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ID Date Authorup Type Category Subject
  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
  15013   Tue Nov 5 12:37:50 2019 gautamUpdatePEMT240 interface unit pulled out

I removed the Trillium T240 DAQ interface unit from 1X4 for investigation.

It was returned to the electronics rack and all the connections were re-made. Some details:

  1. The board is indeed a D1000749-v2 as Koji said it is. There is just an additional board (labelled D1001872 but for which there is no schematic on the DCC) inside the 1U box that breaks out the D37 connector of the v2 into 3 D15 connectors. I took photos.
  2. Armed with the new cable Chub got, and following the manual, I ran the re-centering routine.
    • Now all the mass-monitoring position voltages are <0.3 V DC, as the manual tells me they should be.
    • I noticed that when the seismometer is just plugged in and powered, it takes a few minutes for the mass monitoring voltages to acquire their steady state values.
    • The V indicator reported ~-2V DC, and the W indicator reported -3.9V DC.
    • While running the re-centering routine, I monitored the mass-position indicator voltages (via the backplane D15 connector) on an oscilloscope. See Attachment #1 for the time series. The data was rather noisy, I don't know why this is, so I plot the raw data in light colors and a filtered version in darker colors. Also, there seems to be a gain of x2 in the voltages on the backplane relative to what the T240 manual tells me I should expect, and the values reported when I query the unit via the serial port.
    • We should ideally just install another Acromag ADC in the c1susaux box and acquire these and other available diagnostic information, since the signals are available.
    • We should also probably check the mass position indicator values in a few days to see if they've drifted off again.
    • Looking at the raw time series / spectra of the BS channels, I see no obvious signatures of any change. 
    • I will run a test by locking the PRC and looking for coherence between the seismometer data and angular motion witnessed by the POP QPD, as this was what signalled my investigation in the first place.

Update 445pm: Seems to have done something good - the old feedforward filters reduce the YAW RMS motion by a factor of a few. Pitch performance is not so good, maybe the filter needs re-training, but I see coherence, see Attachment #2 for the frequency domain WF.

Attachment 1: T240_recenter.pdf
T240_recenter.pdf
Attachment 2: ffPotential.pdf
ffPotential.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. 

  15017   Wed Nov 6 19:26:57 2019 gautamUpdatePSLSome PSL cable admin

Koji and I taked about cleaning up some of the flaky cable situation on the PSL table a while ago. The changes were implemented and are documented in Attachment #1. Now the Pomona box between the Thorlabs HV Driver and the NPRO head is sitting on the PSL table (sandwiched between some teflon pieces I found in cabinet S4 along the south arm), and the cables between these two devices are better strain relieved. I turned off the Thorlabs HV supply while working on the PMC table. The IMC could be locked after this work. Probably won't solve the long standing FSS mysteries but probably can't hurt.

Unrelated to this work: I also removed a Bias tee that was just hanging out on top of the FSS electronics, which was used for the modeSpec project.

Attachment 1: PSLcableAdmin.jpg
PSLcableAdmin.jpg
  15022   Wed Nov 13 19:34:45 2019 gautamUpdatePEMFollow-up on seismometer discussion

Attachment #1 shows the spectra of our three available seismometers over a period of ~10ksec.

  • I don't understand why the z-axis motion reported by the T240 is ~10x lower at 10 mHz compared to the X and Y motions. Is this some electronics noise artefact?
  • The difference in the low frequency (<100mHz) shapes of the T240 compared to the Guralps is presumably due to the difference in the internal preamps / readout boxes (?). I haven't checked yet.
  • There is almost certainly some issue with the EX Guralp. IIRC this is the one that had cabling issues in the past, and also is the one that was being futzed around for Tctrl, but also could be that its masses need re-centering, since it is EX_X that is showing the anomalous behaviour.
  • The coherence structure between the other pairs of sensors is consistent.

Attachment #2 shows the result of applying frequency domain Wiener filter subtraction to the POP QPD (target) with the vertex seismometer signals as witness channels.

  • The dataset was PRMI locked with the carrier resonant, ETMs misaligned.
  • The dashed lines in these plots correspond to the RMS for the solid line with the same color.
  • For both PIT and YAW, I am using BS_X and BS_Y seismometer channels for the MISO filter inputs.
  • In particular for PIT, I notice that I am unable to get the same level of performance as in the past, particularly around ~2-3 Hz.
  • The BS seismometer health indicators don't signal any obvious problems with the seismometer itself - so something has changed w.r.t. how the ground motion propagates to the PR2/PR3? Or has the seismometer sensing truly degraded? I don't think the dataset I collected was particularly bad compared to the past, and I confirmed similar performance with a separate PRMI lock from a different time period.
Attachment 1: seisAll_20191111.pdf
seisAll_20191111.pdf
Attachment 2: ffPotential.pdf
ffPotential.pdf
  15024   Wed Nov 13 23:40:15 2019 gautamUpdatePEMFollow-up on seismometer discussion

Here is some disturbance in the spacetime curvature, where the local gradient of the metric seems to have been modulated (in the "downward" as well as in the other two orthogonal Cartesian directions) at ~1 Hz - seems real as far as I can tell, all the suspensions were being shaken about and all the seismometers witnessed it, though the peak is pretty narrow. A broader, less prominent peak also shows up around 0.5 Hz. We couldn't identify any clear source (no LN2 fill-up / obvious CES activity). This event lasted for ~45 mins, and stopped around 2315 local time. Shortly (~5min) after the ~1 Hz peak died down, however, the 3-10 Hz BLRMS channel reports an increase by ~factor of 2. 

Onto trying some locking now that the suspensions have settled down somewhat.

Quote:

this is due to the Equivalence Principle: local accelerations are indistinguishable from spacetime curvature. On a spherical Earth, the local gradient of the metric points in the direction towards the center of the Earth, which is colloquially known as "down".

Attachment 1: seisAll_20191111_1Hz.pdf
seisAll_20191111_1Hz.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?
  15030   Fri Nov 15 12:16:48 2019 gautamUpdatePEMFollow-up on seismometer discussion

Attachment #1 is a spectrogram of the BS sesimometer signals for a ~24 hour period (from Wednesday night to Thursday night local time, zipped because its a large file). I've marked the nearly pure tones that show up for some time and then turn off. We need to get to the bottom of this and ideally stop it from happening at night because it is eating ~1 hour of lockable time.

We considered if we could look at the phasing between the vertex and end seismometers to localize the source of the disturbance.

Attachment 1: BS_ZspecGram.pdf.zip
  15032   Mon Nov 18 14:32:53 2019 gautamUpdatePEMFollow-up on seismometer discussion

The nightly seismic activity enhancement continued during the weekend. It always shows up around 10pm local time, persists for ~1 hour, and then goes away. This isn't a show stopper as long as it stops at some point, but it is annoying that it is eating up >1 hour of possible locking time. I walked over to CES, no one there admitted to anything - there is an "Earth Surface Dynamics Laboratory" there that runs some heavy equipment right next to us, but they claim they aren't running anything post ~530pm. Rick (building manager ?) also doesn't know of anything that turns on with the periodicity we see. He suggested contacting Watson but I have no idea who to talk to there who has an overview of what goes on in the building. 😢 

  15033   Mon Nov 18 16:32:15 2019 gautamUpdateComputersZITA: started upgrade from Ubuntu 14 LTS to 18 LTS

the upgrade seems to have been successfully executed - the machine was restarted at ~430pm local time. Projector remains off and diagnostic striptools are on the samsung.

Quote:

and so it begins...until this is finished I have turned off the projector and moved the striptools to the big TV (time to look for Black Friday deals to replace the projector with a 120 inch LED TV)

  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.
  15035   Tue Nov 19 15:08:48 2019 gautamUpdateCDSVertex models rebooted

Jon and I were surveying the CDS situation so that he can prepare a report for discussion with Rolf/Rich about our upcoming BHD upgrade. In our poking around, we must have bumped something somewhere because the c1ioo machine went offline, and consequently, took all the vertex models out. I rebooted everything with the reboot script, everything seems to have come back smoothly. I took this opportunity to install some saturation counters for the arm servos, as we have for the CARM/DARM loops, because I want to use these for a watch script that catches when the ALS loses lock and shuts stuff off before kicking optics around needlessly. See Attachment #1 for my changes.

Attachment 1: armSat.png
armSat.png
  15036   Tue Nov 19 21:53:57 2019 gautamUpdatePEMFollow-up on seismometer discussion

The shaking started earlier today than yesterday, at ~9pm local time.

While the IFO is shaking, I thought (as Jan Harms suggested) I'd take a look at the cross-spectra between our seismometer channels at the dominant excitation frequency, which is ~1.135 Hz. Attachment #1 shows the phase of the cross spectrum taken for 10 averages (with 30mHz resolution) during the time period when the shaking was strong yesterday (~1500 seconds with 50% overlap). The logic is that we can use the relative phasing between the seismometer channels to estimate the direction of arrival and hence, the source location. However, I already see some inconsistencies - for example, the relative phase between BS_Z and EX_Z suggests that the signal arrives at the EX seismometer first. But the phasing between EX_Y and BS_Y suggests the opposite. So maybe my thinking about the problem as 3 co-located sensors measuring plane-wave disturbances originating from the same place is too simplistic? Moreover, Koji points out that for two sensors separated by ~40m, for a ground wave velocity of 1.5 km/s, the maximum phase delay we should see between sensors is 30 msec, which corresponds to ~10 degrees of phase. I guess we have to undo the effects of the phasing in the electronics chain.

Does anyone have some code that's already attempted something similar that I can put the data through? I'd like to not get sucked into writing fresh code.

🤞 this means that the shaking is over for today and I get a few hours of locking time later today evening.

Another observarion is that even after the main 1.14 Hz peak dies out, there is elevated seismic acitivity reported by the 1-3 Hz BLRMS band. This unfortunately coincides with some stack resonance, and so the arm cavity transmission reports greater RIN even after the main peak dies out. Today, it seems that all the BLRMS return to their "nominal" nighttime levels ~10 mins after the main 1.14 Hz peak dies out.

Attachment 1: seisxSpec.pdf
seisxSpec.pdf
  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
  15038   Wed Nov 20 12:14:17 2019 gautamUpdateLSCLocking - progress

I took a look at the TRX/TRY RIN reported in the single arm POX/POY lock, and compared the performance of the two available PDs at each of the two ends. Attachment #1 shows the results. Some remarks:

  1. The noise performance of the two QPDs at each end isn't identical - is there some transimpedance gain difference?
  2. The lower plot shows the angular motion reported by each QPD when the arm cavity is locked. The EX QPD seems much more sensitive than the EY QPD.
  3. I estimate that in this condition, each photodiode is receiving ~20uW of power, corresponding to a shot noise limited RIN of ~10^-7. None of the photodiodes saturate this bound.
  4. There are some ND filters placed in front of the QPDs at both ends. Do we really need these ND filters? I estimate that for the highest buildups, we will have ~10kW * 15ppm * 0.5 ~75mW of power incident on the QPD, so ~20mW per segment. Assuming silicon responsivity of 0.2 A/W, a transimpedance of 1kohm would give us 4V of signal. But the schematic shows higher transimpedance. Do we still have the switching capability for this QPD?
Quote:

Next steps:

  1. Check the EX QPD / TRX situation.
Attachment 1: TRX_TRY_comparison.pdf
TRX_TRY_comparison.pdf
  15040   Wed Nov 20 17:52:00 2019 gautamUpdateLSCQPD MEDM screen update

Koji and I had noticed that there was some discrepancy between the switchable gain stages of the EX and EY QPDs. Sadly, there was no indication that these switches even exist on the QPD MEDM screen. Yehonathan and I rectified this today. Both EX and EY Transmon QPDs now have some extra info (see Attachment #1). We physically verified the indicated quadrant mapping for the EX QPD (see previous elog in this thread for the details), and I edited the screen accordingly. EY QPD still has to be checked. Note also that there is an ND=0.4 + ND=0.2 filter and some kind of 1064nm light filter installed in series on the EX QPD. The ND filters have a net transmission T~25%.

After making the EX and EY QPDs have the same switchable gain settings (I also reset the trans normalization gains), the angular motion witnessed is much more consistent between the two now - see Attachment #2. The high-frequency noise of the sum channel is somewhat higher for EX than EY - maybe the ND filters are different on the two ends?

Note that there was an extra factor of 40 gain on the EX QPD relative to EY during the lock yesterday. So really, the signals were probably just getting saturated. Now that the gains are consistent between the ends, it'll be interesting to see how balanced the buildup of the two arms is. There still remains the problem that the MICH loop was too unstable, which probably led to to excess arm power fluctuations.

There is a mark on the QPD surface that is probably dirt (since we never have such high power transmitted through the ETM to damage the QPD). I'll try cleaning it up at the next opportunity.

Attachment 1: newLookQPD.png
newLookQPD.png
Attachment 2: TRX_TRY_comparison.pdf
TRX_TRY_comparison.pdf
Attachment 3: IMG_8186.JPG
IMG_8186.JPG
  15041   Wed Nov 20 21:29:28 2019 gautamUpdateLSCPRG ~13

After the QPD fix, both arms report consistent buildup - see Attachment #1. The peak values touch ~250, corresponding to a PRG of ~13. The IFO becomes critically coupled at PRG=15. I am finding that the 3f signal offsets are changing as a function of the CARM offset, and this could be responsible for the lock breaking as I approach 0 CARM offset. I found that I could maintain a more stable and deterministic transition to zero CARM offset by dynamically adjusting the 3f PRCL error signal offset to keep the REFL11 signal approximately at 0. Some shaking seems to have commenced so I am breaking for now.

Note that I find scattered throughout the elog references to a similar problem of the PRMI losing lock as the CARM offset is reduced, e.g. here. But haven't stumbled across what the resolution was, the PRFPMI could be locked pretty easily in 2015 I remember.

Attachment 1: PRG13.pdf
PRG13.pdf
  15042   Thu Nov 21 12:46:22 2019 gautamUpdateLSCCM board study

In preparation for trying out some high-bandwidth Y arm cavity locking using the CM board, I hooked up the POY11_Q_Mon channel of the POY11 demod board to the IN1 of the CM board (and disconnected the usual REFL11 cable that goes to IN1). The digital phase rotation for usual POY Yarm locking is 106 degrees, so the analog POY11_Q channel contains most of the signal. I then set the IN1 gain of the servo to 0dB, and looked at the CM_Slow signal - I changed the whitening gain of this channel to +18dB (to match that used for POY11_I and POY11_Q), and found that I had to apply a digital gain of 0.5 to get the PDH horns in the usual POY11_I signal and the CM_Slow signal to line up. There was also a sign inversion. Then I was able to use the digital LSC system and lock the Y arm cavity length to the PSL frequency by actuating on ETMY, using CM_Slow as an error signal. A comparison of the in-loop POY11_I ASD when the arm is locked is shown in Attachment #1 - CMslow seems to be dominated by some kind of electroncis noise above ~100 Hz, so possibly needs more whitening (even though the nominal whitening filter is engaged)?

Anyway, now that I have this part of the servo working, the next step is to try and engage the AO path and achieve a higher BW lock of the Y arm cavity to the PSL frequency (= IMC length). Maybe it makes more sense to actuate on MC2 for the slow path.

Attachment 1: YARM_CMslow.pdf
YARM_CMslow.pdf
  15044   Thu Nov 21 19:08:58 2019 gautamUpdateLSCHigh BW lock of Y arm length to PSL frequency

Summary:

The Y arm cavity length was locked to the PSL frequency with ~26kHz UGF, and 25 degrees phase margin. Slow actuation was done on ETMY using CM_Slow as an error signal, while fast actuation was done on the IMC error point via the IN2 input of the IMC servo board. Attachment #1 shows the comparison of the in-loop error signal spectra with only slow actuation and with the full CM loop engaged.

Details:

  1. LSC enable OFF.
  2. Configure the CM board for locking:
    • CM board IN1 gain = 25dB.
    • CM_Slow whitening gain = +18dB, make sure the offsets are correctly set. CM_Slow filter bank = -0.015.
    • CM_Slow-->YARM matrix element in LSC input matrix is -2.5.
    • YARM-->ETMY matrix element in LSC Output matrix is 1.
    • AO gain set to +5dB. IMC Servo board IN2 gain starts at -32dB, the path is disabled. The polarity is Plus.
    • Usual YARM FM triggers are set (FM1, FM2, FM3, FM6, FM8), usual YARM servo gain is used (0.01), usual triggering conditions (ON @ TRY>0.3, OFF @ TRY < 0.1), usual power normalization by TRY.
  3. Enable LSC mode, wait for the arm to acquire lock.
  4. Once the digital boosts are engaged, enable the IMC IN2 path, ramp up the gain to -2 dB. Note that this IN2 path is AC coupled, according to this elog. The corner frequency is 1/2/pi/2e3/6.8uF ~11 Hz. This was confirmed by measurement, see Attachment #3. I couldn't find a 2-pin LEMO-->BNC adaptor so I measured at the BNC connector for the IN2 input, which according to the schematic is shorted to the LEMO (which is what we use for the AO path).
  5. Enable the CM board boost.
  6. Ramp up the CM board IN1 gain to +31dB. In this config, the CM_Slow signal is ~18,000 cts pk (with the +18dB whitening gain), so not saturating the ADC.
  7. Ramp up the IMC IN2 gain to 3dB, engage 2 Super Boosts (can't turn on the third). Limiter is always ON.
  8. Use the CM board error point offset adjust to zero the POY11_I error signal average value - there seems to be some offsets when engaging the boosts. The value I used was 0.9 V (this is internally divided by 40 on the CM board).
  9. Whiten the CM_Slow signal - this doesn't seem to have any impact on the noise anywhere.

I hypothesize that the high-frequency noise (>100 Hz) is higher for POY than POX in Attachment #1 because I am using the "MON" port of the demod board - this has a gain of 2, and there could also be some flaky components in this path, hence the high frequency noise is a factor of a few greater in the POY spectrum relative to the POX spectrum (which is using the main demodulated output). For REFL11, we have a low noise preamp generating the input signal so I don't think we need to worry about this too much.

The PC Drive RMS didn't look any stranger than it usually does for the duration of the lock.

Attachment #2 shows the OLTF of the locking servo with the final gains / settings, which are in bold. The loop is maybe a bit marginal, could possibly benefit from backing off one of the super boosts. But the arm has stayed locked for >1 hour.

The purpose of this test was to verify the functionality of the CM board and also the IN2 of the IMC servo board in a low-pressure environment. Once I confirm that the modelled OLTF lines up with the measured, I will call this test a success, and move on to looking at REFL11 in the arms on ALS, PRMI on 3f config. I am returning the REFL11 signal to the input of the CM board, but the SR785 remains hooked up.

Unrelated to this work - PMC alignment was tweaked to improve input power to IMC by ~5%.

Attachment 1: highBW_POY.pdf
highBW_POY.pdf
Attachment 2: CM_UGF.pdf
CM_UGF.pdf
Attachment 3: IN2_ACcoupling.pdf
IN2_ACcoupling.pdf
  15045   Fri Nov 22 00:54:14 2019 gautamUpdateLSClocking notes

[KA, GV]

There was no shaking (that disturbed the locking) tonight!

  1. REFL165 Demod phase was adjusted from -111deg to -125deg. To minimize coherence b/w MICH and PRCL.
  2. MICH 3f loop gain changed to 0.3.
  3. If the POP mode shape looks weird, it probably means that the PRM is sligntly misaligned. Tweaking the alignment improves PRMI stability and also makes the arm buildup higher.
  4. Ditto for MICH - slightly touching up the BS alignment can lower ASDC.
  5. Main finding tonight was that the ALS noise seems to get degraded as a function of the CARM offset! As a result of this, CARM goes through several linewidths, and the arm transmission fluctuates wildly.
    • We suspect some scattered light shenanigans. It is not clear to me why this is happening. Possibilities:
    • Scattered ETM transmission somehow makes it into the fiber coupler and degrades the ALS noise.
    • Sacttered ETM transmission makes it onto the Green PDH photodiode and degrades the ALS noise.
    • Backscatter into the PSL degrades the ALS noise.
    • Shadow sensors of either the ITMs, ETMs, BS, or PRM don't have 1064nm filters and get scatterd light, making the cavity length noise worse.
    • Other possibilities?

The problem is hard to debug because we are feeding back on the ETMs, BS and PRM, and at the low CARM offset (= high PRG), all the DoFs are cross coupled strongly so just by looking at error/control signals, I can't directly determine where the noise is originating. The fact that the ALS CARM spectrum shows a noise increase suggests that the problem has to do with the test masses, PSL, IMC, or end green PDH setups.

My plan is to do a systematic campaign and eliminate some of these possibilities - e.g. install some baffling around the fiber coupler and the end green PDH photodiodes and see if there is any improvement in the situation.

* In attachment #1, the "Ref" traces are when the CARM offset is 0, and the arms are buzzing in and out of resonance. The non-reference traces are for when the CARM offset is ~28kHz (so several linewidths away from resonance).

Attachment 1: ALSnoiseIncrease.pdf
ALSnoiseIncrease.pdf
  15046   Mon Nov 25 19:11:22 2019 gautamUpdateLSCALS noise re-look

I re-checked the ALS noise in the following configurations:

  • PRM is misaligned.
  • Michelson is not locked.
  • TRX/TRY is maintained at ~1.
  1. Arm lengths are controlled using POX/POY as a sensor, and the ETMs as actuators [orange traces in Attachment #1].
    • EX laser frequency is locked to the arm cavity length using the end PDH servo.
    • ALS beat note frequency fluctuations are read out using the calibrated DFD channels.
    • In this config, the DFD outputs are the out-of-loop sensor.
  2. Arm lengths are controlled using the ALS beat frequencies as a sensors [blue traces in Attachment #1]
    • The control is no longer in the XARM/YARM basis, but in the CARM/DARM basis.
    • The CARM actuator is MC2, the DARM actuator is an admixture of the ETMs (equal magnitude of output matrix element, opposite sign).
    • The calibrated POX/POY photodiodes are used as the out-of-loop sensor in this config.

The RMS noise sensed by POX/POY is ~20pm, which is somewhat higher than the best I've seen (maybe the arms are moving more at the time of measurement or the AUX PDH loops need a bit of touching up). But the orange traces in the top row of Attachment #1 are already ~x2 better than the equivalent traces from when we were using the green beams to make the beats. So it's hard to explain the 0-300 fluctuations in the arm powers when the CARM offset is reduced to 0 - i.e. the ALS noise is becoming worse as I reduce the CARM offset (= have more circulating power compared to the conditions of this test). I assume the transmission is Lorentzian, in which case even if we have 5x the CARM linewidth worth of ALS noise, we should see the arm power fluctuate between ~10 and 300. 

* I notice that a big jump in the RMS sensed by POX/POY comes from the 24 Hz peak, which is presumably the Roll mode coupling to length - maybe a ResG can make the situation better. The high frequency noise can also be probably rolled off better.

Attachment 1: ALSnoise.pdf
ALSnoise.pdf
  15049   Tue Nov 26 17:07:41 2019 gautamUpdateLSCPOX / POY calibration

Summary:

Since we are using the POX and POY photodiodes as out-of-loop sensors for measuring the ALS noise, I decided to double-check their calibrations. I determined the following numbers (for the single arm lock):

POX_I [with 30dB whitening gain]: (8 +/- 1)e-13 m/ct

POY_I [with 18dB whitening gain]: (0.9 +/- 0.1)e-13 m/ct

With this calibration, I measured the in-loop spectra of the XARM and YARM error-points when they are locked - they line up well, see Attachment #1. Note that these numbers are close to what we determined some time ago using the same method (I drove the ITMs then, but yesterday I drove the ETMs, so maybe the more accurate measure of uncertainty is the difference between the two measurements).

Attachment #2 shows the out-of-loop spectra sensed by these photodiodes with this calibration applied, when the arms are under control using ALS beat frequencies as the error signals, and controlled in the CARM/DARM basis. Need to think about why there is such a difference between the two signals.

Methodology:

The procedure used was the same as that outlined here.

  • I started by calibrating the AS55_Q output with the free-swinging Michelson.
  • Next, I lock the Michelson and calibrate the BS and ITM actuators using the newly calibrated AS55_Q.
  • Next, I calibrate the ETM actuator gains by measuring the ratio of response in POX/POY of driving the (unknown) ETMs and the (known) ITMs.
  • Finally, I calibrate the POX/POY photodiodes by driving the ETMs by a known amount of meters (at ~310 Hz where the loop gain is negligible because of the sensing matrix measurement notches).

Summary of DC actuator gains:

Optic Series resistance [ohms] x3 Analog gain? x3 Digital gain? DC gain [nm/ct]
BS 100 No Yes 9.48 +/- 0.01
ITMX 400 No Yes 2.42 +/- 0.01
ITMY 400 No Yes 2.41 +/- 0.01
ETMX 2.2k Yes No 1.23 +/- 0.02
ETMY 400 Yes No 6.62 +/- 0.12

The quoted values of the DC gain are for counts seen at the output of the LSC filter bank. I've attempted to show that once we account for the different series resistance and some extra gains between the output of the LSC filter bank and the actual coil, things are fairly consistent.

Some remarks:

  • I do not understand why we need an extra 12dB of whitening gain on the POX channel to get similar PDH fringe height as the POY channel. The light level on these photodiodes is the same, and the RF transimpedances at 11 MHz are also close according to the wiki (3kohm for POX, 2kohm for POY).
  • At night-time, the ALS noise did indeed get reduced compared to what I measured earlier in the evening.
  • Even assuming 50% error in the calibration factors, it's hard to explain the swing of TRX/TRY when the CARM offset is brought to zero.
  • The increase in (admittedly in-loop) CARM noise as the offset is reduced still seems to me to be correlated with the buildup of IR power in the arm cavities.
Attachment 1: POX_POY_sensorNoise.pdf
POX_POY_sensorNoise.pdf
Attachment 2: ALSnoise_20191125.pdf
ALSnoise_20191125.pdf
  15051   Wed Nov 27 12:16:52 2019 gautamUpdateLSCITMX and ITMY OSEMs with low and high circulating power

Summary:

The ITMX OSEMs report elevated noise in the 10-100 Hz band when we have high circulating power in the arm cavities, see Attachment #1. Since there is no LSC actuation on the ITMs in this state, this could be a radiation presssure effect, or could be scattered 1064nm light entering the OSEMs. The Oplevs don't report any elevated noise however. ITMY has the OSEM whitening broken for two channels, but the other two channels don't report as significant an increase as ITMX, see Attachment #2. I can't find the status of which OSEMs have the 1064nm blocking filters installed. The local damping loops are rolled off by ~100dB at 30 Hz, so the sensing noise re-injection should be attenuated by this factor, so maybe the OSEM sensor noise isn't the likely culprit. But radiation pressure didn't worsen the length noise in the past, even after our mirror cleaning and the increased PRG.

Quote:

...maybe the opto-mechanical CARM plant is changing as a function of the CARM offset...

Attachment 1: ITMXshadowSensors.pdf
ITMXshadowSensors.pdf
Attachment 2: ITMYshadowSensors.pdf
ITMYshadowSensors.pdf
  15053   Wed Nov 27 16:10:29 2019 gautamUpdateLSCAOM reconnected

i reconnected the AOM driver to the AOM in the main beam path (it was hijacked for the AOM in the AUX laser path for Anjali's MZ experiment). I also temporarily hooked up the AOM to a CDS channel to facilitate some swept-sine measurements. This was later disconnected. The swept sine will need some hardware to convert the bipolar drive signal from the CDS system to the unipolar input that the AOM driver wants (DTT swept sine wont let me set an offset for the excitation, although awggui can do this).

Quote:

if the RP don't fit

u must acquit

sweep the laser amplitude

to divine the couplin w certitude

  15054   Wed Nov 27 17:51:52 2019 gautamUpdateWienerMCL FF status

The old MCL filters are not completely useless - I find a factor of ~2 reduction in the MCL RMS when I turn the FF on. It'd be interesting to see how effective the FF is during the periods of enhanced seismic activity we see. I also wonder if this means the old PRC angular FF filters are also working, it'd help locking, tbc with PRMI carrrier...

Update: The PRC angular FF loops also do some good it seems - though the PIT loop probably needs some retuning.

Attachment 1: MCL_FF.pdf
MCL_FF.pdf
Attachment 2: PRC_FF.pdf
PRC_FF.pdf
  15056   Wed Nov 27 23:24:01 2019 gautamUpdateLSCNo shaking but no inspiration either

Summary:

I tried the following minor changes to the locking procedure to see if there were any differences in the ALS noise performance:

  1. Actuate DARM only on one ETM (tried both ETMX and ETMY)
  2. Enable MCL and PRC seismic feedforward
  3. DC couple the ITM Oplevs for better angular stability during the lock acquisition

None of these changes had any effect - the ALS noise still goes up with arm buildup. 

I think a good way to determine if the problem is to do with the IR part of the new ALS system is to resurrect the green beat setup - I expect this to be less invasive than installing attenuators/beam dumps in front of the fiber couplers at the ends. We should at the very least recover the old ALS noise levels and we were able to lock the PRFPMI with that config. If the excess noise persists, we can rule out the problem being IR scatter into the beat-mouth fibers. Does this sound like a reasonable plan?

  15058   Mon Dec 2 00:27:20 2019 gautamUpdateALSGreen ALS resurrection

Attachment #1 - comparison of phase tracker servo angle fluctuations for the green beat vs IR beat.

  • To convert to Hz, I used the PT servo calibration detailed here.
  • This is only a function of the delay line length and not the signal strength, so shouldn't be affected by the difference in signal strength between the IR and green beats.
  • For the green beat - I divided the measured spectra by 2 to convert the green beat frequency fluctuations into equivalent IR frequency fluctuations.
  • There is no whitening before digitization. I believe the measured spectra are dominated by ADC noise above ~50 Hz. See this elog for the frequency discriminant as a funtion of signal strength, so 5uV/rtHz ADC noise would be ~2 Hz/rtHz for a -5dBm signal, which is what I expect for the Y beat, and ~0.5 Hz/rtHz for a +5dBm signal, which is what I expect for the X beat. Hence the brown (Green beat, XARM) being lower than the green trace (IR beat, XARM) isn't real, it is just because of my division of 2. So I guess that calibration factor I applied is misleading.
  • I did not yet check the noise in the other configuration - arm lengths controlled using ALS, and POX/POY as the OOL sensors. To be tried tonight.

Attachment #2 - RIN of the DCPDs.

  • I noticed that over 10s of seconds, the GTRY level was fluctuating by ~5%. 
  • This was much more than any drift seen in the GTRX level.
  • Measuring the RIN on the DCPDs (Thorlabs PDA36A) supports this observation (spectra were divided by DC value to convert into RIN units).
  • There is ~120uW (1.6 VDC, compatible with 30dB gain setting) incident on the GTRX PD, and ~6uW (170 mVDC, compatible with 40dB gain setting) incident on the GTRY PD.
  • Not sure what is driving this drift - I don't see any coherence with the IR TRY signal, so doesn't seem like it's the cavity.

Characterization of the green beat setup [past numbers]:

  • With some patient alignment effort (usual near-field/far-field matching), I was able to recover the green beat signals.
  • Overall, the numbers I measured today are consistent with what was seen in the past when we had the ability to lock using green ALS.
  • The mode-matching between the PSL and AUX green beams are still pretty abysmal, ~40-50%. The mode shapes are clearly different, but for now, I don't worry about this.
  • I saw some strong AM of the beat signal (for both EX and EY beats) while I was looking at it on a scope, see Attachment #3. This AM is not visible in the IR beat, not sure what to make of it. The frequency of the AM is ~1 MHz, but it's hard to nail this down because the scope doesn't have a very long buffer, and I didn't look at the frequency content on the Agilent (yet).

o BBPD DC output (mV), all measured with Fluke DMM

             XARM   YARM 
 V_DARK:     +1.0    +2.0
 V_PSL:      +8.0    +13.0
 V_ARM:      +157.0  +8.0


o BBPD DC photocurrent (uA)
I_DC = V_DC / R_DC ... R_DC: DC transimpedance (2kOhm)
 I_PSL:       3.5    5.5
 I_ARM:      78.0    3.0


o Expected beat note amplitude
I_beat_full = I1 + I2 + 2 sqrt(e I1 I2) cos(w t) ... e: mode overlap (in power)
I_beat_RF = 2 sqrt(e I1 I2)

V_RF = 2 R sqrt(e I1 I2) ... R: RF transimpedance (2kOhm)

P_RF = V_RF^2/2/50 [Watt]
     = 10 log10(V_RF^2/2/50*1000) [dBm]

     = 10 log10(e I1 I2) + 82.0412 [dBm]
     = 10 log10(e) +10 log10(I1 I2) + 82.0412 [dBm]

for e=1, the expected RF power at the PDs [dBm]
 P_RF:      -13.6  -25.8


o Measured beat note power (measured with oscilloscope, 50 ohm input impedance)      
 P_RF:      -17.95dBm (80 mVpp)  -28.4dBm (24mVpp)   (40MHz and 42MHz)  
    e:        37%                    55  [%]                                             

I also measured the various green powers with the Ophir power meter (filter off): 

o Green light power (uW) [measured just before PD, does not consider reflection off the PD]
 P_PSL:       18    24
 P_ARM:       400     13

The IR beat is not being made at the moment because I blocked the PSL beam entering the fiber.

Attachment 1: ALSnoiseComparison.pdf
ALSnoiseComparison.pdf
Attachment 2: ALS_TR_RIN.pdf
ALS_TR_RIN.pdf
Attachment 3: GreemAM.pdf
GreemAM.pdf
  15059   Mon Dec 2 18:20:29 2019 gautamUpdateALSEY uPDH post mixer LPF

As part of characterization, I wanted to calibrate the EY uPDH error point monitor into units of Hz. So I thought I'd measure the PDH horn-to-horn voltage with the cable to the laser PZT disconnected. However, I saw no clean PDH fringe while monitoring the signal after the LPF that is immediately downstream of the mixer IF output. I then decided to measure the low pass filter OLTF, and found that it seems to have some complex poles (f0~57kHz, Q~5), that amplify the signal by ~x6 relative to the DC level before beginning to roll-off (see Attachment #1). Is this the desired filter shape? Can't find anything in the elog/wiki about such a filter shape being implemented...

The actual OLTF looks alright to me though, see Attachment #2.

Attachment 1: EY_uPDH_LPF.pdf
EY_uPDH_LPF.pdf
Attachment 2: EY_uPDH_OLTF.pdf
EY_uPDH_OLTF.pdf
  15061   Mon Dec 2 23:01:47 2019 gautamUpdateCDSFrequent DTT crashes on pianosa

I have been experiencing frequent crashes of DTT on pianosa in the past few weeks. This is pretty annoying to deal with when trying to characterize the interferometer loops. I attach the error log dumped to console. The error has to do with some kind of memory corruption. Recall that we aren't using a GDS version that is packaged with the SL7 lscsoft packages, we are using a pretty ancient (2.15) version that is built from source. I have been unable to build a newer version from source (though I didn't spend much time trying). pianosa is the only usable workstation at the moment, but perhaps someone can make this work on donatella / rossa for general improvement in quality of life.

Attachment 1: DTTerrorLog.tgz
  15062   Tue Dec 3 00:03:57 2019 gautamUpdateLSCGreen ALS also shows elevated noise with high arm buildup

Summary:

  1. While noisier, I was able to control the arm lengths to ~30pm RMS(!) using the green ALS beats as error signals (cf. ~10 pm RMS with the IR ALS system).
  2. The PRMI could be locked with a CARM offset applied.
  3. When lowering the CARM offset, I saw an increase in the in-loop ALS error signal, just as I had with the IR beat.
  4. IR TRX / TRY unsurprisingly did not stabilize in any meaningful way.cool
  5. The noise increase seems to have some periodicity along the frequency axis - need to think about what this means.
  6. Since there is no apparent benefit to using the green ALS beats, I restored the IR system. The green PDs should still retain somewhat good alignment if one wishes to do a comparison measurement.
  7. While the shadow sensors of the ITMs report elevated noise, it is unlikely to be responsible for the cavity moving by the amount suggested by the elevated ALS error signals because of the digital low-pass filtering and 1/f^2 of the pendulum.
  8. I confirmed that the ITM shadow sensors do not report elevated noise when the PRMI is locked such that the carrier is resonant. In this config, there is comparable circulating power in the PRC as to when the CARM offset is reduced to ~0.
  9. The fact that the IR and green beats both show similar increase in noise suggestes that the cavity length / laser frequency is in fact being modulated, but I still don't know what the exact mechanism is.

was worth a shot i guess.

Trawling through some elogs, I see that this kind of feature showing up in the ALS CARM is not a new problem, see for example here. But I can't find out what the resolution was.

Attachment 1: ALSnoiseIncrease_greenBeat.pdf
ALSnoiseIncrease_greenBeat.pdf
  15064   Tue Dec 3 00:51:25 2019 gautamUpdateALSEY uPDH post mixer LPF

I'm not sure - maybe it was measurement error on my part, I will double check. Moreover, the EX and EY boxes don't seem to use identical designs, if one believes the schematics drawn on the Pomona boxes. The EY design has a 50ohm input impedance in the stopband, whereas the EX doesn't. Maybe the latter needs a Tee + 50ohm terminator at the input?

Judging by the schematics, the servo inputs to both boxes are driving the non-inverting input of an opamp, so they see high-Z.

Quote:

I got confused. Why don't we see that too-high-Q pole in the OLTF?

  15066   Tue Dec 3 18:15:42 2019 gautamUpdateALSEY uPDH post mixer LPF

Rana and I discussed this alogrythym a bit today - here are some bullet points, I'll work on preparing a notebook. We are still talking about a post-mixer low pass filter.

  • We want to filter out the 2f component - attenuation relative to the 1f content and be well below the slew-rate of the first post-mixer opamp (OP27).
  • We don't want to lose much phase due to the corner of the LPF, so that we can have a somewhat high UGF - let's shoot for 30kHz.
  • What should the order of the filter be such that we achieve these goals?
  • We will use a numerical optimization routine, that makes a filter that has
    • yy dB attenuation at high frequencies
    • sufficient stability margin
    • sufficiently small phase lag at 30 kHz so that we can realize ~30kHz UGF with the existing servo electronics.
Quote:

                   filter Q seems too high,

but what precisely is the proper way to design the IF filter?

   seems like we should be able to do it using math instead of feelins

                              Izumi made this one so maybe he has an algorythym

  15068   Tue Dec 3 21:28:24 2019 gautamUpdateALSEY uPDH post mixer LPF

Here are some loop transfer functions. I basically followed the decomposition of the end PDH loop as was done in the multi-color metrology paper. There is no post-mixer low pass filter at the moment (in my model), but already you can see that the top of the phase bubble is at ~10 kHz. Probably there is still sufficient phase available at 30 kHz, even after we add an LPF. In any case, I'll use this model and set up a cost function minimization problem and see what comes out of it. For the PZT discriminant, I used 5 MHz/V, and for the PDH discriminant, I used 40 uV/Hz, which are numbers that should be close to what's the reality at EY.

(i) Note that there could be some uncertainty in the overall gain (VGA stage in the servo).

(ii) For the cavity pole, I assumed the single pole response, which Rana points out isn't really valid at ~1 MHz, which is close to the next FSR

(ii) The PZT response is approximated as a simple LPF whereas there are likely to be several sharp features which may add/eat phase. 

Quote:

 I'll work on preparing a notebook.

Attachment 1: uPDH.pdf
uPDH.pdf
  15072   Wed Dec 4 12:13:10 2019 gautamUpdateCDSReboot script

It was way more annoying without a script and took longer than the 4 minutes it does now.

You can fix the requirement to enter password by changing the sshd settings on the FEs like I did for pianosa.

After running the script, you should verify that there are no red flags in the output to console. Yesterday, some of the settings the script was supposed to reset weren't correctly reset, possibly due to python/EPICS problems on donatella, and this cost me an hour of searching last night because the locking wasn't working. Anyway, best practise is to not crash the FEs.

Quote:

The script is a bit annoying in that it requires entering the CDSs' passwords multiple times over the time it runs which is long.

  15073   Wed Dec 4 19:54:27 2019 gautamUpdateLSCA look to the past

Trawling through some past elogs, I saw that the ALS noise increase as a function of CARM offset reduction is not really a new thing (see e.g. this elog). In the past, when we were able to lock, when the CARM offset is reduced to zero, the arms would "buzz" through resonance. It just wasn't clear to me how much the buzzing was - in all the plots we presented, we were not looking at the fast 16k output, so it looked like the arm powers had stabilized. But today, looking at the frame data at 16k from back in 2016, it is clear to me that the arm transmission was in fact swinging all the way from 0 to some maximum. Once the IR signal (=REFL11) blending is turned on, we were able to stabilize the arm power somewhat. What this means is that we are in a comparable state as to when we were able to lock in the past (since I'm able to sit at 0 CARM offset with the PRMI locked almost indefinitely).

So, I think what I'll try for the next 3 days is to get this blending going, I think I couldn't enable the CM_slow path because when I was experimenting with the high bandwidth Y arm cavity locking, I had increased the whitening gain of this channel, but REFL11 has much more optical gain (=larger signal) than POY11, and so I'll start from 0dB whitening gain and see if I can turn the magic integrator on. Long term, we should try and compensate the optomechanical plant that changes as our CARM offset gets reduced, as this would further reduce the lock acquisition time and simplify the procedure (no need to fiddle with the integrator, offsets etc). A relevant thread from the past.

Attachment 1: DRFPMI_2016March.pdf
DRFPMI_2016March.pdf
  15074   Wed Dec 4 20:32:43 2019 gautamUpdateGeneralPLL for PM measurement

Were some cables from the ALS beat setup modified? I can't see the beat on the scope, and this elog doesn't say anything about cable connection rearrangement. At ~2311, I am reverting the setup to as it should be.

  15075   Thu Dec 5 01:54:39 2019 gautamUpdateLSCPartial CM board path engaged
  • The arm powers could be stabilized somewhat once the CM_SLOW path to MC2 was engaged.
  • However, I was never able to get the AO path to do anything good.
  • Took a bunch of CM board TFs, need to think about what I need to do differently to get this next bit to work.
  • An SR785 is sitting next to the LSC rack hooked up to the CM board. I also borrowed the GPIB unit from the AG4395 to grab data from said SR785.
  • One thing I noticed that the CARM_B (=CM_SLOW) and DARM_B (=AS55_Q) signals both had a DC offset, so maybe this is indicative of some DC offset in the PRMI 3f signals? Right now, I lock the PRMI without any offsets, and as I reduce the CARM offset, I can see the DC value of REFL11_I and AS55_Q changing significantly. To be investigated in tonight's locking.
Attachment 1: AOengaged.pdf
AOengaged.pdf
  15077   Thu Dec 5 14:54:15 2019 gautamUpdateGeneralSymlink to SRmeasure and AGmeasure

I symlinked the SRmeasure and AGmeasure commands to /usr/bin/ on donatella (as it is done on pianosa) so that these scripts are in $PATH and may be run without having to navigate to the labutils directory.

  15078   Thu Dec 5 15:09:50 2019 gautamUpdateCDSc1oaf crashed c1lsc

I tried starting the c1oaf model, but got a DQ error (I want the option of running feedforward during locking even if the filters aren't particularly well tuned yet). Note that this isn't "just a warning light" - some channels are initialized to +/- 1e20, so if you try turning some filters on, you will deliver a massive kick to the optics. Restarting it crashed c1lsc (this is not unexpected behavior - the only way to clear the DQ error is to restart the model, and empirically, the success rate is ~50%). The reboot script brought everything back online smoothly, and the second, time, c1oaf started without any issues.

While looking at the CDS overview screen, I noticed that the c1scy model was reporting frequent RFM errors for the C1:SCY-RFM_ETMY_LSC channel (but none of the others). On the sender model (c1rfm), no errors were being reported. The diag reset button / mxstream restart didn't really work either. See Attachment #1. Just restarting the c1scy model didn't fix the error - I had to reboot the machine and restart the models, and now no errors are being reported.

Attachment #2 shows the current nominal CDS status - the red light on c1lsc is due to some missing c1dnn channels (I'll remove these at the next c1lsc model change because I don't want to un-necessarily reboot the vertex FEs), and the c1omc model is obsolete I guess. c1daf isn't running right now but once I get the new fiber (ordered), I'm gonna restart this model as well.

P.S. The ALS temperature sliders are not SDF-ed. So when the model was restarted, I had to change the sliders back to their old values to get the beat back in the usable range.

Attachment 1: SCYerrors.png
SCYerrors.png
Attachment 2: CDSnormal.png
CDSnormal.png
  15079   Thu Dec 5 18:15:01 2019 gautamUpdateOptical LeversITM, PRM and BS Oplevs re-centered

In preparation for locking tonight, I re-centered the spots on the Oplev QPDs for the ITMs, BS and PRM after locking and running the dither alignment for the arms and also the PRMI carrier. In the past, DC coupling the ITM Oplevs helped the angular stability a bit, let's see if it still does.

  15080   Fri Dec 6 00:02:48 2019 gautamUpdateLSCWhat is the correct way to set the 3f offsets?

Summary:

I made it to 0 CARM offset, PRMI locked a bunch of times today. However, I could not successfully engage the AO path.

Details:

Much of the procedure is scripted, here is the rough set of steps:

  • Transition control of the arms from IR signals to ALS signals.
  • DC couple the ITM oplev servos
  • Burt-restore the settings for PRMI locking with REFL165I-->PRCL, REFL165Q-->MICH, and then enable the MICH_B / PRCL_B locking servos.
  • Add some POPDC to the PRMI triggering (nominally only POP22_I) to let these loops be locked while POP22_I fluctuates wildly when we are near the CARM=0 point.
  • Zero the CARM offset.
  • Adjust the CARM_A/DARM_A offsets such that CARM_B/DARM_B are fluctuating symmetrically about 0.
  • CARM_B gain --> 1.0, to begin the RF blend.
  • Prepare to hand the DC control authority to ALS by turning off FM1 in the CARM filter bank, and turning ON an integrator in the CARM_B filter.

As I type this out, I realized that I was incorrectly setting offsets to maximize the arm powers by adjusting CARM/DARM offsets as opposed to CARM_A / DARM_A offsets. Tried another round of locking, but this time, I can't even turn the integrator on to get the arms to click into somewhat stable powers. 

One thing I noticed is that depending on the offsets I put into the 3f locking loops, the mean value of REFL11 and AS55 when the ALS CARM/DARM offsets are zeroed changes quite significantly. What is the correct condition to set these offsets? They are different when locking the PRC without arm cavities, and also seem to change continuously with CARM offset. I am wondering if I have too much offset in one of the vertex locking loops?

  15081   Fri Dec 6 15:22:01 2019 gautamFrogsLSCDAFI system revived

[Jordan, gautam]

We did the following:

  • Route the fiber from the control room to 1Y2.
  • Plug fiber in to FiBox at either end, turned FiBoxes ON.
  • Tested the optical connection by driving a 1Vpp 440 Hz sine wave from a function generator - Yehonathan hears it loud and clear in the control room.
  • Tested that both CH1 and CH2 work - only CH1 is connected to the speakers in the control room at the moment.
  • There is some cross-coupling between the channels - not sure if this is happening in the multi-mode fiber or in the electroncis, but I estimate the isolation to be >30dB.
  • Connected CH8 and CH9 of DAC0 in the c1lsc expansion chassis to CH1 and CH2 respectively of the FiBox in 1Y2. 
  • Restarted the c1daf model on c1lsc, came up smooth.
  • Routed the POY11 error signal through the various matrices in c1daf, and we could 👂 the Y-arm cavity 🔐 😎 
  • Channels are muted for now - I'll give this a whirl while doing the PRFPMI locking.
  15107   Tue Dec 31 03:03:02 2019 gautamUpdatePSLPMC cavity ringdown measurement

When I was looking at this, the AOM shutdown time was measured to be ~120 ns, and while I wasn't able to do a ringdown measurement with the PMC (it'd just stay locked because at the time i was using the zeroth order beam), the PMC transmission decayed in <200 ns. 

  15108   Wed Jan 1 04:53:11 2020 gautamUpdatePSLMapping the PSL electronics

For the IMC servo board, it'd be easiest to copy the wiring scheme for the BIO bits as is configured for the CM board (i.e. copy the grouping of the BIO bits on the individual Acromag units). This will enable us to use the latch code with minimal modifications (it was a pain to debug this the first time around). I don't see any major constraint in the wiring assignment that'd make this difficult.

Quote:

PSL wiring spreadsheet is ready. (But the link was stripped. Koji)

Link to a wiki page  with the link to the wiring spreadsheet (Yehonathan)

  15112   Mon Jan 6 16:07:12 2020 gautamUpdatePSLAssembly underway for c1psl upgrade

RTFE. Where did the spares go?

Quote:

I began setting up the host server, but immediately hit a problem: We seem to have no more memory cards or solid-state drives, despite having two more SuperMicro servers. I ordered enough RAM cards and drives to finish both machines. They will hopefully arrive tomorrow.

  15121   Tue Jan 14 20:17:09 2020 gautamSummaryGeneralIFO recovery

Summary:

There was no light entering the IFO. I worked on a few things to bring the interferometer to a somewhat usable state. The goal is to get back to PRFPMI locking ASAP.

Details:

Problem: All fast models report a "0x4000" DC error. See Attachment #1.

Solution: I think this is a "known" issue that happened last new year too. The fix was to add a hard-coded 1 second offset to the daqd config files. However, incrementing/decreasing this offset by +/- 1 second did not fix the errors for me today. I'll reach out to JH for more troubleshooting tips.

Update 15 Jan 2020 830am: The problem is now fixed. See here.

Problem: c1susaux and c1auxey were unresponsive.

Solution: Keyed c1auxey. Rebooted c1susaux and as usual, manually started the eth0/eth1 subnets. The Acromag crate did not have to be power-cycled. ITMY got stuck in this process - I released it using the usual bias jiggling. Why did c1susaux fail? When did it fail? Was there some un-elogged cable jiggling in that part of the lab?

Problem: IMC autolocker and FSS slow processes aren't running on megatron after the upgrade.

Solution: Since no one bothered to do this, I setup systemd infrastructure for doing this on megatron. To run these, you do:

sudo systemctl start MCautolocker.service
sudo systemctl start FSSSlow.service

and to check their status, use:

sudo systemctl status MCautolocker.service
sudo systemctl status FSSSlow.service

The systemd setup is currently done in a naive way (using the bash executable to run a series of commands rather than using the systemd infrastructure itself to setup variables etc) but it works. I confirmed that the autolocker can re-acquire IMC lock, and that the FSS loop only runs when the IMC is locked. I also removed the obsolete messages printed to megatron's console (by editing /etc/motd) on ssh-login, advising the usage of initctl - the updated message reflects the above instructions.

In order to do the IMC locking, I changed the DC voltage to the AOM to +1V DC (it was +0.8 V DC). In this setting, the IMC refl level is ~3.6 V DC. When using the undiffracted AOM beam, we had more like +5.6 V DC (so now we have ~65% of the nominal level) from the IMC REFL PD when the IMC was unlocked. IIRC, the diffraction efficiency of the AOM should be somewhat better, at ~85%. Needs investigation, or better yet, let's just go back to the old configuration of using the undiffracted beam.

There was also an UN-ELOGGED angry change of the nominal value of the PMC servo gain to 12.8, and no transfer function measurement. There needs to be a proper characterization of this loop done to decide what the new nominal value should be.

I'm going to leave the PSL shutter open and let the IMC stay locked for stability investigations. Tomorrow, I'll check the single-arm locking and the ALS system.

Attachment 1: DCerrors.png
DCerrors.png
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