40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log, Page 224 of 339  Not logged in ELOG logo
ID Date Author Type Categoryup Subject
  15352   Tue May 26 03:06:59 2020 gautamUpdateLSCPRFPMI sensing matrix


The response of the PRFPMI length degrees of freedom as measured in the LSC PDs was characterized. Two visualizations are in Attachment #1 and Attachment #2.


  • The sensing matrix infrastructure in the c1cal model was used.
  • The oscillator frequencies are set between 300 - 315 Hz.
  • Notch filters at these frequencies were enabled in the CDS filter banks, to prevent actuation at these frequencies (except for CARM, in which case the loop gain is still non-negligible at ~300 Hz, this correction has not yet been applied).
  • Mainly, I wanted to know what the DARM sensing response in AS55_Q is. 
    • The measurement yields 2.3e13 cts/m. This is a number that will be used in the noise budget to convert the measured DARM spectrum to units of m/rtHz.
    • We have to multiply this by 10/2^15 V/ct, undo the 6dB whitening gain on the AS55_Q channel, and undo the ~5x gain from V_RF to V_IF (see Attachment #4 of this), to get ~0.69 GV/m from the RFPD.
    • The RF transimpedance of AS55_Q is ~550 ohms, and accounting for the InGaAs responsivity, I get an optical gain of 1.8 MW/m. Need to check how this lines up with expectations from the light levels, but seems reasonable.
    • Note that T_SRM is 10%, we dump 70% of the output field into the unused OMC, and there is a 50/50 BS splitting the light between AS55 and AS110 PDs. Assuming 90% throughput from the rest of the chain, we are only sensing ~1.3 % of the output DARM field.
  • Apart from this, I can also infer what the matrix elements / gains need to be for transitioning the PRMI control from 3f to 1f signals. To be done...
  • I found these histograms in Attachment #2 to be a cute way of (i) visualizing the variance in the magnitude of the sensing element and (ii) visualizing the separation between the quadratures, which tells us if the (digital) demod phase needs to be modified.
    • The sensing lines were on for 5 minutes (=300 seconds) and the FFT segment length is 5 seconds, so these histograms are binning the 60 different values obtained for the value of the sensing element.
    • The black dashed lines are "kernel density estimates" of the underlying PDFs
    • I haven't done any rigorous statistical analysis on the appropriateness of using this technique for error estimation, so for now, they are just lines...
Attachment 1: PRFPMI_20200524sensMat.pdf
Attachment 2: PRFPMI_20200524sensMatHistograms.pdf
  15353   Tue May 26 03:26:58 2020 gautamUpdateLSCPreliminary noise budget


This isn't meant to be a serious budget, mainly it was to force myself to write the code for generating this more easily in the future.


  • DARM OLTF model from here was used to undo the loop to convert the in-loop measurement to a free-running estimate.
  • The AS55 PD channels were whitened to reduce the effect of ADC noise.
  • To measured channel was 'C1:LSC-DARM_IN1_DQ'.
    • Some care needs to be taken when applying the conversion from counts to meters using the sensing element measured here.
    • This is because the sensing matrix measurement was made using the response in the channel 'C1:LSC-AS55_Q_ERR_DQ'.
    • Between 'C1:LSC-DARM_IN1_DQ' and 'C1:LSC-AS55_Q_ERR_DQ' there is a scalar gain of 1e-4, and a z:p = 20:0 filter.
    • These have to be corrected for when undoing the loop, since the measurement point is 'C1:LSC-DARM_IN1_DQ'. 
  • The "Dark noise" trace was measured with the PSL shutter closed, but all CDS filters up to 'C1:LSC-DARM_IN1_DQ' enabled as they were when the DARM measurement was taken.
  • It would be interesting to see what the budget looks like once the DARM loop gain has been turned down a bit, some low-pass filtering is enabled, and the vertex DoFs are transitioned to 1f control which is hopefully lower noise.
Attachment 1: PRFPMI_NB.pdf
  15355   Tue May 26 14:32:44 2020 gautamUpdateLSCArm transmission RIN


The measured RIN of the arm cavity transmission when the PRFPMI is locked is ~10x in RMS relative to the single arm POX/POY lock. It is not yet clear to me where the excess is coming from.


Attachment #1 shows the comparison.

  • For the PRFPMI lock, the ITM Oplev Servos are DC coupled, and the ETM QPD ASC servos are also enabled.
  • Admittedly, the PD used in the POX/POY lock case is the Thorlabs PD while when the PRFPMI is locked, it is the QPD.
  • I found that there isn't really a big difference in the RIN if we normalize by the IMC transmission or not (this is what the "un-normalized" in the plot legend is referring to).  A scatter plot of TRX vs TRY and TRX/MCtrans vs TRY/MCtrans have nearly identical principal components. 
  • To convert to RIN, I divided the ocmputed spectra by the mean value of the data stream. For the POX/POY lock, the arm transmission is normalized to 1, so no further manipulation is required.
  • The spectra are truncated to 512 Hz because the IMC sum channel is DQ-ed at 1 kHz, but because of the above bullet point, in principle, I could calculate this out to higher frequencies.
Attachment 1: armRIN.pdf
  15356   Tue May 26 16:00:06 2020 gautamUpdateLSCPower buildup diagnostics


I looked at some DC signals for the buildup of the carrier and sideband fields in various places. The results are shown in Attachments #1 and #2.


  • A previous study may be found here.
  • For the carrier field, REFL, POP and TRX/TRY all show the expected behavior. In particular, the REFL/TRX variation is consistent with the study linked in the previous bullet.
  • There seems to be some offset between TRX and TRY - I don't yet know if this is real or just some PD gain imbalance issue.
  • The 1-sigma variation in TRX/TRY seen here is consistent with the RMS RIN of 0.1 evaluated here.
  • For the sideband powers, I guess the phasing of the POP22 and AS110 photodiodes should be adjusted? These are proxies for the buildup of the 11 MHz and 55 MHz sidebands in the vertex region, and so shouldn't depend on the arm offset, and so adjusting the digital demod phases shouldn't affect the LSC triggering for the PRMI locking, I think.
  • Based on this data, the recycling gain for the carrier is ~12 +/- 2, so still undercoupled. In fact, at some points, I saw the transmitted power exceed 300, which would be a recycling gain of ~17, which is then nearly the point of critical coupling. REFLDC doesn't hit 0 because of the mode mismatch I guess.
Attachment 1: PRFPMIcorner_DC_1274419354_1274419654.pdf
Attachment 2: PRFPMIcorner_SB_1274419354_1274419654.pdf
  15358   Wed May 27 17:41:57 2020 KojiUpdateLSCPower buildup diagnostics

This is very interesting. Do you have the ASDC vs PRG (~ TRXor TRY) plot? That gives you insight on what is the cause of the low recycling gain.

  15359   Wed May 27 19:36:33 2020 KojiUpdateLSCArm transmission RIN

My speculation for the worse RIN is:

- Unoptimized alignment -> Larger linear coupling of the RIN with the misalignment
- PRC TT misalignment (~3Hz)

Don't can you check the correlation between the POP QPD and the arm RIN?

  15360   Wed May 27 20:14:51 2020 KojiUpdateLSCLock acquisition sequence

I see. At the 40m, we have the direct transition from ALS to RF. But it's hard to compare them as the storage time is very different.

  15361   Thu May 28 18:36:45 2020 gautamUpdateLSCArm transmission RIN

I agree, I think the PRC excess angular motion, PIT in particular, is a dominant contributor to the RIN. Attachments #1-#3 support this hypothesis. In these plots, "XARM" should really read "COMM" and "YARM" should really read "DIFF", because the error signals from the two end QPDs are mixed to generate the PIT and YAW error signals for these ASC servos - this is some channel renaming that will have to be done on the ASC model. The fact that the scatter plot between these DoFs has some ellipticity probably means the basis transformation isn't exactly right, because if they were truly orthogonal, we would expect them to be uncorrelated?

  • In the corner plots, I am plotting the error signals of the ASC servos and the arm transmission. POP feedback is not engaged, but some feedback control to the ETMs based on the QPD signals is engaged.
  • In the coherence plot, I show the coherence of the ASC error signals with the POP and TR QPD based error signals, under the same conditions. The coherence is high out to ~20 Hz.

I guess what this means is that the stability of the lock could be improved by turning on some POP QPD based feedback control, I'll give it a shot.


- PRC TT misalignment (~3Hz)

Don't can you check the correlation between the POP QPD and the arm RIN

Attachment 1: PRFPMIcorner_ASC_PIT_1274419354_1274419654.pdf
Attachment 2: PRFPMIcorner_ASC_YAW_1274419354_1274419654.pdf
Attachment 3: PRFPMIcorner_ASC_coherence_1274419354_1274419654.pdf
  15362   Fri May 29 00:34:57 2020 ranaUpdateLSCArm transmission RIN

how bout corner plot with power signals and oplevs? I think that would show not just linear couplings (like your coherence), but also quadratic couplings (chesire cat grin)devil

  15364   Wed Jun 3 01:34:53 2020 gautamUpdateLSCLock acquisition update portal


  • With better ASC servos implemented, I think the lock stability has been improved. 
  • Arm transmission of ~350 was stably maintained (PRG~20, overcoupled). It went as high as 410, so this is now very close to the highest (~425) I've ever managed to get.
  • I was trying to get the vertex transitioned to 1f control but it remains out of reach for now.  The noise at ~100 Hz is dominated by MICH-->DARM coupling (as judged by coherence, I haven't yet done the broadband noise injection characterization). I figured I'd try the 1f transition before thinking about feedforward.
  • The biggest problems remain flaky electronics (poor IMC duty cycle, jumping RF offsets, newly glitchy seismometer, ...)


  15366   Wed Jun 3 01:46:14 2020 gautamUpdateLSCCARM loop


The CARM loop now has a UGF of ~12 kHz with a phase margin of ~60 degrees. These values of conventional stability indicators are good. The CARM optical gain that best fits the measurements is 9 MW/m.

I've been working on understanding the loop better, here are the notes.


Attachment #1 shows a block diagram of the loop topology.

  • The "crossover" measurement made at the digital CARM error point, and the OLG measurement at the CM board error point are shown.
  • I've tried to include all the pieces in the loop, and yet, I had to introduce a fudge gain in the digital path to get the model to line up with the measurement (see below).

Attachment #2 shows the OLGs of the two actuation paths.

  • Aforementioned fudge factor for the digital path is included.
  • For the AO path, I assumed a value of the PDH discriminant at the IMC error point to be 13 kHz/V, per my earlier measurement. 
  • I trawled the elog for the most up-to-date info about the IMC servo (elog9457, elog13696, elog15044) and CM board, to build up the model. 

Attachment #3 and #4 show the model, overlaid with measurements of the loop OLG and crossover TF respectively.

  • No fitting is done yet - the next step would be to add the delay of the CDS system for the digital path, and the analog electronics for the AO path. Though these are likely only small corrections.
  • For the crossover TF - I've divided out the digital filters in the CARM_B filter bank, because the injection is made downstream of it (see Attachment #1).
  • There is reasonably good agreement between model and measurement.
  • I think the biggest source of error is the assumed model for the IMC OLTF.

Attachment #5 shows the evolution of the CARM OLG at a few points in the lock acquisition sequence.

  • "Before handoff" corresponds to the state where the primary control is still done by the ALS leg, but the REFL11 signal has begun to enter the picture via the CARM_B path.
  • "IN2 ramped" corresponds to the state where the AO path gain (=IN2 gain on the IMC servo board) has been ramped up to its final value (+0 dB), but the overall loop gain (=IN1 gain on the CM board) is still low. So this is preparation for high bandwidth control. Typically, the arm powers will have stabilized in this state, but ALS control is still on.
  • "Pre-boost" corresponds to an intermediate state - ALS control is off, but the low frequency boosts have not yet been enabled. I typically first engage some ASC to stabilize things somewhat, and then turn on the boosts.
  • "Final" - self explanatory.

Next steps:

Now the I have a model I believe, I need to think about whether there is any benefit to changing some of these loop shapes. I've already raised the possibility of changing the shape of the boosts on the CM board, with which we could get a bit more suppression in the 100 Hz - 1kHz region (noise budget of laser frequency noise --> DARM required to see if this is necessary). 

Attachment 1: CM_loop_topology.pdf
Attachment 2: CARM_TFs.pdf
Attachment 3: CARM_OLTF.pdf
Attachment 4: CARM_xover.pdf
Attachment 5: CARM_OLG_evolution.pdf
  15367   Wed Jun 3 02:08:00 2020 gautamUpdateLSCPower buildup diagnostics

Attachments #1 and Attachments #2 are in the style of elog15356, but with data from a more recent lock. It'd be nice to calibrate the ASDC channel (and in general all channels) into power units, so we have an estimate of how much sideband power we expect, and the rest can be attributed to carrier leakage to ASDC.

On the basis of Attachments #1, the PRG is ~19, and at times, the arm transmission goes even higher. I'd say we are now in the regime where the uncertainty of the losses in the recycling cavity - maybe beamsplitter clipping? is important in using this info to try and constrain the arm cavity losses. I'm also not sure what to make of the asymmetry between TRX and TRY. Allegedly, the Y arm is supposed to be lossier.


This is very interesting. Do you have the ASDC vs PRG (~ TRXor TRY) plot? That gives you insight on what is the cause of the low recycling gain.

Attachment 1: PRFPMIcorner_DC_1275190251_1275190551.pdf
Attachment 2: PRFPMIcorner_SB_1275190251_1275190551.pdf
  15369   Wed Jun 3 03:29:26 2020 KojiUpdateLSCLock acquisition update portal

Woo hoo!

Which 1f signals are you going to use? PRCL has sign flipping at the carrier critical coupling. So if the IFO is close to that condition, 1f PRCL suffers from the sign flipping or large gain variation.

  15371   Wed Jun 3 11:40:56 2020 gautamUpdateLSCLock acquisition update portal

For these initial attempts, I was just trying to transition MICH to REFL55Q. I agree, the PRCL situation may be more complicated.

Which 1f signals are you going to use? PRCL has sign flipping at the carrier critical coupling. So if the IFO is close to that condition, 1f PRCL suffers from the sign flipping or large gain variation.

  15372   Wed Jun 3 18:49:47 2020 gautamUpdateLSCPRG and CARM signal sign


I am inclined to believe that the arm cavity losses are such that the IFO is overcoupled. Some calculations, validated with Finesse modeling also suggest that there isn't a sign change for the CARM error signal when the IFO goes from being undercoupled to overcoupled, but I may have made a mistake here?


  • We’d like to gain some insight into whether the interferometer is undercoupled, critically coupled, or overcoupled. Factors that determine which of these is true include:
    • Arm cavity losses
    • Recycling cavity losses
  • The proxy by which we determine the recycling gain is usually the arm cavity transmission. Assuming T_PRM = 5.637 % according to the wiki, and assuming the arm cavity transmission is normalized to 1 when locked in the POX/POY state, we can say that the PRG is given by G_PRC = TRX × T_PRM, assuming that the (i) the RF sideband fields are perfectly rejected by the arm cavities and (ii) mode-matching efficiency between the input beam and the arm mode is the same as that between the input beam and the CARM mode.
  • Apart from this, the other measurement we have available to us is the buildup of the sideband fields, namely POP22 and POP110. We can compare the values in the PRMI lock vs the PRFPMI to make some inference.
  • I started off with an analytic calculation of the reflectivity of the compound arm cavity mirror.
    • Attachment #1 suggests we will have an over-coupled IFO for arm cavity losses below ~200 ppm, which is a regime we are almost certainly in now.
  • Then, I repeat the analysis for the coupled CARM cavity, with the end mirror as the compound arm mirror and the input mirror as the PRM.
    • I assume 2 % loss in the PRC.
    • Attachment #2 shows that while the carrier field goes through a sign change in amplitude reflectivity (as expected), the sideband fields dont.
    • Per equation 4.2 of Koji's thesis, the error signal for CARM depends on the (signed) IFO reflectivity, and the absolute value of the derivative of the arm cavity reflectivity for the carrier w.r.t. CARM phase.
    • So, we don't expect the REFL11 signal to show a sign change.
    • The situation is more complicated for PRCL in REFL11, because as explicitly evaluated in Eq 4.3 of Koji's thesis, there are two terms that contribute, and their relative magnitudes will dictate the overall sign. 
  • For a Finesse validation, I use a simplified 3 mirror coupled cavity to approximate the PRFPMI. I also retained the RF sidebands for diagnostic purposes. The idea was to study these PRG proxies and what their expected behavior is.
    • Attachment #3 shows the PDH error signal in the (arbitrarily defined) REFL11 I quadrature. While the optical gain changes as a function of the arm cavity loss, the actual slope does not change sign. The fact that the zero crossing doesn't happen at exactly 0 CARM offset is because of higher order mode light at the REFL port (in my model, I tried to preserve the flipped folding mirror situation so the mode matching between the arm cavity and PRC in my model is ~96%).
    • In fact, this may explain why a CARM_B offset is required to do the ALS-->IR handoff - the ALS servo wants to keep the arm offset to zero, but at that point, the PDH error signal isn't zero, and so the two loops end up fighting each other?
    • Attachment #4 is a more detailed study of the recycling gain as a function of arm cavity loss, but now including losses in the recycling cavity.


  1. I think the arm cavity losses are in the 60-80 ppm round-trip region. I don't see how we can explain the arm cavity transmission of ~350 otherwise.
  2. The fact that REFLDC decreases as the arm transmission increases is because the input beam is getting better matched to the CARM mode, and there is less junk carrier light. 

Thoughts from others?

Attachment 1: armCavReflectivities.pdf
Attachment 2: IFOreflectivities.pdf
Attachment 3: PDHerrSigs.pdf
Attachment 4: PRGvsLoss_finesse.pdf
  15419   Fri Jun 19 17:06:50 2020 gautamUpdateLSCWhat should the short-term commissioning goals be?


I want some input about what the short-term (next two weeks) commissioning goals should be.


Before the vacuum fracas, the locking was pretty robust. With some human servoing of the input beam, I could maintain locks for ~1 hour. My primary goals were:

  1. Transition the vertex length DoF control from 3f signals to 1f signals.
  2. Turn on some MICH-->DARM feedforward cancellation, because the noise between ~100 Hz and ~1kHz is dominated by this cross-coupling.

I didn't succeed in either so far.

  1. I find that there is poor separation of the length DoFs in the 1f sensors, which makes this transition hopeless.
    • Why should this be? I can't get any sensing matrix in Finesse to line up with what I measure in-lock.
    • One hypothesis I came up with (but haven't yet tested) is that the offsets from the 3f photodiodes are changing from time to time, which somehow changes the projection of the various DoFs onto the photodiode quadratures. 
    • The attached GIF shows the variation in the measured sensing matrix on two days - while the sensing of MICH/PRCL in the 3f photodiodes have hardly changed, they are significantly different in the 1f photodiodes. Note that the I and Q have changed for REFL11 and REFL55 between the two days because I changed the demod phase.
    • I also thought that maybe the CARM suppression isn't sufficient for REFL11 to be used as a PRCL sensor - but even after engaging a CM board SuperBoost, I was unable to realize the PRCL 3f-->1f transition, even though the CARM-->REFL11 coupling did get smaller in the measured sensing matrix (red line in the GIF). I don't think we can juice up the CARM gain much more without modifying the CM board boosts, see Attachment #1.
  2. I was able to measure the MICH CTRL --> DARM ERR transfer function with somewhat high coherence (~0.98).
    • I then used the infrastructure available in the LSC model to try and implement some cancellation, but didn't really see any effect.
    • Perhaps the TF needs to be measured with higher coherence.
    • It may also be that if I am able to successfully execute the 3f-->1f transition, the coupling gets smaller because the 1f sensing noise is lower?

I guess apart from this, we want to run the ALS scan to try and infer something about the absorption-induced thermal lens. I guess at this point, the costs outweigh the benefits in trying to bring in the SRC as well, since we will be changing the SRC config?

Attachment 1: CARM_superBoost.pdf
  15427   Wed Jun 24 17:20:16 2020 gautamUpdateLSCWhat should the short-term commissioning goals be?

Per the discussion at the meeting today, the plan of action is:

  1. Lock the PRMI on carrier and measure the sensing matrix, see if the MICH and PRCL signals look sensible in 1f and 3f photodiodes.
  2. Try locking CARM on POP55 (since there is currently no POP55 photodiode, can we use POX/POY as an intermediary?).
  3. For the ASC, can we hijack one of the IMC WFS heads to study what the AS port WFS signals would look like, and maybe close a feedback loop on the ETMs?
    • My guess is no, because currently, the L2A is so poorly tuned on MC2 that the CARM length control messes with the alignment of the IMC significantly.
    • So we need the IMC WFS loops to maintain the pointing.
    • Of course, the MC2 L2A can be tuned to mitigate this problem. 
    • I also believe there is something funky going on with the WFS heads. More to follow on that in a later elog.
    • Apart from these issues, for this scheme to be tested, some mods to the c1ioo model will have to be made so that we can route the servo output to the ETMs (as opposed to the IMC mirrors as is the usual case).

If I missed something, please add here.



I want some input about what the short-term (next two weeks) commissioning goals should be.

  15442   Tue Jun 30 10:59:16 2020 gautamUpdateLSCThree sensing matrices


I injected some sensing lines and measured their responses in the various photodiodes, with the interferometer in a few different configurations. The results are summarized in Attachments #1 - #3. Even with the PRMI (no arm cavities) locked on 1f error signals, the MICH and PRCL signals show up in nearly the same quadrature in the REFL port photodiodes, except REFL165. I am now thinking if the output (actuation) matrix has something to do with this - part of the MICH control signal is fed back to the PRM in order to minimize the appearance of the MICH dither in the PRCL error signal, but maybe this matrix element is somehow horribly mistuned?


Attachment #1:

  • ETMs were misaligned and the PRMI was locked with the carrier resonant in the cavity (i.e. sidebands reflected).
  • The locking scheme was AS55_Q --> MICH and REFL11_I --> PRCL.

Attachment #2:

  • The PRFPMI was locked. The vertex DoFs were still under control using 3f error signals (REFL165_I for PRCL and REFL165_Q for MICH).
  • Still, the MICH/PRCL degeneracy in all photodiodes except REFL165 persists.

Attachment #3:

  • Nearly identical configuration to Attachment #2.
  • The main difference here is that I applied some offsets to the MICH and PRCL error points.
  • The offsets were chosen so that the appearance of a ~300 Hz dither in the length of MICH/PRCL was nulled in the AS110_Q / POP22_I signals respectively.
  • For the latter, the appearance of this peak in the POP110_I signal was also nulled, as it should be if our macroscopic PRC length is set correctly.
  • The offsets that best nulled the peak were 110 cts for PRCL, 25 cts for MICH. The measured sensing response is 1e12 cts/m for PRCL in REFL165_I and 9.2e11 cts/m for MICH in REFL165_Q. So these offsets, in physical units, are: 110 pm for PRCL and 27 pm for MICH. They seem like reasonable numbers to me - the PRC linewidth is ~7.5 nm, so the detuning without any digital offset applied is only 1.5% of the linewidth.
  • Note that I changed the POP22/POP110 demod phases to maximize the signal in the I quadrature. The final numbers were -124 degrees / -10 degrees respectively.
  • Yet another piece of evidence suggesting these were the correct offsets is that the DC value of POX and POY were zero on average after these offsets were applied.
  • However, the MICH/PRCL responses in the 1f REFL port photodiodes remain nearly degenerate.

Some other mysteries that I will investigate further:

  1. While POP22 indicated stable buildup of 11 MHz power in the PRC, I couldn't make any sense of the AS110 signals at the dark port - there was large variation of the signal content in the two quadratures, so unlike the POP signals, I couldn't find a digital demod phase that consistently had all the signal in one of the two quadratures. This is all due to angular fluctuations?
  2. My ASC simulations suggest that the POP QPD is a poor sensor of PRM motion when the PRFPMI is locked. However, I find that turning on a feedback loop with the POP QPD as a sensor and the PRM as the actuator dramatically reduces the low-frequency fluctuations of the arm cavity carrier buildup. 🤔

I blew the long lock last night because I forgot to not clear the ASS offsets when trying to find the right settings for running the ASS system at high power. Will try again tonight...


Lock the PRMI on carrier and measure the sensing matrix, see if the MICH and PRCL signals look sensible in 1f and 3f photodiodes.

Attachment 1: PRMI_1f_20200625sensMat.pdf
Attachment 2: PRFPMI_20200629sensMat.pdf
Attachment 3: PRFPMI_20200629sensMat_wOffset.pdf
  15457   Mon Jul 6 17:41:19 2020 gautamUpdateLSCAn LSC puzzler

Last Tuesday evening, while attempting the PRFPMI locking, I noticed a strange feature in the LSC signals, which is shown in Attachment #1 (the PDF exported by dataviewer is 14MB so I upload the jpeg instead). As best as I can tell, the REFL33 and POP22 channels show an abrupt jump in the signal levels, while the other channels do not. POP110 shows a slight jump at around the same time, and the large excursion in AS110_Q actually occurs a few seconds later, and is probably some angular excursion of the PRC/BS. I'm struggling to interpret how this can be explained by some interferometric mechanism, but haven't come up with anything yet. The LO for the 3f error signals is the 2f field, but then why doesn't the POP110 channel show a similar jump if there is some abrupt change in the resonant condition? Is such a change even feasible from a cavity length change point of view? Or did the sideband frequency somehow abruptly jump? But if so, why is the jump much more clearly visible in one sideband than the other?

Does anyone have any ideas as to what could be going on here? This may give some clue as to what's up with the weird sensing matrices, but may also be something boring like broken electronics... 

Attachment 1: LSCsignals.jpg
  15466   Fri Jul 10 01:25:28 2020 gautamUpdateLSCLocking notes

More tomorrow, but I tried the following tonight:

  1. Dither alignment for PRC / MICH seems to work when the PRFPMI is locked. Unfortunately, the correct settings for the arm cavity dither alignment loops continue to elude me.
  2. I tried some arm ASC loop characterization by stepping the error points of these loops - I saw some weird cross coupling between the loops that needs investigation.
  3. I'm unable to turn an integrator on for the "Common YAW" QPD loop - unclear why this is, but every time I attempt to engage said integrator, the lock is immediately blown. Needs some investigation of the signals.
  4. With the PRC / MICH angular DoFs aligned with the dither alignemnt, and the arm ASC loops hand tuned, I was able to get the darkest dark port I've seen. In terms of ASDC counts, it was ~ 200, which after undoing all the digital gains etc corresponds to ~100 uW of light. I think we can get a rough estimate of the contrast defect by accounting for (i) T_SRM, (ii) OMC pickoff fraction (iii) other losses between the BS dark port and the AP table (iv) 50/50 BS between AS55 and AS110 PD (the ASDC signal is derived from the former) and (v) the throughput of the 55 MHz sideband to the dark port, although there are many uncertainties. 
  15468   Fri Jul 10 15:26:28 2020 gautamUpdateLSCMC2 coils need DC balancing?

I was looking at some signals from last night, see Attachment #1.

  • It looks like as the DC control signal to the MC2 suspension increases, the MC transmission decreases.
    • I confirmed that the IMC REFL level doesn't correspondingly trend up, but didn't include it here for plot compactness, so I think the cavity length isn't being detuned.
    • So the problem is suggestive of some L2A coupling, and the MC2 coil actuators need to be balanced better at DC?
    • You can see from the IMC WFS control signals that the WFS servo is presumably trying to counteract this L2A action, but doesn't succeed, probably because the servo isn't tuned correctly.
    • This is a problem that is distinct from the drifting TT alignment. So it complicates the alignment situation.
    • Ideally, if the dither alignment servos could be made to work for the arm cavities when locked in the PRFPMI config, this wouldn't be so much of a problem, as the TTs would just adjust the beam pointing to match the cavity axes of the arms. But since I haven't managed to get that servo working yet...
  • But why should MC2 need such a large DC control signal ever?
    • In the PRFPMI lock, the CARM servo is supposed to match the laser frequency to the average length of the two arm cavities.
    • The MC2 suspension is used as a frequency actuator in order to realize this matching.
    • But, as you can see, the digital CARM control signal picks up a significant DC offset the deeper we go into the lock.
    • Can't we offload this DC signal to the laser crystal temperature servo? Is there going to be some weird interaction with the existing slow loop? Or is the idea itself flawed?

Attachment #2 shows some ASC metrics. My conclusion here is that running the PRCL and MICH dither alignment servos (former demodulating REFLDC and latter demodulating ASDC to get an error signal) that running the dither alignment servo and hand tuning the arm ASC loop offsets improves the mode matching to the IFO, because:

  1. The arm transmission increases.
  2. POPDC increases.
  3. ASDC decreases.

The REFLDC behavior needs a bit more interpretation I think, because if the IFO is overcoupled (as I claim it is), then better alignment would at some point actually result in REFLDC increasing. 

All the DC signals recorded by the fast system come from the backplane P2 connector of the PD interface boards. According to the schematic, these signals have a voltage gain of 2. The LSC photodiodes themselves have a nominal DC gain of 50 ohms. So, the conversion from power to digital counts is: 0.8 A/W * 50 V/A * 2 * 3276.8 cts/V * whtGain. Inverting, I get 3.8 uW/ct for a whitening gain of 1. This is power measured at the photodiode - optical losses upstream of the photodiode will have to be accounted for separately.

Assuming a modulation depth of 0.2, the 55 MHz sideband power should be ~20 mW. The Schnupp asymmetry is supposed to give us O(1) transmission of this field to the AS port. Then, the SRM will attenuate the field by a factor of 10, so we expect ~2 mW at the AS port. Let's assume 80 % throughput of this field to the AP table, and then there is a 50/50 beamsplitter dividing the light between the AS55 and AS110 photodiodes. So, we expect there to be ~700 uW of power in the TEM00 mode 55 MHz sideband field. This corresponds to 1600 cts according to the above calibration (the ASDC whitening gain is set to 18 dB). The fact that much smaller numbers were seen for ASDC indicates that (i) the schnupp asymmetry is not so perfectly tuned and the actual transmission of the sideband field to the dark port is smaller, or (ii) one or more optical splitting fractions assumed above is wrong. If the former is true, we can still probably infer the contrast defect if we can somehow get an accurate measurement of the sideband transmission to the dark port.

Attachment 1: MC2_balancing.png
Attachment 2: ASDC.png
  15470   Sat Jul 11 18:24:30 2020 KojiUpdateLSCMC2 coils need DC balancing?

> Can't we offload this DC signal to the laser crystal temperature servo?
No. PSL already follows the MC length. So this offset is coming from the difference between the MC length and the CARM length.
What you can do is to offload the MC length to the CARM DC if this helps.

  15471   Sun Jul 12 02:42:01 2020 gautamUpdateLSCLocking (on rossa)

Main goals tonight were:

  1. Check if I can lock the interferometer by working on rossa - indeed, I could! It is much snappier than the ageing pianosa. The viewing angle of the CRT monitors from this corner isn't so good though.
  2. Measure step responses for the arm ASC loops to see if any insight can be gained into these loops. Analysis forthcoming...
Attachment 1: ASCsteps.png
  15474   Mon Jul 13 11:36:08 2020 ranaUpdateLSCMC2 coils need DC balancing?
  1. if IMC REFL is not increasing, I don't think its a mis-alignment. Usually, REFL is a more sensitive indicator of alignment than TRANS since its usually near zero. Maybe the MC2 TRANS PD is not centered or doesn't have enough lens action.
  2. to reduce the DC load on MC2, you could have a slow releif drive the ETMs and DC and minimize LSC-MCL
  15476   Tue Jul 14 00:06:09 2020 gautamUpdateLSCLocking with POX for CARM

I tried using the POX_I error signal for the DC CARM_B path today a couple of times. Got to a point where the AO path could be engaged and the arm powers stabilized somewhat, but I couldn't turn the CARM_A path off without blowing the lock. Now the IMC has entered a temperemental state, so I'm abandoning efforts for tonight, but things to try tomorrow are:

  1. Check that the demod phase is set correctly
  2. With the CARM_B path engaged, measure some CARM OLTFs. Tonight, I was a bit over-optimistic I think, by expecting the scripted transition to take me all the way, but I think I'll have to fiddle around with the gains a bit.
  3. Check for offsets. The AO path should be AC coupled, but maybe the POX signal has some offset?

I have some data from a couple of days ago when the PRFPMI was locked as usual (CARM_B on REFL for both DC and AO paths), and the sensing lines were on, so I can measure the relative strength of the sensing lines in POX/REFL and get an estimate of what the correct digital gain should be.

The motivation here is to see if the sensing matrix looks any different with a modified locking scheme.

  15477   Tue Jul 14 01:55:03 2020 KojiUpdateLSCLocking with POX for CARM

The usual technique is that keeping the IFO locked with the old set of the signals and the relative gain/TF between the conventional and new signals are measured in-lock so that you can calibrate the new gain/demod-phase setting.

  15481   Tue Jul 14 17:28:29 2020 gautamUpdateLSCLocking with POX for CARM

From Attachment #1, looks like the phasing and gain for CARM on POX11 is nearly the same as CARM of REFL11, which is probably why I was able to execute a partial transition last night. The response in POY11 is ~10 times greater than POX11, as expected - though the two photodiodes have similar RF transimpedance, there is a ZFL-500-HLN at the POY11 output. The actual numerical values are 2.5e10 cts/m for CARM-->REFL11_I, 2.6e10 cts/m for CARM-->POX11_I, and 3.2e11 cts/m for CARM-->POY11_I.

So I think I'll just have to fiddle around with the transition settings a little more tonight. 

One possible concern is that the POX and POY signals are digitized without preamplificatio, maybe this explains the larger uncertainty ellipse for the POX and POY photodiodes relative to the REFL11 photodiode? Maybe the high frequency noise is worse and is injecting junk in the AO path? I think it's valid to directly compare the POX and REFL spectra in Attachment #2, without correcting for any loops, because this signal is digitized from the LSC demodulator board output (not the preamplified one, which is what goes to the CM board, and hence, is suppressed by the CARM loop). Hard to be sure though, because while the heads are supposed to have similar transimpedance, and the POX photodiode has +12dB more whitening gain than REFL11, and I don't know what the relative light levels on these photodiodes are in lock.


I have some data from a couple of days ago when the PRFPMI was locked as usual (CARM_B on REFL for both DC and AO paths), and the sensing lines were on, so I can measure the relative strength of the sensing lines in POX/REFL and get an estimate of what the correct digital gain should be

Attachment 1: PRFPMI_2020712sensMat.pdf
Attachment 2: LSCerrSigs.pdf
  15714   Mon Dec 7 14:32:02 2020 gautamUpdateLSCNew demod phases for POX/POY locking

In favor of keeping the same servo gains, I tuned the digital demod phases for the POX and POY photodiode signals to put as much of the PDH error signal in the _I quadrature as possible. The changes are summarized below:

POX / POY demod phases
PD Old demod phase [deg] New demod phase [deg]
POX11 79.5 -75.5
POY11 -106.0 116.0

The old locking settings seem to work fine again. This setting isn't set by the ifoconfigure scripts when they do the burt restore - do we want it to be?

Attachments #1 and #2 show some spectra and TFs for the POX/POY loops. In Attachment #2, the reference traces are from the past, while the live traces are from today. In fact, to have the same UGF as the reference traces (from ~1 year ago), I had to also raise the digital servo loop gain by ~20%. Not sure if this can be put down to a lower modulation depth - at least, at the output on the freq ref box, I measured the same output power (at the 0dB variable attenuator gain setting we nominally run in) before and after the changes. But I haven't done an optical measurement of the modulation depth yet. There is also a hint of lesser phase available at ~100 Hz now compared to a year ago.

Attachment 1: POX_POY_OLTF.pdf
Attachment 2: POX_POY_spectra.pdf
  15715   Mon Dec 7 22:54:30 2020 gautamUpdateLSCModulation depth measurement


I measured the modulation depth at 11 MHz andf 55 MHz using an optical beat + PLL setup. Both numbers are ~0.2 rad, which is consistent with previous numbers. More careful analysis forthcoming, but I think this supports my claim that the optical gain for the PDH locking loops should not have decreased.


  • For this measurement, I closed the PSL shutter between ~4pm and ~9pm local time. 
  • The photodiode used was the NF1611, which I assumed has a flat response in the 1-200 MHz band, and so did not apply any correction/calibration.
Attachment 1: modDepth.pdf
  15718   Wed Dec 9 12:02:04 2020 gautamUpdateLSCPOX locking still unsatisfactory

Continuting the IFO recovery - I am unable to recover similar levels of TRX RIN as I had before. Attachment #1 shows that the TRX RIN is ~4x higher in RMS than TRY RIN (the latter is commensurate with what we had previously). The excess is dominated by some low frequency (~1 Hz) fluctuations. The coherence structure is confusing - why is TRY RIN coherent with IMC transmission at ~2 Hz but not TRX? But anyways, doesn't look like its intensity fluctuations on the incident light (unsurprisingly, since the TRY RIN was okay). I thought it may be because of insufficient low-frequency loop gain - but the loop shape is the same for TRX and TRY. I confirmed that the loop UGF is similar now (red trace in Attachment #2) as it was ~1 month ago (black trace in Attachment #2). Seismometers don't suggest excess motion at 1 Hz. I don't think the modulation depth at 11 MHz is to blame either. As I showed earlier, the spectrum of the error point is comparable now as it was previously.

What am I missing?

Attachment 1: armRIN.pdf
Attachment 2: POX_OLTF.pdf
  15750   Wed Jan 6 19:00:04 2021 gautamUpdateLSCPhase loss in POX/POY loops

I've noticed that there is some phase loss in the POX/POY locking loops - see Attachment #1, live traces are from a recent measurement while the references are from Nov 4 2018. Hard to imagine a true delay being responsible to cause so much phase loss at 100 Hz. Attachment #2 shows my best effort loop modeling, I think I've got all the pieces, but maybe I missed something (I assume the analog whitening / digital anti-whitening are perfectly balanced, anyway this wasn't messed with anytime recently)? The fitter wants to add 560 us (!) of delay, which is almost 10 clock cycles on the RTS, and even so, the fit is poor (I constrain the fitter to a maximum of 600 us delay so maybe this isn't the best diagnostic). Anyway, how can this change be explained? The recent works I can think of that could have affected the LSC sensing were (i) RF source box re-working, and (ii) vent. But I can't imagine how either of these would introduce phase loss in the LSC sensing. Note that the digital demod phase has been tuned to put all the PDH signal in the "I" quadrature, which is the condition in which the measurement was taken.

Probably this isn't gonna affect locking efforts (unless it's symptomatic of some other larger problem).

Attachment 1: POXloop.pdf
Attachment 2: loopFit.pdf
  15768   Fri Jan 15 17:04:45 2021 gautamUpdateLSCMessed up LSC sensing

I want to lock the PRFPMI again (to commission AS WFS). Have had some success - but in doing characterization, I find that the REFL port sensing is completely messed up compared to what I had before. Specifically, MICH and PRCL DoFs have no separation in either the 1f or 3f photodiodes. 

  • A sensing line driven in PRCL doesn't show up in the AS55 photodiode signal - this is good and as expected.
  • For MICH - I set the MICH--->PRM actuation matrix element so as to minimize the height of the peak at the MICH drive frequency that shows up at the PRCL error point. My memory is that I used to be able to pretty much null this signal in the past, but I can't find a DTT spectrum in the elog as evidence. Anyways, the best effort nulling I can achieve now still results in a large peak at the PRCL error point. Since the sensing matrix doesn't actually make any sense, idk if it is meaningful to even try and calibrate the above qualitative statement into quantitative numbers of cross coupling in meters.
  • With the PRMI locked on 1f error signals (ETMs misaligned, PRCL sensed with REFL11_I, MICH sensed with AS55_Q) - I tried tweaking the digital demod phase of the REFL33 and REFL165 signals. But I find that the MICH and PRCL peaks move in unison as I tweak the demod phase. This suggests to me that both signals are arriving optically in phase at the photodiode, which is weird.
  • The phenomenon is seen also in the REFL11 signal.

I did make considerable changes to the RF source box, and so now the relative phase between the 11 MHz and 55 MHz signals is changed compared to what it was before. But do we really expect any effect even in the 1f signal? I am not able to reproduce this effect in simulation (Finesse), though I'm using a simplified model. I attach two sensing matrices to illustrate what i mean:

  1. Attachment #1 is in the PRFPMI state, with the IFO on RF control (CARM on REFL11, PRCL on REFL165_I, MICH on REFL165_Q, DARM on AS55_Q). 
  2. Attachment #2 is between the transition to RF control (CARM and DARM on ALS, PRCL on REFL165_I, MICH on REFL165_Q). The CARM offset is ~4nm (c.f. the linewidth of ~20pm), so the circulating power in the arm cavities is low.
Attachment 1: PRFPMI_Jan12sensMat.pdf
Attachment 2: PRMI3f_ALS_Jan11_largeOffsetsensMat.pdf
  15769   Sat Jan 16 18:59:44 2021 gautamUpdateLSCModulation depth measurement

I decided to analyze the data I took in December more carefully to see if there are any clues about the weird LSC sensing.

Attachment #1 shows the measurement setup.

  • The PSL shutter was closed. All feedback to both lasers was disconnected during the measurement. I also disabled the input switch to the FSS Box - so the two laser beams being interfered shouldn't have any modulations on them other than the free running NPRO noise and the main IFO modulations.
  • Everything is done in fiber as I had the beams already coupled into collimators and this avoided having to optimize any mode matching on the beat photodiode.
  • The pickoff of the PSL is from the collimator placed after the triply resonant EOM that was installed for the air BHD experiment.
  • The other beam is the EX laser beam, arriving at the PSL table via the 40m long fiber from the end (this is the usual beam used for ALS).
  • I didn't characterize precisely the PLL loop shape. But basically, I wasn't able to increase the SR560 gain any more without breaking the PLL lock. Past experience suggests that the UGF is ~20 kHz, and I was able to get several averages on the AG4395 without the lock being disturbed.

Attachment #2 shows the measured spectrum with the PSL and EX laser frequency offset locked via PLL.

  • The various peaks are identified.
  • There are several peaks which I cannot explain - any hypothesis for what these might be? Some kind of Sorensen pollution? They aren't any multiples of any of the standard RF sources. They are also rather prominent (and stationary during the measurement time, which I think rules out the cause being some leakage light from the EY beam, which I had also left connected to the BeatMouth during the measurement).
  • In the previous such characterization done by Koji, such spurious extra peaks aren't seen.
  • Also, I can't really explain why some multiples of the main modulation are missing (could also be that my peak finding missed the tiny peaks)?
  • The measuremet setup is very similar to what he had - important differences are 
    • Much of the optical path was fiber coupled.
    • Beat photodiode is NF1611, which is higher BW than the PDA10CF.
    • The second laser source was the Innolight EX NPRO as opposed to the Lightwave that was used.
    • The RF source has been modified, so relative phasing between 11 MHz and 55 MHz is different.

Fitting the measured sideband powers (up to n=7, taking the average of the measured upper and lower sideband powers to compute a least squares fit if both are measured, else just that of the one sideband measured) agains those expected from a model, I get the following best fit parameters:

\begin{align*} \Gamma_1 &= 0.193 \pm 0.004 \\ \Gamma_2 &= 0.246 \pm 0.008 \\ \phi &= 75.5^{\circ +17.5^{\circ}}_{\, -40.3^{\circ}} \end{align*}

To be explicit, the residual at each datapoint was calculated as

\Delta = \bigg| \frac{\rm{model}-\rm{measurement}}{\rm{model}}\bigg|^2.

The numbers compare favourably with what Koji reported I think - the modulation depths are slightly increased, consistent with the RF power out of the RF box being slightly increased after I removed various attenuators etc. Note the large uncertainty on the relative phase between the two modulations - I think this is because there are relatively few sidebands (one example is n=3) which has a functional dependence that informs on phi - most of the others do not directly give us any information about this parameter (since we are just measuring powers, not the actual phase of the electric field). 

Attachment #3 shows a plot of the measured modulation profile, along with the expected heights plugging the best fit parameters into the model. The size of the datapoint markers is illustrative only - the dependence on the model parameters is complicated and the full covariance would need to be taken into account to put error bars on those markers, which I didn't do. 

Attachment #4 shows a time domain measurement of the relative phasing between the 11 MHz and 55 MHz signals at the EOM drive outputs on the RF source box. I fit a model there and get a value for the relative phase that is totally inconsistent from what I get with this fit. 

Attachment 1: PLL.pdf
Attachment 2: modDepth.pdf
Attachment 3: modProfile.pdf
Attachment 4: EOMpath_postMod.pdf
  15822   Fri Feb 19 13:38:26 2021 gautamUpdateLSCPRFPMI

I forgot that I had already done some investigation into recovering the PRFPMI lock after my work on the RF source. I don't really have any ideas on how to explain (or more importantly, resolve) the poor seperation of MICH and PRCL sensed in our 3f (but also 1f) photodiodes, see full thread here. Anyone have any ideas? I don't think my analysis (=code) of the sensing matrix can be blamed - in DTT, just looking the spectra of the _ERR_DQ channels for the various photodiodes while a ssingle frequency line is driving the PRM/BS suspension, there is no digital demod phase that decouples the MICH/PRCL peak in any of the REFL port photodiode spectra.

  15827   Fri Feb 19 18:22:42 2021 ranaUpdateLSCPRFPMI sensiing matrix woes

I would:

  1. look at the free swingin michelson. Should be able tu null that siggnal in all ports to define the Q phase.
  2. If things are weird, put an RF signal nto the demod board mhich is offset from the LO by ~100 Hz and verify the demod/whitening chain is kosher.
  3. Lock PRMI and drive lines > 200 Hz. If PRC/MICH are not orthogonal, then there may be a mis tuning of RF SB wavelength and cavity lengths.
  4. IF PRMI is sort of healthy, we could be having a weird SB resonance in the arms.
  15849   Sun Feb 28 16:59:39 2021 rana, gautamUpdateLSCmore PRMI checks here: what it is ain't exactly clear

On Friday evening we checked out a few more things, somewhat overlapping with previous tests. All tests done with PRMI on carrier lock (REFL11_I -> PRC, AS55_Q-> MICH):

  • check that PRC drive appropriately minimizes in REFL55_Q. I:Q ratio is ~100:1; good enough.
  • put sine waves around 311 and 333 Hz into PRCL and MICH at the LSC output matrix using awggui and LSC osc. not able to adjust LSC/OSC output matrix to minimize the MICH drive in REFL_I.
  • measured the TF from BS & PRM LSC drive to the REFL55_I/Q outputs. very nearly the same audio frequency phase, so the problem is NOT in the electronics || mechanical transfer functions of the suspensions.


Further questions:

  1. is this something pathological in the PRMI carrier lock? we should check by locking on sidebands to REFL55 and REFL165 and repeat tests.
  2. Can it be a severe mode mismatch from IMC output to PRMI mode? the cavity should be stable with the flipped folding mirrors, but maybe something strange happening. How do we measure the mode-matching to the PRC quantitatively?
  3. huge RAM is ruled out by Gautam's test of looking at REFL demod signals: dark offset vs. offset with a single bounce off of PRM (with ITMs mis-aligned)
  4. if there is a large (optical) offset in the AS55_Q lock point, how big would it have to be to mess up the REFL phase so much?
  5. what is going on with the REFL55 whitening/AA electronics?

unrelated note: Donatella the Workstation was ~3 minutes ahead of the FE machines (you can look at the C0:TIM-PACIFIC_STRING on many of the MEDM screens for a rough simulacrum). When the workstation time is so far off, DTT doesn't work right (has errors like test timed out, or other blah blah). I installed NTP on donatella and started the service per SL7 rules. Since we want to migrate all the workstations to Debian (following the party line), lets not futz with this too much.

gautam, 1 Mar 1600: In case I'm being dumb, I attach the screen grab comparing dark offset to the single bounce off PRM, to estimate the RAM contribution. The other signals are there just to show that the ITMs are sufficiently misaligned. The PRCL PDH fringe is usually ~12000 cts in REFL11, ~5000cts in REFL55, and so the RAM offset is <0.1% of the horn-to-horn PDH fringe.

P.S. I know generally PNGs in the elog are frowned upon. But with so many points, the vector PDF export by NDS (i) is several megabytes in size and (ii) excruciatingly slow. I'm proposing a decimation filter for the export function of ndscope - but until then, I claim plotting with "rasterized=True" and saving to PDF and exporting to PNG are equivalent, since both yield a rasterized graphic.

Attachment 1: RAMestimate.png
  15850   Sun Feb 28 22:53:22 2021 gautamUpdateLSCmore PRMI checks here: what it is ain't exactly clear

I looked into this a bit more and crossed off some of the points Rana listed. In order to use REFL 55 as a sensor, I had to fix the frequent saturations seen in the MICH signals, at the nominal (flat) whitening gain of +18 dB. The light level on the REFL55 photodiode (13 mW), its transimpedance (400 ohm), and this +18dB (~ x8) gain, cannot explain signal saturation (0.7A/W * 400 V/A * 8 ~ 2.2kV/W, and the PRCL PDH fringe should be ~1 MW/m, so the PDH fringe across the 4nm linewidth of the PRC should only be a couple of volts). Could be some weird effect of the quad LT1125. Anyway, the fix that has worked in the past, and also this time, is detailed here. Note that the anomalously high noise of the REFL55_Q channel in particular remains a problem. After taking care of that, I did the following:

  1. PRMI (ETMs misaligned) locking with sidebands resonant in the PRC was restored - REFL55_I was used for PRCL sensing and REFL55_Q was used for MICH sensing. The locks are acquired nearly instantaneously if the alignment is good, and they are pretty robust, see Attachment #1 (the lock losses were IMC related and not really any PRC/MICH problem).
  2. Measured the loop OLTFs using the usual IN1/IN2 technique. The PRCL loop looks just fine, but the MICH loop UGF is very low apparently. I can't just raise the loop gain because of the feature at ~600 Hz. Not sure what the origin of this is, it isn't present in the analogous TF measurement when the PRMI is locked with carrier resonant (REFL11_I for PRCL sensing, AS55_Q for MICH sensing). I will post the loop breakdown later. 
  3. Re-confirmed that the MICH-->PRCL coupling couldn't be nulled completely in this config either.
    • The effect is a geometric one - then 1 unit change in MICH causes a 1/sqrt(2) change in PRCL. 
    • The actual matrix element that best nulls a MICH drive in the PRCL error point is -0.34 (this has not changed from the PRMI resonant on carrier locking). Why should it be that we can't null this element, if the mechanical transfer functions (see next point) are okay?
  4. Looked at the mechanical actuator TFs are again (since we forgot to save plots on Friday), by driving the BS and PRM with sine waves (311.1 Hz), one at a time, and looking at the response in REFL55_I and REFL55_Q. Some evidence of some funkiness here already. I can't find any configuration of digital demod phase that gives me a PRCL/MICH sensing ratio of ~100 in REFL55_I, and simultaneously, a MICH/PRCL sensing ratio of ~100 in REFL55_Q. The results are in Attachments #5
  5. Drove single frequency lines in MICH and PRCL at 311.1 and 313.35 Hz respectively, for 5 minutes, and made the radar plots in Attachments #2 and #3. Long story short - even in the "nominal" configuration where the sidebands are resonant in the PRC and the carrier is rejected, there is poor separation in sensing. 
    • Attachments #2 is with the digital REFL55 demod phase set to 35 degrees - I thought this gave the best PRCL sensing in REFL55_I (eyeballed it roughly by looking at ndscope free-swinging PDH fringes).
    • But the test detailed in bullet #4, and Attachments #2 itself, suggested that PRCL was actually being sensed almost entirely in the Q phase signal.
    • So I changed the digital demod phase to -30 degrees (did a more quantitative estimate with free-swinging PDH fringes on ndscope, horn-to-horn voltages etc).
    • The same procedure of sine-wave-driving now yields Attachments #3. Indeed, now PRCL is sensed almost perfectly in REFL55_I, but the MICH signal is also nearly in REFL55_I. How can the lock be so robust if this is really true? 
  6. Attachments #4 shows some relevant time domain signals in the PRMI lock with the sidebands resonant. 
    • REFL11_I hovers around 0 when REFL55_I is used to sense and lock PRCL - good. The m/ct calibration for REFL11_I and REFL55_I are different so this plot doesn't directly tell us how good the PRCL loop is based on the out-of-loop REFL11_I sensor.
    • ASDC is nearly 0, good.
    • POP22_I is ~200cts (and POP22_Q is nearly 0) - I didn't see any peak at the drive frequency when driving PRCL with a sine wave, so no linear coupling of PRCL to the f1 sideband buildup, which would suggest there is no PRCL offset.
    • Couldn't do the analogous test for AS110 as I removed that photodiode for the AS WFS - it is pretty simple to re-install it, but the ASDC level already doesn't suggest anything crazy here.

Rana also suggested checking if the digital demod phase that senses MICH in REFL55_Q changes from free-swinging Michelson (PRM misaligned), to PRMI aligned - we can quantify any macroscopic length mismatch in the PRC length using this measurement. I couldn't see any MICH signal in REFL55_Q with the PRM misaligned and the Michelson fringing. Could be that +18dB is insufficient whitening gain, but I ran out of time this afternoon, so I'll check later. But not sure if the double attenuation by the PRM makes this impossible.

Attachment 1: PRMI_SBres_REFL55.png
Attachment 2: PRMI1f_noArmssensMat.pdf
Attachment 3: PRMI1f_noArmssensMat.pdf
Attachment 4: PRMI_locked.png
Attachment 5: actTFs.pdf
  15853   Mon Mar 1 16:27:17 2021 gautamUpdateLSCPRM violin filter excessive?

The PRM violin filter seems very suboptimal - the gain peaking shows up in the MICH OLTF, presumably due to the MICH-->PRM LSC output matrix. I plot the one used for the BS in comparison in Attachment #1, seems much more reasonable. Why does the PRM need so many notches? Is this meant to cover some violin modes of PR2/PR3 as well? Do we really need that? Are the PR2/PR3 violin modes really so close in frequency to that for the 3" SOS? I suppose it could be since the suspension wire is thinner and the mass is lighter, and the two effects nearly cancel, but we don't actuate on PR2/PR3? According to the earlier elog in this thread, this particular filter wasn't deemed offensive and was left on.

Indeed, as shown in Attachment #2, I can realize a much healthier UGF for the MICH loop with just a single frequency notch (black reference trace) rather than using the existing "PRvio1,2" filter (FM2), (live red trace). The PR violins are eating so much phase at ~600 Hz.


We turned off many excessive violin mode bandstop filters in the LSC.

Attachment 1: violins.pdf
Attachment 2: PRviolin.pdf
  15854   Tue Mar 2 13:39:31 2021 ranaUpdateLSCPRM violin filter excessive?

agreed, seems excessive. I always prefer bandstop over notch in case the eigenfrequency wanders, but the bandstop could be made to be just a few Hz wide.


  15855   Tue Mar 2 19:52:46 2021 gautamUpdateLSCREFL55 demod board rework

There were multiple problems with the REFL55 demod board. I fixed them and re-installed the board. The TFs and noise measured on the bench now look more like what is expected from a noise model. The noise in-situ also looked good. After this work, my settings for the PRMI sideband lock don't work anymore so I probably have to tweak things a bit, will look into it tomorrow.

  15859   Wed Mar 3 22:13:05 2021 gautamUpdateLSCREFL55 demod board rework

After this work, I measured that the orthogonality was poor. I confirmed on the bench that the PQW-2-90 was busted, pin 2 (0 degree output) showed a sensible signal half of the input, but pin 6 had far too small an output and the phase difference was more like 45 degrees and not 90 degrees. I can't find any spares of this part in the lab - however, we do have the equivalent part used in the aLIGO demodulator. Koji has kindly agreed to do the replacement (it requires a bit of jumper wiring action because the pin mapping between the two parts isn't exactly identical - in fact, the circuit schematic uses a transformer to do the splitting, but at some unknown point in time, the change to the minicircuits part was made. Anyway, until this is restored, I defer the PRMI sideband locking.


There were multiple problems with the REFL55 demod board. I fixed them and re-installed the board. The TFs and noise measured on the bench now look more like what is expected from a noise model. The noise in-situ also looked good. After this work, my settings for the PRMI sideband lock don't work anymore so I probably have to tweak things a bit, will look into it tomorrow.

  15861   Thu Mar 4 10:54:12 2021 Paco, AnchalSummaryLSCPOY11 measurement, tried to lock Green Yend laser

[Paco, Anchal]

- First ran burtgooey as last time.

- Installed pyepics on base environment of donatella

- Clicked on ON in the drop down of "! More Scripts" below "! Scripts XARM" in C1ASS.adl
- Clicked on "Freeze Outputs" in the same menu after some time.
- Noticed that the sensing and output matrix of ASS on XARM and YARM look very different. The reason probably is because the YARM outputs have 4 TT1/2 P/Y dof instead of BS P/Y on the XARM. What are these TT1/2?

(Probably, unrelated but MC Unlocked and kept on trying to lock for about 10 minutes attaining the lock eventually.)

Locking XARM:
- From scripts/XARM we ran lockXarm.py from outside any conda environment using python command.
- Weirdly, we see that YARM is locked??? But XARM is not. Maybe this script is old.
- C1:LSC-TRY-OUTPUT went to around 0.75 (units unknown) while C1:LSC-TRX-OUTPUT is fluctuating around 0 only.

POY11 Spectrum measurement when YARM is locked:
- Created our own template as we couldn't find an existing one in users/Templates.
- Template file and data in Attachment 2.
- It is interesting to see most of the noise is in I quadrature with most noise in 10 to 100 Hz.
- Given the ARM is supposed to be much calmer than MC, this noise should be mostly due to the mode cleaner noise.
- We are not sure what units C1:LSC-POY11_I_ERR_DQ have, so Y scale is shown with out units.

Trying to lock Green YEND laser to YARM:
- We opened the Green Y shutter.
- We ensured that when temperature slider og green Y is moved up, the beatnote goes up.
- ARM was POY locked from previous step.
- Ran script scripts/YARM/Lock_ALS_YARM.py from outside any conda environment using python command.
- This locked green laser but unlocked the YARM POY.

Things moving around:
- Last step must have made all the suspension controls unstable.
- We see PRM and SRM QPDs moving a lot.
- Then we did burt restore to /opt/rtcds/caltech/c1/burt/autoburt/today/08:19/*.snap to go back to the state before we started changing things today.

[Paco left for vaccine appointment]

- However the unstable state didn't change from restore. I see a lot of movement in ITMX/Y. PRM and BS also now. Movement in WFS1 and MC2T as well.
 - I closed PSL shutter as well to hopefully disengage any loops that are still running unstably.
 - But at this point, it seems that the optics are just oscillating and need time to come back to rest. Hopefully we din't cause too much harm today :(.

My guess on what happened:

  • Us using the Lock_ALS_YARM.py probably created an unstable configuration in LSC matrix and was the start of the issue.
  • On seeing PRM fluctuate so much, we thought we should just burst restore everything. But that was a hammer to the problem.
  • This hammer probably changed the suspension position values suddenly causing an impulse to all the optics. So everything started oscillating.
  • Now MC WFS is waiting for MC to lock before it stablizes the mode cleaner. But MC autolocker is unable to lock because the optics are oscillating. Chicken-egg issue.
  • I'm not aware of how manually one can restore the state now. My only known guess is that if we wait for few hours, everything should calm back enough that MC can be locked and WFS servo can be switched on.
Attachment 1: 20210304_POY11_Spectrum_YARMLocked.pdf
Attachment 2: 20210304_POY11_Spectrum_YARMLocked.tar.gz
  15862   Thu Mar 4 11:59:25 2021 Paco, AnchalSummaryLSCWatchdog tripped, Optics damped back

Gautam came in and noted that the optics damping watchdogs had been tripped by a >5 magnitude earthquake somewhere off the coast of Australia. So, under guided assistance, we manually damped the optics using following:

  • Using the scripts/SUS/reEnableWatchdogs.py script we re-enabled all the watchdogs.
  • Everything except SRM was restored to stable state.
  • Then we clicked on SRM in SUS-> Watchdogs, disabled the Oplevs, shutdown the watchdog.
  • We changed the threshold for watchdog temporarily to 1000 to allow damping.
  • We enabled all the coil outputs  manually. Then enabled watchdog by clicking on Normal.
  • Once the SRM was damped, we shutdown the watchdog, brought back the threshold to 215 and restarted it.

Gautum also noticed that MC autolocker got turned OFF by me (Anchal), we turned it back on and MC engaged the lock again. All good, no harm done.

  15864   Thu Mar 4 23:16:08 2021 KojiUpdateLSCREFL55 demod board rework

A new hybrid splitter (DQS-10-100) was installed. As the amplification of the final stage is sufficient for the input level of 3dBm, I have bypassed the input amplification (Attachment 1). One of the mixer was desoldered to check the power level. With a 1dB ATTN, the output of the last ERA-5 was +17.8dBm (Attachment 2). (The mixer was resoldered.)

With LO3dBm. RF0dBm, and delta_f = 30Hz, the output Vpp of 340mV and the phase difference is 88.93deg. (Attachment 3/4, the traces were averaged)

Attachment 1: D990511-00_REFL55.pdf
Attachment 2: P_20210304_215602.jpg
Attachment 3: P_20210304_222400.jpg
Attachment 4: P_20210304_222412.jpg
Attachment 5: 20210304234400_IMG_0526.jpg
  15867   Fri Mar 5 13:53:57 2021 gautamUpdateLSCREFL55 demod board rework

0 dBm ~ 0.63 Vpp. I guess there is ~4dB total loss (3dB from splitter and 1dB from total excess loss above theoretical from various components) between the SMA input and each RF input of the JMS-1-H mixer, which has an advertised conversion loss of ~6dB. So the RF input to each mixer, for 0dBm to the front panel SMA is ~-4dBm (=0.4 Vpp), and the I/F output is 0.34Vpp. So the conversion loss is only ~-1.5 dB? Seems really low? I assume the 0.34 Vpp is at the input to the preamp? If it's after the preamp, then the numbers still don't add up, because with the nominal 6dB conversion loss, the output. should be ~2Vpp? I will check it later.


With LO3dBm. RF0dBm, and delta_f = 30Hz, the output Vpp of 340mV and the phase difference is 88.93deg. (Attachment 3/4, the traces were averaged)

  15869   Fri Mar 5 15:31:23 2021 KojiUpdateLSCREFL55 demod board rework

Missed to note: The IF test was done at TP7 and TP6 using pomona clips i.e. brefore the preamp.


  15871   Fri Mar 5 16:24:24 2021 gautamUpdateLSCREFL55 demod board re-installed in 1Y2

I don't have a good explanation why, but I too measured similar numbers to what Koji measured. The overall conversion gain for this board (including the +20dB gain from the daughter board) was measured to be ~5.3 V/V on the bench, and ~16000 cts/V in the CDS system (100Hz offset from the LO frequency). It would appear that the effective JMS-1-H conversion loss is <2dB. Seems fishy, but I can't find anything else obviously wrong with the circuit (e.g. a pre-amp for the RF signal that I missed, there is none).

I also attach the result of the measured noise at the outputs of the daughter board (i.e. what is digitized by the ADC), see Attachment #2. Apart from the usual forest of lines of unknown origin, there is still a significant excess above the voltage noise of the OP27, which is expected to be the dominant noise source in this configuration. Neverthelesss, considering that we have only 40dB of whitening gain, it is not expected that we see this noise directly in the digitized signal (above the ADC noise of ~1uV/rtHz). Note that the measured noise today, particularly for the Q channel,  is significantly lower than before the changes were made

Attachment 1: REFL55.pdf
Attachment 2: demodNoise.pdf
  15873   Fri Mar 5 22:25:13 2021 gautamUpdateLSCPRMI 1f SB locking recovered

Now that the REFL55 signal chain is capable of providing balanced, orthogonal readout of the two quadratures, I was able to recover the 1f SB resonant lock pretty easily. Ran sensing lines for ~5mins, still looks weird. But I didn't try to optimize anything / do other checks (e.g. actuate MICH using ITMs instead of BS) tonight, and I'm craving the Blueberry pie Rana left me. Will continue to do more systematic tests in the next days.

Attachment 1: PRMI1f_noArmssensMat.pdf
  15874   Sat Mar 6 12:34:18 2021 gautamUpdateLSCSensing matrix settings messed with

To my dismay, I found today that somebody had changed the oscillator frequencies for the sensing matrix infrastructure we have. The change happened 2 days and 2 hours ago (I write this at ~1230 on Saturday, 3/6), i.e. ~1030am on Thursday. According to the elog, this is when Anchal and Paco were working on the interferometer, but I can find no mention of these settings being changed. Not cool guys 😒 .

This was relatively easy to track down but I don't know what else may have been messed with. I don't understand how anything that was documented in the elog can lead to this weird doubling of the frequencies.

I have now restored the correct settings. The "sensing matrix" I posted last night is obviously useless.

Attachment 1: sensMat.png
  15875   Sun Mar 7 15:26:10 2021 gautamUpdateLSCHousekeeping + more PRMI
  1. Beam pointing into PMC was tweaked to improve transmission.
  2. AS110 photodiode was re-installed on the AS table - I picked off 30% of the light going to the AS WFS using a beamsplitter and put it on the AS110 photodiode.
  3. Adjusted ASDC whitening gain - we have been running nominally with +18dB, but after Sept 2020 vent, there is ~x3 amount of light incident on the AS55 RFPD (from which the ASDC signal is derived). I want to run the dither alignment servos that use this PD using the same settings as before, hence this adjustment.
  4. Adjusted digital demod phases of POP22, POP110 and AS110 signals with the PRMI locked (sideband resonant). I want these to be useful to debug the PRMI. the phases were adjusted so that AS110_Q, POP22_I and POP110_I contain the signal (= sideband buildup) when the PRMI is locked.
  5. Ran the actuator calibration routine for BS, ITMX and ITMY - i'll try and do the PRM and ETMs as well later.
  6. With the PRMI locked (sidebands resonant), looked at the sideband power buildup. POP22 and POP110 remain stable, but there is some low frequency variation in the AS110_Q channel (but not the I channel, so this is really a time varying transmission of the f2 sideband to the dark port). What's that about? Also unsure about those abrupt jumps in the POP22/POP110 signals, see Attachment #1 (admittedly these are slow channels). I don't see any correlation in the MICH control signal.
  7. Measured the loop shapes of the MICH (UGF ~90 degrees, PM~30 degrees) and PRCL (UGF~110 Hz, PM~30 degreees) loops - stability margins and loop UGFs seem reasonable to me.
  8. Tried nulling the MICH-->PRCL coupling by adjusting the MICH-->PRM matrix element - as has been the case for a while, unable to do any better, and I can't null that line as we expect to be able to.
  9. Not expecting to get anything sensible, but ran some sensing matrix lines (at the correct frequencies this time).
  10. Tried locking the PRMI with MICH actuation to an ITM instead of the BS - I can realize the lock but the loop OLTF I measure with this configuration is very weird, needs more investigation. I may look into this later today evening.

I was also reminded today of the poor reliability of the LSC whitening electronics. Basically, there may be hidden saturations in all the channels that have a large DC value (e.g. the photodiode DC mon channels) due to the poor design of the cascaded gain stages. I was thinking about using the REFL DC channel to estimate the mode-matching into the PRC, but this has a couple of problems. Electronically, there may be some signal distortion due to the aforementioned problem. But in addition, optically, the estimation of mode-matching into the PRC by comparing REFL DC levels in single bounce off the PRM and the PRMI locked has the problem that the mode-matching is degenerate with the intra-cavity loss, which is of the same order as the mode mismatch (a percent or two I claiM). If Koji or someone else can implement the fix suggested by Hartmut for all the LSC whitening channels, that'd give us more faith in the signals. It may be less work than just replacing all the whitening filters with a better design (e.g. the aLIGO ISC whitening filter which implements the cascaded gain stages using single OP27s and more importantly has a 1 kohm series resistance with the input to the op amp (so the preceeding stage never has to drive > 10V/1kohms ~10mA of DC current) would presumably reduce distortion.

Attachment 1: PRMI_SBres.png
Attachment 2: MICH_act_calib.pdf
ELOG V3.1.3-