); ctn:1480
Cavity pole
116(10) kHz 135(3) kHz, 139(10) kHz νFSR/(2F)

 

  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.

  1473   Thu Aug 14 15:27:36 2014 EvanDailyProgressPMCPMC encap measurements

OD: 1.63"

Depth: ca. 0.9"

Minimum clearance between cap and mount: ca. 0.5"

  1472   Thu Aug 14 15:24:14 2014 Emily NotesopticTemporarily changed angle on half wave plate

(Laser going to ACAV) I changed the angle of the half-waveplate before the PBS in order to increase the amount of power going into the fiber that goes to gyro lab.  Its original position was at 277 degrees.  I put a beam dump behind the lens (PLCX-25.4-38.6-UV-1064) so the higher power does not reach the photodiode.  The new position is at 248 degrees.  I will move it back before I leave.

  1471   Thu Aug 14 15:23:36 2014 Emily, EvanNotesopticAOM fiber noise cancellation

 New setup for fiber phase noise cancellation with one AOM

 
We re-did mode-matching calculations and replaced the lenses before the fiber input in order to optimize the amount of power that comes out of the fiber.  The waist coming out of the PMC is 370 microns.  Following the PMC are the following lenses: placed 7 inches away is a PLCX-25.4.128.8-UV-1064 with a focal length of 250mm, placed 29 inches away is a PLCX-25.4-64.4-C-1064 with a focal length of 125 mm, and placed 35 inches away is a KBX052 with a focal length of 50.2mm. This yields a waist of 69 microns going into the fiber.  Going into the fiber is about 1.1 mw and coming out is approximately 500 micro watts.  We replaced the VCO driver since it was not driving the AOM and had a deformed signal.  Now we are using a Marconi and low-noise amplifier to drive the AOM.  We also replaced the AOM with an Isomet AOM 1205c-843.  
 
We re-did mode-matching calculations into the AOM and to the mirror.  After the fiber output is a waist of 50 microns.  Placed 2 inches away is a: PLCX-25.4-33.7-UV-1064 with a focal length of 50mm, placed 10 inches away is a: PLCX-25.4-77.3-UV-1064 with a focal length of 150mm and placed 18 inches away is a: PLCX-25.4-36.1-UV-1064 with a focal length of 70mm.  The first two lenses before the AOM yield a was it of 150 microns going through the AOM (recommended waist from the Isomet AOM 1205c-843 manual) and the third lens yields a waist of 156 microns at the mirror.  We used a beam dump to block the zeroth order beam, so the only the first order beam is double passed through the fiber.  
 
We are using the same setup to beat the double passed beam with the original beam onto a new focus 1811 photodiode.  The original beam has a power of 850 micro-watts and the double-passed beam has a power of 10 micro-watts. While the efficiency can be improved, for now we will work with what we have in order to prove that our new setup with 1 AOM will cancel the noise in the system.  
 Final_AOM_Setup.pdf
In this setup, we lock the optical beat to the marconi in a PLL.  
The AC signal optical beat fluctuation was 198-428mV.
Once the optical beat was locked to the marconi, we measured the error signal and control signal.  We also measured the control signal without cancellation to make sure that this works.  In order to do the measurement without cancellation, we locked the marconi to the optical beat.  We also measured the open loop transfer function with and without cancellation.  The following data was obtained: 
 
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  1470   Sun Aug 10 18:42:48 2014 EvanNotesPMCPMC heater, v2

I'm building this instead:

pmc_heater_2.jpg

  1469   Wed Aug 6 17:29:30 2014 EvanDailyProgressRefCavOptical contacting

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.

Attachment 1: 114.jpg
114.jpg
  1468   Tue Aug 5 18:10:31 2014 EvanDailyProgressopticAlGaAs mirror transmissions; optical contacting

I used the ThorLabs power meter to get the transmission coefficients for the five AlGaAs mirrors.

For each measurement, I wrote down the incident power (20 mW nominal), the transmitted power (≈3.5 µW, depending on the mirror and background light level), and the transmitted power with the beam blocked (to get the dark power).

Mirror
Transmission (ppm) Average (ppm)
#114 142(6) 142(6)
#132 162.4(1.4), 159.8(2.1), 163.0(2.1) 161.7(1.9)
#137 149.8(3.4), 149.5(2.0), 148.0(2.0) 149.1(2.5)
#141

154.9(2.0), 155.4(2.1), 155.4(2.1)

155.2(2.1)
#143 155.6(2.1), 154.7(2.1) 155.2(2.1)

In other news, Tara bonded mirror #114 to spacer #95. The contacting seems to be tough going because of some recalcitrant smudges on the substrate surfaces.

Attachment 1: almost.jpg
almost.jpg
Attachment 2: done.jpg
done.jpg
  1467   Tue Aug 5 14:54:36 2014 EvanDailyProgressopticBRDF of AlGaAs mirror 132 after cleaning

Quote:

Incident power: 20.0(1) mW

Exposure times used: 25 ms, 50 ms, 200 ms, 500 ms, 1000 ms

Transmitted power: 3.34(2) µW. This gives a transmission of 167(1) ppm for this mirror.

TIS from 16° to 73° is 18(1) ppm.

Data and code are on the SVN at CTNLab/measurements/2014_08_05.

 Basically the same story with 132.

Attachment 1: 132brdf.pdf
132brdf.pdf
  1466   Tue Aug 5 08:14:14 2014 EvanDailyProgressopticBRDF of AlGaAs mirror 141 after cleaning

Incident power: 20.0(1) mW

Exposure times used: 25 ms, 50 ms, 200 ms, 500 ms, 1000 ms

Transmitted power: 3.34(2) µW. This gives a transmission of 167(1) ppm for this mirror.

TIS from 16° to 73° is 18(1) ppm.

Data and code are on the SVN at CTNLab/measurements/2014_08_05.

Attachment 1: 141brdf.pdf
141brdf.pdf
  1465   Mon Aug 4 15:23:06 2014 EvanDailyProgressopticBRDF of AlGaAs mirror 114 after cleaning

[Tara, Evan]

Tara also took a BRDF measurement of #114 after cleaning it.

After cleaning, TIS from 14° to 71° is 2.7(5) ppm. Much improved.

Data and code are on the SVN at CTNlab/measurements/2014_07_31.

Attachment 1: 114_cleaned_brdf.pdf
114_cleaned_brdf.pdf
  1464   Sat Aug 2 23:07:39 2014 EvanNotesNoiseBudgetAlGaAs python noise budget

I have started a python implementation of the AlGaAs noise budget. All parameters, functions, etc. are defined in a single notebook, and this same notebook generates the plot. The python uncertainties package facilitates estimation of uncertainties in material parameters, optical parameters, etc.

Currently, the coating thermo-optic trace is not an actual calculation; it is just a flat line culled from figure 5.9 of Tara's thesis.

The PDH shot noise trace is shown assuming an incident power of 1 mW on each cavity, a PDH modulation index of 0.2 rad, and a cavity visibility of 0.92.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1463   Thu Jul 31 09:40:24 2014 EvanDailyProgressopticBRDF of AlGaAs mirror 114

[Tara, Evan]

Tara took a BRDF measurement yesterday of AlGaAs mirror #114.

In this measurement, the return beam is dumped using black anodized foil instead of a razor blade dump. This seems to make the peak at 20° disappear, and now we get a more or less monotonic falloff in scattered power.

TIS from 14° to 71° is 39(6) ppm.

Data and code are on the SVN at CTNlab/measurements/2014_07_30.

Attachment 1: 114brdf.pdf
114brdf.pdf
  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.

 

  1461   Tue Jul 29 11:40:09 2014 EvanDailyProgressopticBRDF of AlGaAs mirror 143

[Tara, Evan]

Yesterday we took a scatter measurement of AlGaAs mirror #143. Instead of one bright scattering center, we saw 3.

The procedure is identical to the procedure used for mirror #137, although the exposure settings and choice of angles are a bit different (see the attached plot). Also, we used 20 mW of incident power instead of 10 mW.

Total integrated scatter from 14° to 82° is 80(8) ppm.

Data, images, and plot-generating code are on the SVN at CTNlab/measurements/2014_07_28.

Attachment 1: 143brdf.pdf
143brdf.pdf
  1460   Sun Jul 27 19:46:16 2014 EvanDailyProgressopticBRDF of AlGaAs mirror 137B1

[Tara, Evan]

We replaced the Lambertian diffuser with AlGaAs mirror 137B1. We intentionally induced a nonzero AOI of the incident beam, so that the reflected beam could be dumped cleanly. At a distance of 25.7(3) cm back from the mirror, the reflected and incident beams were separated by 1.3(1) cm, giving an AOI of 1.45(11)°.

  1. We measured the incident laser power as 9.94(2) mW.
  2. We set the exposure time of the camera to 250 ms.
  3. We swung the boom to 13°, 16°, 19°, 22°, 25°, 28°, 31°, and 34°. At each angle, we took 5 CCD images with the beam incident, and 1 CCD image with the beam blocked.
  4. We measured the incident laser power as 9.95(2) mW.
  5. Because the scattered power had fallen off sharply by 30°, we turned up the exposure time to 1.00 s.
  6. We swung the boom to 31°, 34°, 37°, 40°, and 43°. At each angle, we took 5 CCD images with the beam incident, and 1 CCD image with the beam blocked.
  7. We measured the incident laser power as 10.08(2) mW.
  8. We swung the boom to 46°, 49°, 52°, 55°, 58°, 61°, 64°, 67°, and 70°. At each angle, we took 5 CCD images with the beam incident, and 1 CCD image with the beam blocked.
  9. We measured the incident laser power as 10.06(2) mW.

For all of these measurements, the two ND filters (OD1.5+OD3.0) were not attached; just the RG1000. With the ThorLabs power meter, we measured the combined transmissivity of these two ND filters to be 1865(14) ppm.

The first attachment shows an example CCD image. The second attachment shows the raw counts, the inferred scattered power, and the BRDF.

Attachment 1: ccdImage.pdf
ccdImage.pdf
Attachment 2: 137brdf.pdf
137brdf.pdf
  1459   Fri Jul 25 14:21:34 2014 EvanDailyProgressopticCalibration for scattered light measurement

Quote:

 Do we need to improve this before moving on to the AlGaAs BRDF measurement?

Yes.

We added an OD1.5, an OD3.0, and an RG1000 in front of the camera lens (note that these ODs are probably specked for something other than 1064 nm). Then we increased the exposure time to 20 ms. For the AlGaAs measurement, we may need to increase it even further in order to get good statistics.

Then we fixed the boom at 25° and varied the power using the upstream HWP + PBS combo.

For each power level, we took a measurement with the power meter, then 10 CCD images, then another measurement with the power meter. From this we are able to extract nominal values and uncertanties for the power level and the counts. The result is attached. The calibration has about a 4% uncertainty.

 

Note (Tara): The power measurement includes the solid angle of 3.375 x10^-3 str ( detector diameter = 0.4 inch, distance from the sample = 15.5 cm)

Attachment 1: cal.pdf
cal.pdf
  1458   Fri Jul 25 08:12:40 2014 EvanDailyProgressopticCalibration for scattered light measurement

Quote:

The first attachment is the BRDF of the diffuser based on the power data. The second is the inferred calibration between total CCD counts (with background counts subtracted) and scattered power. The correlation is not great. We may want to retake this data with the room lights off, and also we may want to take multiple exposures per angle setting (that way we can make some estimate of the uncertainty in the CCD counts).

 I put the boom at 15° and took four sets of five exposures. Then I ran my image processing code again to get an uncertainty in the count values. I get the following:

  • Beam incident, room lights on: 546(31) × 103 cts
  • Beam blocked, room lights on: 417(9) × 103 cts
  • Beam incident, room lights off: 547(34) × 103 cts
  • Beam blocked, room lights off: 410(2) × 103 cts

For each set of five, the nominal value is the mean and the uncertainty is the standard deviation of the total counts within the 200×200 pixel region around the beam. Again the exposure time is 100 µs and there was an RG1000 filter in front of the camera lens.

Using a fractional uncertainty of 31/546 = 0.057 for yesterday's background-subtracted total counts, I reran the calibration code. The new plot is attached. The calibration slope (and its uncertainty) doesn't change much, but we can see that the uncertainties in the total counts are quite large. Do we need to improve this before moving on to the AlGaAs BRDF measurement?

Attachment 1: ccdCal.pdf
ccdCal.pdf
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