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ID Date Author Type Categoryup Subject
  14240   Tue Oct 9 23:03:43 2018 yukiConfigurationLSCYarm Green locking was recovered

[ Yuki, Gautam, Steve ]

To align the green beam in Y-end these hardware were installed:

  • PZT mirrors in Y-end table
  • PZT driver in 1Y4 rack
  • Anti-Imaging board in 1Y4 rack
  • cables (DAC - AIboard - PZTdriver - PZT)
  • high voltage supplier 

I made sure that DAC CH9~16 and cable to AI-board worked correctly. 

When we applied +100V to PZT driver and connected DAC, AI-board and PZT drive, the output voltage of the driver was not correct. I'll check it tomorrow.

Attachment 1: Pic_1Y4.jpg
Pic_1Y4.jpg
Attachment 2: Pic_PZTcable.jpg
Pic_PZTcable.jpg
  14241   Wed Oct 10 12:38:27 2018 yukiConfigurationLSCAll hardware was installed

I connected DAC - AIboard - PZTdriver - PZT mirrors and made sure the PZT mirrors were moving when changing the signal from DAC. Tomorrow I will prepare alignment servo with green beam for Y-arm.

  14285   Wed Nov 7 23:07:11 2018 gautamUpdateLSCDRMI locking recovered

I had some success today. I hope that the tweaks I made will allow working with the DRMI during the day as well, though it looks like the main limiting factor in lock duty cycle is angular stability of the PRC.

  • Since there has been some change in the light levels / in vacuum optical paths, I decided to be a bit more systematic.
  • Initial guess of locking gains / demod phases was what I had last year.
  • Then I misaligned SRM, and locked PRMI, for the sideband resonant in the PRC (but still no arm cavities, and using 1f Refl error signals).
  • Measured loop TFs, adjusted gains, re-enabled boosts.
  • Brought the SRM back into the picture. Decided to trigger SRCL loop on AS110I rather than the existing POP22I (because why should 2f1 signal buildup carry information about SRCL?). New settings saved to the configure script. Reduced MICH gain to account for the SRC cavity gain.
  • Re-measured loop TFs, re-adjusted gains. More analysis about the state of the loops tomorrow, but all loops have UGF ~100-120 Hz.
  • Ran some sensing lines - need to check my sensing matrix making script, and once I get the matrix elements, I can correct the error signal demod phasing as necessary.

[Attachment #1]: Repeatable and reliable DRMI locks tonight, stability is mainly limited by angular glitches - I'm not sure yet if these are due to a suspect Oplev servo on the PRM, or if they're because of the tip-tilt PR2/PR3/SR2/SR3.

[Attachment #2]: A pass at measuring the TF from SRCL error point to MICH error point via control noise re-injection. I was trying to measure down to 40 Hz, but lost the lock, and am calling it for the night.

[Attachment #3]: Coherence between PRM oplev error point and beam spot motion on POP QPD.

Note that the MICH actuation is not necessarily optimally de-coupled by actuating on the PRM and SRM yet (i.e. the latter two elements of the LSC output matrix are not precisely tuned yet).

What is the correct way to make feedforward filters for this application? Swept-sine transfer function measurement? Or drive broadband noise at the SRCL error point and then do time-domain Wiener filter construction using SRCL error as the witness and MICH error as the target? Or some other technique? Does this even count as "feedforward" since the sensor is not truly "outside" the loop?

Attachment 1: Screenshot_from_2018-11-07_23-05-58.png
Screenshot_from_2018-11-07_23-05-58.png
Attachment 2: SRCL2MICH_crosscpl.pdf
SRCL2MICH_crosscpl.pdf
Attachment 3: PRCangularCoh_rot.pdf
PRCangularCoh_rot.pdf
  14288   Sat Nov 10 17:32:33 2018 gautamUpdateLSCNulling MICH->PRCL and MICH->SRCL

With the DRMI locked, I drove a line in MICH using the sensing matrix infrastructure. Then I looked at the error points of MICH, PRCL and SRCL. Initially, the sensing line oscillator output matrix for MICH was set to drive only the BS. Subsequently, I changed the --> PRM and --> SRM matrix elements until the line height in the PRCL and SRCL error signals was minimized (i.e. the change to PRCL and SRCL due to the BS moving, which is a geometric effect, is cancelled by applying the opposite actuation to the PRM/SRM respectively. Then I transferred these to the LSC output matrix (old numbers in brackets).

MICH--> PRM = -0.335 (-0.2655)

MICH--> SRM = -0.35 (+0.25)

I then measured the loop TFs - all 3 loops had UGFs around 100 Hz, coinciding with the peaks of the phase bubbles. I also ran some sensing lines and did a sensing matrix measurement, Attachment #1 - looks similar to what I have obtained in the past, although the relative angles between the DoFs makes no sense to me. I guess the AS55 demod phase can be tuned up a bit.

The demodulation was done offline - I mixed the time series of the actuator and sensor signals with a "local oscillator" cosine wave - but instead of using the entire 5 minute time series and low-passing the mixer output, I divvied up the data into 5 second chunks, windowed with a Tukey window, and have plotted the mean value of the resulting mixer output.

Unrelated to this work: I re-aligned the PMC on the PSL table, mostly in Pitch.

Attachment 1: sensMat_2018-11-10.pdf
sensMat_2018-11-10.pdf
  14292   Tue Nov 13 18:09:24 2018 gautamUpdateLSCInvestigation of SRCL-->MICH coupling

Summary:

I've been looking into the cross-coupling from the SRCL loop control point to the Michelson error point.

[Attachment #1] - Swept sine measurement of transfer function from SRCL_OUT_DQ to MICH_IN1_DQ. Details below.

[Attachment #2] - Attempt to measure time variation of coupling from SRCL control point to MICH error point. Details below.

[Attachment #3] - Histogram of the data in Attachment #2.

[Attachment #4] - Spectrogram of the duration in which data in #2 and #3 were collected, to investigate the occurrance of fast glitches.

Hypothesis: (so that people can correct me where I'm wrong - 40m tests are on DRMI so "MICH" in this discussion would be "DARM" when considering the sites)

  • SRM motion creates noise in MICH.
  • The SRM motion may be naively decomposed into two contributions -
    • Category #1: "sensing noise induced" motion, which comes about because of the SRCL control loop moving the SRM due to shot noise (or any other sensing noise) of the SRCL PDH photodiode, and
    • Category #2: all other SRM motion.
  • We'd like to cancel the former contribution from DARM.
  • The idea is to measure the transfer function from SRCL control point to the MICH error point. Knowing this, we can design a filter so that the SRCL control signal is filtered and summed in at the MICH error point to null the SRCL coupling to MICH.
  • Caveats/questions:
    • Introducing this extra loop actually increases the coupling of the "all other" category of SRM motion to MICH. But the hypothesis is that the MICH noise at low frequencies, which is where this increased coupling is expected to matter, will be dominated by seismic/other noise contributions, and so we are not actually degrading the MICH sensitivity.
    • Knowing the nosie-budget for MICH and SRCL, can we AC couple the feedforward loop such that we are only doing stuff at frequencies where Category #1 is the dominant SRCL noise?

Measurement details and next steps:

Attachment #1

  • This measurement was done using DTT swept sine.
  • Plotted TF is from SRCL_OUT to MICH_IN, so the SRCL loop shape shouldn't matter.
  • I expect the pendulum TF of the SRM to describe this shape - I've overlaid a 1/f^2 shape, it's not quite a fit, and I think the phase profile is due to a delay, but I didn't fit this.
  • I had to average at each datapoint for ~10 seconds to get coherence >0.9.
  • The whole measurement takes a few minutes.

Attachments #2 and #3

  • With the DRMI locked, I drove a sine wave at 83.13 Hz at the SRCL error point using awggui.
  • I ramped up the amplitude till I could see this line with an SNR of ~10 in the MICH error signal.
  • Then I downloaded ~10mins of data, demodulated it digitally, and low-passed the mixer output.
  • I had to use a pretty low corner frequency (0.1 Hz, second order butterworth) on the LPF, as otherwise, the data was too noisy.
  • Even so, the observed variation seems too large - can the coupling really change by x100?
  • The scatter is huge - part of the problem is that there are numerous glitches while the DRMI is locked.
  • As discussed at the meeting today, I'll try another approach of doing multiple swept-sines and using Craig's TFplotter utility to see what scatter that yields.

Attachments #2

  • Spectrogram generated with 1 second time strides, for the duration in which the 83 Hz line was driven.
  • There are a couple of large fast glitches visible.
Attachment 1: TF_sweptSineMeas.pdf
TF_sweptSineMeas.pdf
Attachment 2: digitalDemod.pdf
digitalDemod.pdf
Attachment 3: digitalDemod_hist.pdf
digitalDemod_hist.pdf
Attachment 4: DRMI_LSCspectrogram.pdf
DRMI_LSCspectrogram.pdf
  14298   Fri Nov 16 00:47:43 2018 gautamUpdateLSCMore DRMI characterization

Summary:

  • More DRMI characterization was done.
  • I was working on trying to improve the stability of the DRMI locks as this is necessary for any serious characterization.
  • Today I revived the PRC angular feedforward - this was a gamechanger, the DRMI locks were much more stable. It's probably worth spending some time improving the POP LSC/ASC sensing optics/electronics looking towards the full IFO locking.
  • Quantitatively, the angular fluctuations as witnessed by the POP QPD is lowered by ~2x with the FF on compared to offyes [Attachment #1, references are with FF off, live traces are with FF on].
  • The first DRMI lock I got is already running 15 mins, looking stable.
    • Update: Out of the ~1 hour i've tried DRMI locking tonight, >50 mins locked!
  • I think the filters can be retrained and this performance improved, something to work on while we are vented.
  • Ran sensing lines, measured loop TFs, analysis tomorrow, but I think the phasing of the 1f PDs is now okay.
    • MICH in AS55 Q, demod phase = -92deg, +6dB wht gain.
    • PRCL in REFL11 I, demod phase = +18 deg, +18dB wht gain.
    • SRCL in REFL55 I, demod phase = -175 deg, +18dB wht gain.
  • Also repeated the line in SRCL-->witness in MICH test.
    • At least 10 minutes of data available, but I'm still collecting since the lock is holding.
    • This time I drove the line at ~124 Hz with awggui, since this is more a regime where we are sensing noise dominated.

Prep for this work:

  • Reboots of c1psl, c1iool0, c1susaux.
  • Removed AS port PD loss measurement PD.
  • Initial alignment procedure as usual: single arms --> PRMI locked on carrier --> DRMI

I was trying to get some pics of the optics as a zeroth-level reference for the pre-vent loss with the single arms locked, but since our SL7 upgrade, the sensoray won't work anymore no. I'll try fixing this during the daytime.

Attachment 1: PRCff.pdf
PRCff.pdf
Attachment 2: DRMI_withPRCff.png
DRMI_withPRCff.png
  14313   Wed Nov 21 09:59:26 2018 gautamUpdateLSCLSC feedforward block diagram

Attachment #1 is a block diagram depicting the pathway by which the vertex DOF control signals can couple into DARM (adapted from a similar diagram in Gabriele's Virgo note on the subject). I've also indicated some points where noise can couple into either loop. In general, there are sensing noises that couple in at the error point of the loop, and actuation noises that couple in at the control point. In this linear picture, each block represents a (possibly time varying) transfer function. So we can write out the node-to-node transfer functions and evaluate the various couplings.

The motivation is to see if we can first simulate with some realistic noise and time-varying couplings (and then possibly test on the realtime system) the effectiveness of the filter denoted by "FF" in canceling out the shot noise from the auxiliary loop being re-injected into the DARM loop via the DARM sensor. Does this look correct?

Attachment 1: IMG_7173.JPG
IMG_7173.JPG
  14326   Fri Nov 30 19:37:47 2018 gautamUpdateLSCLSC feedforward block diagram

I wanted to set up an RTCDS model to understand this problem better. Attachment #1 is the simulink diagram of the signal flow. The idea will be to put in the appropriate filter shapes into the various filter blocks denoting the DARM and auxiliary DoF plants, controllers and actuators, and then use awggui / diaggui to inject some noises and see if in this idealized model I can achieve good subtraction. Then we can build up to applying a time varying cross coupling between DARM and the vertex DoF, and see how good the adaptive FF works. Still need to setup some MEDM screens to make working with the test system easier.

I figured c1omc would be the least invasive model to set this upon without risking losing any of our IR/green alignment references. Compile and install went smooth, see Attachment #2. The c1omc model was clocking 4us before, now it's using 7us.

Attachment #3 shows the top level of the OMC model, while Attachment #4 shows the MEDM screen.

* Note to self: when closing a loop inside the realtime model, there has to be a delay block somewhere in the loop, else a compilation error is thrown.

Attachment 1: LSC_FF_tester.png
LSC_FF_tester.png
Attachment 2: Screenshot_from_2018-11-30_19-41-07.png
Screenshot_from_2018-11-30_19-41-07.png
Attachment 3: Screenshot_from_2018-12-10_15-31-23.png
Screenshot_from_2018-12-10_15-31-23.png
Attachment 4: SimLSC.png
SimLSC.png
  14339   Mon Dec 10 15:53:16 2018 gautamUpdateLSCSwept-sine measurement with DTT

Disclaimer: This is almost certainly some user error on my part.

I've been trying to get this running for a couple of days, but am struggling to understand some behavior I've been seeing with DTT.

Test:

I wanted to measure some transfer functions in the simulated model I set up.

  • To start with, I put a pendulum (f0 = 1Hz, Q=5) TF into one of the filter modules
  • Isolated it from the other interconnections (by turning off the MEDM ON/OFF switches).
  • Set up a DTT swept-sine measurement
    • EXC channel was C1:OMC-TST_AUX_A_EXC
    • Monitored channels were C1:OMC-TST_AUX_A_IN2 and C1:OMC-TST_AUX_A_OUT.
    • Transfer function being measured was C1:OMC-TST_AUX_A_OUT/C1:OMC-TST_AUX_A_IN2.
    • Coherence between the excitation and output were also monitored.
  • Sweep parameters:
    • Measurement band was 0.1 - 900 Hz
    • Logarithmic, downward.
    • Excitation amplitude = 1ct, waveform = "Sine"

Unexplained behavior:

  • The transfer function measurement fails with a "Synchronization error", at ~15 Hz.
    • I don't know what is special about this frequency, but it fails repeatedly at the same point in the measurement.
  • Coherence is not 1 always
    • Why should the coherence deviate from 1 since everything is simulated? I think numerical noise would manifest when the gain of the filter is small (i.e. high frequencies for the pendulum), but the measurement and coherence seem fine down to a few tens of Hz.

To see if this is just a feature in the simulated model, I tried measuring the "plant" filter in the C1:LSC-PRCL filter bank (which is also just a pendulum TF), and run into the same error. I also tried running the DTT template on donatella (Ubuntu12) and pianosa (SL7), and get the same error, so this must be something I'm doing wrong with the way the measurement is being run / setup. I couldn't find any mention of similar problems in the SimPlant elogs I looked through, does anyone have an idea as to what's going on here?

* I can't get the "import" feature of DTT to work - I go through the GUI prompts to import an ASCII txt file exported from FOTON but nothing selectable shows up in DTT once the import dialog closes (which I presume means that the import was successful). Are we using an outdated version of DTT (GDS-2.15.1)?  But Attachment #1 shows the measured part of the pendulum TF, and is consistent with what is expected until the measurement terminates with a synchronization error.


the import problem is fixed - when importing, you have to give names to the two channels that define the TF you're importing (these can be arbitrary since the ASCII file doesn't have any channel name information). once i did that, the import works. you can see that while the measurement ran, the foton TF matches the DTT measured counterpart.


11 Dec 2pm: After discussing with Jamie and Gabriele, I also tried changing the # of points, start frequency etc, but run into the same error (though admittedly I only tried 4 combinations of these, so not exhaustive).

Attachment 1: SimTF.pdf
SimTF.pdf
  14445   Fri Feb 8 20:48:52 2019 gautamUpdateLSCIFO recovery

Several housekeeping tasks were carried out today in preparation for the Y-arm loss measurement.

  1. The mess around the OMC rack was cleared a bit. The vertex laptop paola now lives there, instead of on the ITMY optical table.
  2. Centering of beam on AS photodiodes on AS table (starting from the first optic in this path at the exit point from the vacuum), adjusted AS camera to bring the spot roughly to the center.
  3. POX/POY locking was restored, GTRY/GTRX levels are healthy. TRY was centered on the Thorlabs PD by triggering the LSC lock on AS110.
  4. Oplevs on all four TMs and BS were centered for post-vent alignment.
  5. ETMY OL transfer function was checked since we have swapped the HeNe during the vent, 4.5 Hz UGF for both DoFs and ~30 deg phase margin. The calibration of the error point to urad needs to be double checked.
  6. There are some huge 60 Hz harmonics in the TRY signal - hunting down the source of this. The one thing I can think of that was changed is that we plugged the c1auxey eurocrate into the ethernet powerstrip, I wonder if this created some kind of ground loop.
    • I checked the signal from the PD with a battery powered scope, no evidence of any 60 Hz in the time domain or scope FFT (Attachment #1, FFT in red and time domain signal in green can be seen).
    • Restored the power of c1auxey eurocrate to its original socket in the back of 1Y4 - harmonics still present --> points to the problem being in the whitening board / ADC electronics?
    • The harmonics only seem to show up when TRY > ~0.5
    • Some elog hunting revealed that this signal is being digitized through a modified D990399. So somehow the signal pollution is happening inside this board? Because from the output of this board, the signal is going straight into the ADC.
    • To confirm, I will temporarily hijack another ADC channel and look at the spectrum. There is apparently some kind of daughter board (D040060), but how 60 Hz is coupling at this stage is unknown to me.
  7. The ASS system for both arms still isn't working properly, to be investigated. The dirty TRY signal probably isn't helping the situation.
Attachment 1: IMG_7307.JPG
IMG_7307.JPG
  14446   Mon Feb 11 15:41:49 2019 gautamUpdateLSCTRY 60 Hz solved - but clipping persists

Rich came by the 40m to photocopy some pages from Hobbs, and saw me working on the 60 Hz hunting. As I suspected, the problem was being generated in the D040060. This board receives the photodiode signal single-ended, but has a different power ground than the photodiode (even though the PD is plugged into a power strip that claims to come from 1Y4). The mechanism is not entirely clear - the presence of these 60 Hz features seemed to be dependent on the light level on the TRY photodiode (i.e. they were absent when the PSL shutter is closed, and were more prominent when TRY was 0.9 rather than 0.5) but the PD certainly wasn't saturated - the DC signal was only ~100 mV when viewed on a scope. In any case, Rich suggested the simplest test would be to ground the BNC shield bringing TRY to the rack, to the local ground on the board, which I did using a crocodile clip. This did the trick, the TRY signal RMS is now dominated by the ~1 Hz seismic-driven variation.

 On a more pessimistic note - it looks like the elliptical reflector moving did not work, and the clipping in the Y arm persists no. I am able to recover TRY~1 with the yaw offset on the ETM (which is still lower than the 1.06-1.07 Koji reported in Aug 2018, but I can believe that being down to the MC transmission being a few % lower at 15000cts rather than 15500), while the maximum I see without it is ~0.9. This is puzzling, because when the chamber was open, we saw that there was ~1.5" clearance between the edge of the reflector and the beam on an IR card. I suppose the input pointing could have been off by a small amount. So one of the primary vent objectives wasn't acheieved... But I will push ahead with the loss measurement.

  14447   Mon Feb 11 16:38:34 2019 gautamUpdateLSCETMY OL calibration updated

Since we changed the HeNe, I updated the calibration factors, and accepted the changes in the SDF.

DOF OLD [urad/ct] NEW [urad/ct]
PITCH 140 176
YAW 143

193

Attachment 1: OL_calib_ETMY_PERROR.pdf
OL_calib_ETMY_PERROR.pdf
Attachment 2: OL_calib_ETMY_YERROR.pdf
OL_calib_ETMY_YERROR.pdf
  14459   Fri Feb 15 18:42:57 2019 gautamUpdateLSCTRY 60 Hz solved

A more permanent fix than a crocodile clip was implemented. Should probably look to do this for the X end unit as well.

Attachment 1: IMG_7323.JPG
IMG_7323.JPG
  14704   Wed Jun 26 21:01:26 2019 gautamUpdateLSCPOX and POY locking

Now that the IMC is remaining locked for extended periods of time, the next problem to attack is the ASS dither alignment system. For a start, I decided to try and get the POX and POY locking working again, as we have not fully recovered the interferometer alignment after the most recent pumpdown. I spent a couple of hours tweaking the alignment of the arm cavity mirrors, BS, and TTs to try and recover the maximum possible TRX and TRY - however, my best efforts only yielded TRX~0.8, TRY~0.75. Moreover, the beam axis is such that the spot is significantly off in YAW on both ETMs, as evidenced by the camera views (also true but less obvious on the ITMs). However, trying to bring the beam back to the center of the optics yields TRY and TRX values lower than the above reported maxima. The EX green beam is currently unavailable to verify the arm cavity alignment because of my hijacking the EX NPROs PZT control for PLL investigations, but with the Y arm, I'm able to lock a TEM00 mode. Probably just needs more careful systematic alignment, but I'm not pursuing this tonight.

  14705   Thu Jun 27 14:28:12 2019 gautamUpdateLSCPOX and POY locking

After a more systematic alignment effort, I was able to get the spots better centered on the optics (judged by eye from the analog camera views). TRY ~0.7, TRX~1.15. The X-arm dither alignment system seems to work out-of-the-box with the existing settings, I was able to run it and maximize the X-arm transmission.

Other work: I also cleaned up the area around MC2 a litte - laptop from on top of the vacuum chamber was removed and a rogue ethernet cable was also removed. The resulted in some misalignment of the IMC, which I corrected by manual alignment. Now the IMC is locked again with nominal transmission levels.

On the PSL table, I re-routed the RF output from the BeatMouth to the regular IR-ALS electronics chain (it was hijacked for PLL investigations). At EX, I disconnected the cable running from the LB1005 to the EX NPRO laser PZT (again was being used for PLL locking), and re-connected the output from the Green uPDH box to allow for some ALS tests to be done. I could then lock the EX green beam to the X-arm, and achieved GTRY ~ 0.35 using the ASX system. More to follow on ALS tests later today.

  14720   Tue Jul 2 17:34:54 2019 gautamUpdateLSCIrides opened up on EY table

In preparation for the ASS debugging, I decided to check out the beam path on the EY table. In order to be able to do this, I had to setup the POY locking to trigger on AS110 instead of TRY (as is usual for this kind of debugging). Then I could poke an IR card in the beam path without destroying the lock.

There are two irides in the beam path immediately between the vacuum window and the harmonic separator that splits off the IR and green beams. I found that the beam was in fact getting clipped on both of them. It was also somewhat off center on a 2" beamsplitter that sends half of the light to the QPD (currently decommissioned). The purpose of these irides are (I think) to eliminate some ghost reflections of the green beam and also the Oplev beam. I opened up the irides until I felt that there wasn't any more clipping of the IR beam, but the appropriate ghost beams were still getting caught.

I also re-aligned the beam onto the TRY Thorlabs PD so as to better center it on the active area. In summary, the result of this work was that the TRY level went from ~0.6 to ~0.93. There may still be some scope for optimizing this - I tried running the Y-arm ASS scripts, and already, the loops don't run away any more. I'll do the systematic analysis of the servo anyways. But given that the IMC Trans lev el used to be ~15,500 counts and is now ~14,500 counts, I think ~7% drop in TRY level is in line with what we "expect" (assuming the pre-power-degradation TRY level was 1.000).

Note that these irides were installed (I think) by Yuki, and so cannot explain the ASS anomalies of July 2018 (i.e. it does not exonerate in-vacuum clipping of the beam, as Koji had already verified that the in-air path was clean back then).

  14738   Tue Jul 9 18:06:05 2019 gautamUpdateLSCY-arm ASS in a workable state

The Y-arm ASS was tuned to be in a workable state. Basically, I followed Koji's recipe.

The SNR of the dither lines in the TRY and YARM control signals were checked - Attachment #1. The dither frequencies are marked with vertical dashed lines (can't figure out how to add 4 cursors in DTT so there's two in each row for a total of 4). A couple of days ago, when I was doing some preliminary checks, I found that the oscillator at 24.91 Hz caused a broadband increase in the TRY noise between DC and ~100 Hz. But today I saw no evidence of such behaviour. So I decided against changing the frequency.

The linearity of the demodulated error signals around the quadratic maxima of the TRY level was checked. I did not, however, investigate in detail the frequency-dependent offset Koji has reported in his elog. 

After this work, the TRY level is at 0.95. This is commensurate with the MC trans level being lower by ~7% relative to July 2018. Furthermore, the ASS servo is able to return to TRY~0.95 with a time-constant of ~5 seconds in response to misalignment of the cavity optics. After I investigate the X-arm ASS, I will reset the normalization for TRX and TRY.

Update 645pm: In the spirit of general IFO recovery, I re-centered the ITM and ETM oplev spots, and also the IR beam on the IPPOS QPD to mark the new input pointing alignment (the spot is slightly lower on the AS camera than what I remember). I then tweaked the XARM transmission to maximize it, and re-set the TransMon normalization. I edited the normalization script to comment out the normalizing of the TransMon QPD gains as the QPDs are in some kind of indeterminate state now. Attachment #2 shows the current status, you can also see the normalization being reset. LSC mode disabled for overnight.

Once the XARM ASS is also checked out, I propose moving back to locking the DRMI / PRFPMI configs. 

Attachment 1: ditherFreqs.pdf
ditherFreqs.pdf
Attachment 2: transRenorm.png
transRenorm.png
  14904   Fri Sep 20 18:28:34 2019 gautamUpdateLSCY arm locking attempt

I tried to lock the Y arm cavity length to the PSL frequency using POY11_I as an error signal. Even though I think the cavity alignment is good (I see TRY flashes ~0.8), I am unable to achieve a lock. I checked the signal conditioning, and as far as I can tell, all the settings are correct, but there may be some settings that have not been re-assigned correct values. The other possibility is that something is not quite right with the new c1iscaux. The PDH error signal and arm cavity flashes all seem good though (see Attachment #1), so I'm not sure what obvious thing I'm missing.

To be continued...

Attachment 1: POYlocking.png
POYlocking.png
  14910   Sun Sep 29 15:58:19 2019 gautamUpdateLSCPOX locking attempt

Summary:

There is no visible PDH error signal on the POX11 channels. As a result, I am unable to lock the XARM length to the laser frequency. See Attachment #1 - the Y arm length is locked to the PSL frequency, and control is disabled for the XARM servo.

Details:

Now that several of the c1iscaux functionality tests have been completed, I wanted to push ahead with some locking. However, I was foiled at this early stage, for reasons as yet unknown. One possibility is that the

  • I am able to see TRX cavity flashes >0.8, which suggest to me that the cavity is well aligned.
  • Moreover, I am able to lock some (admittedly high TEM order) mode of the green laser, which further supports the above hypothesis.
  • However, there are no visible PDH-like features in the POX11_I or POX11_Q channels.
  • I checked that the cables from the output of the POX11 demod board are in fact going to the correct channels on WF1 (#5 and #6 respectively), and that the whitening gain for this channel is set to the nominal +30 dB.
  • Next, I went to the POX table and looked for the POX IR beam. I couldn't see anything, but this beam is expected to be weak (of the order of 1 W * T_PRM * R_AR_ITM ~ 30 uW), which is probably not so easily visible.

Next steps:

  • Look for the POX beam with an IR viewer.
  • Confirm that everything is order on the LSC Demod board for POX 11 - maybe the LO isn't connected (somehow)?
Attachment 1: POXlockAttempt.png
POXlockAttempt.png
  14915   Mon Sep 30 14:16:43 2019 gautamUpdateLSCPOX PD checkout - solved

I confirmed that there is light incident on the POX photodiode. So the problem must lie downstream in the demod / whitening / AA electronics. With the PRM aligned (i.e. PRFPMI config with all DoFs uncontrolled), I could see the flashing beam on an IR card. I could also see the spikes in DC power incident on the photodiode using the "DC Monitor" port on the photodiode head and an oscilloscope.

Update 245 pm: I confirmed that I could see a 11 MHz sine wave by connecting the POX11 RFPD output cable at the 1Y2 end to an oscilloscope. The amplitude of this signal was also changing, corresponding to the cavity fringing in and out of resonance. I couldn't, however, see any signal on the RFPDmon port, or the I/Q demodulated output ports. So as of now, the culprit seems to be something on the Demod board. Further investigations underway...

Update 315pm: I did the following checks:

  1. Checked the LO signal level into a 50ohm input scope - it was ~720 mVpp, which was compatible with the LO level into the POY Demod board, so the LO signal level couldn't be to blame.
  2. Connected an RF funcgen to the PD input of the demod board. Drove it at 11.066210 MHz, 50 mVpp, and saw a signal 400 cts-pp in the CDS system - so the demod + digitizaiton electronics also seemed fine.
  3. #2, coupled with the fact I could see no signal at the RF-mon port of the demod board (even though there was a signal visible at the cable coming to 1Y2) suggested that the cable routing the POX11 PD output from the Heliax-breakout in 1Y2 to the demod board was busted - indeed this was the case!
  4. Koji replaced the cable without changing its length, and now the XARM locks readily 👏 . I ran ASS and got TRX ~ 0.95. See Attachment #1
Quote:

Look for the POX beam with an IR viewer.

  14920   Tue Oct 1 21:19:51 2019 gautamUpdateLSCPRMI locked on carrier

Summary:

The PRMI was locked with the carrier field resonant in the PRC 🙌. The lock is pretty stable (I only let it stay locked for ~10mins and then deliberately unlocked to see if I could readily re-lock, but it has stayed locked for the last ~20mins while I typed this up). See Attachment #1 for the DC power monitor StripTool for a short section of lock.

Details:

  • This is the opposite of the config we'd want usually for locking the IFO, but it is a useful configuration for setting the alignment of the vertex optics, and also to train angular feedforward filters, so I decided to try it out.
  • Some patient alignment work was required. I started with the single arm locks, maximized TRX/TRY with ASS, and then misaligned the ETMs and brought the PRM into alignment.
  • The PRM Oplev spot was roughly centerd on its QPD once I judged I was getting decent PRMI cavity flashes on the POP camera. The PRMI Oplev servo needs some tuning, it is currently susceptible to oscillations in Pitch.
  • The error signals used were: REFL11_I ---> PRCL and AS55_Q ---> MICH.
  • The whitening gains were: REFL11 --> +18 dB, AS55 ---> +6 dB.
  • Triggering was done using POPDC, this worked better for me than any of the RF signals (e.g. POP22/POP110). Trigger ON --> 200cts, Trigger OFF --> 100 cts.
  • The DCPD whitening gains may not be set correctly - I think I remember POPDC being ~4000 cts in this configuration, but it may also be that we are not well centered on the POP photodiode.
  • The dominant cause of the POP circulating power seems to be the usual angular instability ascribed to the TTs. The OAF model wasn't running tonight (and I didn't want to try starting it and have to do a full vertex FE reboot tonight) so I didn't get a chance to engage the angular FF.

Next (for LSC activities):

  • PRMI locking with the sidebands resonant in the PRC.
  • DRMI locking

I'm leaving the LSC mode off for tonight, but with the PRMI optics aligned and ETMs misaligned.

Attachment 1: PRMIlocked.png
PRMIlocked.png
  14924   Wed Oct 2 11:52:16 2019 gautamUpdateLSCPRMI Oplev loop checkout

I measured the OLTF of both the PRM Oplev loops. Nothing odd sticks out as odd to me in this measurement - there seems to be ~40 degrees of phase margin and >10 dB gain margin for both loops, see Attachment #1. I didn't measure down to the second UGF at ~0.2 Hz (the Oplev loops are AC coupled), so there could be something funky going on there. The problem still persists - if I misalign and realign the PRM using the ifoalign scripts, the automatic engagement of Oplev loops causes the loop to oscillate. Could be that the script doesn't wait for long enough for the alignment transient to die out.

Update 1230pm: Indeed, this was due to the integrator transient. It dies away after a couple of seconds.

Quote:

The PRMI Oplev servo needs some tuning, it is currently susceptible to oscillations in Pitch.

Attachment 1: PRM_OLTF.pdf
PRM_OLTF.pdf
  14926   Wed Oct 2 23:15:02 2019 gautamUpdateLSCFPMI locking

Summary:

I was able to lock the FPMI. The lock was quite stable. However, the fluctuations in the ASDC power suggest that it will be difficult to make a DC measurement of the contrast defect in this configuration. This problem can be circumvented in part by some electronics tuning. However, the alignment jitter couples some HOM light which is an independent effect. Can this be a good testbed for the proposed AS WFS system? 

Details:

  • First, the arm cavities were locked and TRX/TRY were maximized using ASS.
  • Next, AS55_Q-->MICH_A (MICH-->BS) matrix element was set to 1 in the LSC input (output) matrix. The trigger was set to always on.
  • AS55 digital demod phase was -37 degrees.
  • I was then able to increase the gain on the MICH servo and turn on some integrators without any problem.
  • Some guesswork had to be done to get the correct sign. Final servo gain used was -0.8. 

I didn't do any serious budgeting yet - need to think about / do some modeling on how this configuration can be made useful.

Attachment 1: FPMIlocked.png
FPMIlocked.png
  14928   Thu Oct 3 11:01:18 2019 ranaUpdateLSC(PR)FPMI locking

wonder if its possible to do variable finesse locking

Gabriele mentioned that Virgo used arm trans PDH for this, but I guess we could possibly use POX/POY to start and bring in the PRM with 50% MICH trans

  14933   Thu Oct 3 19:40:18 2019 gautamUpdateLSCPOX/POY imbalance

Summary:

There is an imbalance between the POX and POY detector outputs reported in the CDS system. Possibilities are (i) the POX PD has a uncoated glass window whereas POY does not or (ii) there is some problem in the elctronics.

Details:

  1. Nominally, we run the POX/POY locking with +18dB whitening gain on POY and +30 dB on POX. This is a factor of 4 difference.
  2. The DC levels reported in C1:LSC-POXDC_OUT and C1:LSC-POYDC_OUT differ by a factor of 10 (24 cts for POY vs 2.4 cts for POX with 0dB whitening gain). These channels come from the P2 connector on the back of the PD Interface board into the fast CDS system.
  3. The levels reported by the Acromag system (which come out of the P1 connector) are 60mV for POY  vs 15 mV for POX.
  4. I confirmed that this imbalance is not due to clipping on the POX photodiode - I tweaked the steering mirror and observed the plateau (I did not, however, look at the beam on the PD active area with an IR viewew which would be a more conclusive test).
  5. I measured the power incident on either PD (using Ophir power meter, filter OFF). They were both ~10uW, as expected since the beam extraction for POY and POX are identical - a single HR mirror and the vacuum viewport.

Update 820pm: 

  1. I checked that there is no glass window on the PD.
  2. It is hard to see the beam on a viewer - but with the PRM aligned, I think I convinced myself that the beam is pretty well centered on the PD. 

So increasingly, it looks like the electronics are the source of the problem.

  14954   Tue Oct 8 18:35:09 2019 gautamUpdateLSCLocking prep

In preparation for some locking work tonight, I did the following at the POP in air table with the PRMI locked on carrier:

  1. Raised the POP camera by ~5mm. The POP spot is now well centered on the CCD view.
  2. Tweaked alignment onto the PDA10CF photodiode that serves as (i) POP22, (ii) POP110, and (iii) POP DC. In lock the POPDC level went from ~800 cts to ~1200 cts.
  3. Moved the QPD that witnesses part of the POP beam such that the spot was centered on the photodiode. This may be useful for collecting some FF data or if we want to try feedback to stabilize the PRMI.

TBC...

  14960   Wed Oct 9 18:15:26 2019 gautamUpdateLSCPRMI 3f locking

After making sure the beams were hitting the 3f photodiodes on the "AP" table, I was able to lock the PRMI with the sidebands resonant inside the RC using 3f error signals. This would be the config we run in when trying to lock some more complicated configuration, such as the PRFPMI (i.e. start with the arms controlled by ALS, held off resonance). Tonight, I will try this (even though obviously I am not ready for the CARM transition step). The 3f lock is pretty robust, I was able to stay locked for minutes at a time and re-acquisition was also pretty quick. See Attachment #1. Not sure how significant it is, but I set the offsets to the 3f paths by averaging the REFL33_I and REFL33_Q signals when the PRMI was locked with the 1f error signals.

As usual, there's a lot of angular motion of the POP spot on the CCD monitor, but the lock seems to be able to ride it out.

Lock-settings (I modified the .snap file accordingly):

REFL33_I --> PRCL, loop gain = -0.019, Trigger on POP22, ON @ 20cts, OFF@0.5cts.

REFL33_Q --> MICH, loop gain = +1.4, Trigger on POP22, ON @ 20cts, OFF@0.5 cts.

Attachment 1: PRMI_1f.png
PRMI_1f.png
  14961   Wed Oct 9 22:02:58 2019 gautamUpdateLSCREFL55 whitening issue

This problem has re-surfaced. Is this indicative of some problem with the on-board VGA? Even with 0dB of whitening gain, I see PDH horns that are 10,000 ADC counts in amplitude, whereas the nominal whitening gain for this channel is +18dB. I'll look at it in the daytime, not planning to use REFL55 for any locking tonight.

  14962   Thu Oct 10 01:12:56 2019 gautamUpdateLSCLocking studies

Summary:

  1. ALS control of arms in the CARM/DARM basis seems pretty robust - I was able to hold lock for >40mins tonight. The scripted transition from POX/POY control to ALS control is pretty deterministic now.
  2. The PRMI could be locked with the arms detuned from resonance by applying an offset to the CARM loop error point.
  3. Much daytime work remains to be done before attempting any sort of reliable locking.

Hardware issues that need addressing:

  1. Both EX and EY Trans QPDs need a look. I believe the one at EY is simply blocked (on account of the mode spectroscopy project), while the one at EX shows a weird discontinuity between the Thorlabs PD and the QPD. Could be just a gain/normalization issue I guess. See Attachment #1.
  2. While the PRMI stayed locked, I don't think I was using anywhere close to optimal settings. Need to run some sensing lines, measure transfer functions etc, to make the PRMI + arms lock more robust. The PRMI always lost lock when I brought the CARM offset to 0. Could also benefit from some finesse modeling I guess. I could not get a reliable estimate of what the PRG is tonight, because the PRMI didn't stay locked as I approached 0 CARM offset.
  3. REFL 55 whitening board needs a checkup.
Attachment 1: PRFPMIstudies.png
PRFPMIstudies.png
  14963   Thu Oct 10 22:11:53 2019 gautamUpdateLSCTrans QPD checkout
  1. I removed the flip-mount that was installed on the EY in-air table for the mode-spectroscopy project (see Attachment #1). The Transmon QPD at EY sees IR light again.
  2. Dark noise checkout - see Attachment #2.
  3. Light-level expectations:
    • For the current config, let's say 0.8 W reaches the PRM, and we will have a PRG of 50. 
    • This implies ~5.5 kW circulating power in the arms.
    • This implies ~70mW will get transmitted through the ETM, of which at most half makes it to the QPD. 
    • In the nominal operating condition, we expect more like 6 W circulating in the arm cavity. So something like 30uW is expected to make it out onto the Trans QPDs.
    • But in this condition, we expect to run with the high-gain Thorlabs PD.
    • In reality the number is likely to be somewhat smaller. But we should set the transimpedance gain of this photodiode accordingly. Currently, there are a bunch of ND filters installed on this photodiode, which probably should be removed.
  4. Angular control
    • The other purpose these QPDs are expected to serve is to stabilize the angular motion of the cavities when locked with high circulating power.
    • Need to calculate what the sensing noise requirement is.
Attachment 1: EY_table_20191010.jpeg
EY_table_20191010.jpeg
Attachment 2: darkNoise.pdf
darkNoise.pdf
  14973   Wed Oct 16 11:42:17 2019 gautamUpdateLSCPoor separation of PRCL/MICH in 3f signals

Summary:

There is poor separation of the PRCL and MICH length error signals as sensed in the 3f photodiodes. I don't know why this is so - one possibility is that the MICH-->PRM matrix element in the LSC output matrix needs to be tuned to minimize the MICH -->PRCL coupling.

Details:

Over the last few days, I've been trying to make the 3f locking of the PRMI more reliable. Turns out that while I was able to lock the PRMI on 3f error signals, it was just a fluke. So I set about trying to be more systematic. Here are the steps I followed:

  1. Lock the PRMI (i.e. ETMs misaligned) using REFL11 for PRCL, AS55 for MICH.
    • This is the so-called 1f scheme.
    • The servo signs are chosen such that the carrier field is resonant in the PRC.
    • Run the dither alignment to maximize POPDC, minimize ASDC. This is the main purpose of locking in this config.
    • Measure some loop TFs, make sure the servo gains are giving us ~100 Hz UGF on these loops.
  2. Change the sign of the servo loops to make the sidebands resonant in the PRC.
    • The error signals are still sourced from the 1f photodiodes.
    • Measure loop TFs, and also the TF between the 1f and 3f error signals. 
    • This allowed me to determine how the servo gains (and signs) that would be appropriate when using the 3f signals in place of the 1f.
    • Determine the offsets in the 3f error signals when the PRMI is locked on 1f error signals. This allows me to set the error point offsets for the PRCL_B and MICH_B paths, which are what is used for the 3f locking.
  3. Change the error signals from 1f to 3f. 
    • After much trial and error, I finally managed to get a stable (>10 mins) lock going.
    • Measured some loop TFs.
    • Turned on the notch filters in the control filter bank at the sensMat oscillator frequencies, and ran some sensing lines.

Attachment #1 is the result. I don't know what is the reason for such poor separation of the MICH and PRCL error signals in REFL165. The situation seems very different from when I had the DRMI locked in Nov last year.

After this exercise, I tried for some hours to get the 3f PRMI locking going with the arm cavities held off resonance under ALS control, but had no success. The angular motion of the PRC isn't helping, but I feel this shouldn't be a show stopper.

Attachment 1: sensMat.pdf
sensMat.pdf
  14974   Thu Oct 17 11:19:28 2019 gautamUpdateLSCLocking activity last night
  1. Tuning the MICH-->PRM output matrix element
    • Locked the PRMI with the carrier field resonant in the PRC.
    • REFL11 used to control PRCL, AS55 for MICH.
    • Turned on the sensing notches in the control filter bank. Drove a line in MICH at 311.10 Hz.
    • Tweaked the MICH-->PRM matrix element to minimize the coupling witnessed.
    • As shown in Attachment #1, the minimum coupling was found to be at the value -0.34 (the old value was -0.2655).
    • The minimum was very sharp. A 1% change from the optimum value increased the peak height by > x2. Is this reasonable?
  2. Some sensing matrix measurements: After tuning the output matrix element, I locked the PRMI (ETMs misaligned) in four configurations:
    • PRMI locked with carrier resonant. REFL11_I used for PRCL control, AS55_Q used for MICH control.
    • PRMI locked with sidebands resonant. REFL11_I used for PRCL control, AS55_Q used for MICH control.
    • PRMI locked with sidebands resonant. REFL11_I used for PRCL control, REFL165_I used for MICH control (based on sensing matrix measurement and offsets from previous config).
    • PRMI locked with sidebands resonant. REFL33_I used for PRCL control, AS55_Q used for MICH control.
    • The attached GIF shows the evolution of the demodulated sensing lines as we move through configurations.
       
    • The actual PDFs are attached as a zip, Attachment #2.
  3. PRMI locking with arms under ALS control
    • The arm cavity lengths were controlled as usual with ALS. This system needs some noise budgeting.
    • I set the CARM offset to -8 (arbitrarily chosen, approximately equal to 20nm, but anyways well above the cavity linewidth).
    • Then I re-aligned the PRM, and attemped to lock the PRMI using the 3f settings determined with no arm cavities --> no success.
    • Tried locking using the 1f error signals instead - in this config, the lock could be established.
    • However, I saw that there was significant light on the AS camera, and I had to put in an offset into the MICH loop to make ASDC go as low as possible.
    • I guess it is possible that the ALS control wasn't precise enough and the leaked light to the dark port was because of differential reflectivity of the arm cavities?
    • Anyways, I ran a sensing matrix measurement with the interferometer in this configuration, and I found that the MICH signal in REFL165 had rotated significantly.
    • I also found that the 3f DC offsets in this configuration were ~5x greater than what was the case for the lock with no arm cavities.

This is as far as I got last night. The first step is to see how reliable the settings determined last night are, today. I don't understand how changing the output matrix element can have brought about such a significant change in the MICH/PRCL separation in all the RF photodiodes.

Attachment 1: MICH2PRCLnulling.pdf
MICH2PRCLnulling.pdf
Attachment 2: consolidatedSensingMatrices.pdf.zip
  14983   Tue Oct 22 00:52:27 2019 gautamUpdateLSCLocking updates
  1. Transition of arms from POX/POY to CARM/DARM was much smoother today - a change was made at the EX PDH setup, see here.
  2. Reliable settings for 3f locking with arms held off resonance seem to have been found.
  3. Took sensing matrix in this condition, measured loop TFs.
  4. Reduced CARM offset - reached arm powers ~50 at which point the PRMI lost lock. Reacquisition was quick though.
    • The POP22_I level seemed to decay as I reduced the CARM offset.
    • This would suggest that somehow the PRCL lock point is getting shifted as I reduce the CARM offset.
    • Tonight, I will investigate this more.
Attachment 1: PRMI3f_ALS_Oct21sensMat.pdf
PRMI3f_ALS_Oct21sensMat.pdf
  14996   Tue Oct 29 01:24:45 2019 gautamUpdateLSCMore locking updates

Summary:

  1. The two arm lengths can be controlled reliably in the CARM/DARM basis using ALS error signals.
  2. With a CARM offset to keep the arm cavitites off resonance, the PRMI can be locked using 3f error signals.
  3. On attempting to reduce the CARM offset, I see a drop in the POP22 buildup in the PRC (correlated with the arm powers increasing). Not entirely clear why this is happening.

I ran some sensing measurements at various CARM offsets to check if the PRCL-->REFL33 and MICH-->REFL165 signals were being rotated out of the sensing quadrature as I lowered the CARM offset - there was no evidence of this happening. See Attachment #2. Other possibilities:

  • CARM offset dependant offsets in the MICH/PRCL error points?
  • Check the RAM due to the EOM? Perhaps the pointing / polarization control into the EOM got degraded.
  • Angular stability of the PRC is still pretty poor, getting the angular feedforward back up and running would help the duty cycle enormously.

The IMC went into some crazy state so I'm calling it for the night, need to think about what could be happening and take a closer look at more signals during the CARM offset reduction period for some clues...

Attachment 1: POP22_feature.png
POP22_feature.png
Attachment 2: PRMI3f_ALS_Oct21sensMat.pdf
PRMI3f_ALS_Oct21sensMat.pdf
  14997   Tue Oct 29 15:13:19 2019 gautamUpdateLSCMore locking updates

I looked at some signals for a 10 second period when the PRMI was locked with at some CARM offset, just before the PRMI lost lock, to see if there are any clues. I don't see any obvious signatures in this set of signals - if anything, the PRM is picking up some pitch offset, this is seen both at the Oplev error point and also in the POP QPD spot position. But why should this be happening as I reduce the CARM offset? The arm transmission is only ~5, so it's hard to imagine that the radiation pressure is somehow torquing the PRM. There are no angular feedback loops actuating on the PRM in this state except the local damping and Oplev loops.

The 1f signals are also changing their mean DC offset values, which may be a signature of a changing offset in the 3f MICH and PRCL error points? The MICH error signal is pretty noisy (maybe I can turn on some LPF to clean this up a bit), but I don't see any DC drift in the PRCL control signal.

Attachment 1: PRMI_lockloss.png
PRMI_lockloss.png
  14998   Tue Oct 29 17:40:48 2019 gautamUpdateLSCMore locking updates

I set up a photodiode (PDA10CF) in the IFO REFL beampath and the Agilent NA is sitting on the east side of the PSL enclosure. This was meant to be just a first look, maybe the PDA10CF isn't suitable for this measurement. The measurement condition was - PRM aligned so we have a REFL beam (DC level = 8.4V measured with High-Z). Both ITMs and ETMs were macroscopically misaligned so that there isn't any cavity effects to consider. I collected noise around 11 and 55 MHz, and also a dark measurement, plots to follow. The optics were re-aligned to the nominal config but I left the NA on the east side of the PSL enclosure for now, in anticipation of us maybe wanting to tune something while minimizing a peak.

Attachment #1: Results of a coarse sweep from 5 MHz to 100 MHz. The broadband RIN level is not resolvable above the dark noise of the photodiode, but the peaks at the modulation frequencies (11 MHz, 55 MHz and 29.5 MHz) are clearly visible. Not sure what is the peak at ~44 MHz or 66 MHz. Come to think of it, why is the 29.5 MHz peak so prominent? The IMC cavity pole is ~4kHz so shouldn't the 29.5 MHz be attenuated by 80dB in transmission through the cavity?

Attachment #2: Zoomed in spectra with finer IF bandwidth around the RF modualtion frequencies. From this first measurement, it seems like the RIN/rad level is ~10^5, which I vaguely remember from discussions being the level which is best achieved in practise in the 40m in the past.

Quote:
 

Check the RAM due to the EOM? Perhaps the pointing / polarization control into the EOM got degraded.

Attachment 1: broadSweep.pdf
broadSweep.pdf
Attachment 2: zoomSweep.pdf
zoomSweep.pdf
  14999   Wed Oct 30 01:27:00 2019 gautamUpdateLSCMore locking updates

Tried a bunch of things tonight.

  1. Modified the "ELP300" filter module in the MICH filter bank - this was really a 4th order elliptic low pass with corner at 80 Hz, which was much too low. I tried upping the corner to 500 Hz, and reducing the order, while I was able to enable the filter, there was clearly a gain-peaking feature visible after engaging this module, so the exercise of reducing the high frequency MICH actuation requires more careful (daytime) loop optimization.
  2. Tried adding some POPDC to the MICH/PRCL trigger once the PRMI was locked - I thought this would help if the problem was just with POP22 triggering turning off the MICH/PRCL loops, but the problem seems to persist with the mixed matrix trigger as well, once I reach a CARM offset where the arm powers exceed ~10, the PRMI loses lock.
  3. One strange feature I don't understand is that with the PRMI locked with the carrier field resonant, when running the dither alignment servo to minimize REFLDC (= more carrier coupled into the PRC), the POPDC level also goes down, but TRX and TRY go up slightly. I confirmed that the beam isn't falling off the POP22 photodiode (Thorlabs PDA10CF), but I don't understand why these two DC powers should fall simultaneously - if I couple more carrier into the PRC, shouldn't the POPDC level also increase?

One possibility is that the arm buildup is exerting some torque on the ITMs, which can also change the PRC cavity axis - as the buildup increases, the dominant component of the circulating field in the PRC comes from the leakage from the overcoupled arm cavity. We used to DC couple the ITM Oplev servos when locking the PRMI. The TRX level of 1 corresponds to ~5W of circulating power in the arm cavity, and the static radiation pressure force due to this circulating power is ~30 nN, rising up to 300nN as the TRX level hits 10. So for 1mm offset of the spot position on the ITM, we'd still only exert 300 pN m of torque. I don't see any transient in the Oplev error signals when locking the arm cavity as usual with POX/POY, but on timescales of several seconds, the Oplev error point shows ~3-5 urad of variation.

Attachment 1: POP_ASS.png
POP_ASS.png
  15000   Wed Oct 30 11:53:41 2019 gautamUpdateLSCMICH loop shape tuning

I changed the shape of the low pass filter to reduce high frequency sensor noise injection into the MICH control signal. The loop stability isn't adversely affected (lost ~5 degrees of phase margin but still have ~50 degrees), while the control signal RMS is reduced by ~x10. This test was done with the PRMI locked on the carrier, need to confirm that this works with the arms controlled on ALS and PRMI lcoked on sideband.

Attachment 1: MICH_ELP.pdf
MICH_ELP.pdf
Attachment 2: MICH_ELP_TFs.pdf
MICH_ELP_TFs.pdf
  15001   Wed Oct 30 17:08:40 2019 gautamUpdateLSCPOP22 investigation

The POP beam coming out of the vacuum chamber is split by a 50/50 BS and half is diverted to the POP22/POP110/POPDC photodiode (Thorlabs PDA10CF) and the other half goes to the POP QPD. This optical layout is still pretty accurate. I looked at the data of the POPDC and POP QPD SUM channels while the dither alignment was running, to see if I could figure out what's up with the weird correlated dip in REFLDC and POPDC. While the POPDC channel shows some degradation as the REFLDC level goes down (=alignment gets better), the QPD sum channel shows the expected light level increase. So it could yet be some weird clipping somewhere in the beampath - perhaps at the 50/50 BS? I will lock the PRMI (no arms) and check...

Attachment 1: POP22anomaly.pdf
POP22anomaly.pdf
  15009   Mon Nov 4 15:29:47 2019 gautamUpdateLSCPOP signal path

There are many versions of the POP22 signal path I found on the elog, e.g. this thread. But what I saw at the LSC rack was not quite in agreement with any of those. So here is the latest greatest version.

Since the 2f signals are mainly indicators of power buildups and are used for triggering various PDH loops, I don't know how critical some of these things are, but here are some remarks:

  1. There is no Tee + 50 ohm terminator after the minicircuits filters, whose impedance in the stopband are High-Z (I have been told but never personally verified).
  2. The RF amplifier used is a Minicircuits ZFL-1000-LN+. This has a gain of 20dB and 1dB compression output power spec of 3dBm. So to be safe, we want to have not more than -20dBm of signal at the input. On a 50-ohm scope (AC coupled), I saw a signal that has ~100mVpp amplitude (there is a mixture of many frequencies so this is not the Vpp of a pure sinusoid). This corresponds to -16dBm. Might be cutting it a bit close even after accounting for cable loss and insertion loss of the bias tee.
  3. We use a resistive power splitter to divide the power between the POP22 and POP110 paths, which automatically throws away 50% of the RF power. A better option is the ZAPD-2-252-S+.
  4. The Thorlabs PDA10CF photodiode (not this particular one) has been modelled to have a response that can be approximated by a complex pole pair with Q=1 at ~130 MHz. But we are also using this PD for measuring the 110 MHz PD which is a bit close to the band edge?
Attachment 1: POPchain.pdf
POPchain.pdf
  15010   Mon Nov 4 16:06:58 2019 gautamUpdateLSCPOP optical path

I did some re-alignment of the POP beam on the IX in air table. Here are the details:

  1. Attachment #1 - optical layout.
  2. With the PRC locked with the carrier resonant (no arm cavities), there is ~300uW of DC power incident on the Thorlabs PDA10CF, which serves as POP22, POP110 and POPDC photosensor.
    • See this elog for the signal paths.
    • On a scope, this corresponded to ~1.8 V DC of voltage. This is in good agreement with the expected transimpedance gain of 10 kOhms and responsivity of ~0.65 A/W given on the datasheet.
    • This is also in agreement with the ~6000 ADC counts I see in the CDS system (although there are large fluctuations). 
  3. These was significant misalignment of the beam on this photodiode at some point:
    • Previously, I had used the CDS system to walk the beam on thde photodiode to try and maximize the power.
    • Today I took a different approach - triggered the MICH and PRCL loops on REFLDC (instead of the usual POPDC / POP22) so I could freely block the beam.
    • I found that there is a fast (f=35mm) lens to make the beam small enough for the PDA10CF. The beam was somewhat mis-centered on this strongly curved optic, and I suspect it was amplifying small misalignments. Anyway it is much better centered now (see Attachment #2) and I have a much stronger POPDC signal (by a factor of ~2-3, see Attachment #3).
    • The ASS dither alignment now shows much more consistent behavior - minimizing REFLDC maximises POPDC, see Attachment #4.
    • I took this opportunity to take some spectra/time-series of the PD output with the interferometer in this configuration. 

Tangentially related to this work - I took the nuclear option and did a hard reboot of the c1susaux Acromag crate on Sunday to fix the EPICS issue - it seems to be gone for now, see Attachment #5.

Attachment 1: IMG_8027.JPG
IMG_8027.JPG
Attachment 2: lensRealignment.jpg
lensRealignment.jpg
Attachment 3: POPrealigned.png
POPrealigned.png
Attachment 4: POPdither.png
POPdither.png
Attachment 5: PRMfixed.png
PRMfixed.png
  15012   Tue Nov 5 11:52:27 2019 gautamUpdateLSCLocking notes

Summary:

I am still unable to achieve arm powers greater than TRX/TRY ~10 while keeping the PRMI locked. A couple of times, I was able to get TRY ~50, but TRX stayed at ~10, or even dropped a little, suggestive of a DARM offset? On the positive side, the ALS system seems to work pretty reliably, and I can keep the arms controlled by ALS for several tens of minutes.

Details:

  • Despite my POP beam path improvements, I saw the POP22 level drop as I lowered the CARM offset.
  • One strange feature last night was that with the arms held off resonance using ALS, I had to flip the sign and increase the gain by ~x2 of the REFL33_I-->PRCL loop in order to lock the PRMI. This was confirmed by locking on the 1f error signals and measuring the ratio of the response between the 1f and 3f signals while shaking PRCL using DTT swept sine.
  • At different CARM offsets, I noted that the DC offset level on the 1f photodiodes (i.e. REFL11 and AS55) were changing significantly.
  • I ran a measurement of the sensing matrix with the arm powers hovering around ~10, which is just before I lose the PRMI lock - managed to stay locked for >5 minutes, but the sensing matrix seems to suggest that the REFL33 demod angle needs to be rotated - maybe this is the reason why the PDH horn-to-horn voltage of REFL33 is lower now than it was last week? No idea why that should be, I was around the LSC rack but if the situation is so fragile, seems hopeless.
  • MICH sensed by REFL165_Q still seems stable, so that's good...
  • So my best hypothesis at the moment is that the PRCL optical gain is falling as I reduce the CARM offset (due to DC offset? or something else?). Needs some detailed modeling for more insight, I'm out of ideas for tests to run while locking as I've gone through the full gamut of OLTF and sensing matrix measurements at various CARM offsets without getting any clues as to what's going on.
Attachment 1: PRMI3f_ALS_Nov4sensMat.pdf
PRMI3f_ALS_Nov4sensMat.pdf
  15014   Wed Nov 6 02:08:48 2019 gautamUpdateLSCLocking updates

Summary:

There seems to be stronger-than-expected coupling between CARM and the 3f sensors. 

Details:

Full analysis tomorrow, but I collected sensing matrix measurements with lines driven in PRCL,MICH and CARM at a couple of CARM offsets. I also wanted to calibrate the CARM offset to physical units so I ran some scans of the CARM offset and collected the data so I can use the arm cavity FSR to calibrate CARM. Koji suggested using REFL165_I for PRCL and REFL165_Q for MICH control - this would allow us to see if the problem was with the 1f sideband only. While the lock could be established, we still couldn't push the arm powers above 10 without breaking the PRMI lock. While changing the CARM offset, we saw a significant shift in the DC offset level of the out-of-loop REFL33_I signal. Need to think about what this means...

  15015   Wed Nov 6 17:05:45 2019 gautamUpdateLSCCARM calibration

Summary:

A coarse calibration of the CARM error point (when on ALS control) is 7.040 +/- 0.030 kHz/ct. This corresponds to approximately 0.95nm/ct. I typically lose the PRMI lock when the CARM offset is ~0.2 cts, which means I am about 1kHz away from the resonance. This is >10 CARM linewidths.

Details:

The calibration was done by sweeping the CARM offset (no PRM) and identifying the arm cavity FSRs by looking for peaks in TRX / TRY. Attachment #1 shows the scan, while Attachment #2 shows a linear fit to the FSRs. In Attachment #2, the frequency axis is taken from the phase tracker servo, which was calibrated by injecting a "known" frequency with the Marconi, and there is good agreement to the expected FSR with 37.79 m long arm cavities. There is much more info in the scan (e.g. modulation depths, mode matching to the arm cavities etc) which I will extract later, but if anyone wants the data (pre-downsampled by me to have a managable filesize), it's attached as a .zip file in Attachment #3.

Attachment 1: CARMscan.pdf
CARMscan.pdf
Attachment 2: CARMcalib.pdf
CARMcalib.pdf
Attachment 3: scan.hdf5.zip
  15016   Wed Nov 6 17:45:34 2019 gautamUpdateLSC~

Here is a comparison of the response of various DoFs in our various RFPD sensors for two different CARM offsets. Even in the case of the smaller CARM offset of ~1kHz, we are several linewidths away from the resonance. Need to do some finesse modeling to make any meaningful statement about this - why is the CARM response in REFL11 apparently smaller for the smaller CARM offset?

If you mistrust my signal processing, the GPS times for which I ran the sensing lines are:

CARM offset = ~30kHz (arm transmission <0.02) --- 1257064777+5min

CARM offset = ~1kHz (arm transmission ~5) --- 1257065566+5min

Quote:

Summary:

There seems to be stronger-than-expected coupling between CARM and the 3f sensors. 

  15026   Thu Nov 14 23:56:18 2019 ranaUpdateLSCoff the bad Violin filters

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

Due to some feedforward work by Jenne or EQ some years ago, we have had ~10 violin notches on in the LSC between the output matrix and the outputs to the SUS.

They were eating phase, computation time, and making ~3 dB gain peaking in places where we can't afford it. I have turned them off and Gautam SDF safed it.

Offensive BS shown in brown and cooler BS shown in blue.

To rotate the DTT landscape plot to not be sideways, use this command (note that the string is 1east, not least):
pdftk in.pdf cat 1east output out.pdf

Attachment 1: out.pdf
out.pdf
  15028   Fri Nov 15 11:58:12 2019 gautamUpdateLSCoff the bad Violin filters

The clue was that the loop X arm POX loop looked to have low (<3dB)) gain margin around 600 Hz (and again at 700 Hz). Attachment #1 shows a comparison of the OLTF for this loop (measured using the IN1/IN2 method) before and after our change. We hypothesize that one of the violin filters that were turned off had non-unity DC gain, because I had to lower the loop gain by 20% after these turn-offs to have the same UGF. I updated the snap files called by the arm locking scripts.

I think I caught all the places where the FM settings are saved, but some locking scripts may still try and turn on some of these filters, so let's keep an eye on it.

Quote:

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

Attachment 1: violinFix.pdf
violinFix.pdf
Attachment 2: newViolinConfig.png
newViolinConfig.png
  15029   Fri Nov 15 12:08:04 2019 gautamUpdateLSCPOPDC whitening board

The DC port of the Bias-Tee is routed to (a modified version of) the iLIGO whitening board. This has the well-known problem of the protection diodes of the LT1125 quad-op-amp lowering the (ideally infinite) input impedance of the first gain stage (+24 dB). To be sure as to how much signal we can put into this port (in anticipation of trying some variable finesse PRFPMI locking but also for general book-keeping), I tested the usable input range by driving a triangle wave at ~3 Hz and changing the amplitude of the signal until we observed saturation. We found that we could drive a 10 Vpp signal at which point there was evidence of some clipping (it was asymmetric, the top end of the signal was getting clipped at +14,000 cts while the bottom end still looked like a triangle wave at -16,000 counts). Anyway we probably don't want to exceed +/- 10,000 counts on this channel. This is consistent with Hartmut's statement of having +/- 4V of usable range (although the counts he mentions are twice what I saw yesterday).

Other discussion points between Rana, Koji and Gautam:

  1. Conside putting an in-vacuum (Silicon ?) QPD for the PRC angular motion sensing
    • In-vacuum will yield lower acoustic noise coupling
    • Bring the photocurrent out and do the transimpedance amplification in air 
    • Use a large area QPD so as to be more tolerant to alignment drifts without having to introduce picomotors (but how much does the POP spot actually drift and is this feasible?)
  2. Is there some better telescope configuration for the existing in-air QPD?
    • What is the correct Gouy-phase for this to be able to best sense the PRC cavity axis motion?
  15034   Mon Nov 18 21:04:38 2019 gautamUpdateLSCLocking - some ideas

Some ideas Koji suggested:

  1. Try approaching the CARM offset zero point "from the other side" - i.e. start with a CARM offset of the opposite sign (I typically use negative CARM offset).
  2. With the PRMI locked, try bringing one arm onto resonance while the other arm is held off resonance. 

For the second idea, it is convenient to be able to control the arms in the XARM/YARM basis as opposed to the CARM/DARM basis as we usually do when going through the locking sequence. After some fiddling, I am able to reliably execute this transition, and achieve a state where the FP arm cavities are resonant for the IR with the ALS beat note frequency being the error signal being used. Some important differences:

  1. The frequency actuator (ETM) is weaker in this case than in the CARM/DARM basis (where MC2 is the frequency actuator) due to the longer length of the arm cavity (and for ETMX, the higher coil driver series resistance). I had to twiddle the limits of the servo banks to accommodate this. 
  2. The ALS error signal is significantly noisier than POX/POY. Hence, the control signal RMS is often in danger of saturating the DAC range. I implemented a partial fix by adding a 1st order Butterworth LPF with 1kHz corner frequency. According to the model, this eats <5 degrees of phase at the desired UGF of ~150 Hz.
  15037   Wed Nov 20 01:07:18 2019 gautamUpdateLSCLocking - progress

Summary:

  1. CARM offset was reduced to 0 with the PRMI locked.
  2. TRY levels touched ~200 (Recycling gain ~10, IFO is still undercoupled).
  3. TRX level never went so high - I suspect QPD issues or clipping in the beampath.

Details:

  • Attachment #1 is a StripTool summary of the lock - encouragingly, the PRMI stayed locked for several 10s of minutes even when the CARM offset was brought to 0.
  • The MICH signal was pretty glitchy - we increased the gain of the MICH and PRCL loops and thought we saw some improvement, but needs more quantitative investigation.
  • Main differences in locking procedure today were:
    • Added some POPDC to the MICH/PRCL trigger elements in addition to POP22
    • Tried adding a DARM offset before doing the final steps of CARM offset reduction, and then zerod the DARM offset too.
  • The TRX level never went as high as TRY - even though on the CRT monitors, both arms seemed to saturate somewhat more evenly. Potentially the EX QPD needs a checkout, or there is some clipping in the in-air TRX path. Although, puzzilingly, the POXDC level never goes as high as POYDC either. So maybe the buildup is really lower in the XARM? For the daytime tomorrow.

Next steps:

  1. Check the EX QPD / TRX situation.
  2. Figure out how to make the PRMI lock more stable as I reduce the CARM offset.
  3. Start figuring out the CM board, as we'd want to do the handoff to RF at some point.
Attachment 1: PRFPMI.png
PRFPMI.png
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