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ID Date Author Type Category Subject
15278   Tue Mar 17 01:22:03 2020 gautamUpdateLSCLocking updates

Summary:

No real progress tonight - I made it a bunch of times to the point where CARM was RF only, but I never got to run a measurement to determine what the DARM_B loop gain should be to make the control fully RF.

Details:

• Touched up PMC alignment.
• There were very few BNC cables available at the rack near SW corner of the PSL table - the short BNC cables are NOT meant to be daisy chained to make long cables to run along the arm, I removed all those.
• Restored SR785 at LSC rack for CARM TF measurements.
• I was able to get the CARM UGF ~5 kHz, but everytime I was trying to run a DTT swept sine to measure the ratio of DARM_B_IN1 / DARM_A_IN1, the lock was lost - not sure if this is because of the excitation injected or something else.
• I'll probably give this another shot Wednesday eve.
15281   Thu Mar 19 03:33:28 2020 gautamUpdateLSCMore locking updates

Some short notes, more details tomorrow.

1. I was able to make it to CARM on RF only ~10 times tonight.
2. Highest stable circulating power was ~200 (recycling gain ~10) but the control scheme is still not finalized in terms of offsets etc.
3. DARM to RF transition was never fully engaged - I got to a point where the ALS gain was reduced to <half its nominal value, but IMC always lost lock.
4. CARM loop UGF of ~5 kHz was realized. I was also able to turn on a regular boost. But couldn't push the gain up much more than this. Should probably modify the boosts on this board, their corner frequencies are pretty high.
5. The increased FSS flakiness post c1psl upgrade is definitely hurting this effort, there are periods of ~20-30mins when the IMC just wont lock.

Attachment #1 shows time series of some signals, from the time I ramp of ALS CARM control to a lockloss. With this limited set of signals, I don't see any clear indication of the cause of lockloss, but I was never able to keep the lock going for > a couple of mins.

Attachment #2 shows the CARM OLTF. Compared to last week, I was able to get the UGF a little higher. This particular measurement doesn't show it, but I was also able to engage the regular boost. I did a zeroth order test looking at the CM_SLOW input to make sure that I wasn't increasing the gain so much that the ADC was getting saturated. However, I did notice that the pk-to-pk error signal in this locked, 5kHz UGF state was still ~1000 cts, which seems large?

Attachment #3 shows the DTT measurement of the relative gains of DARM A and B paths. This measurement was taken when the DARM_A gain was 1, and DARM_B gain was 0.015. On the basis of this measurement, DARM_B (=AS55) sees the excitation injected 16dB above the ALS signal, and so the gain of the DARM_B path should be ~0.16 for the same UGF. But I was never able to get the DARM_B gain above 0.02 without breaking the lock (admittedly the lockloss may have been due to something else).

Attachment #4 shows a zoomed in version of Attachment #1 around the time when the lock was lost. Maybe POP_YAW experienced too large an excursion?

Some other misc points:

• It was much quicker to acquire the PRMI lock with CARM held off resonance using the 1f signals rather than 3f - so I did that and then once the lock is acquired, transfer control to 3f signals (using CDS ramptime) before zeroing the CARM offset.
• The whole process is pretty speedy - it takes <5mins to get to the CARM on RF only stage provided the PRMI lock doesn't take too long (the transition from POX/POY to ALS sequence takes <1min).
• I am wondering what the correct way to set the offsets for the 3f error signals is?
• The arm buildup is strongly dependent on the DC alignment of the PRMI - the best buildups I got were when I tweaked the BS alignment after the CARM offset was zeroed.
Attachment 1: PRFPMI_lock.png
Attachment 2: CARMTF.pdf
Attachment 3: DARM_AvB.pdf
Attachment 4: lockLoss.png
15324   Mon May 11 00:12:34 2020 gautamUpdateLSCRF only PRFPMI

Finally - Attachment #1. This plot uses 16 Hz EPICS data. All y-axes are uncalibrated for now, but TRX/TRY are normalized such that the POX/POY lock yields a transmission of 1. CARM UGF is only ~3 kHz, no boosts were turned on yet.

Attachment #2 and Attachment #3 are phone photos of the camera images of the various ports. After some alignment work, the transmitted arm powers were ~200, i.e. PRG ~10. fwiw, this is the darkest i've ever seen the 40m dark port. c.f. 2016. Of course, the exposure time / ND filter / light levels could all have changed.

This work was possible during the daytime (~6pm PDT), but probably only because it was Sunday. The other rate limiting factor here is the franky terrible IMC duty cycle. TBH, I didn't honestly expect to get so far and ran out of time, but I think the next steps are:

1. Turn on some sensing lines and calibrate CARM/DARM.
2. Transition vertex control to 1f signals.
3. Whiten DARM.
4. Turn on some ASC for better power stabilization.
5. Scan the CARM offset and check that we are truly on resonance
6. Noise budget.

As usual, I would like to request that we don't change the IFO as far as possible until the BHD vent, i found it pretty difficult to get here.

Attachment #4 now shows the measured DARM OLTF when DARM is entirely on AS55_Q control. UGF is ~120 Hz and the phase margin is ~30 deg, seems okay for a first attempt. I'll now need to infer the OLTF over a wider range of frequencies by lining this measurement up with some model, so that I can undo the loop in plotting the DARM ASD.

Attachment 1: PRFPMIlock.pdf
Attachment 2: IMG_8549.JPG
Attachment 3: IMG_8548.JPG
Attachment 4: DARM_OLTF.pdf
15326   Tue May 12 18:16:17 2020 gautamUpdateLSCRelative importance of losses in the arm and PRC

Attachment #1 is meant to show that having a T=500ppm PR2 optic will not be the dominant contributor to the achievable recycling gain. Nevertheless, I think we should change this optic to start with. Here, I assume:

• \eta_A denotes the (average) round trip loss per arm cavity (i.e. ITM + ETM). Currently, I guess this is ~100ppm.
• Fixed 0.5% loss from mode mismatch between the CARM mode and the PRC mode (the x-axis does NOT include this number).
• No substrates/AR coatings inside the cavity.
• For the nominal case, let's say the intracavity loss sums to 100 ppm.
• For the T=500ppm PR2, I assumed a total of 550 ppm loss in the PRC.

In relaity, I don't know how good the MM is between the PRC and the arms. All the scans of the arm cavity under ALS control and looking at the IR resonances suggest that the mode-matching into the arm is ~92%, which I think is pretty lousy. Kiwamu and co. claim 99.3% matching into the interferometer, but in all the locks, the REFL mode looks completely crazy, so idk

Attachment 1: armLossVSPRCloss.pdf
15327   Tue May 12 20:16:31 2020 KojiUpdateLSCRelative importance of losses in the arm and PRC

Is \eta_A the roundtrip loss for an arm?

Thinking about the PRG=10 you saw:
- What's the current PR2/3 AR? 100ppm? 300ppm? The beam double-passes them. So (AR loss)x4 is added.
- Average arm loss is ~150ppm?

Does this explain PRG=10?

15328   Tue May 12 22:47:49 2020 gautamUpdateLSCRelative importance of losses in the arm and PRC

Yes, \eta_A is the (average) round-trip loss for an arm cavity. I'd estimate this is ~100ppm currently. I edited the original elog to fill in this omission.

The RC mirror specs require some guesswork - the available specs for the Laseroptik mirrors (PR3) are for a 48 degree angle of incidence, and could be as high as 0.5 %. According to the poster, the spec is 2.6% loss inside the recycling cavity but I don't know where I got the number for the AR surface of the G&H PR2, and presumably that includes some guess I made for the MM between the PRC and the arm. Previously, assuming ~1-2% loss inside the RC gave good agreement between model and measurement. Certainly, if we assume similar numbers, a recycling gain of ~11 (200 * T_P=5.637%) is reasonable. But I think we need more data to make a stronger statement.

 Quote: Is \eta_A the roundtrip loss for an arm? Thinking about the PRG=10 you saw: - What's the current PR2/3 AR? 100ppm? 300ppm? The beam double-passes them. So (AR loss)x4 is added. - Average arm loss is ~150ppm? Does this explain PRG=10?
15330   Thu May 14 00:21:03 2020 gautamUpdateLSCCM board boosts

Summary:

I think the boosts that are currently stuffed on the CM board are too aggressive to be usable for locking the interferometer. I propose some changes.

Details:

[Measurement of the AO path TF]. Empirically, I have observed that the CARM OLTF has ~90 degrees phase margin available at the UGF when no boosts are engaged, which is consistent with Koji's measurement. Assuming we want at least 30 degrees phase margin in the final configuration, and assuming a UGF to be ~10 kHz, the current boosts eat up way too much phase at 10 kHz. Attachment #1 shows the current TFs (dashed lines), as the boosts are serially engaged. I have subtracted the 180 degrees coming from the inverting input stage. The horizontal dash-dot line on the lower plot is meant to indicate the frequency at which the boost stages eat up 60 degrees of phase, which tells us if we can meet the 30 degree PM requirement.

In solid lines on Attachment #1, I have plotted the analogous TFs, with the following changes:

• R52, R54: 1.21k --> 3.16k (changes 4 kHz zero to 1.5 kHz).
• R61, R62: 82.5 --> 165 (changes 20 kHz zero to 10 kHz).
• R63: 165 --> 300 (changes 10 kHz zero to 5 kHz).

These changes will allow possibly two super boosts to be engaged if we can bump up the CARM UGF to ~15 kHz. We sacrifice some DC gain - I have not yet done the noise analysis of the full CARM loop, but it may be that we don't need 120 dB gain at DC to be sensing noise limited. I suppose the pole frequencies can also be halved if we want to keep the same low frequency gain. In any case, in the current form, we can't access all that gain anyways because we can't enable the boosts without the loop going unstable.

The input referred noise gets worse by a factor of 2 as a result of these changes, but the IN1 gain stage noise is maybe already higher? If this sounds like a reasonable plan, I'll implement it the next time I'm in the lab.

Attachment 1: boosts.pdf
Attachment 2: boosts_noise.pdf
15348   Tue May 26 02:15:36 2020 gautamUpdateLSCLock acquisition portal entry

Summary:

Provided the IMC is cooperative, the input pointing isn't drifting, and the RF offsets aren't jumping around too much, the locking sequence is now pretty robust.

Details:

Most of the analysis uses data between the GPS times 1274418176 and 1274419654 that are recorded to frames.

15349   Tue May 26 02:31:00 2020 gautamUpdateLSCLock acquisition sequence

Here, I provide some details of the sequence. Obviously, I am presenting one of the quickest transitions to the fully locked state, I don't claim that every attempt is so smooth. But it is pretty cool that the whole thing can be done in ~3 minutes.

See Attachment #1 for the labels.

• A --- Arms are locked on POX/POY, and EX/EY lasers are also locked to their respective arms. The phase tracker outputs are averaged in preparation for transitioning control from POX/POY to ALS.
• B --- Aforementioned transition has been realized. CARM offset of -4 is applied. Based on this calibration, this is ~ 4 nm.
• C --- PRM is aligned in preparation for 3f vertex locking. Between C and D, the long pause is because I also use this time to DC couple the ITM Oplev servos, which requires some averaging.
• D --- PRMI is locked. CARM offset reduction begins. Between D and E, I scan CARM through a resonance, and look at the necessary offset requried in the CARM_B (=RF) path. It is a mystery to me why this is required.
• E --- Ramp CARM offset completely to 0. Twiddle CARM_A and DARM_A offsets (=ALS path) to maximize the arm transmitted powers. Between E and F, you can see that the arm powers stabilize somewhat before any RF control is engaged (more on this later), which means we are approximately in the linear regime of the CARM PDH signal, and the switchover can be effected. As I write this, I wonder if there is any benefit to normalizing the REFL_11 error signal (=CM_SLOW) by the arm transmission for a broader capture range?
• F --- CARM_B and DARM_B (=RF) paths engaged. I ramp off the ALS servos between F and G using a 10 second ramptime.
• G --- IFO is now under RF control, ALS control has been turned off completely.
• H --- Rudimentary ASC is enabled. The ITMs are already running with DC coupled Oplev servos, and for the ETMs, I use the Transmon QPDs. The loop shapes/gains for this part haven't been finalized yet, but some improvement in the stability is seen.

This particular lock held for ~20 minutes so I could run some loop characterization measurements etc.

I am struggling to explain:

1. Why POP22 goes to 0 when we zero the CARM offset? The arm length is such that the 2f fields don't experience any abrupt changes in reflectivity from the arm cavity for a wide range of offsets. This signal is the trigger signal for the PRMI LSC control - right now, I get around this problem by mixing in some amount of POP DC once the PRMI is locked. But if the lock is lost, this requires some EPICS button gynmastics to try and salvage the lock... I guess the 1f field components experience a different phase on reflection at various offsets, so maybe I should be looking at sqrt(POP22_I^2 + POP22_Q^2) instead of just POP22_I.
2. Why is an error point offset required in the CARM RF path?
Attachment 1: PRFPMIlock_1274418200_1274418550.pdf
15350   Tue May 26 02:37:19 2020 gautamUpdateLSCDARM loop measurement and fitting

Summary:

In order to estimate the free-running DARM displacement noise, I measured the DARM OLTF using the usual IN1/IN2 prescription. The measured data was then used to fit some model paramters for a loop model that can be used over a larger frequency range.

Details:

• Attachment #1 shows an overlay of the measured and modelled TFs.
• Attachment #2 shows the various components that went into building up this model.
• The digital AA and AI filter coefficients were taken from the RTCDS code.
• The analog AA and AI filter zpks were taken from here and here respectively.
• CDS filters taken from the banks enabled. The 20Hz : 0Hz z:p filter in the CARM_B path is also accounted for, as have the violin-mode notches.
• Pendulum TF is just 1/f^2, the overall scaling is unimportant because it will be fitted (in combination with the overall scaling uncertainty on the DC optical gain), but I used a value of 10 nm/f^2 which should be in the right ballbark.
• The optical gain includes the DARM pole at ~4.5 kHz for this config.
• With all these components, to make the measurement and fit line up, I added two free parameters - an overall gain, and a delay.
• NLSQ minimizer was used to find the best-fit values for these parameters.
• I'm not sure what to make of the relatively large disagreement between measurement and model below 100 Hz - I'm pretty sure I got all the CDS filters included...
• Moreover, I don't have a good explanation for why the best-fit delay is 400 us. One RTCDS clock cycle is onyl 60 us, and even with an extra clock cycle for the RFM transfer, I still can't get up to such a high delay...

In summary, the UGF is ~150 Hz and phase margin is ~30 deg. This loop would probably benefit from some low-pass filter being turned on.

Attachment 1: DARM_TF.pdf
Attachment 2: DARM_TF_breakdown.pdf
15351   Tue May 26 03:01:35 2020 gautamUpdateLSCCARM loop

Summary:

I am able to realize ~8 kHz UGF with ~60 degrees of phase margin on the CARM loop OLTF (combination of analog and digital signal paths).

Details:

• Attachment #1 shows the measured OLTF.
• The measurement is made by using the "EXC A" bank on the CM board, with an SR785. With this technique, the measurement will be poor where the loop gain is high, as the excitation will be squished. Nevertheless, we can estimate the behavior in those regimes by using a model, and fitting it to the regions where the measurement is valid (in this case, above ~1 kHz).
• This measurement was made with IN1 Gain = +4 dB, AO gain = 0 dB, and IMC IN2 gain = 0 dB.
• The regular boost has been enabled, but no super-boosts yet, mainly because I think they consume too much phase close to the UGF.
• The modeling/fitting of this, including a more thorough characterization of the crossover, will follow...
Attachment 1: CARM_OLTF.pdf
15352   Tue May 26 03:06:59 2020 gautamUpdateLSCPRFPMI sensing matrix

Summary:

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.

Details:

• 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

Summary:

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.

Details:

• 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

Summary:

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.

Details:

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

Summary:

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.

Details:

• 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.

 Quote: - 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)

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

Highlights:

• 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, ...)

Details:

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

Summary:

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.

Details:

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.

 Quote: 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

Summary:

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?

Details:

• 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.

Conclusions:

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?

Summary:

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

Details:

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).

 Quote: Summary: 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

Summary:

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?

Details:

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...

 Quote: 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.

 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
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

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
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.

 Quote: 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.

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