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  16089   Wed Apr 28 10:56:10 2021 Anchal, PacoUpdateSUSIMC Filters diagnosed

Good morning!

We ran the f2a filter test for MC1, MC2, and MC3.


The new filters differ from previous versions by a adding non-unity Q factor for the pole pairs as well.

\frac{f^2 - i \frac{f_z}{Q}f + f_z^2}{f^2 - i \frac{f_0}{Q}f + f_0^2}
This in terms of zpk is: [ [zr + i zi, zr - i zi], [pr + i pi, pr - i pi], 1] where
z_r = -\frac{f_z}{2Q}, \quad z_i = f_z \sqrt{1 - \frac{1}{4Q^2}}, \quad p_r = -\frac{f_0}{2Q}, \quad p_i = f_0 \sqrt{1 - \frac{1}{4Q^2}}, \quad f_z = f_0 \sqrt{G_{DC}}

  • Attachment #1 shows the filters for MC1 evaluated for Q=3, 7,and 10.
  • Attachment #2 shows the filters for MC2 evaluated for Q=3, 7, and 10.
  • Attachment #3 shows the filters for MC3 evaluated for Q=3, 7, and 10.
  • Attachment #4 shows the bode plots generated by foton after uploading for Q=3 case.

We uploaded all these filters using foton, into the three last FM slots on the POS output gain coil.


We ran tests on all suspended optics using the following (nominal) procedure:

  1. Upload new input matrix
  2. Lower the C1:IOO-WFS_GAIN to 0.05.
  3. Upload AC coil balancing gains.
  4. Take ASD for the following channels:
    • C1:IOO-MC_WFS1_PIT_IN1
    • C1:IOO-MC_WFS1_YAW_IN1
    • C1:IOO-MC_WFS2_PIT_IN1
    • C1:IOO-MC_WFS2_YAW_IN1
  5. For the following combinations:
    • No excitation** + no filter
    • No excitation + filter
    • Excitation + no filter
    • Excitation + filter

** Excitation = 0.05 - 3.5 Hz uniform noise, 100 amplitude, 100 gain


  • Attachment 5-7 give the test results for MC1, MC2 and MC3.
  • In each pdf, the three pages show ASD of TRANS QPD, WFS1 and WFS2 channels' PIT and YAW, respectively.
  • Red/blue correspond to data taken while F2A filters were on. Pink/Cyan correspond to data taken with filters off.
  • Solid curves were taken with excitation ON and dashed curves were taken with excitation off.
  • We see good suppression of POS-> PIT coupling in MC2 and MC3. POS->YAw is minimally affected in all cases.
  • MC1 is clearly not doing good with the filters and probably needs readjustement. Something to do later in the future.
Attachment 1: IMC_F2A_Params_MC1.pdf
IMC_F2A_Params_MC1.pdf IMC_F2A_Params_MC1.pdf IMC_F2A_Params_MC1.pdf
Attachment 2: IMC_F2A_Params_MC2.pdf
IMC_F2A_Params_MC2.pdf IMC_F2A_Params_MC2.pdf IMC_F2A_Params_MC2.pdf
Attachment 3: IMC_F2A_Params_MC3.pdf
IMC_F2A_Params_MC3.pdf IMC_F2A_Params_MC3.pdf IMC_F2A_Params_MC3.pdf
Attachment 4: IMC_F2A_Foton.pdf
IMC_F2A_Foton.pdf IMC_F2A_Foton.pdf IMC_F2A_Foton.pdf
Attachment 5: MC1_POS2ANG_Filter_Test.pdf
MC1_POS2ANG_Filter_Test.pdf MC1_POS2ANG_Filter_Test.pdf MC1_POS2ANG_Filter_Test.pdf
Attachment 6: MC2_POS2ANG_Filter_Test.pdf
MC2_POS2ANG_Filter_Test.pdf MC2_POS2ANG_Filter_Test.pdf MC2_POS2ANG_Filter_Test.pdf
Attachment 7: MC3_POS2ANG_Filter_Test.pdf
MC3_POS2ANG_Filter_Test.pdf MC3_POS2ANG_Filter_Test.pdf MC3_POS2ANG_Filter_Test.pdf
  15902   Thu Mar 11 08:13:24 2021 Paco, AnchalUpdateSUSIMC First Free Swing Test failed due to typo, restarting now

[Paco, Anchal]

The triggered code went on at 5:00 am today but a last minute change I made yesterday to increase number of repititions had an error and caused the script to exit putting everything back to normal. So as we came in the morning, we found the mode cleaner locked continuously after one free swing attempt at 5:00 am. I've fixed the script and ran it for 2 hours starting at 8;10 am. Our plan is to get some data atleast to play with when we are here. If the duration is not long enough, we'll try to run this again tomorrow morning. The new script is running on same tmux session 'MCFreeSwingTest' on Rossa

10:13 the script finished and IMC recovered lock.

Thu Mar 11 10:58:27 2021

The test ran succefully with the mode cleaner optics coming back to normal in the end of it. We wrote some scripts to read data and analyze it. More will come in future posts. No other changes were made today to the systems.

  16679   Thu Feb 24 19:26:32 2022 AnchalUpdateGeneralIMC Locking

I think I have aligned the cavity, including MC1 such that we are seeing flashing of fundamental mode and significant transmission sum value as well.However, I'm unable to catch lock following Koji's method in 40m/16673. Autolocker could not catch lock either. Maybe I am doing something wrong, I'll pickup again tomorrow, hopefully the cavity won't drift too much in this time.

  16685   Sun Feb 27 00:37:00 2022 KojiUpdateGeneralIMC Locking Recovery


- IMC was locked.
- Some alignment change in the output optics.
- The WFS servos working fine now.
- You need to follow the proper alignment procedure to recover the good alignment condition.

- Basically followed the previous procedure 40m/16673.
- The autolocker was turned off. Used MC2 and MC3 for the alignment.
- Once I hit the low order modes, increased the IN1 gain to acquire the lock. This helped me to bring the alignment to TEM00
- Found the MC2 spot was way too off in pitch and yaw.
- Moved MC1/2/3 to bring the MC2 spot around the center of the mirror.
- Found a reasonably good visibility (<90%) at a MC2 spot. Decided this to be the reference (at least for now)

SP Table Alignment Work
- Went to the SP table and aligned the WFS1/2 spots.
- I saw no spot on the camera. Found that the beam for the camera was way too weak and a PO mirror was useless to bring the spot on the CCD.
- So, instead, I decided to catch an AR reflection of the 90% mirror. (See Attachment 1)
- This made the CCD vulnerable to the stronger incident beam to the IMC. Work on the CCD path before increasing the incident power.

MC2 end table alignment work
- I knew that the focusing lens there and the end QPD had inconsistent alignment.
- The true MC2 spot needs to be optimized with A2L (and noise analysis / transmitted beam power analysis / etc)
- So, just aligned the QPD spot using today's beam as the temporary target of the MC alignment. (See Attachment 2)

Resulting CCD image on the quad display (Attachment 3)

WFS Servo
- To activate the WFS with the low transmitted power, the trigger threshold was reduced from 5000 to 500. (See Attachment 4)
- WFS offset was reset with /opt/rtcds/caltech/c1/scripts/MC/WFS/WFS_RF_offsets
- Resulting working state looks like Attachment 5

Attachment 1: PXL_20220226_093809056.jpg
Attachment 2: PXL_20220226_093854857.jpg
Attachment 3: PXL_20220226_100859871.jpg
Attachment 4: Screenshot_2022-02-26_01-56-31.png
Attachment 5: Screenshot_2022-02-26_01-56-47.png
  11795   Sat Nov 21 00:46:33 2015 KojiUpdateIOOIMC OLTF

Here is the comparison before and after the fix.

Before the work, the UGF was ~40kHz. The phase margin was ~5deg. This caused huge bump of the frequency noise.

After the LO power increase, I had to reduce the MC loop gain (VCO Gain) from 18dB to 6dB. This resulted 4dB (x2.5) increase of the OLTF. This means that my fix increased the optical gain by 16dB (x6.3). The resulting UGF and phase mergin were measured to be 117kHz and 31deg, respectively.


Now I was curious to see if the PMC err shows reasonable improvement when the IMC is locked. Attachment 2 shows the latest comparison of the PMC err with and without the IMC locked. The PMC error has been taken up to 500kHz. The errors were divided by 17.5kHz LPF and 150kHz LPF to compensate the sensing response. The PMC cavity pole was ignored in this calculation. T990025 saids the PMC finesse is 4400 and the cavity pole is 174kHz. If this is true, this also needs to be applied.


1. Now we can see improvement of the PMC error in the region between 10kHz to 70kHz.

2. The sharp peak at 8kHz is due to the marginally stable PMC servo. We should implement another notch there. T990025 suggests that the body resonance of the PMC spacer is somewhere around there. We might be able to damp it by placing a lossy material on it.

3. Similarly, the features at 12kHz and 28kHz is coming from the PMC. They are seen in the OLTF of the PMC loop.

4. The large peak at 36kHz does not change with the IMC state. This does mean that it is coming from the laser itself, or anything high-Q of the PMC. This signal is seen in the IMC error too.

5. 72kHz, 108kHz, 144kHz: Harmonics of 36kHz?

6. Broad feature from 40kHz to 200kHz. The IMC loop is adding the noise. This is the frequency range of the PC drive. Is something in the PC drive noisy???

7. The feature at 130kHz. Unknown. Seems not related to IMC. The laser noise or the PMC noise.

Remaining IMC issues:

Done (Nov 23, 2015) - 29.5MHz oscillator output degraded. Possibly unstable and noisy. Do we have any replacement? Can we take a Marconi back from one of the labs?

Done (Nov 23, 2015) - Too high LO?

- Large 36kHz peak in the IMC

- IMC loop shape optimization

- IMC locking issue. The lock streatch is not long.

- IMC PC drive issue. Could be related to the above issue.

Maybe not relevant - PC drive noise?

Attachment 1: IMC_OLTF.pdf
Attachment 2: PMC_noise_comparison.pdf
  11798   Sun Nov 22 12:12:17 2015 KojiUpdateIOOIMC OLTF

Well. I thought a bit more and now I think it is likely that this is just the servo bump as you can see in the closed-loop TF.


6. Broad feature from 40kHz to 200kHz. The IMC loop is adding the noise. This is the frequency range of the PC drive. Is something in the PC drive noisy???

  17194   Mon Oct 17 17:42:35 2022 JCHowToOPLEV TablesIMC Reflected beam sketch

I sketched up a quick drawing with estimated length for the IMC reflected beam. This includes the distances and focal length. Distances are from optic to optic. 

Attachment 1: Screenshot_2022-10-18_093033.png
  7256   Thu Aug 23 12:17:39 2012 ManasaUpdate IMC Ringdown

The ringdown measurements are in progress. But it seems that the MC mirrors are getting kicked everytime the cavity is unlocked by either changing the frequency at the MC servo or by shutting down the input to the MC. This means what we've been observing is not the ringdown of the IMC alone. Attached are MC sus sensor data and the observed ringdown on the oscilloscope.  I think we need to find a way to unlock the cavity without the mirrors getting kicked....in which case we should think about including an AOM or using a fast shutter before the IMC.

P.S. The origin of the ripples at the end of the ringdown still are of unknown origin. As of now, I don't think it is because of the mirrors moving but something else that should figured out.

Attachment 1: mozilla.pdf
Attachment 2: MC_sus.pdf
  7257   Thu Aug 23 15:35:33 2012 ranaUpdate IMC Ringdown


 It is HIGHLY unlikely that the IMC mirrors are having any effect on the ringdown. The ringdowns take ~20 usec to happen. The mirrors are 0.25 kg and you can calculate that its very hard to get enough force to move them any appreciable distance in that time.

  7260   Thu Aug 23 17:51:25 2012 ManasaUpdate IMC Ringdown



 It is HIGHLY unlikely that the IMC mirrors are having any effect on the ringdown. The ringdowns take ~20 usec to happen. The mirrors are 0.25 kg and you can calculate that its very hard to get enough force to move them any appreciable distance in that time.

The huge kick observed in the MC sus sensors seem to last for ~10usec; almost matching the observed ringdown decay time. We should find a way to record the ringdown and the MC sus sensor data simultaneously to know when the mirrors are exactly moving during the measurement process. It could also be that the moving mirrors were responsible for the ripples observed later during the ringdown as well.

* How fast do the WFS respond to the frequency switching (time taken by WFS to turn off)? I think this information will help in narrowing down the many possible explanations to a few.

  15183   Mon Feb 3 13:54:10 2020 YehonathanUpdateIOOIMC Ringdowns extended data analysis

I extended the ringdown data analysis to the reflected beam following Isogai et al.

The idea is that measuring the cavity's reflected light one can use known relationships to extract the transmission of the cavity mirrors and not only the finesse.

The finesse calculated from the transmission ringdown shown in the previous elog is 1520 according to the Zucker model, 1680 according to the first exponential and 1728 according to the second exponential.

Attachment 1 shows the measured reflected light during an IMC ringdown in and out of resonance and the values that are read off it to compute the transmission.

The equations for m1 and m3 are the same as in Isogai's paper because they describe a steady-state that doesn't care about the extinction ratio of the light.

The equation for m2, however, is modified due to the finite extinction present in our zeroth-order ringdown.

Modelling the IMC as a critically coupled 2 mirror cavity one can verify that:

m_2=P_0KR\left[T-\alpha\left(1-R\right)\right]^2+\alpha^2 P_1

Where P_0 is the coupled light power 

P_1 is the power rejected from the cavity (higher-order modes, sidebands)

K=\left(\mathcal{F} /\pi \right )^2 is the cavity gain.

R and T are the power reflectivity and transmissivity per mirror, respectively.

\alpha^2 is the power attenuation factor. For perfect extinction, this is 0.

Solving the equations (m1 and m3 + modified m2), using Zucker model's finesse, gives the following information:

Loss per mirror = 84.99 ppm
Transmission per mirror = 1980.77 ppm
Coupling efficiency (to TEM00) = 97.94%
Attachment 1: IMCTransReflAnalysis_anotated.pdf
  15190   Wed Feb 5 21:13:17 2020 YehonathanUpdateIOOIMC Ringdowns extended data analysis

I translate the results obtained in the previous elog to the IMC 3 mirror cavity. I assume the loss in each mirror in the IMC is equal and that M2 has a negligible transmission.

I find that to a very good approximation the loss per IMC mirror is 2/3 the loss per mirror in the 2 mirror cavity model. That is the loss per mirror in the IMC is 56 ppm. The transmission per mirror in the IMC is the same as in the 2 mirror model, which is 1980 ppm.

The total transmission is the same as in the 2 mirror model and is given by:

\frac{P_0}{P_0+P1}KT^2\approx 90\%

where \frac{P_0}{P_0+P1} is the coupling efficiency to the TEM00 mode.

  15175   Wed Jan 29 12:40:24 2020 YehonathanUpdateIOOIMC Ringdowns preliminary data analysis

I analyze the IMC ringdown data from last night. 

Attachment 1 shows the normalized raw data. Oscillations come in much later than in Gautam's measurement. Probably because the IMC stays locked.

Attachment 2 shows fits of the transmitted PD to unconstrained double exponential and the Zucker model.

Zucker model gives time constant of 21.6us

Unconstrained exponentials give time constants of 23.99us and 46.7us which is nice because it converges close to the Zucker model.

Attachment 1: IMCRingdownNormalizedRawdata.pdf
Attachment 2: IMCTransPDFits.pdf
  15912   Fri Mar 12 11:44:53 2021 Paco, AnchalUpdatetrainingIMC SUS diagonalization in progress

[Paco, Anchal]

- Today we spent the morning shift debugging SUS input matrix diagonalization. MC stayed locked for most of the 4 hours we were here, and we didn't really touch any controls.

  15258   Fri Mar 6 01:12:10 2020 gautamUpdateElectronicsIMC Servo IN2 path looks just fine

It seems like the AO path gain stages on the IMC Servo board work just fine. The weird results I reported earlier were likely a measurement error arising from the fact that I did not disconnect the LEMO IN2 cable while measuring using the BNC IN2 connector, which probably made some parasitic path to ground that was screwing the measurement up. Today, I re-did the measurement with the signal injected at the IN2 BNC, and the TF measured being the ratio of TP3 on the board to a split-off of the SR785 source (T-eed off). Attachments #1, #2 shows the result - the gain deficit from the "expected" value is now consistent with that seen on other sliders.

Note that the signal from the CM board in the LSC rack is sent single-ended over a 2-pin LEMO cable (whose return pin is shorted to ground). But it is received differentially on the IMC Servo board. I took this chance to look for evidence of extra power line noise due to potential ground loops by looking at the IMC error point with various auxiliary cables connected to the board - but got distracted by some excess noise (next elog).

Attachment 1: AO_inputTFs_5Mar.pdf
Attachment 2: sliderCal_5Mar.pdf
  15257   Thu Mar 5 19:51:14 2020 gautamUpdateElectronicsIMC Servo board being tested

I am running some tests on the IMC servo board with an extender card so the IMC will not be locking for a couple of hours.

  16174   Wed Jun 2 09:43:30 2021 Anchal, PacoSummarySUSIMC Settings characterization

Plot description:

  • We picked up three 10 min times belonging to the three different configurations:
    • 'Old Settings': IMC Suspension settings before Paco and I changed anything. Data taken from Apr 26, 2021, 00:30:42 PDT (GPS 1303457460).
    • 'New Settings': New input matrices uploaded on April 28th, along with F2A filters and AC coil balancing gains (see 16091). Data taken from May 01, 2021, 00:30:42 PDT (GPS 1303889460).
    • 'New settings with new gains' Above and new suspension damping gains uploaded on May5th, 2021 (see 16120). Data taken from May 07, 2021, 03:10:42 PDT (GPS 1304417460).
  • Attachment 1  shows the RMS seismic noise along X direction between 1 Hz and 3 Hz picked from C1:PEM-RMS_BS_X_1_3 during the three time durations chosen. This plot is to establish that RMS noise levels were similar and mostly constant. Page 2 shows the mean ampltidue spectral density of seismic noise in x-direction over the 3 durations.
  • Attachment 2 shows the transfer function estimate of seismic noise to MC_F during the three durations. Page 1 shows ratio of ASDs taken with median averaging while page 2 shows the same for mean averaging.
  • Attachment 3 shows the transfer function estimate of seismic noise to MC_TRANS_PIT during the three durations. Page 1 shows ratio of ASDs taken with median averaging while page 2 shows the same for mean averaging.
  • Attachment 4 shows the transfer function estimate of seismic noise to MC_TRANS_YAW during the three durations. Page 1 shows ratio of ASDs taken with median averaging while page 2 shows the same for mean averaging.


  • From Attachment 2 Page 1:
    • We see that 'old settings' caused the least coupling of seismic noise to MC_F signal in most of the low frequency band except between 1.5 to 3 Hz where the 'new settings' were slightly better.
    • 'new settings' also show less coupling in 4 Hz to 6 Hz band, but at these frequencies, seismix noise is filtered out by suspension, so this could be just coincidental and is not really a sign of better configuration.
    • There is excess noise coupling seen with 'new settings' between 0.4 Hz and 1.5 Hz. We're not sure why this coupling increased.
    • 'new settings with new gains' show the most coupling in most of the frequency band. Clearly, the increased suspension damping gains did not behaved well with rest of the system.
  • From Attachment 3 Page 1:
    • Coupling to MC_TRANS_PIT error signal is reduced for 'new settings' in almost all of the frequency band in comparison to the 'old settings'.
    • 'new settings with new gains' did even better below 1 Hz but had excess noise in 1 Hz to 6 Hz band. Again increased suspension damping gains did not help much.
    • But low coupling to PIT error for 'new settings' suggest that our decoupling efforts in matrix diagonalization, F2A filters and ac coil balancing worked to some extent.
  • From Attachment 4 Page 1:
    • 'new settings' and 'old settings' have the same coupling of seismic noise to MC_TRANS_YAW in all of the frequency band. This is in-line witht eh fact that we found very little POS to YAW couping in our analysis before and there was little to no change for these settings.
    • 'new settings with new gains' did better below 1 Hz but here too there was excess coupling between 1 Hz to 9 Hz.
  • Page 1 vs Page 2:
    • Mean and median should be same if the data sample was large enough and noise was stationary. A difference between the two suggests existence of outliers in the data set and median provides a better central estimate in such case.
    • MC_F: Mean and median are same below 4 hz. There are high frequency outliers above 4 Hz in 'new settings with new gains' and 'old settings' data sets, maybe due to transient higher free running laser frequency noise. But since, suspension settigns affect below 1 Hz mostly, the data sets chosen are stationary enough for us.
    • MC_TRANS_PIT: Mean ratio is lower for 'new settings' and 'old settings' in 0.3 hz to 0.8 Hz band. Same case above 4 Hz as listed above.
    • MC_TRANS_YAW:  Same as above.
  • Conclusion 1:  The 'new settings with new gains' cause more coupling to seismic noise, probably due to low phase margin in control loops. We should revert back the suspension damping gains.
  • Conclusion 2: The 'new settings' work as expected and can be kept when WFS loops are optimized further.
  • Conjecture: From our experience over last 2 weeks, locking the arms to the main laser with 'new settings with new gains' introduces noise in the arm length large enough that the Xend green laser does not remain locked to the arm for longer than tens of seconds. So this is definitely not a configuration in which we can carry out other measurements and experiments in the interferometer.
Attachment 1: seismicX.pdf
seismicX.pdf seismicX.pdf
Attachment 2: seismicXtoMC_F_TFest.pdf
seismicXtoMC_F_TFest.pdf seismicXtoMC_F_TFest.pdf
Attachment 3: seismicXtoMC_TRANS_PIT_TFest.pdf
seismicXtoMC_TRANS_PIT_TFest.pdf seismicXtoMC_TRANS_PIT_TFest.pdf
Attachment 4: seismicXtoMC_TRANS_YAW_TFest.pdf
seismicXtoMC_TRANS_YAW_TFest.pdf seismicXtoMC_TRANS_YAW_TFest.pdf
  16102   Thu Apr 29 18:53:33 2021 AnchalUpdateSUSIMC Suspension Damping Gains Test

With the input matrix, coil ouput gains and F2A filters loaded as in 16091, I tested the suspension loops' step response to offsets in LSC, ASCPIT and ASCYAW channels, before and after applying the "new damping gains" mentioned in 16066 and 16072. If these look better, we should upload the new (higher) damping gains as well. This was not done in 16091.

Note that in the plots, I have added offsets in the different channels to plot them together, hence the units are "au".

Attachment 1: MC1_SUSDampGainTest.pdf
MC1_SUSDampGainTest.pdf MC1_SUSDampGainTest.pdf MC1_SUSDampGainTest.pdf
Attachment 2: MC2_SUSDampGainTest.pdf
MC2_SUSDampGainTest.pdf MC2_SUSDampGainTest.pdf MC2_SUSDampGainTest.pdf
Attachment 3: MC3_SUSDampGainTest.pdf
MC3_SUSDampGainTest.pdf MC3_SUSDampGainTest.pdf MC3_SUSDampGainTest.pdf
  16110   Mon May 3 16:24:14 2021 AnchalUpdateSUSIMC Suspension Damping Gains Test Repeated with IMC unlocked

We repeated the same test with IMC unlocked. We had found these gains when IMC was unlocked and their characterization needs to be done with no light in the cavity. attached are the results. Everything else is same as before.


With the input matrix, coil ouput gains and F2A filters loaded as in 16091, I tested the suspension loops' step response to offsets in LSC, ASCPIT and ASCYAW channels, before and after applying the "new damping gains" mentioned in 16066 and 16072. If these look better, we should upload the new (higher) damping gains as well. This was not done in 16091.

Note that in the plots, I have added offsets in the different channels to plot them together, hence the units are "au".

Edit Tue May 4 14:43:48 2021 :

  • Adding zoomed in plots to show first 25s after the step.
  • MC1:
    • Our improvements by new gains are only modest.
    • This optic needs a more careful coil balancing first.
    • Still the ring time is reduced to about 5s for all step responses as opposed to 10s at old gains.
  • MC2:
    • The first page of MC2 might be bit misleading. We have not changed the damping gain for SUSPOS channel, so the longer ringing is probably just an artifact of somthing else. We didn't retake data.
    • In PIT and YAW where we increased the gain by a factor of 3, we see a reduction in ringing lifetime by about half.
  • MC3:
    • We saw the most optimistic improvement on this optic.
    • The gains were unusually low in this optic, not sure why.
    • By increasing SUSPOS gain from 200 to 500, we saw a reduction of ringing halftime from 7-8s to about 2s. Improvements are seen in other DOFs as well.
    • You can notice rightaway that YAW of MC3 keeps oscillating near resonance (about 1 Hz). Maybe more careful feedback shaping is required here.
    • In SUSPIT, we increased gain from 12 to 35 and saw a good reduction in both ringing time and initial amplitude of ringing.
    • In SUSYAW, we only increased the gain to 12 from 8, which still helped a lot in reducing big ringing step response to below 5s from about 12s.

Overall, I would recommend setting the new gains in the suspension loops as well to observe long term effects too.

Attachment 1: MC1_SusDampGainTest.pdf
MC1_SusDampGainTest.pdf MC1_SusDampGainTest.pdf MC1_SusDampGainTest.pdf
Attachment 2: MC2_SusDampGainTest.pdf
MC2_SusDampGainTest.pdf MC2_SusDampGainTest.pdf MC2_SusDampGainTest.pdf
Attachment 3: MC3_SusDampGainTest.pdf
MC3_SusDampGainTest.pdf MC3_SusDampGainTest.pdf MC3_SusDampGainTest.pdf
  16175   Wed Jun 2 16:20:59 2021 Anchal, PacoSummarySUSIMC Suspension gains reverted to old values

Following the conclusion, we are reverting the suspension gains to old values, i.e.

IMC Suspension Gains
  MC1 MC2 MC3
SUSPOS 120 150 200
SUSPIT 60 10 12
SUSYAW 60 10 8

While the F2A filters, AC coil gains and input matrices are changed to as mentioned in 16066 and 16072.

The changes can be reverted all the way back to old settings (before Paco and I changed anything in the IMC suspensions) by running python scripts/SUS/general/20210602_NewIMCOldGains/restoreOldConfigIMC.py on allegra. The new settings can be uploaded back by running python scripts/SUS/general/20210602_NewIMCOldGains/uploadNewConfigIMC.py on allegra.

Change time:

Unix Time = 1622676038

UTC Jun 02, 2021 23:20:38 UTC
Central Jun 02, 2021 18:20:38 CDT
Pacific Jun 02, 2021 16:20:38 PDT

GPS Time = 1306711256

  • Conclusion 1:  The 'new settings with new gains' cause more coupling to seismic noise, probably due to low phase margin in control loops. We should revert back the suspension damping gains.
  • Conclusion 2: The 'new settings' work as expected and can be kept when WFS loops are optimized further.
  • Conjecture: From our experience over last 2 weeks, locking the arms to the main laser with 'new settings with new gains' introduces noise in the arm length large enough that the Xend green laser does not remain locked to the arm for longer than tens of seconds. So this is definitely not a configuration in which we can carry out other measurements and experiments in the interferometer.


  16094   Thu Apr 29 10:52:56 2021 AnchalUpdateSUSIMC Trans QPD and WFS loops step response test

In 16087 we mentioned that we were unable to do a step response test for WFS loop to get an estimate of their UGF. The primary issue there was that we were not putting the step at the right place. It should go into the actuator directly, in this case, on C1:SUS-MC2_PIT_COMM and C1:SUS-MC2_YAW_COMM. These channels directly set an offset in the control loop and we can see how the error signals first jump up and then decay back to zero. The 'half-time' of this decay would be the inverse of the estimated UGF of the loop. For this test, the overall WFS loops gain,  C1:IOO-WFS_GAIN was set to full value 1. This test is performed in the changed settings uploaded in 16091.

I did this test twice, once giving a step in PIT and once in YAW.

Attachment 1 is the striptool screenshot for when PIT was given a step up and then step down by 0.01.

  • Here we can see that the half-time is roughly 10s for TRANS_PIT and WFS1_PIT corresponding to roughly 0.1 Hz UGF.
  • Note that WFS2 channels were not disturbed significantly.
  • You can also notice that third most significant disturbance was to TRANS_YAW actually followed by WF1 YAW.

Attachment 2 is the striptool screenshot when YAW was given a step up and down by 0.01. Note the difference in x-scale in this plot.

  • Here, TRANS YAW got there greatest hit and it took it around 2 minutes to decay to half value. This gives UGF estimate of about 10 mHz!
  • Then, weirdly, TRANS PIT first went slowly up for about a minutes and then slowly came dome in a half time of 2 minutes again. Why was PIT signal so much disturbed by the YAW offset in the first place?
  • Next, WFS1 YAW can be seen decaying relatively fast with half-life of about 20s or so.
  • Nothing else was disturbed much.

  • So maybe we never needed to reduce WFS gain in our measurement in 16089 as the UGF everywhere were already very low.
  • What other interesting things can we infer from this?
  • Should I sometime repeat this test with steps given to MC1 or MC3 optics?
Attachment 1: PIT_OFFSET_ON_MC2.png
Attachment 2: YAW_STEP_ON_MC2_complete.png
  15215   Sat Feb 15 12:56:24 2020 YehonathanUpdateIOOIMC Transfer function measurement

{Yehonathan, Meenakshi}

We measure the IMC transfer function using SR785.

We hook up the AOM driver to the SOURCE OUT, Input PD to CHANNEL ONE and the IMC transmission PD to CHANNEL TWO.

We use the frequency response measurement feature in the SR785. A swept sine from 100KHz to 100Hz is excited with an amplitude of 10mV.

Attachment 1 shows the data with a fit to a low pass filter frequency response.

IMC pole frequency is measured to be 3.795KHz, while the ringdowns predict a pole frequency 3.638KHz, a 4% difference.

The closeness of the results discourages me from calibrating the PDs' transfer functions.

I tend to believe the pole frequency measurement a bit more since it coincides with a linewidth measurement done awhile ago Gautam was telling me about.


I think of trying to try another zero-order ringdown but with much smaller excitation than what used before (500mV) and than move on to the first-order beam.

Also, it seems like the reflection signal in zero-order ringdown (Attachment 2,  green trace) has only one time constant similar to the full extinction ringdown. The reason is that due to the fact the IMC is critically coupled there is no DC term in the electric field even when the extinction of light is partial. The intensity of light, therefore, has only one time constant.

Fitting this curve (Attachment 3) gives a time constant of 18us, a bit too small (gives a pole of 4.3KHz). I think a smaller extinction ringdown will give a cleaner result.

Attachment 1: IMCFrequencyResponse.pdf
Attachment 2: IMCRingdownNormalizedRawdata.pdf
Attachment 3: IMCREFLPDFits.pdf
  11529   Tue Aug 25 16:09:54 2015 ericqUpdateIOOIMC Tweak

A little more information about the IMC loop tweak...

I increased the overall IMC loop gain by 4dB, and decreased the FAST gain (which determines the PZT/EOM crossover) by 3dB. This changed the AO transfer function from the blue trace to the green trace in the first plot. This changed the CARM loop open loop TF shape from the unfortunate blue shape to the more pleasing green shape in the second plot. The red trace is the addition of one super boost. 


Oddly, these transfer functions look a bit different than what I measured in March (ELOG 11167), which itself differed from the shaping done December of 2014 (ELOG 10841). 

I haven't yet attempted any 1F handoff of the PRMI since relocking, but back when Jenne and I did so in April, the lock was definitely less stable. My suspicion is that we may need more CARM supression; we never computed the loop gain requirement that ensures that the residual CARM fluctuations witnessed by, say, REFL55 are small enough to use as a reliable PRMI sensor.

I should be able to come up with this with data from last night. 

Attachment 1: imcTweak.pdf
Attachment 2: CARM_TF.pdf
  11538   Fri Aug 28 19:05:53 2015 ranaUpdateIOOIMC Tweak

Well, green looks better than blue, but it makes the PCDRIVE go high, which means its starting to saturate the EOM drive. So we can't just maximize the phase margin in the PZT/EOM crossover. We have to take into account the EOM drive spectrum and its RMS.

Also, your gain bump seems suspicious. See my TF measurements of the crossover in December. Maybe you were saturating the EOM in your TF ?

Lets find out what's happening with FSS servos over in Bridge and then modify ours to be less unstable.

  15318   Tue May 5 23:44:14 2020 gautamUpdateASCIMC WFS


I've been thinking about the IMC WFS. I want to repeat the sort of analysis done at LLO where a Finesse model was built and some inferences could be made about, for example, the Gouy phase separation b/w the sensors by comparing the Finesse sensing matrix to a measured sensing matrix. Taking the currently implemented output matrix as a "measurement" (since the IMC WFS stabilize the IMC transmission), I don't get any agreement between it and my Finesse model. Could be that the model needs tweaking, but there are several known issues with the WFS themselves (e.g. imbalanced segment gains).

Building the finesse model:

  • I pulled the WFS telescopes from Andres elogs/SURF report, which I think was the last time the WFS telescopes were modified.
  • The in-vacuum propagation distances were estimated from CAD diagrams.
  • According to my model, the Gouy phase separation between the two WFS heads is ~70 degrees, whereas Andres' a la mode simulations suggest more like 90 degrees. Presumably, some lengths/lenses are different between what I assume and what he used, but I continue the analysis anyway...
  • The appropriate power attenuations were placed in each path - one thing I noticed is that the BS that splits light between WFS1 and WFS2 is a 30/70 BS and not a 50/50, I don't see any reason why this should be (presumably it was to do with component availability). see below for Rana's comments.


  • The way the WFS servos are set up currently, the input matrix is diagonal while the output matrix encodes the sensing information.
  • In finesse, I measured the input matrix (i.e. response sensed in each sensor when an optic is dithered in angle). The length is kept resonant for the carrier (but not using a locking signal), which should be valid for small angular disturbances, which is the regime in which the error signals will be linear anyways.
  • Then I inverted the simulated sensing matrix so as to be able to compare with the CDS output matrix. Note that there is a relative gain scaling of 100 between the WFS paths and the MC2T QPD paths which I added to the simulation. I also normalized the columns of the matrix by the largest element in the column, in an attempt to account for the various other gains that are between the optical sensing and the digitizaiton (e.g. WFS demod boards, QPD transimpedance etc etc).
  • Attachment #1 shows the comparison between simulation and measurement. The two aren't even qualitatively similar, needs more thought...

Some notes about the WFS heads:

  • The transimpedance resistor is 1.5 kohms. With the gain stages, the transimpedance gain is nominally 37.5 kohms, and 3.75 kohms when the attenuation setting is engaged (as it is for 2/4 quadrants on each head).
  • Assuming a modulation depth of 0.1, the Johnson noise of the transimpedance resistor dominates (with the MAX4106 current noise a close second), and these heads cannot be shot noise limited when operating at 1 W input power (though of course the situation will change if we have 25 W input).
  • The heads are mounted at a ~45 deg angle, mixing PIT/YAW, but I assume we can just use the input matrix to rotate back to the natural PIT/YAW basis.

Update 215 pm 5/6: adding in some comments from Rana raised during the meeting:

  1. The transimpedance is actually done by the RLC network (L6 and C38 for CH 3), and not 1.5 kohms. It just coincidentally happens that the reactance is ~1.5 kohms at 29.5 MHz. Note that my LTspice simulation using ideal inductors and capacitors still predicts ~4pA/rtHz noise at 29.5 MHz, so the conclusion about shot noise remains valid I think... One option is to change the attenuation in this path and send more light onto the WFS heads.
    The transimpedance gain and noise are now in Attachment #2. I just tweaked the L values to get a peak at 29.5 MHz and a notch at twice that frequency. For this I assumed a photodiode capacitance of 225pF and the shown transimpedance gain has the voltage gain of the MAX4106 stages divided out. The current noise is input referred.
  2. The imbalanced power on WFS heads may have some motivation - it may be that the W/rad TF for one of the two modes we are trying to sense (beam plane tilt vs beam plane translation) is not equal, so we want more light on the head with weaker response.
  3. The 45 degree mounting of the heads is actually meant to decouple PIT and YAW.
Attachment 1: WFSmatrixComparison.pdf
Attachment 2: WFSheadNoise.pdf
  15320   Thu May 7 09:43:21 2020 ranaUpdateASCIMC WFS

This is the doc from Keita Kawabe on why the WFS heads should be rotated.

  15321   Thu May 7 10:58:06 2020 gautamUpdateASCIMC WFS

OK so the QPD segments are in the "+" orientation when the 40m IMC WFS heads are mounted at 45 deg. I thought "+" was the natural PIT/YAW basis but I guess in the the LIGO parlance, the "X" orientation was considered more natural.


This is the doc from Keita Kawabe on why the WFS heads should be rotated.

  16990   Tue Jul 12 09:25:09 2022 ranaUpdateIOOIMC WFS

MC WFS Demod board needs some attention.

Tomislav has been measuring a very high noise level in the MC WFS demod output (which he promised to elog today!). I thought this was a bogus measurement, but when he, and Paco and I tried to measure the MC WFS sensing matrix, we noticed that there is no response in any WFS, although there are beams on the WFS heads. There is a large response in MC2 TRANS QPD, so we know that there is real motion.

I suspect that the demod board needs to be reset somehow. Maybe the PLL is unlocked or some cable is wonky. Hopefully not both demod boards are fried.

Please leave the WFS loops off until demod board has been assessed.

  17177   Fri Oct 7 20:00:46 2022 KojiUpdateIOOIMC WFS / MC2 SUS glitch

After the CDS upgrade team called for a day (their work TBD), I took over the locked IMC to check how it looked like.

The lock was robust but the IMC REFL spot and the WFS DC/MC2 QPD were moving too much.
I wondered if there were something wrong with the damping. I thought MC3 damping seemed weak, but this was torelable level.LR
During the ring down check of MC2, I saw that the OSEM signals were glitchy. In the end I found it was LR sensor which was glitchy and fluctuating.

I went into the lab and pushed the connectors on the euro card modules and the side connectors as well as the cables on the MC2 sat amp.
I came back to the control room and found the MC2 LR OSEMs had the jump and it seems more stable now.

I leave the IMC locked & WFS running. This sus situation is not great at all and before we go too far, we'll work on the transition to the new electronics (but not today or next week).

By the way the unit of the signals on the dataviewer didn't make sense. Something seemed wrong with them.

Attachment 1: Screenshot_2022-10-07_19-59-45.png
  17179   Sun Oct 9 13:49:49 2022 KojiUpdateIOOIMC WFS / MC2 SUS glitch

The IMC and the IMC WFS kept running for ~2days. 👍

I wanted to look at the trand of the IMC REFL DC, but the dataviewer showed that the recorded values are zero. And this slow channel is missing in the channel list.

I checked the PSL PMC signals (slow) as an example, and many channels are missing in the channel list.

So something is not right with some part of the CDS.

Note that I also reported that the WFS plot in the above refered previous elog has the value like 1e39


Attachment 1: Screen_Shot_2022-10-09_at_13.49.12.png
  17180   Mon Oct 10 00:05:24 2022 ChrisUpdateIOOIMC WFS / MC2 SUS glitch

Thanks for pointing out that EPICS data collection (slow channels) was not working. I started the service that collects these channels (standalone_edc, running on c1sus), and pointed it to the channel list in /opt/rtcds/caltech/c1/chans/daq/C0EDCU.ini, so this should be working now.

controls@c1sus:~$ systemctl status rts-edc_c1sus
● rts-edc_c1sus.service - Advanced LIGO RTS stand-alone EPICS data concentrator
   Loaded: loaded (/etc/systemd/system/rts-edc_c1sus.service; enabled; vendor preset: enabled)
   Active: active (running) since Sun 2022-10-09 23:30:15 PDT; 10h ago
 Main PID: 32154 (standalone_edc)
   CGroup: /system.slice/rts-edc_c1sus.service
           ├─32154 /usr/bin/standalone_edc -i /etc/advligorts/edc.ini -l
           └─32159 caRepeater



The IMC and the IMC WFS kept running for ~2days. 👍

I wanted to look at the trand of the IMC REFL DC, but the dataviewer showed that the recorded values are zero. And this slow channel is missing in the channel list.

I checked the PSL PMC signals (slow) as an example, and many channels are missing in the channel list.

So something is not right with some part of the CDS.

Note that I also reported that the WFS plot in the above refered previous elog has the value like 1e39



  12641   Sat Nov 26 19:16:28 2016 KojiUpdateIOOIMC WFS Demod board measurement & analysis

[Rana, Koji]

1. The response of the IMC WFS board was measured. The LO signal with 0.3Vpp@29.5MHz on 50Ohm was supplied from DS345. I've confirmed that this signal is enough to trigger the comparator chip right next to the LO input. The RF signal with 0.1Vpp on the 50Ohm input impedance was provided from another DS345 to CH1 with a frequency offset of 20Hz~10kHz. Two DS345s were synced by the 10MHz RFreference at the rear of the units. The resulting low frequency signal from the 1st AF stage (AD797) and the 2nd AF stage (OP284) were checked.

Attachment 1 shows the measured and modelled response of the demodulator with various frequency offsets. The value shows the signal transfer (i.e. the output amplitude normalized by the input amplitude) from the input to the outputs of the 1st and 2nd stages. According to the datasheet, the demodulator chip provides a single pole cutoff of 340kHz with the 33nF caps between AP/AN and VP. The first stage is a broadband amplifier, but there is a passive LPF (fc=~1kHz). The second stage also provides the 2nd order LPF at fc~1kHz too. The measurement and the model show good agreement.

2. The output noise levels of the 1st and 2nd stages were meausred and compared with the noise model by LISO.
Attachment 2 shows the input referred noise of the demodulator circuit. The output noise is basically limited by the noise of the first stage. The noise of the 2nd stage make the significant contribution only above the cut off freq of the circuit (~1kHz). And the model supports this fact. The 6.65kOhm of the passive filter and the input current noise of AD797 cause the large (>30nV/rtHz) noise contribution below 100Hz. This completely spoils the low noiseness (~1nV/rtHz) of AD797. At lower frequency like 0.1Hz other component comes up above the modelled noise level.

3. Rana and I had a discussion about the modification of the circuit. Attachment 4 shows the possible improvement of the demod circuit and the 1st stage preamplifier. The demodulator chip can have a cut off by the attached capacitor. We will replace the 33nF caps with 1uF and the cut off will be pushed down to ~10kHz. Then the passive LPF will be removed. We don't need "rodeo horse" AD797 for this circuit, but op27 is just fine instead. The gain of the 1st stage can be increased from 9 to 21. This should give us >x10 improvement of the noise contribution from the demodualtor (Attachment 3). We also can replace some of the important resistors with the thin film low noise resistors.

Attachment 1: WFS_demod_response.pdf
Attachment 2: WFS_demod_noise.pdf
Attachment 3: WFS_demod_noise_plan.pdf
Attachment 4: Screen_shot_2011-07-01_at_11.13.01_AM.png
  12645   Tue Nov 29 17:45:06 2016 KojiUpdateIOOIMC WFS Demod board measurement & analysis

Summary: The demodulator input noise level was improved by a factor of more than 2. This was not as much as we expected from the preamp noise improvement, but is something. If this looks OK, I will implement this modification to all the 16 channels.

The modification shown in Attachment 1 has actually been applied to a channel.

  • The two 1.5uF capacitors between VP and AN/AP were added. This decreases the bandwidth of the demodulator down to 7.4kHz
  • The offset trimming circuit was disabled. i.e. Pin18 of AD831 was grounded.
  • The passive low pass at the demodulator output was removed. (R18, C34)
  • The stage1 (preamp) chip was changed from AD797 to OP27.
  • The gain of the preamp stage was changed from 9 to 21. Also the thin film resistors are used.

Attachment 2 shows the measured and expected output signal transfer of the demodulator. The actual behavior of the demodulator is as expected, and we still keep the over all LPF feature of 3rd order with fc=~1kHz.

Attachment 3 shows the improvement of the noise level with the signal reffered to the demodulator input. The improvement by a factor >2 was observed all over the frequency range. However, this noise level could not be explained by the preamp noise level. Actually this noise below 1kHz is present at the output of the demodulator. (Surprisingly, or as usual, the noise level of the previous preamp configuration was just right at the noise level of the demodulator below 100Hz.) The removal of the offset trimmer circuit contributed to the noise improvement below 0.3Hz.

Attachment 1: demod.pdf
Attachment 2: WFS_demod_response.pdf
Attachment 3: WFS_demod_noise.pdf
  12647   Tue Nov 29 18:35:32 2016 ranaUpdateIOOIMC WFS Demod board measurement & analysis

more U4 gain, lesssss U5 gain

  12661   Fri Dec 2 18:02:37 2016 KojiUpdateIOOIMC WFS Demod board measurement & analysis

ELOG of the Wednesday work.

It turned out that the IMC WFS demod boards have the PCB board that has a different pattern for each of 8ch.
In addition, AD831 has a quite narrow leg pitch with legs that are not easily accessible.
Because of these, we (Koji and Rana) decided to leave the demodulator chip untouched.

I have plugged in the board with the WFS2-Q1 channel modified in order to check the significance of the modification.

WFS performance before the modification

Attachment 1 shows the PSD of WFS2-I1_OUT calibrated to be referred to the demodulator output. (i.e. Measured PSDs (cnt/rtHz) were divided by 8.9*2^16/20)
There are three curves: One is the output with the MC locked (WFS servos not engaged). The second is the PSD with the PSL beam blocked (i.e. dark noise). The third is the electronics noise with the RF input terminated and the nominal LO supplied.

This tells us that the measured PSD was dominated by the demodulator noise in the dark condition. And the WFS signal was also dominated by the demod noise below 0.1Hz and above 20Hz. There are annoying features at 0.7, 1.4, 2.1, ... Hz. They basically impose these noise peaks on the stabilized mirror motion.

WFS performance after the modification

Attachment 2 shows the PSD of WFS2-Q1_OUT calibrated to be referred to the demodulator output. (i.e. Measured PSDs (cnt/rtHz) were divided by 21.4*2^16/20)
There are three same curves as the other plot. In addition to these, the PSD of WFS2-I1_OUT with the MC locked is also shown as a red curve for comparison.

This figure tells us that the measured PSD below 20Hz was dominated by the demodulator noise in the dark condition. And the WFS signal is no longer dominated by the electronics noise. However, there still are the peaks at the harmonics of 0.7, 1.4, 2.1, ... Hz. I need further inspection of the FWS demod and whtening boards to track down the cause of these peaks.

Attachment 1: WFS_demod_noise_orig.pdf
Attachment 2: WFS_demod_noise_mod.pdf
  12662   Sat Dec 3 13:27:35 2016 KojiUpdateIOOIMC WFS Demod board measurement & analysis

ELOG of the work on Thursday

Gautam suggested looking at the preamplifier noise by shorting the input to the first stage. I thought it was a great idea.

To my surprise, the noise of the 2nd stage was really high compared to the model. I proceeded to investigate what was wrong.

It turned out that the resistors used in this sallen-key LPF were thick film resistors. I swapped them with thin film resistors and this gave the huge improvement of the preamplifier noise in the low frequency band.

Attachment 1 shows the summary of the results. Previously the input referred noise of the preamp was the curve in red. We the resistors replaced, it became the curve in magenta, which is pretty close to the expected noise level by LISO model above 3Hz (dashed curves). Unfortunately, the output of the unit with the demodulator connected showed no improvement (blue vs green), because the output is still limited by the demodulator noise. There were harmonic noise peaks at n x 10Hz before the resistor replacement. I wonder if this modification also removed the harmonic noise seen in the CDS signals. I will check this next week.

Attachment 2 shows the current schematic diagram of the demodulator board. The Q of the sallen key filter was adjusted by the gain to have 0.7 (butter worth). We can adjust the Q by the ratio of the capacitance. We can short 3.83K and remove 6.65K next to it. And use 22nF and 47nF for the capacitors at the positive input and the feedback, respectively. This reduces the number of the resistors.

Attachment 1: WFS_demod_noise.pdf
Attachment 2: demod.pdf
  12668   Tue Dec 6 13:37:02 2016 KojiUpdateIOOIMC WFS Demod board measurement & analysis

I have implemented the modification to the demod boards (Attachment 1).
Now, I am looking at the noise in the whitening board. Attachment 2 shows the comparison of the error signal with the input of the whitening filter shorted and with the 50ohm terminator on the demodulator board. The message is that the whitening filter dominates the noise below 3Hz.

I am looking at the schematic of the whitening board D990196-B. It has an VGA AD602 at the input. I could not find the gain setting for this chip.
If the gain input is fixed at 0V, AD602 has the gain of 10dB. The later stages are the filters. I presume they have the thick film resistors.
Then they may also cause the noise. Not sure which is the case yet.

Also it seems that 0.7Hz noise is still present. We can say that this is coming from the demod board but not on the work bench but in the eurocard crate.

Attachment 1: demod.pdf
Attachment 2: WFS_error_noise.pdf
  17332   Sat Dec 3 17:42:25 2022 AnchalUpdateASCIMC WFS Fixed for now

Today I did a lot of steps to eventually reach to WFS locking stably for long times and improving and keeping the IMC transmission counts to 14400. I think the main culprit in thw WFS loop going unstable was the offset value set on MC_TRANS_PIT filter module  (C1:IOO-MC_TRANS_PIT_OFFSET). This value was roughly correct in magnitude but opposite in sign, which created a big offset in MC_TRANS PIT error signal which would integrate by the loops and misalign the mode cleaner.

WFS offsets tuning

  • I ran C1:IOO-WFS_MASTER > Actiona > Correct WFS DC offsets script while the two WFS heads were blocked.
  • Then I aligned IMC to maximize transmission. I also made PMC transmission better by walking the input beam.
  • Then, while IMC is locked and WFS loops are off, I aligned the beam spot on WFS heads to center it in DC (i.e. zeroing C1:IOO-WFS1_PIT_DC, C1:IOO-WFS1_YAW_DC, C1:IOO-WFS2_PIT_DC, C1:IOO-WFS2_YAW_DC)
  • Then I ran C1:IOO-WFS_MASTER > Actiona > Correct WFS DC offsets script while keeping IMC locked (note the script says to keep it unlocked, but I think that moves away the beam). If we all agree this is ok, I'll edit this script.
  • Then I checked the error signals of all WFS loops and still found that C1:IOO-MC_TRANS_PIT_OUTPUT and C1:IOO-MC_TRANS_YAW_OUTPUT have offsets. I relieved these offsets by averaging the input to these filter moduels for 100s and updating the offset. This is where I noticed that the PIT offset was wrong in sign.

WFS loops UGF tuning

  • Starting with only YAW loops, I measured the open loop transfer functions (OLTFs) for each loop by simultaneously injecting gaussian noise from 0.01 Hz to 0.5 Hz using diaggui at the loop filter module excitation points and taking ration of IN1/IN2 of the filter modules.
  • Then I scaled the YAW output matrix columns to get UGF of 0.1 Hz when YAW loop was along turned on.
  • Then I tried to do this for PIT as well but it failed as even with overall gain of 0.1, the PIT loops actuate a lot of YAW motion causing the IMC to loose lock eventually.
  • So I tried locking PIT loops along with YAW loops but with 0.1 overall gain. This worked for long enough that I could get a rough estimate of the OLTFs. I scaled the columns of PIT output matrix and slowly increased the overall gain while repeating this step to get about 0.1 Hz UGF for all PIT loops too.
  • Note though that the PIT loop shape did not come out as expected with a shallower slope and much worse coherence for same amount of excitation in comparison to YAW loops. See attached plots.
  • Never the less, I was able to reach to an output matric which works at overall gain of 1. I tested this configuration for atleast 15 minutes but the loop was working even with 6 excitations happening simultaneously for OLTF measurement.
  • We will need to revisit PIT loop shapes, matrix diagonalization, and sources of noise.

OLTF measurements were done using this diaggui file. The measurement file got deleted by me by mistake, so I recreated the template. Thankfully, I had saved the pdf of the measurements, but I do not have same measurement results in the git repo.


Attachment 1: IMC_WFS_OLTF.pdf
  17334   Sun Dec 4 16:44:04 2022 AnchalUpdateASCIMC WFS Fixed for now

Today, I worked on WFS loop output matrix for PIT DOFs.

  • I began with the matrix that was in place before Nov 15.
  • I followed the same method as last time to fist get all UGFs around 0.06 Hz with overall gain of 0.6 on the WFS loops.
  • This showed me that MC2_TRANS_PIT loop shape matches well with the nice working YAW loops, but the WFS1 and WFS2 loops still looked flat like before.
  • This indicated that output matrix needs to be fixed for cross coupling between WFS1 and WFS2 loops.
  • I ran this script WFSoutMatBalancing.py which injects low frequency (<0.5 Hz) oscillations when the loops are open, and measures sensing matrix using error signals. I used 1000s duration for this test.
  • The direct inverse of this sensing matrix fixed the loop shape for WFS1 indicating WFS1 PIT loop is disentangled from WFS2 now.
  • Note this is a very vague definition of diagonalization, but I am aiming to reach to a workign WFS loop asap with whatever means first. Then we can work on accurate diagonalization later.
  • I simply ran the script WFSoutMatBalancing.py again for another 1000s and this time the sensing matrix mostly looked like an identity.
  • I implemented the new output matrix found by direct inversion and took new OLTF.Again though, the WFS2_PIT loop comes out to be flat. See Attachment 1.
  • Then noting from this elog post, I reduced the gain values on MC2 TRANS loops to 0.1 I think it is better to use this place to reduce loop UGF then the output matrix as this will remind us that MC2 TRANS loops are slower than others by 10 times.
  • I retook OLTF but very unexpected results came. The overall gain of WFS1_YAW and WFS2_YAW seemed to have increased by 6. All other OLTFs remained same as expected. See attachment 2.
  • To fulfill the condition that all UGF should be less than 0.1 Hz, I reduced gains on WFS1_YAW and WFS2_YAW loops but that made the YAW loops unstable. So I reverted back to all gains 1.
  • We probably need to diagonalize Yaw matrix better than it is for letting MC2_TRANS_YAW loop to be at lower UGF.
  • I'm leaving the mode cleaner in this state and would come back in an hour to see if it remains locked at good alignment. See attachment 3 for current state.

Sun Dec 4 17:36:32 2022 AG: IMC lock is holding as strong as before. None of the control signals or error signals seem to be increasing monotonously over the last one hour. I'll continue monitoring the lock.

Mon Dec 5 11:11:08 2022 AG: IMC was locked all night for past 18 hours. See attachment 4 for the minute trend.

Attachment 1: IMC_WFS_OLTF_All_Gains_1.pdf
IMC_WFS_OLTF_All_Gains_1.pdf IMC_WFS_OLTF_All_Gains_1.pdf
Attachment 2: IMC_WFS_OLTF_Nom_Gain.pdf
IMC_WFS_OLTF_Nom_Gain.pdf IMC_WFS_OLTF_Nom_Gain.pdf
Attachment 3: WFS_Loop_Configuration.png
Attachment 4: WFS_Loop_Performance.png
  12748   Tue Jan 24 01:04:16 2017 gautamSummaryIOOIMC WFS RF power levels


I got around to doing this measurement today, using a minicircuits bi-directional coupler (ZFBDC20-61-HP-S+), along with some SMA-LEMO cables.

  • With the IMC "well aligned" (MC transmission maximized, WFS control signals ~0), the RF power per quadrant into the Demod board is of the order of tens of pW up to a 100pW.
  • With MC1 misaligned such that the MC transmission dropped by ~10%, the power per quadrant into the demod board is of the order of hundreds of pW.
  • In both cases, the peak at 29.5MHz was well above the analyzer noise floor (>20dB for the smaller RF signals), which was all that was visible in the 1MHz span centered around 29.5 MHz (except for the side-lobes described later).
  • There is anomalously large reflection from Quadrant 2 input to the Demod board for both WFS
  • The LO levels are ~-12dBm, ~2dBm lower than the 10dBm that I gather is the recommended level from the AD831 datasheet

We should insert a bi-directional coupler (if we can find some LEMO to SMA converters) and find out how much actual RF is getting into the demod board.


I first aligned the mode cleaner, and offloaded the DC offsets from the WFS servos.

The bi-directional coupler has 4 ports: Input, Output, Coupled forward RF and Coupled Reverse RF. I connected the LEMO going to the input of the Demod board to the Input, and connected the output of the coupler to the Demod board (via some SMA-LEMO adaptor cables). The two (20dB) coupled ports were connected to the Agilent spectrum analyzer, which have input impedance 50ohms and hence should be impedance matched to the coupled outputs. I set the analyzer to span 1MHz (29-30MHz), IF BW 30Hz, 0dB input attenuation. It was not necessary to turn on averaging to resolve the peaks at ~29.5MHz since the IF bandwidth was fine enough.

I took two sets of measurements, one with the IMC well aligned (I maximized the MC Trans as best as I could to ~15,000 cts), and one with a macroscopic misalignment to MC1 such that the MC Trans fell to 90% of its usual value (~13,500 cts). The peak function on the analyzer was used to read off the peak height in dBm. I then converted this to RF power, which is summarized in the table below. I did not account for the main line loss of the coupler, but according to the datasheet, the maximum value is 0.25dB so there numbers should be accurate to ~10% (so I'm really quoting more S.Fs than I should be).

WFS Quadrant Pin (pW) Preflected(pW) Pin-demod board (pW)

IMC well aligned

1 1 50.1 12.6 37.5
2 20.0 199.5 -179.6
3 28.2 10.0 18.2
4 70.8 5.0


2 5 100 19.6 80.0
6 56.2 158.5 -102.3
7 125.9 6.3 11.5
8 17.8 6.3


WFS Quadrant Pin (pW) Preflected(pW) Pin-demod board (pW)

MC1 Misaligned

1 1 501.2 5.0 496.2
2 630.6 208.9 422
3 871.0 5.0 866
4 407.4 16.6


2 5 407.4 28.2 379.2
6 316.2 141.3 175.0
7 199.5 15.8 183.7
8 446.7 10.0 436.7


For the well aligned measurement, there was ~0.4mW incident on WFS1, and ~0.3mW incident on WFS2 (measured with Ophir power meter, filter out).

I am not sure how to interpret the numbers for quadrants #2 and #6 in the first table, where the reverse coupled RF power was greater than the forward coupled RF power. But this measurement was repeatable, and even in the second table, the reverse coupled power from these quadrants are more than 10x the other quadrants. The peaks were also well above (>10dBm) the analyzer noise floor 

I haven't gone through the full misalginment -> Power coupled to TEM10 mode algebra to see if these numbers make sense, but assuming a photodetector responsivity of 0.8A/W, the product (P1P2) of the powers of the beating modes works out to ~tens of pW (for the IMC well aligned case), which seems reasonable as something like P1~10uW, P2 ~ 5uW would lead to P1P2~50pW. This discussion was based on me wrongly looking at numbers for the aLIGO WFS heads, and Koji pointed out that we have a much older generation here. I will try and find numbers for the version we have and update this discussion.


  1. For the sake of completeness, the LO levels are ~ -12.1dBm for both WFS demod boards (reflected coupling was negligible)
  2. In the input signal coupled spectrum, there were side lobes (about 10dB lower than the central peak) at 29.44875 MHz and 29.52125 MHz (central peak at 29.485MHz) for all of the quadrants. These were not seen for the LO spectra.
  3. Attached is a plot of the OSEM sensor signals during the time I misaligned MC1 (in both pitch and yaw approximately by equal amounts). Assuming 2V/mm for the OSEM calibration, the approximate misalignment was by ~10urad in each direction.
  4. No IMC suspension glitching the whole time I was working today yes


Attachment 1: MC1_misalignment.png
  12759   Fri Jan 27 00:14:02 2017 gautamSummaryIOOIMC WFS RF power levels

It was raised at the Wednesday meeting that I did not check the RF pickup levels while measuring the RF error signal levels into the Demod board. So I closed the PSL shutter, and re-did the measurement with the same measurement scheme. The detailed power levels (with no light incident on the WFS, so all RF pickup) is reported in the table below.

IMC WFS RF Pickup levels @ 29.5MHz
WFS Quadrant Pin (pW) Preflected
1 1 0.21 10.
2 1.41 148
3 0.71 7.1
4 0.16 3.6
2 1 0.16 10.5
2 1.48 166
3 0.81 5.1
4 0.56 0.33

These numbers can be subtracted from the corresponding columns in the previous elog to get a more accurate estimate of the true RF error signal levels. Note that the abnormal behaviour of Quadrant #2 on both WFS demod boards persists.

  14709   Sun Jun 30 19:47:09 2019 ranaUpdateIOOIMC WFS agenda

we are thinking of doing a sprucing up of the input mode cleaner WFS (sensors + electronics + feedback loops)

  1. WFS Heads
    1. it has been known since ~2002 that the RF circuits in the heads oscillate. 
    2. in the attached PDF you can see that 2 opamps (U3 & U4; MAX4106) are used to amplify the tank circuit made up of the photodiode capacitance and L6.
    3. due to poor PCB layout (the output of U4 runs close to the input of U3) the opamps oscillate if the if the Reed relay (RY2) is left open (not attenuating)
    4. we need to remove/disable the relay
    5. also remove U3 for each quadrant so that it has a fixed gain of (TBD) and a 50 Ohm output
    6. also check that all the resonances are tuned to 1f, 2f, & 3f respectively
  2. Demod boards
  3. DC quadrant readouts
  4. Whitening
  5. Noise budget of sensors, including electronics chain
  6. diagonalization of sensors / actuators
  7. Requirements -
  8. Optical Layout
  9. What does the future hold ?

  1. what is our preferred pin-for-pin replacement for the MAX4106/MAX4107? internet suggests AD9632. Anyone have any experience with it? The Rabbott uses LMH6642 in the aLIGO WFSs. It has a lower slew rate than 9632, but they both have the same distortion of ~ -60 dB for 29.5 MHz.
  2. the whole DC current readout is weird. Should have a load resistor and go into the + input of the opamp, so as to decouple it from the RF stuff. Also why such a fast part? Should have used a OP27 equivalent or LT1124.
  3. LEMO connectors for RF are bad. Wonder if we could remove them and put SMA panel mount on there.
  4. as usual, makes me feel like replacing with better heads...and downstream electronics...
Attachment 1: WFS-Head.pdf
  15747   Sun Jan 3 16:26:06 2021 KojiUpdateSUSIMC WFS check (Yet another round of Sat. Box. switcharoo)

I wanted to check the functionality of the IMC WFS. I just turned on the WFS servo loops as they were. For the past two hours, they didn't run away. The servo has been left turned on. I don't think there is no reason to keep it turned off.

Attachment 1: Screen_Shot_2021-01-03_at_17.14.57.png
  10728   Thu Nov 20 22:43:15 2014 KojiUpdateIOOIMC WFS damping gain adjustment

From the measured OLTF, the dynamics of the damped suspension was inferred by calculating H_damped = H_pend / (1+OLTF).
Here H_pend is a pendulum transfer function. For simplicity, the DC gain of the unity is used. The resonant frequency of the mode
is estimated from the OLTF measurement. Because of inprecise resonant frequency for each mode, calculated damped pendulum
has glitches at the resonant frequency. In fact measurement of the OLTF at the resonant freq was not precise (of course). We can
just ignore this glitchiness (numerically I don't know how to do it particularly when the residual Q is high).

Here is my recommended values to have the residual Q of 3~5 for each mode.

MC1 SUS POS current  75   -> x3   = 225
MC1 SUS PIT current   7.5 -> x2   =  22.5
MC1 SUS YAW current  11   -> x2   =  22
MC1 SUS SD  current 300   -> x2   = 600

MC2 SUS POS current  75   -> x3   = 225
MC2 SUS PIT current  20   -> x0.5 =  10
MC2 SUS YAW current   8   -> x1.5 =  12
MC2 SUS SD  current 300   -> x2   = 600

MC3 SUS POS current  95   -> x3   = 300
MC3 SUS PIT current   9   -> x1.5 =  13.5
MC3 SUS YAW current   6   -> x1.5 =   9
MC3 SUS SD  current 250   -> x3   = 750

This is the current setting in the end.

MC1 SUS SD  450

MC2 SUS SD  450

MC3 SUS SD  500

Attachment 1: MC_OLTF_CLTF.pdf
  17337   Mon Dec 5 20:02:06 2022 AnchalUpdateASCIMC WFS heads electronic feasibility test for using for Arm ASC

I took transfer function measurement of WFS2 SEG4 photodiode between 1 MHz to 100 MHz in a linear sweep.

Measurement details:

  • The reincarnated Jenne laser head was used for this test. The laser diode is 950 nm though, which should just mean a different responsivity of the photodiode while we are mainly interested in relative response of the WFS heads at 11 MHz and 55 MHz with respect to 29.5 MHz.
  • See attachment 2 for how the laser was placed on AP table.
  • The beam was injected in between beam splitter for MC reflection camera and beam splitter for beam dump.
  • The input was aligned such that all the light of the laser was falling on Segment 4 of WFS2.
  • Using moku, I took RF transfer function from 1 MHz to 100 MHz, 512 points, linearly spaced, with excitation amplitude of 1 V and 100,000 cycles of averaging.
  • Measurement data and settings are stored here.


Relative to 29.5 MHz, teh photodiode response is:

  • At 11 MHz: -20.4 dB
  • At 55 MHz: -36.9 dB
  • At 71.28 MHz: -5.9 dB

I'm throwing in an extra number at the end as I found a peak at this frequency as well. This means to use these WFS heads for arm ASC, we need to have 10 times more light for 11 MHz and roughly 100 times more light for 55 MHz. According to Gautam's thesis Table A.1 and this elog post, the modulation depth for 11 MHz is 0.193 and for 55 MHz is 0.243 in comparison to 0.1 for 29.5 MHz., so the sideband TEM00 light available for beating against carrier TEM01/TEM10 is roughly twice as much for single arm ASC. That would mean we would have 5 times less error signal for 11 MHz and 40 times less error signal for 55 MHz. These are rough calculations ofcourse.


Attachment 1: 20221205_193105_WFS2_SEG4_RF_TF_Screenshot.png
Attachment 2: PXL_20221206_033419110.jpg
  10561   Thu Oct 2 20:54:45 2014 KojiUpdateIOOIMC WFS measurements

[Eric Koji]

We made sensing matrix measurements for the IMC WFS and the MC2 QPD.

The data is under further analysis but here is some record of the current state to show
IMC Trans RIN and the ASC error signals with/without IMC ASC loops

The measureents were done automatically running DTT. This can be done by


The analysis is in preparation so that it provides us a diagnostic report in a PDF file.

Attachment 1: IMC_RIN_141002.pdf
Attachment 2: IMC_WFS_141002.pdf
  10564   Fri Oct 3 13:03:05 2014 ericqUpdateIOOIMC WFS measurements

Yesterday, Koji and I measured the transfer function of pitch and yaw excitations of each MC mirror, directly to each quadrant of each WFS QPD. 

When I last touched the WFS settings, I only used MC2 excitations to set the individual quadrant demodulation phases, but Koji pointed out that this could be incomplete, since motion of the curved MC2 mirror is qualitatively different than motion of the flat 1&3. 

We set up a DTT file with twenty TFs (the excitation to I & Q of each WFS quadrant, and the MC2 trans quadrants), and then used some perl find and replace magic to create an xml file for each excitation. These are the files called by the measurement script Koji wrote. 

I then wrote a MATLAB script that uses the magical new dttData function Koji and Nic have created, to extract the TF data at the excitation frequency, and build up the sensing elements. I broke the measurements down by detector and excitation coordinate (pitch or yaw).

The amplitudes of the sensing elements in the following plots are normalized to the single largest response of any of the QPD's quadrants to an excitation in the given coordinate, the angles are unchanged. From this, we should be able to read off the proper digital demodulation angles for each segment, confirm the signs of their combinations for pitch and yaw, and construct the sensing matrix elements of the properly rotated signals. 



The axes of each quadrant look consistent across mirrors, which is good, as it nails down the proper demod angle. 

The xml files and matlab script used to generate these plots is attached. (It requires the dttData functions however, which are in the svn (and the dttData functions require a MATLAB newer than 2012b))

Attachment 5: analyzeWfs.zip
  10565   Sun Oct 5 10:09:49 2014 ranaUpdateIOOIMC WFS measurements

It seems clever, but I wonder why use DTT and command line perl, instead of using the FE lockins or just demod the offline data or all of the other sensing matrix scripts made for the LSC (at 40m) or ASC (at LLO) ?

  10566   Sun Oct 5 23:43:08 2014 KojiUpdateIOOIMC WFS measurements

There are several non scientific reasons.

  16108   Mon May 3 09:14:01 2021 Anchal, PacoUpdateLSCIMC WFS noise contribution in arm cavity length noise

Lock ARMs

  • Try IFO Configure ! Restore Y Arm (POY) and saw XARM lock, not YARM. Looks like YARM biases on ITMY and ETMY are not optimal, so we slide C1:SUS-ETMY_OFF from 3.0 --> -14.0 and watch Y catch its lock.
  • Run ASS scripts for both arms and get TRY/TRX ~ 0.95
    • We ran X, then Y and noted that TRX dropped to ~0.8 so we ran it again and it was well after that. From now on, we will do Y, then X.

WFS1 noise injection

  • Turn WFS limits off by running switchOffWFSlims.sh
  • Inject broadband noise (80-90 Hz band) of varying amplitudes from 100 - 100000 counts on C1:IOO-WFS1_PIT_EXC
  • After this we try to track its propagation through various channels, starting with
    • C1:IOO-MC_F_DQ
    • C1:IOO-WFS1_PIT_IN2

** denotes [UL, UR, LL, LR]; the output coils.

  • Attachment 1 shows the power spectra with IMC unlocked
  • Attachment 2 shows the power spectra with the ARMs (and IMC) locked
Attachment 1: WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf
WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf WFS1_PIT_Noise_Inj_Test_IMC_unlocked.pdf
Attachment 2: WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf
WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf WFS1_PIT_Noise_Inj_Test_ARM_locked.pdf
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