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ID Date Author Type Categorydown Subject
  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
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.

Quote:

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
PRFPMI_2020712sensMat.pdf
Attachment 2: LSCerrSigs.pdf
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
POX_POY_OLTF.pdf
Attachment 2: POX_POY_spectra.pdf
POX_POY_spectra.pdf
  15715   Mon Dec 7 22:54:30 2020 gautamUpdateLSCModulation depth measurement

Summary:

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.

Details:

  • 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
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
armRIN.pdf
Attachment 2: POX_OLTF.pdf
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
POXloop.pdf
Attachment 2: loopFit.pdf
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
PRFPMI_Jan12sensMat.pdf
Attachment 2: PRMI3f_ALS_Jan11_largeOffsetsensMat.pdf
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
PLL.pdf
Attachment 2: modDepth.pdf
modDepth.pdf
Attachment 3: modProfile.pdf
modProfile.pdf
Attachment 4: EOMpath_postMod.pdf
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
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
PRMI_SBres_REFL55.png
Attachment 2: PRMI1f_noArmssensMat.pdf
PRMI1f_noArmssensMat.pdf
Attachment 3: PRMI1f_noArmssensMat.pdf
PRMI1f_noArmssensMat.pdf
Attachment 4: PRMI_locked.png
PRMI_locked.png
Attachment 5: actTFs.pdf
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.

Quote:

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

Attachment 1: violins.pdf
violins.pdf
Attachment 2: PRviolin.pdf
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.

Quote:

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

ASS XARM:
- 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
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
D990511-00_REFL55.pdf
Attachment 2: P_20210304_215602.jpg
P_20210304_215602.jpg
Attachment 3: P_20210304_222400.jpg
P_20210304_222400.jpg
Attachment 4: P_20210304_222412.jpg
P_20210304_222412.jpg
Attachment 5: 20210304234400_IMG_0526.jpg
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.

Quote:

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
REFL55.pdf
Attachment 2: demodNoise.pdf
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
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
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
PRMI_SBres.png
Attachment 2: MICH_act_calib.pdf
MICH_act_calib.pdf
  15876   Sun Mar 7 19:56:27 2021 AnchalUpdateLSCSensing matrix settings messed with

I understand this mst be frustrating for you. But we did not change these settings, knowingly atleast. We have documented all the things we did there. The only thing I can think of which could possibly change any of those channels are the scripts that we ran that are mentioned and the burt restore that we did on all channels (which wasn't really necessary). We promise to be more vigilant of changes that occur when we are present in future.

Quote:

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.

 

  15883   Mon Mar 8 22:01:26 2021 gautamUpdateLSCMore PRMI

There are still many mysteries remaining - e.g. the MICH-->PRCL contribution still can't be nulled. But for now, I have the settings that keep the PRMI locked fairly robustly with REFL55I/Q or REFL165I/Q (I quadrature for PRCL, Q for MICH in both cases), see Attachment #1 and Attachment #2 respectively. For the 1f locking, the REFL55 digital demod phase was fine-tuned to minimize the frequency noise (generated by driving MC2) coupling to the Michelson readout (as the Michelson is supposed to be immune) - the coupling was measured to be ~60dB larger at the PRCL error point than MICH. There was still nearly unity coherence between my MC2 drive and the MICH error point demodulated at the drive frequency, but I was not able to null it any better than this. With the PRMI (ETMs misaligned) locked on the 1f signals, I measured Attachment #1 and used it to determine the demod phase that would best enable REFL165_I to be a PRCL sensor. Rana thinks that if there is some subtle effect in the marginally stable PRC, we would not see it unless we do a mode scan (time consuming to set up and execute). So I'm just going to push on with the PRFPMI locking - let's see if the clean arm mode forces a clean TEM00 mode to be resonant in the PRC, and if that can sort out the lack of orthogonality between MICH/PRCL in the 1f sensors (after all, we only care about the 3f signals in as much as they allow us to lock the interferometer). I'll try the PRMI with arms on ALS tomorrow eve.

I have no idea what to make of how the single frequency lines I am driving in MICH and PRCL show up in REFL11 and REFL33 - the signals are apparently completely degenerate (in optical quadrature). How this is possible even though the PRMI remains stably locked, POP22/POP110/AS110 report stable sideband buildup is not clear to me.

Attachment 1: PRMI1f_noArmssensMat.pdf
PRMI1f_noArmssensMat.pdf
Attachment 2: PRMI3f_noArmssensMat.pdf
PRMI3f_noArmssensMat.pdf
  15892   Wed Mar 10 00:32:03 2021 gautamUpdateLSCPRFPMi

The interferometer can nearly be locked again. I was unable to fully hand off control from ALS-->RF, I suspect I may be using the wrong sign on the AO path (or some such other sub-optimal CM board settings). I'll hook up the SR785 and take some TFs tomorrow, that should give more insight into what's what. With the arms held off resonance, the PRMI acquires lock nearly instantly (REFL165 I for PRCL, REFL165 Q for MICH), and can stay locked nearly indefinitely, which is what I need so I can get the RF lock going. However the sensing matrix (for vertex DoFs, arms held off resonance) still makes no sense to me. The MICH loop has ~50 Hz UGF and the PRCL loop ~150 Hz. I think the MICH loop shape can be optimized a little for better low frequency suppression, but this isn't the show-stopper at the moment. For record-keeping, the ALS performance was excellent and other subsystems were nominal tonight.

Attachment 1: PRMI3f_ALSsensMat.pdf
PRMI3f_ALSsensMat.pdf
  15899   Wed Mar 10 19:58:27 2021 gautamUpdateLSCSR785 hooked up to CM board

In preparation for later today evening. The TT alignment wasn't visibly disturbed.

  15900   Thu Mar 11 01:45:42 2021 gautamUpdateLSCPRFPMi
  1. PRM satellite box indeed seems to have been the culprit - shortly after I swapped it to the SRM, its shadow sensors went dark. I leave the watchdog tripped.
  2. I still was unable to realize the RF only IFO
    • Clearly my old settings don't work, so I tried to go about it systematically. First, try and transition CARM to RF, leave DARM on ALS.
    • As usual, I can realize the state were the arm powers are ~100, and the two paths are blended. 
    • But I'm not able to completely turn off the CARM_A path without blowing the lock.

Pity really, I was hoping to make it much further tonight. I think I'll have to go back to the high BW POX/POY lock, and also check out the conversion efficiency / noise of the daughter board on the REFL11 demod board. Compared to before my work on the RF source, the demod phase for the PRMI lock using REFL11 as an error signal has basically necessitated a change of the digital demod phase by 180 degrees - so I made the appropriate polarity changes in the CM_SLOW and AO paths (the assumption is that CARM in REFL11 would require the same change in digital demod phase, and I think this is a reasonable assumption - indeed, with the arm powers somewhat stable ~100, if I look at the PDH signal in REFL11 I and Q, it does seem to show up largely in the I quadrature (pre digital phase rotation). Anyway, with so many weird effects (wonky PRM suspension, strange PRMI sensing etc etc, who knows what's going on. This will take a systematic effort.

I defer the electronics characterization for the daytime (if I feel like I need it tomorrow I'll do it, else. Koji has said he can do it on Friday).

Quote:

 I was unable to fully hand off control from ALS-->RF, I suspect I may be using the wrong sign on the AO path (or some such other sub-optimal CM board settings). I'll hook up the SR785 and take some TFs tomorrow, that should give more insight into what's what. 

  15903   Thu Mar 11 14:03:02 2021 gautamUpdateLSCAO path

There is some evidence of weird saturation but the gain balancing (0.8dB) and orthogonality (~89 deg) for the daughter board on the REFL11 demod board that generates the AO path error signal seem reasonable. This board would probably benefit from the AD797-->Op27 and thick-film-->thin film swap but i don't think this is to blame for being unable to execute the RF transition.

Attachment 1: IMG_9127.HEIC
  15906   Thu Mar 11 20:18:00 2021 gautamUpdateLSCHigh bandwidth POY

I repeated the high bandwidth POY locking experiment.

  1. The "Q" demod output (SMA) was routed to the common mode board (it appears in the past I used the LEMO "MON" output instead but that shouldn't be a meaningful change).
  2. As usual, slow actuation --> ETMY, fast actuation --> IMC error point.
  3. Loop UGF measurement suggests that bandwidth ~25kHz, with ~25 degrees phase margin. Anyway the lock was pretty stable.

One thing I am not sure is - when looking at the in-loop error point spectra, the Y-arm error point did not get suppressed to the CM board's sensing noise floor - I would have thought that with the huge amount of gain at ~16 Hz, the usual structure we see in the spectra between 10-30Hz would be completely squished. Need to think about if this is signalling something wrong, because the loop TF measurements seemed as expected to me.

1020pm: plots uploaded. As I made the plot of the spectrum, I realized that I don't have the calibration for the Y-arm error point into displacement noise units, so it's in unphysical units for now. But I think the comment about the hump around 16 Hz not being crushed to some sort of flat electronics noise floor. For the TF plots, when the loop gain is high, this IN1/IN2 technique isn't the best (due to saturation issues) but I don't think there's anything controversial about getting the UGF this way, and the fact that the phase evolves as expected when the various gains are cranked up / boosts enabled makes me think that the CM board is itself just fine.


10am 12 March: i realized that the "Y-arm error point" plotted below is not the true error point - that would be the input to the CM board (before boosts etc), which we don't monitor digitally. The spectra are plotted for the CM_SLOW input which already has some transfer function applied to it. In the past, I routed the LEMO "MON" connector on the demod board to the CM board input, and hence, had the usual SMA outputs from the demod board going to the digital system. I hypothesize that plotting the spectra for that signal would have showed this expected suppression to the electronics noise floor.

In summary, on the basis of this test, I don't see any red flags with the CM board.

Attachment 1: OLGevolution.pdf
OLGevolution.pdf
Attachment 2: inLoopSpec.pdf
inLoopSpec.pdf
  15917   Fri Mar 12 19:44:31 2021 gautamUpdateLSCDelay line

I may want to use the delay line phase shifter in 1Y2 to allow remote actuation of the REFL11 demod phase (for the AO path, not the low bandwidth one). I had this working last Feb, but today, I am unable to remotely change the delay. @Koji, it would be great if you could fix this the next time you are in the lab - I bet it's a busted latch IC or some such thing. I did the non-invasive tests - cable is connected, control bits are changing (at least according to the CDS BIO indicators) and the switch controlling remote/local switching is set correctly. The local switching works just fine.

In the meantime, I will keep trying - I am unconvinced we really need the delay line.

  15918   Fri Mar 12 21:15:19 2021 gautamUpdateLSCcoronaversary PRFPMi

Attachment #1 - proof that the lock is RF only (A paths are ALS, B paths are RF).

Attachment #2 - CARM OLTF.

Some tuning can be done, the circulating power can be made ~twice as high with some ASC. The vertex is still on 3f control. I didn't get any major characterization done tonight but it's nice to be back here, a year on i guess.

Attachment 1: PRFPMI.png
PRFPMI.png
Attachment 2: CARM_OLTF.pdf
CARM_OLTF.pdf
  15923   Tue Mar 16 16:02:33 2021 KojiUpdateLSCREFL11 demod retrofitting

I'm going to remove REFL11 demod for the noise check/circuit improvement.

----

  • The module was removed (~4pm). Upon removal, I had to loosen AS110 LO/I out/Q out. Check the connection and tighten their SMAs upon restoration of REFL11.
  • REFL11 configuration / LO: see below, PD: a short thick SMA cable, I OUT: Whitening CH3, Q OUT: Whitening CH4, I MON daughterboard: CM board IN1 (BNC cable)
  • The LO cable for REFL11 was made of soft coax cable (Belden 9239 Low Noise Coax). The vendor specifies that this cable is for audio signals and NOT recommended for RF purposes [Link to Technical Datasheet (PDF)].
    I'm going to measure the delay of the cable and make a replacement.
  • There is a bunch of PD RF Mon cables connected to many of the demo modules. I suppose that they are connected to the PD calibration system which hasn't been used for 8 years. And the controllers are going to be removed from the rack soon.
    I'm going to remove these cables.

----

First I checked the noise levels and the transfer functions of the daughterboard preamp were checked. The CH-1 of the SR785 seemed funky (I can't comprehensively tell yet how it was), so the measurement maybe unreliable.

For the replacement of AD797, I tested OP27 and TLE2027. TLE2027 is similar to OP27, but slightly faster, less noisy, and better in various aspects.

The replacement of the AD797 and whatever-film resistors with LTE2027 and thin-film Rs were straightforward for the I phase channel, while the stabilization of the Q phase channel was a struggle (no matter I used OP27 or TLE2027). It seems that the 1st stage has some kind of instability and I suffered from 3Hz comb up to ~kHz. But the scope didn't show obvious 3Hz noise.

After a quite bit of struggle, I could tame this strange noise by adjusting the feedback capacitor of the 1st stage. The final transfer functions and the noise levels were measured. (To be analyzed later)

----

Now the REFL11 LO cable was replaced from the soft low noise audio coax (Belden 9239) to jacketed solder-soaked coax cable (Belden 1671J - RG405 compatible). The original cable indicated the delay of -34.3deg (@11MHz, 8.64ns) and the loss of 0.189dB.

I took 80-inch 1671J cable and measured the delay to be ~40deg. The length was adjusted using this number and the resulting cable indicated the delay of -34.0deg (@11MHz, 8.57ns) and the loss of 0.117dB.

The REFL11 demod module was restored and the cabling around REFL11 and AS110 were restored, tightened, and checked.

----

I've removed the PD mon cables from the NI RF switch. The open ports were plugged with 50Ohm temirnators.

----

I ask commissioners to make the final check of the REFL11 performance using CDS.

Attachment 1: IMG_0545.jpeg
IMG_0545.jpeg
Attachment 2: IMG_0547.jpeg
IMG_0547.jpeg
Attachment 3: D040179-A.pdf
D040179-A.pdf
Attachment 4: IMG_0548.jpeg
IMG_0548.jpeg
Attachment 5: IMG_0550.jpeg
IMG_0550.jpeg
  15927   Wed Mar 17 00:05:26 2021 gautamUpdateLSCDelay line BIO remote control

While Koji is working on the REFL 11 demod board, I took the opportunity to investigate the non-remote-controllability of the delay line in 1Y2, since the TTs have already been disturbed. Here is what I found today.

  1. First, I brought over the spare delay line from the rack Chiara sits in over to 1Y2. 
    • Connected a Marconi to the input, monitored a -3dB pickoff and the delay line output simultaneously on a 300MHz scope.
    • With the front panel selector set to "Internal", verified that local (i.e. toggling front panel switches) switchability seems to work 👍 
    • Set the front panel switch to "External", and connected the D25 cable from the BIO card in 1Y3 to the back panel of the delay line unit - found that I could not remotely change the delay 😒 
    • I thought it'd be too much of a coincidence if both delay lines have the same failure mode for the remote switching part only, so I decided to investigate further up the signal chain.
  2. BIO switching - the CDS BIO bit status MEDM screen seems to respond, indicating that the bits are getting set correctly in software at least. I don't know of any other software indicator for this functionality further down the signal processing chain. So it would seem the BIO card is not actually switching.
  3. The Contec DO cards don't actually source the voltage - they just provide a path for current to flow (or isolate said path). I checked that pin 12 of the rear panel D25 connector is at +5 V DC relative to ground as indicated in the schematic (see P1 connector - this connector isn't a Dsub, it is IDE24, so the mapping to the Dsub pins isn't one-to-one, but pin 23 on the former corresponds to pin 12 on the latter), suggesting that the pull up resistors have the necessary voltage applied to them.
  4. Made a little LED tester breakout board, and saw no swtiching when I toggled the status of some random bits.
  5. Noted that the bench power supply powering this setup (hacky arrangement from 2015 that never got unhacked) shows a current draw of 0A.
    • I am not sure what the quiescent draw of these boards is - the datasheet says "Power consumption: 3.3VDC, 450mA", but the recommended supply voltage is "12-24V DC (+/-10%)" not 3.3VDC, so not sure what to make of that.
    • To try and get some insight, I took one of the new Contec-32L-PE cards we got from near Jon's CDS test stand (I've labelled the one I took lest there be some fault with it in the future), and connected it to a bench supply (pin 18 = +15V DC, pin1 = GND). But in this condition, the bench supply reports 0A current draw.
  6. Ruled out the wrong cable being plugged in - I traced the cable over the cable tray, and seems like it is in fact connecting the BIO card in the c1lsc expansion chassis to the delay line.

So it would seem something is not quite right with this BIO card. The c1lsc expansion chassis, in which this card sits, is notoriously finicky, and this delay line isn't very high priority, so I am deferring more invasive investigation to the next time the system crashes.

* I forgot we have the nice PCB Contec tester board with LEDs - the only downside is that this board has D37 connectors on both ends whereas the delay line wants a D25, necessitating some custom ribbon cable action. But maybe there is a way to use this.

As part of this work, I was in various sensitive areas (1Y3, chiara rack, FE test stand etc) but as far as I can tell, all systems are nominal.

  15935   Thu Mar 18 01:12:31 2021 gautamUpdateLSCPRFPMi
  1. Integrated >1 hour at RF only control, high circulating powers tonight.
    • All of the locklosses were due to me typing a wrong number / turning on the wrong filter.
    • So the lock seems pretty stable, at least on the 20 minute timescale.
    • No idea why given the various known broken parts.
  2. Did a bunch of characterization.
    • DARM OLTF - Attachment #1. The reference is when DARM is under ALS control.
    • CARM OLTF - Attachment #2. Seems okay.
    • Sensing matrix - Attachment #3. The CARM and DARM phases seem okay. Maybe the CARM phase can be tuned a bit with the delay line, but I think we are within 10 degrees.
  3. TRX/TRY between 300-400, with large fluctuations mostly angular. So PRG ~17-22, to answer Koji's question in the meeting today.
    • This is similar to what I had before the vent of Sep 2020.
    • Not surprising to me, since I claim that we are in the regime where the recycling gain is limited by the flipped folding mirrors.
  4. Tried to tweak the ASC (QPD only) by looking at the step responses, but I could never get the loop gains such that I could close an integrator on all the loops.

I need to think a little bit about the ASC commissioning strategy. On the positive side

  1. REFL11 board seems to perform at least as well as before.
  2. ALS performance made me (as Pep would say), so so happy.
  3. Whole lock acquisiton sequence takes ~5mins if the PRMI catches lock quickly (5/7 times tonight).
  4. Process seems repeatable.

Things to think about:

  1. How to get the AS WFS in the picture?
  2. What does the (still) crazy sensing matrix mean? I think it's not possible to transfer vertex control to 1f signals with this kind of sensing.
  3. What does it mean that the PRM actuation seems to work, even though the coils are imabalnced by a factor of 3-5, and the coil resistances read out <2 ohms???
  4. What's going on at the ALS-->CARM transition? The ALS noise is clearly low enough that I can sit inside the CARM linewidth. Yet, there seems to be some offset between what ALS thinks is the resonant point, and what the REFL11 signal thinks is the resonant point. I am kind of able to "power through" this conflict, but the IMC error point (=AO path) is not very happy during the transition. It worked 8/8 times tonight, but would be good to figure out how to make this even more robust.
Attachment 1: DARM_OLTF_20210317.pdf
DARM_OLTF_20210317.pdf
Attachment 2: CARMTF_20210317.pdf
CARMTF_20210317.pdf
Attachment 3: PRFPMI_Mar_17sensMat.pdf
PRFPMI_Mar_17sensMat.pdf
  15936   Thu Mar 18 07:02:27 2021 KojiUpdateLSCREFL11 demod retrofitting

Attachment 1: Transfer Functions

The original circuit had a gain of ~20 and the phase delay of ~1deg at 10kHz, while the new CH-I and CH-Q have a phase delay of 3 deg and 2 deg, respectively.

Attachment 2: Output Noise Levels

The AD797 circuit had higher noise at low frequency and better noise levels at high frequency. Each TLE2027 circuit was tuned to eliminate the instability and shows a better noise level compared to the low-frequency spectrum of the AD797 version.

RXA: AD797 sad, all hail the op-amps ending with 27 !

Attachment 1: TFs.pdf
TFs.pdf
Attachment 2: PSD.pdf
PSD.pdf
  15942   Thu Mar 18 21:37:59 2021 ranaUpdateLSCPRMI investigations: what IS the matrix??
  • Locked PRMI several tmes after Gautam setup. Easy w IFO CONFIG screenheart
  • tuned up alignment
  • Still POP22_I doesn't go above ~111, so not triggering the loops. Lowered triggers to 100 (POP22 units) and it locks fine now. smiley
  • Ran update on zita, and now it lost its mounts (and maybe its mind). Zita needs some love to recover the StripTool plots  sad
  • Put the $600 ebay TDS3052 near the LSC rack and tried to look at the RF power, but found lots of confusing information. Is there really a RF monitor in this demod board or was it disconnected by a crazy Koji cheeky ? I couldn't see any signal above a few mV.angry
  • Put a 20 dB coupler in line with the RF input and saw zip. Then I put the RF signal directly into the scope and saw that the 55 MHz signal is ~30 mVpp into 50 Ohms. I waited a few minutes with triggering to make sure I was getting the largest flashes. Why is the optical/RF signal so puny? surprise This is ~100x smaller than I think we want...its OK to saturate the RF stuff a little during lock acquisition as long as the loop can suppress it so that the RMS is < 3 dBm in the steady state.
Attachment 1: PXL_20210319_045925024.jpg
PXL_20210319_045925024.jpg
  15944   Fri Mar 19 11:18:25 2021 gautamUpdateLSCPRMI investigations: what IS the matrix??

From Finesse simulation (and also analytic calcs), the expected PRCL optical gain is ~1 MW/m (there is a large uncertainty, let's say a factor of 5, because of unknown losses e.g. PRC, Faraday, steering mirrors, splitting fractions on the AP table between the REFL photodiodes). From the same simulation, the MICH optical gain in the Q-phase signal is expected to be a factor of ~10 smaller. I measured the REFL55 RF transimpedance to be ~400 ohms in June last year, which was already a little lower than the previous number I found on the wiki (Koji's?) of 615 ohms. So we expect, across the ~3nm PRCL linewidth, a PDH horn-to-horn voltage of ~1 V (equivalently, the optical gain in units of V/m for PRCL is ~0.3 GV/m).

In the measurement, the MICH gain is indeed ~x10 smaller than the PRCL gain. However, the measured optical gain (~0.1GV/m, but this is after the x10 gain of the daughter board) is ~10 times smaller than what is expected (after accounting for the various splitting fractions on the AS table between REFL photodiodes). We've established that the modulation depth isn't to blame I think. I will check (i) REFL55 transimpedance, (ii) cable loss between AP table and 1Y2 and (iii) is the beam well centered on the REFL55 photodiode.

Basically, with the 400ohm transimpedance gain, we should be running with a whitening gain of 0dB before digitization as we expect a signal of O(1V). We are currently running at +18dB.

Quote:

Then I put the RF signal directly into the scope and saw that the 55 MHz signal is ~30 mVpp into 50 Ohms. I waited a few minutes with triggering to make sure I was getting the largest flashes. Why is the optical/RF signal so puny? surprise This is ~100x smaller than I think we want...its OK to saturate the RF stuff a little during lock acquisition as long as the loop can suppress it so that the RMS is < 3 dBm in the steady state.

  15949   Fri Mar 19 22:24:54 2021 gautamUpdateLSCPRMI investigations: what IS the matrix??

I did all these checks today. 

Quote:

I will check (i) REFL55 transimpedance, (ii) cable loss between AP table and 1Y2 and (iii) is the beam well centered on the REFL55 photodiode.

  1. The transimpedance was measured to be ~420 ohms at 55 MHz (-4.3 dB relative to the assumed 700V/A of the NF1611), so close to what I measured in June (the data download didn't work apparently and so I don't have a plot but it can readily be repeated). The DC levels also checked out - with 20mA drive current for the Jenne laser, I measured ~2.3 V on the NF1611 (10kohm DC transimpedance) vs ~13mV on the DC output of the REFL55 PD (50 ohm DC transimpedance).
  2. Time domain confirmation of the above statement is seen in Attachment #1. The Agilent was used to drive the Jenne laser with 0dBm RF signal @ 55 MHz. Ch1 (yellow) is the REFL55 PD output, Ch2 (blue) is the NF1611 RFPD, measured at the AP table (sorry for the confusing V/div setting).
  3. Re-connected the cabling at the AP table, and measured the signal at 1Y2 using the scope Rana conveniently left there, see Attachment #2. Though the two scopes are different, the cable+connector loss estimated from the Vpp of the signal at the AP table vs that at 1Y2 is 1.5 dB, which isn't outrageous I think.
  4. For the integrated test, I left the AM laser incident on the REFL55 photodiode, reconnected all the cabling to the CDS system, and viewed the traces on ndscope, see Attachment #3. Again, I think all the numbers are consistent. 
    • REFL55 demod board has an overall conversion gain (including the x10 gain of the daughter board preamp) of ~5V I/F per 1V RF.
    • There is a flat 18 dB whitening gain.
    • The digitized signal was ~13000 ctspp - assuming 3276.8 cts/V, that's ~4Vpp. Undoing the flat whitening gain and the conversion efficiency, I get 13000 / 3276.8 / (10^(18/20)) / 5 ~ 100 mVpp, which is in good agreement with Attachment #3 (pardon the thin traces, I didn't realize it looked so bad until I closed everything).

So it would seem that there is nothing wrong with the sensing electronics. I also think we can rule out any funkiness with the modulation depths since they have been confirmed with multiple different measurements.

One thing I checked was the splitting ratios on the AP table. Jenne's diagram is still accurate (assuming the components are labelled correctly). Let's assume 0.8 W makes it through the IMC to the PRM - then, I would expect, according to the linked diagram, 0.8 W * 0.8 * (1-5.637e-2) * 0.8 * 0.1 * 0.5 * 0.9 ~ 22 mW to make it onto the REFL55 PD. With the PRM aligned and the beam centered on the PD (using DC monitor but I also looked through an IR viewer, looked pretty well centered), I measured 500 mV DC level. Assuming 50 ohm DC transimpedance, that's 500 / 50 / 0.8 ~ 12.5 mW of power on this photodiode, which while is consistent with what's annotated on Jenne's diagram, is ~50% off from expectation. Is the uncertainty in the Faraday transmission and IMC transmission enough to account for this large deviation?

If we want more optical gain, we'd have to put more light on this PD. I suppose we could have ~10x the power since that's what is on IMC REFL when the MC is unlocked? If we want x100 increase in optical gain, we'd also have to increase the transimpedance by 10. I'll double check the simulation but I"m inclined to believe that the sensing electronics are not to blame.


Unconnected to this work but I feel like I'm flying blind without the wall StripTool traces so I restored them on zita (ran /opt/rtcds/caltech/c1/scripts/general/startStrip.sh).

Attachment 1: IMG_9140.jpg
IMG_9140.jpg
Attachment 2: IMG_9141.jpg
IMG_9141.jpg
Attachment 3: REFL55.png
REFL55.png
  15956   Wed Mar 24 00:51:19 2021 gautamUpdateLSCSchnupp asymmetry

I used the Valera technique to measure the Schnupp asymmetry to be \approx 3.5 \, \mathrm{cm}, see Attachment #1. The data points are points, and the zero crossing is estimated using a linear fit. I repeated the measurement 3 times for each arm to see if I get consistent results - seems like I do. Subtle effects like possible differential detuning of each arm cavity (since the measurement is done one arm at a time) are not included in the error analysis, but I think it's not controversial to say that our Schnupp asymmetry has not changed by a huge amount from past measurements. Jamie set a pretty high bar with his plot which I've tried to live up to. 

Attachment 1: Lsch.pdf
Lsch.pdf
  15958   Wed Mar 24 15:24:13 2021 gautamUpdateLSCNotes on tests

For my note-taking:

  1. Lock PRMI with ITMs as the MICH actuator. Confirm that the MICH-->PRCL contribution cannot be nulled. ✅  [15960]
  2. Lock PRMI on REFL165 I/Q. Check if transition can be made smoothly to (and from?) REFL55 I/Q.
  3. Lock PRMI. Turn sensing lines on. Change alignment of PRM / BS and see if we can change the orthogonality of the sensing.
  4. Lock PRMI. Put a razor blade in front of an out-of-loop photodiode, e.g. REFL11 or REFL33. Try a few different masks (vertical half / horizontal half and L/R permutations) and see if the orthogonality (or lack thereof) is mask-dependent.
  5. Double check the resistance/inductance of the PRM OSEMs by measuring at 1X4 instead of flange. ✅  [15966]
  6. Check MC spot centering.

If I missed any of the tests we discussed, please add them here.

  15960   Wed Mar 24 22:54:49 2021 gautamUpdateLSCNew day, new problems

I thought I'd get started on some of the tests tonight. But I found that this problem had resurfaced. I don't know what's so special about the REFL55 photodiode - as far as I can tell, other photodiodes at the REFL port are running with comparable light incident on it, similar flat whitening gain, etc etc. The whitening electronics are known to be horrible because they use the quad LT1125 - but why is only this channel problematic? To describe the problem in detail:

  • I had checked the entire chain by putting an AM field on the REFL 55 photodiode, and corroborating the pk-to-pk (counts) value measured in CDS with the "nominal" setting of +18dB flat whitening gain against the voltage measured by a "reference" PD, in this case a fiber coupled NF1611.
  • In the above test, I confirmed that the measured signal was consistent with the value reported by the NF1611.
  • So, at least on Friday, the entire chain worked just fine. The PRMI PDH fringes were ~6000cts-pp in this condition.
  • Today, I found that while trying to acquire PRMI lock, the PDH fringes witnessed in REFL55 were saturating the ADC. I lowered the whitening gain to 0 dB (so a factor of 8). Then the PDH fringes were ~20,000cts-pp. So, overall, the gain of the chain seems to have gone up by a factor of ~25. 
  • Given my NF1611 based test, the part of the chain I am most suspicious of is the whitening filter. But I don't have a good mechanism that explains this observation. Can't be as simple as the input impedance of the LT1125 being lowered due to internal saturations, because that would have the opposite effect, we would measure a tiny signal instead of a huge one

I request Koji to look into this, time permitting, tomorrow. In slightly longer term, we cannot run the IFO like this - the frequency of occurrence is much too high and the "fix" seems random to me, why should sweeping the whitening gain fix the problem? There was some suggestion of cutting the PCB trace and putting a resistor to limit the current draw on the preceeding stage, but this PCB is ancient and I believe some traces are buried in internal layers. At the same time, I am guessing it's too much work to completely replace the whitening electronics with the aLIGO style units. Anyone have any bright ideas?


Anyway, I managed to lock the PRMI (ETMs misaligned) using REFL165I/Q. Then, instead of using the BS as the MICH actuator, I used the two ITMs (equal magnitude, opposite sign in the LSC output matrix).

  • The digital demod phase in this config is different from what is used when the arm cavities are in play (under ALS control). Probably the difference is telling us something about the reflectivity of the arm cavity for various sideband fields, from which we can extract some useful info about the arm cavity (length, losses etc). But that's not the focus here - the correct digital demod phase was 11 degrees. See Attachment #1 for the sensing matrix. I've annotated it with some remarks.
  • The signals appear much more orthogonal when actuating on the ITMs. However, I was still only able to null the MICH line sensed in the PRCL sensor to a ratio of 1/5 (while looking at peaks on DTT). I was unable to do better by fine tuning either the digital demod phase, or the relative actuation strength on each ITM
  • The PRCL loop had a UGF of ~120 Hz, MICH loop ~60 Hz.
  • With the PRMI locked in this config, I tried to measure the appropriate loop gain and sign if I were to use the REFL55 photodiode instead of REFL165 - but I didn't have any luck. Unsurprising given the known electronics issues I guess...

I didn't get around to running any of the other tests tonight, will continue tomorrow.


Update Mar 26: Attachments #2 and #3 show that there is clearly something wrong with the whitening electronics associated with REFL55 channels - with the PSL shutter closed (so the only "signal" being digitized should be the electronics noise at the input of the whitening stage), the I and Q channels don't show similar profiles, and moreover, are not consistent (the two screenshots are from two separate sweeps). I don't know what to make of the parts of the sweep that don't show the expected "steps". Until ndscope gets a log-scaled y-axis option, we have to live with the poor visualization of the gain steps which are dB (rather than linearly) spaced. For this particular case, StripTool isn't an option either because the Q channel as a negative offset, and I opted agains futzing with the cabling at 1Y2 to give a small fixed positive voltage instead. I will emphasize that on Friday, this problem was not present, because the gain balance of the I and Q channels was good to within 1dB.

Attachment 1: PRMI3f_noArmssensMat.pdf
PRMI3f_noArmssensMat.pdf
Attachment 2: REFL55_whtGainStepping.png
REFL55_whtGainStepping.png
Attachment 3: REFL55_whtGainStepping2.png
REFL55_whtGainStepping2.png
  15977   Mon Mar 29 19:32:46 2021 gautamUpdateLSCREFL55 whitening checkout

I repeated the usual whitening board characterization test of:

  • driving a signal (using awggui) into the two inputs of the whitening board using a spare DAC channel available in 1Y2
  • demodulating the response using the LSC sensing matrix infrastructure
  • Stepping the whitening gain, incrementing it in 3dB steps, and checking if the demodulated lock-in outputs increase in the expected 3dB steps.

Attachment #1 suggests that the steps are equal (3dB) in size, but note that the "Q" channel shows only ~half the response of the I channel. The drive is derived from a channel of an unused AI+dewhite board in 1Y2, split with a BNC Tee, and fed to the two inputs on the whitening filter. The impedance is expected to be the same on each channel, and so each channel should see the same signal, but I see a large asymmetry. All of this checked out a couple of weeks ago (since we saw ellipses and not circles) so not sure what changed in the meantime, or if this is symptomatic of some deeper problem.

Usually, doing this and then restoring the cabling returns the signal levels of REFL55 to nominal levels. Today it did not - at the nominal whitening gain setting of +18dB flat gain, when the PRMI is fringing, the REFL55 inputs are frequently reporting ADC overflows. Needless to say, all my attempts today evening to transition the length control of the vertex from REFL165 to REFL55 failed.

I suppose we could try shifting the channels to (physical) Ch5 and Ch6 which were formerly used to digitize the ALS DFD outputs and are currently unused (from Ch3, Ch4) on this whitening filter and see if that improves the situation, but this will require a recompile of the RTCDS model and consequent CDS bootfest, which I'm not willing to undertake today. If anyone decides to do this test, let's also take the opportunity to debug the BIO switching for the delay line.

Attachment 1: REFL55wht.png
REFL55wht.png
  15994   Sat Apr 3 00:42:40 2021 gautamUpdateLSCPRFPMI locking with half input power

Summary:

I wanted to put my optomechanical instability hypothesis to the test. So I decided to cut the input power to the IMC by ~half and try locking the PRFPMI. However, this did not improve the stability of the buildup in the arm cavities, while the control was solely on the ALS error signal

Details:

  1. The waveplate I installed for this purpose was rotated until the MC RFPD DCMON channel reported ~half it's nominal value.
  2. I adjusted the IMC servo gains appropriately to compensate. IMC lock was readily realized.
  3. I increased the whitening gains on the POX, POY and REFL165 photodiodes by 6dB, to compensate for the reduced light levels.
    • One day soon, we will have remote power control, and it'd be nice to have this process be automated.
    • Really, we should have de-whitening filters that undo these flat gains in addition to undoing the frequency dependent whitening.
    • I'm not sure the quality of the electronics is good enough though, for the changing electronics offsets to not be a problem.
    • One possibility is that we can normalize some signals by the DC light level at that port, but I still think compensating the changing optical gain as far upstream as possible is best, and the whitening gain is the convenient stage to do this.
  4. Recovered single arm POX/POY locking. 
  5. Then I decided to try and lock the PRFPMI with the reduced input power.

Basically, with some tweaks to loop gains, it worked, see Attachment #1. Note that the lower right axis shows the IMC transmission and is ~7500 cts, vs the nominal ~15,000 cts.

Discussion:

Cutting the input power did not have the effect I hoped it would. Basically, I was hoping to zero the optical CARM offset while the IFO was entirely under ALS control, and have the arm transmission be stable (or at least, stay in the linear regime of REFL11). However, the observation was that the IFO did the usual "buzzing" in and out of the linear regime. Right now, this is not at all a problem - once the IR error signal is blended in, and DC control authority is transferred to that signal, the lock acquisition can proceed just fine. And I guess it is cool that we can lock the IFO at ~half the input power, something to keep in mind when we have the remote controlled waveplate, maybe we always want to lock at the lowest power possible such that optomechanical transients are not a problem. 

I also don't think this test directly disputes my claim that the residual CARM noise when the arm cavities are under purely ALS control is smaller than the CARM linewidth.

What does this mean for my hypothesis? I still think it is valid, maybe the power has to be cut even further for the optomechanics to not be a problem. In Finesse (see Attachment #2), with 0.3 W input power to the back of the PRM, and with best guesses for the 40m optical losses in the PRC and arms, I still see that considerable phase can be eaten up due to the optomechanical resonance around ~100 Hz, which is where the digital CARM loop UGF is. So I guess it isn't entirely unreasonable that the instability didn't go away?


After this work, I undid all the changes I made for the low power lock test. I confirmed that IMC locking, POX/POY locking, and the dither alignment systems all function as expected after I reverted the system.

Attachment 1: PRFPMIlock_1301464998_1301465238.pdf
PRFPMIlock_1301464998_1301465238.pdf
Attachment 2: CARMplant.pdf
CARMplant.pdf
  15996   Mon Apr 5 22:26:01 2021 gautamUpdateLSCPRMI 1f locking (no ETMs) recovered

Since it seems like the entire electronics chain has no obvious failure, I decided to compensate for the apparent increased optical gain by turning the flat whitening gain down from +18dB to 0dB. Then, after some fiddling around with alignment, settings etc, I was able to lock the PRMI once again, with the ETMs misaligned, using REFL55_I to sense PRCL, and REFL55_Q to sense MICH. Some sensing matrices attached. Some notes:

  1. I made some changes to the presentation so that the units of the sensing matrix are now in [W/m] sensed on a photodiode. 
    • The info printed on the plot is also more verbose, I now indicate explicitly the actuator driven to make the measurement, and also the drive frequency. The various gains used to convert counts to Watts on a photodiode are also indicated.
    • I thought about printing the actual matrix too but haven't arrived at a clean prez style yet.
    • This is to facilitate easier comparison to Finesse models / analytic calcs.
    • I take into account all the gains from the photodetector to the servo error point where the measurement is made. However, the splitting fractions between various photodiodes is not included, so you will have to do that yourself when comparing to a Finesse model.
    • For example, in pg2 of Attachment #1, the measured gain of PRCL sensed in REFL55_I is ~2e6 W/m. But only ~4% of IFO REFL ends up on the REFL55 photodiode. So, this measurement would be consistent with a Finesse simulated optical gain of ~50MW/m, which is in the ballpark of what I do actually see.
  2. There seems to be reasonable agreement between Finesse and these measurements. But why should the old settings / locking have worked at all then?
  3. I tried two schemes for MICH actuation today.
    • The first was the usual BS + PRM combo, and I got the sensing matrix on pg 1 of Attachment #1. With this scheme, the MICH/PRCL orthogonality is a joke.
    • Then I changed the MICH actuator to the ITMs, and got the sensing matrix on pg 2 of Attachment #1. With this scheme, the orthogonality looks much better. I think the slight non-orthogonality in the 11/33 MHz photodiodes may even be reasonable, since the 11 MHz field isn't a good sensor of the anti-symmetric modes, but have to confirm by calculation/simulation. But certainly the separation of signals is much cleaner when the ITMs are used to control MICH.

So there is clearly something funky with the nominal MICH actuation scheme (MICH suspension, PRM suspension or both), which we should get to the bottom of before trying any low noise locking. I think using the ITMs as the MICH actuator in the full lock will not be a good low nosie strategy, as we would then be "polluting" all our suspended optics with our control loops, which seems highly suboptimal for technical noise sources like coil driver noise etc.

Attachment 1: PRMI_Apr5sensMat_consolidated.pdf
PRMI_Apr5sensMat_consolidated.pdf PRMI_Apr5sensMat_consolidated.pdf
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