40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
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ID Date Authorup Type Category Subject
  14422   Tue Jan 29 22:12:40 2019 gautamUpdateSUSAlignment prep

Since we may want to close up tomorrow, I did the following prep work:

  1. Cleaned up Y-end suspension eleoctronics setup, connected the Sat Box back to the flange
    • The OSEMs are just sitting on the table right now, so they are just seeing the fully open voltage
    • Post filter insertion, the four face OSEMs report ~3-4% lower open-voltage values compared to before, which is compatible with the transmission spec for the filters (T>95%)
    • The side OSEM is reporting ~10% lower - perhaps I just didn't put the filter on right, something to be looked at inside the chamber
  2. Suspension watchdog restoration
    • I'd shutdown all the watchdogs during the Satellite box debacle
    • However, I left ITMY, ETMY and SRM tripped as these optics are EQ-stopped / don't have the OSEMs inserted.
  3. Checked IMC alignment
    • After some hand-alignment of the IMC, it was locked, transmission is ~1200 counts which is what I remember it being
  4. Checked X-arm alignment
    • Strictly speaking, this has to be done after setting the Y-arm alignment as that dictates the input pointing of the IMC transmission to the IFO, but I decided to have a quick look nevertheless
    • Surprisingly, ITMX damping isn't working very well it seems - the optic is clearly swinging around a lot, and the shadow sensor RMS voltage is ~10s of mV, whereas for all the other optics, it is ~1mV.
    • I'll try the usual cable squishing voodoo

Rather than try and rush and close up tomorrow, I propose spending the day tomorrow cleaning the peripheral areas of the optic, suspension cage, and chamber. Then on Thursday morning, we can replace the Y-arm optics, try and recover the cavity alignment, and then aim for a Thursday afternoon pumpdown. The main motivation is to reduce the time the optics spend in air after F.C. peeling and going to vacuum.

  14423   Wed Jan 30 11:54:24 2019 gautamUpdateSUSMore alignment prep

[chub, gautam]

  1. ETMY cage was wiped down
    • Targeted potential areas where dust could drift off from and get attracted to a charged HR surface
    • These areas were surprisingly dusty, even left a grey mark on the wipe [Attachment #1] - we think we did a sufficiently thorough job, but unclear if this helps the loss numbers
    • More pictures are on gPhoto
  2. Filters on SD and LR OSEMs were replaced - the open shadow sensor voltages with filters in/out are consistent with the T>95% coating spec.
  3. IPANG beam position was checked 
    • It is already too high, missing the first steering optic by ~0.5 inch, not the greatest photo but conclusion holds [Attachment #2].
    • I think we shouldn't worry about it for this pumpdown, we can fix it when we put in the new PR3.
  4. Cage wiping procedure was repeated on ITMY
    • The cage was much dustier than ETMY
    • However, the optic itself (barrel and edge of HR face) was cleaner
    • All accessible areas were wiped with isopropanol
    • Before/after pics are on gPhoto (even after cleaning, there are some marks on the suspension that looks like dust, but these are machining marks)

Procedure tomorrow [comments / suggestions welcome]:

  1. Start with IY chamber
    • Peel first contact with TopGun jet flowing
    • Inspect optic face with green flashlight to check for residual First Contact
    • Replace ITMY suspension cage in its position, clamp it down
    • Release ITMY from its EQ stops
    • Replace OSEMs in ITMY cage, best effort to recover previous alignment of OSEMs in their holders (I have a photo before removal of OSEMs), which supposedly minimized the coupling of the B-R modes into the shadow sensor signals
    • Best effort to have shadow sensor PD outputs at half their fully open voltages (with DC bias voltage applied)
    • Quick check that we are hitting the center of the ITM with the alignment tool
    • Check that the Oplev HeNe is reasonably centered on steering mirrors
    • Tie down OSEM cabling to the ITMY cage with clean copper wire
    • Replace the OSEM wiring tower
    • Release the SRM from its EQ stops
    • Check table leveling
    • Take pictures of everything, check that we have not left any tools inside the chamber
    • Heavy doors on
  2. Next, EY chamber
    • Repeat first seven bullets from the IY chamber, :%s/ITMY/ETMY/g
    • Confirm sufficient clearance between IFO beam axis and the elliptical reflector
    • Check Oplev beam path
    • Check table leveling
    • Take pictures of everything, check that we have not left any tools inside the chamber
    • Heavy doors on
  3. IFO alignment checks - basically follow the wiki, we want to be able to lock both arms (or at least see TEM00 resonances), and see that the PRC and SRC mode flashes look reasonable.
  4. Tighten all heavy doors up
  5. Pump down

All photos have been uploaded to google photos.

  14424   Wed Jan 30 19:25:40 2019 gautamUpdateSUSXarm cavity alignment

Squishing cables at the ITMX satellite box seems to have fixed the wandering ITM that I observed yesterday - the sooner we are rid of these evil connectors the better.

I had changed the input pointing of the green injection from EX to mark a "good" alignment of the cavity axis, so I used the green beam to try and recover the X arm alignment. After some tweaking of the ITM and ETM angle bias voltages, I was able to get good GTRX values [Attachment #1], and also see clear evidence of (admittedly weak) IR resonances in TRX [Attachment #2]. I can't see the reflection from ITMX on the AS camera, but I suspect this is because the ITMY cage is in the way. This will likely have to be redone tomorrow after setting the input pointing for the Y arm cavity axis, but hopefully things will converge faster and we can close up sooner. Closing the PSL shutter for now...

I also rebooted the unresponsive c1susaux to facilitate the alignment work tomorrow.

  14425   Fri Feb 1 01:24:06 2019 gautamUpdateSUSAlmost ready for pumpdown tomorrow

[koji, chub, jon, rana, gautam]

Full story tomorrow, but we went through most of the required pre close-up checks/tasks (i.e. both arms were locked, PRC and SRC cavity flashes were observed). Tomorrow, it remains to 

  1. Confirm clearance between elliptical reflector and ETMY
  2. Confirm leveling of ETMY table
  3. Take pics of ETMY table
  4. Put heavy door on ETMY chamber
  5. Pump down

The ETMY suspension chain needs to be re-characterized (neither the old settings, nor a +/- 1 gain setting worked well for us tonight), but this can be done once we are back under vacuum.

  14426   Fri Feb 1 13:16:50 2019 gautamUpdateSUSPumpdown 83 underway

[chub, bob, gautam]

  1. Steps described in previous elog were carried out
  2. EY heavy door was put on at about 1130am.
  3. Pumpdown commenced at ~noon. We are going down at ~3 torr/min.
  14427   Fri Feb 1 14:44:14 2019 gautamUpdateSUSY arm FC cleaning and reinstall

[Attachment #1]: ITMY HR face after cleaning. I determined this to be sufficiently clean and re-installed the optic.

[Attachment #2]: ETMY HR face after cleaning. This is what the HR face looks like after 3 rounds of First-Contact application. After the first round, we noticed some arc-shaped lines near the center of the optic's clear aperture. We were worried this was a scratch, but we now believe it to be First-Contact residue, because we were able to remove it after drag wiping with acetone and isopropanol. However, we mistrust the quality of the solvents used - they are not any special dehydrated kind, and we are looking into acquiring some dehydrated solvents for future cleaning efforts.

[Attachment #3]: Top view of ETMY cage meant to show increased clearance between the IFO axis and the elliptical reflector.

Many more photos (including table leveling checks) on the google-photos page for this vent. The estimated time between F.C. peeling and pumpdown is ~24 hours for ITMY and ~15 hours for ETMY, but for the former, the heavy doors were put on ~1 hour after the peeling.

The first task is to fix the damping of ETMY.

  14428   Fri Feb 1 21:52:57 2019 gautamUpdateSUSPumpdown 83 underway

[jon, koji, gautam]

  1. IFO is at ~1 mtorr, but pressure is slowly rising because of outgassing presumably (we valved off the turbos from the main volume)
  2. Everything went smooth -
    • 760 torr to 500 mtorr took ~7 hours (we deliberately kept a slow pump rate)
    • TP3 current was found to rise above 1 A easily as we opened RV2 during the turbo pumping phase, particularly in going from 500 mtorr to 10 mtorr, so we just ran TP2 more aggressively rather than change the interlock condition.
    • The pumpspool is isolated from the main volume - TP1-3 are running (TP2 and TP3 are on Standby mode) but are only exposed to the small pumpspool volume and RGA volume).
    • RP1 and RP3 were turned off, and the manual roughing line was disconnected.
    • We will resume the pumping on Monday.

I'm leaving all suspension watchdogs tripped over the weekend as part of the suspension diagonalization campaign...

  14430   Sun Feb 3 15:15:21 2019 gautamUpdateVACovernight leak rate

I looked into this a bit today. Did a walkthrough of the lab, didn't hear any obvious hissing (makes sense, that presumably would signal a much larger leak rate).

Attachment #1: Data from the 30 ksec we had the main vol valved off on Jan 10, but from the gauges we have running right now (the CC gauges have not had their HV enabled yet so we don't have that readback).

Attachment #2: Data from ~150 ksec from Friday night till now.

Interpretation: The number quoted from Jan 10 is from the cold-cathode gauge (~20 utorr increase). In the same period, the Pirani gauge reports a increase of ~5 mtorr (=250x the number reported by the cold-cathode gauge). So which gauge do we trust in this regime more? Additionally, the rate at which the annuli pressures are increasing seem consistent between Jan 10 and now, at ~100 mtorr every 30 ksec.

I don't think this is conclusive, but at least the leak rates between Jan 10 and now don't seem that different for the annuli pressures. Moreover, for the Jan 10 pumpdown, we had the IFO at low pressure for several days over the chirstmas break, which presumably gave time for some outgassing which was cleaned up by the TPs on Jan 10, whereas for this current pumpdown, we don't have that luxury.

Do we want to do a systematic leak check before resuming the pumpdown on Monday? The main differences in vacuum I can think of are

  1. Two pieces of Kapton tape are now in the EY chamber.
  2. Possible resiudue from cleaning solvents in IY and EY chambers are still outgassing.

This entry by Steve says that the "expected" outgassing rate is 3-5 mtorr per day, which doesn't match either the current observation or that from Jan 10.

  14432   Mon Feb 4 12:23:24 2019 gautamUpdateVACpumpdown 83 - leak tests

[koji, gautam]

As planned, we valved off the main volume and the annuli from the turbo-pumps at ~730 PM PST. At this time, the main volume pressure was 30 uTorr. It started rising at a rate of ~200 uTorr/hr, which translates to ~5 mtorr/day, which is in the ballpark of what Steve said is "normal". However, the calibration of the Hornet gauge seems to be piecewise-linear (see Attachment #1), so we will have to observe overnight to get a better handle on this number.

We decided to vent the IY and EY chamber annular volumes, and check if this made anu dramatic changes in the main volume pressure increase rate, presumably signalling a leak from the outside. However, we saw no such increase - so right now, the working hypothesis is still that the main volume pressure increase is being driven by outgassing of something from the vacuum.

Let's leave things in this state overnight - V1 and V5 closed so that neither the main volume nor the annuli are being pumped, and get some baseline numbers for what the outgassing rate is.

  14433   Mon Feb 4 20:13:39 2019 gautamUpdateSUSETMY suspension oddness

I looked at the free-swinging sensor data from two nights ago, and am struggling with the interpretation. 

[Attachment #1] - Fine resolution spectral densities of the 5 shadow sensor signals (y-axis assumes 1ct ~1um). The puzzling feature is that there are only 3 resonant peaks visible around the 1 Hz region, whereas we would expect 4 (PIT, YAW, POS and SIDE). afaik, Lydia looked into the ETMY suspension diagonalization last, in 2016. Compared to her plots (which are in the Euler basis while mine are in the OSEM basis), the ~0.73 Hz peak is nowhere to be seen. I also think the frequency resolution (<1 mHz) is good enough to be able to resolve two closely spaced peaks, so it looks like due to some reason (mechanical or otherwise), there are only 3 independent modes being sensed around 1 Hz.

[Attachment #2] - Koji arrived and we looked at some transfer functions to see if we could make sense of all this. During this investigation, we also think that the UL coil actuator electronics chain has some problem. This test was done by driving the individual coils and looking for the 1/f^2 pendulum transfer function shape in the Oplev error signals. The ~ 4dB difference between UR/LL and LR is due to a gain imbalance in the coil output filter bank, once we have solved the other problems, we can reset the individual coil balancing using this measurement technique.

[Attachment #3] - Downsampled time-series of the data used to make Attachment #1. The ringdown looks pretty clean, I don't see any evidence of any stuck magnets looking at these signals. The X-axis is in kilo-seconds.

We found that the POS and SIDE local damping loops do not result in instability building up. So one option is to use only Oplevs for angular control, while using shadow-sensor damping for POS and SIDE.

  14434   Tue Feb 5 10:11:30 2019 gautamUpdateVACleak tests complete, pumpdown 83 resumed

I guess we forgot to close V5, so we were indeed pumping on the ITMY and ETMY annuli, but the other three were isolated suggest a leak rate of ~200-300 mtorr/day, see Attachment #1 (consistent with my earlier post).

As for the main volume - according to CC1, the pressure saturates at ~250 uTorr and is stable, while the Pirani P1a reports ~100x that pressure. I guess the cold-cathode gauge is supposed to be more accurate at low pressures, but how well do we believe the calibration on either gauge? Either ways, based on last night's test (see Attachment #2), we can set an upper limit of 12 mtorr/day. This is 2-3x the number Steve said is normal, but perhaps this is down to the fact that the outgassing from the main volume is higher immediately after a vent and in-chamber work. It is also 5x lower rate of pressure increase than what was observed on Feb 2.

I am resuming the pumping down with the turbo-pumps, let's see how long we take to get down to the nominal operating pressure of 8e-6 torr, it ususally takes ~ 1 week. V1, VASV, VASE and VABS were opened at 1030am PST. Per Chub's request (see #14435), I ran RP1 and RP3 for ~30 seconds, he will check if the oil level has changed.


Let's leave things in this state overnight - V1 and V5 closed so that neither the main volume nor the annuli are being pumped, and get some baseline numbers for what the outgassing rate is.

  14436   Tue Feb 5 19:30:14 2019 gautamUpdateVACMain volume at 20 uTorr

Pumpdown looks healthy, so I'm leaving the TPs on overnight. At some point, we should probably get the RGA going again. I don't know that we have a "reference" RGA trace that we can compare the scan to, should check with Steve. The high power (1 W) beam has not yet been sent into the vacuum, we should probably add the interlock condition that shuts off the PSL shutter before that.

  14438   Thu Feb 7 13:55:25 2019 gautamUpdateVACRGA turned on

[chub, steve, gautam]

Steve came by the lab today. He advised us to turn the RGA on again, now that the main volume pressure is < 20 uTorr. I did this by running the RGAset.py script on c0rga - the temperature of the unit was 22C in the morning, after ~3 hours of the filament being turned on, the temperature has already risen to 34 C. Steve says this is normal. We also opened VM1 (I had to edit the interlocks.yaml to allow VM1 to open when CC1 < 20uTorr instead of 10uTorr), so that the RGA volume is exposed to the main volume. So the nightly scans should run now, Steve suggests ignoring the first few while the pumpdown is still reaching nominal pressure. Note that we probably want to migrate all the RGA stuff to the new c1vac machine.

Other notes from Steve:

  • RP1 and RP3 should have their oil fully changed (as opposed to just topped up)
  • VABSSCI adn VABSSCO are NOT vent valves, they are isolating the annuli of the IOO and OMC chambers from the BS chamber annuli. So next time we vent, we should fix this!
  • Leak rate of 3-5 mTorr/day is "normal" once the system has been pumped for a few days. Steve agrees that our observations of the main volume pressure increase is expected, given that we were at atmosphere.
  • Regarding the upcoming CES construction
    • Steve recommends keeping the door along the east arm, as it is useful for bringing equipment into the lab (end door access is limited because of end optical tables)
    • Particle counter data logging should be resumed before the construction starts, so that we can monitor if the lab is getting dirtier
  • OSEM filters (new ones, i.e. made according to spects in D000209) are in the Clean Cabinet (EX). They are individually packaged in little capsules, see Attachment #1. So the ones I installed were actually a 2002 vintage. We have 50pcs, enough to install new ones on all the core optics + spares.
  14440   Thu Feb 7 19:28:46 2019 gautamUpdateVACIFO recovery

[rana, gautam]

The full 1 W is again being sent into the IMC. We have left the PBS+HWP combo installed as Rana pointed out that it is good to have polarization control after the PMC but before the EOM. The G&H mirror setup used to route a pickoff of the post-EOM beam along the east edge of the PSL table to the AUX laser beat setup was deemed too flaky and has been bypassed. Centering on the steering mirror and subsequently the IMC REFL photodiode was done using an IR viewer - this technique allows one to geometrically center the beam on the steering mirror and PD, to the resolution of the eye, whereas the voltage maximization technique using the monitor port and an o'scope doesn't allow the former. Nominal IMC transmission of ~15,000 counts has been recovered, and the IMC REFL level is also around 0.12, consistent with the pre-vent levels.

  14441   Thu Feb 7 19:34:18 2019 gautamUpdateSUSETMY suspension oddness

I did some tests of the electronics chain today.

  1. Drove a sine-wave using awggui to the UL-EXC channel, and monitored using an o'scope and a DB25 breakout board at J1 of the satellite box, with the flange cable disconnected - while driving 3000 cts amplitude signal, I saw a 2 Vpp signal on the scope, which is consistent with expectations.
  2. Checked resistances of the pin pairs corresponding to the OSEMs at the flange end using a breakout board - all 5 pairs read out ~16-17 ohms.
  3. Rana pointed out that the inductance is the unambiguous FoM here: all coils measured between 3.19 and 3.3 mH according to the LCR meter...

Hypothesising a bad connection between the sat box output J1 and the flange connection cable. Indeed, measuring the OSEM inductance from the DSUB end at the coil-driver board, the UL coil pins showed no inductance reading on the LCR meter, whereas the other 4 coils showed numbers between 3.2-3.3 mH. Suspecting the satellite box, I swapped it out for the spare (S/N 100). This seemed to do the trick, all 5 coil channels read out ~3.3 mH on the LCR meter when measured from the Coil driver board end. What's more, the damping behavior seemed more predictable - in fact, Rana found that all the loops were heavily overdamped. For our suspensions, I guess we want the damping to be critically damped - overdamping imparts excess displacement noise to the optic, while underdamping doesn't work either - in past elogs, I've seen a directive to aim for Q~5 for the pendulum resonances, so when someone does a systematic investigation of the suspensions, this will be something to look out for.. These flaky connectors are proving pretty troublesome, let's start testing out some prototype new Sat Boxes with a better connector solution - I think it's equally important to have a properly thought out monitoring connector scheme, so that we don't have to frequently plug-unplug connectors in the main electronics chain, which may lead to wear and tear.

The input and output matrices were reset to their "naive" values - unfortunately, two eigenmodes still seem to be degenerate to within 1 mHz, as can be seen from the below spectra (Attachment #1). Next step is to identify which modes these peaks actually correspond to, but if I can lock the arm cavities in a stable way and run the dither alignment, I may prioritize measurement of the loss. At least all the coils show the expected 1/f**2 response at the Oplev error point now. The coil output filter gains varied by ~ factor of 2 among the 4 coils, but after balancing the gains, they show identical responses in the Oplev - Attachment #2.

  14442   Fri Feb 8 00:20:56 2019 gautamSummaryTip-TIltFive FiveNine Optics Optics delivered

They have been stored on the 3rd shelf from top in the clean optics cabinet at the south end. EX


5 PR3/SR3 optics from FiveNine Optics were delivered. The data sheets were scanned and uploaded to the following wiki page. https://wiki-40m.ligo.caltech.edu/Aux_Optics

  14443   Fri Feb 8 02:00:34 2019 gautamUpdateSUSITMY has tendency of getting stuck

As it turns out, now ITMY has a tendency to get stuck. I found it MUCH more difficult to release the optic using the bias jiggling technique, it took me ~ 2 hours. Best to avoid c1susaux reboots, and if it has to be done, take precautions that were listed for ITMX - better yet, let's swap out the new Acromag chassis ASAP. I will do the arm locking tests tomorrow.

  14444   Fri Feb 8 20:35:57 2019 gautamSummaryTip-TIltCoating spec

[Attachment #1]: Computed spectral power transmissivity (according to my model) for the coating design at a few angles of incidence. Behavior lines up well with what FNO measured, although I get a transmission that is slightly lower than measured at 45 degrees. I suspect this is because of slight changes in the dispersion relation assumed and what was used for the coating in reality.

[Attachment #2]: Similar information as Attachment #1, but with the angle of incidence as the independent parameter in a continuous sweep. 

Conclusion: The coating behaves in a way that is in reasonable agreement with our model. At 41.1 degrees, which is what the PR3 angle of incidence will be, T<50 ppm, which was what we specified. The larger range of angles was included because originally, we thought of using this optic as a substitute for SR3 as well. But I claim that for the shorter SRC (signal recycling as opposed to RSE), we will not end up using the new optic, but rather go for the G&H mirror. In any case, as Koji pointed out, ~50 ppm extra loss in the RC will not severely limit the recycling gain. Such large variation was not seen in the MC analysis because we only varied the angle of incidence by +/- 0.5 degrees about the nominal design value of 41.1 degrees.

  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.
  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


  14448   Mon Feb 11 19:53:59 2019 gautamSummaryLoss MeasurementLoss measurement setup

To measure the Y-arm loss, I decided to start with the classic reflectivity method. To prepare for this measurement, I did the following:

  1. Placed a PDA 520 in the AS beam path on the AS table.
  2. Centered AS beam on above PDA 520.
  3. Monitored signal from PDA520 and the MC transmission simultaneously in the single-bounce from ITMY config (i.e. all other optics were misaligned). Convinced myself that variations in the two signals were correlated, thus ruling out in this rough test any interference from ghost beams from ITMX / PRM etc.
  4. For the DAQ, I decided to use the two ALS Y arm channels in 1Y4, mainly because we have some whitening electronics available there - the OMC model would've been ideal but we don't have free whitening channels available there. So I ran long BNCs to the rack, labelled them.
  5. It'd be nice to have these signals logged to frames, so I added DQ-channels for the IN1 points of the BEATY_FINE filters, recording at 2048 Hz for now. Of course this necessitated restart of the c1lsc model, which caused all the vertex FEs to crash, but the reboot script brought everything back smoothly.
  6. Not sure what to make of the shape of the spectrum of the AS photodiode, see Attachment #1 - looks like some kind of scattering shelf but I checked the centering on the PD itself, looks good. In any case, with the whitening gains I'm using, seems like both channels are measuring above ADC noise.
  7. Found that the existing misalignment to the ETMY does not eliminate signatures of cavity flash in the AS photodiode. So I increased the amount of misalignment until I saw no evidence of flashes in the reflected photodiode.
  8. Johannes' old scripts didn't work out of the box - so I massaged it into a form that works.
  9. Re-centered Oplevs to try and keep them as well centered in the linear range as possible, maybe the DC position info from the Oplevs is useful in the analysis.

I'm running a measurement tonight, starting now (~1130PM), should be done in ~1hour, may need to do more data-quality improvements to get a realistic loss number, but I figured I'd give this a whirl.

I'm rather pleased with an initial look at the first align/misalign cycle, at least there is discernable contrast between the two states - Attachment #2. The data is normalized by MC transmission, and then sig.decimated by x512 (8**3).

  14449   Tue Feb 12 18:00:32 2019 gautamSummaryLoss MeasurementLoss measurement setup

Another arm loss measurement started at 6pm.

  14450   Tue Feb 12 22:59:17 2019 gautamSummaryLoss MeasurementY arm loss


There are still several data quality issues that can be improved. I think there is little point in reading too much into this until some of the problems outlined below are fixed and we get a better measurement.


  1. Mainly, we are plagued by the inability of the ASS system to get back to the good transmission levels - I haven't done a careful diagnosis of the servo, but the ITM PIT output always seems to run away. As a result, the later measurements are poor, as can be seen in Attachment #2.
  2. For this reason, we can't easily sample different spot positions on the ETM.
  3. Data processing:
    • Download AS reflection and MC transmission DQ channels
    • Take their ratio
    • Downsample to 4 Hz by repeated application of scipy.signal.decimate by a factor of 8 each time, thrice, with the filtfilt option enabled
  4. Attachment #1 and #2 are basically showing the same data - the former collects all locked (top left) and misaligned (top right) data segments and plots them with the corresponding TRY values in the bottom row. The second plot shows a pseudo-continuous time series (pseudo because the segments transitioning from locked to misaligned states have been excised).

As an interim fix, I'm going to try and use the Oplevs as a DC reference, and run the dither alignment from zero each time, as this prevents the runaway problem at least. Data run started at 11:20 pm.

  14451   Wed Feb 13 02:28:58 2019 gautamSummaryLoss MeasurementY arm loss

Attachment #1 shows estimated systematic uncertainty contributions due to 

  1. ITM transmission by +/- 0.01 % about the nominal value of 1.384 %
  2. ETM transmission of +/- 3 ppm about the nominal value of 13.7 ppm
  3. Mode matching efficiency into the cavity by +/- 5% about the nominal value of 92%.

In all the measurements so far, the ratio seems to be < 1, so this would seem to set a lower bound on the loss of ~35 ppm. The dominant source of systematic uncertainty is the 5% assumed fudge in the mode-matching

To do: 

  1. Account for uncertainties on modulation depths
  2. To estimate if the amount of fluctuation we are seeing in the reflected signal even after normalizing by the MC transmission, get an estimate of statistical uncertainty in the reflected power due to 
    • Pointing jitter - is there some spec for the damped angular displacement of the TT1/TT2?
    • Cavity length in-loop residual

Bottom line: I think we need to have other measurements and simultaenously analyse the data to get a more precise estimate of the loss.

  14452   Thu Feb 14 15:37:35 2019 gautamUpdateVACVacromag failure

[chub, gautam]


One of the XT1111 units (XT1111a) in the new vacuum system has malfunctioned. So all valves are closed, PSL shutter is also closed, until this is resolved.


  1. Chub alerted me he had changed the main N2 line pressure, but this did not show up in the trend data. In fact, the trend data suggested that all 3 N2 gauges had stopped logging data (they just held the previous value) since sometime on Monday, see Attachment #1.
  2. We verified that the gauges were being powered, and that the analog voltage output of the gauges made sense in the drill press room ---> So this suggested something was wrong at the Vacuum rack electronics rack.
  3. Went to the vacuum rack, saw no obvious indicator lights signalling a fault.
  4. So I restarted the modbus process on c1vac using sudo systemctl restart modbusIOC.service. The way Jon has this setup, this service controls all the sub-processes talking to gauges and TPs, so resatrting this master process should have brought everything back.
  5. This tripped the interlock, and all valves got closed.
  6. Once the modbus service restarted, most things came back normally. However, V1, V3, V4 and V5 readbacks were listed as "UNDEF".
  7. The way the interlock code works, it checks a valve state change request against the monitor channel, so all these valves could not be opened.
  8. We confirmed that the valves themselves were operational, by bypassing the itnerlock logic and directly actuating on the valve - but this is not a safe way of running overnight so we decided to shut everything down.
  9. We also confirmed that the problem is with one particular Acromag unit - switching the readback Dsub connector to another channel (e.g. V1 --> VM2) showed the expected readback.
  10. As a further check - I connected a windows laptop with the Acromag software installed, to the suspected XT1111 - it reported an error message saying "USB device may be damaged". Plugging into another XT111 in the crate, I was able to access the unit in the normal way.
  11. The phoenix connector architecture of the Acromags makes it possible to replace this single unit (we have spare XT1111 units) without disturbing the whole system - so barring objections, we plan to do this at 9am tomorrow. The replacement plan is summarized in Attachment #2.

Pressure of the main volume seems to have stabilized - see Attachment #3, so it should be fine to leave the IFO in this state overnight.


  1. What caused the original failure of the writing to the ADC channels hooked up to the N2 gauges? There isn't any logging setup from the modbus processes afaik.
  2. What caused the failure of the XT1111? What is the failure mode even? Because some other channels on the same XT1111 are working...
  3. Was it user error? The only operation carried out by me was restarting the modbus services - how did this damage the readback channels for just four valves? I think Chub also re-arranged some wires at the end, but unplugging/re-connecting some cables shouldn't produce this kind of response...

The whole point of the upgrade was to move to a more reliable system - but seems quite flaky already.

  14454   Thu Feb 14 21:29:24 2019 gautamSummaryLoss MeasurementInferred Y arm loss


From the measurements I have, the Y arm loss is estimated to be 58 +/- 12 ppm. The quoted values are the median (50th percentile) and the distance to the 25th and 75th quantiles. This is significantly worse than the ~25 ppm number Johannes had determined. The data quality is questionable, so I would want to get some better data and run it through this machinery and see what number that yields. I'll try and systematically fix the ASS tomorrow and give it another shot.

Model and analysis framework:

Johannes and I have cleaned up the equations used for this calculation - while we may make more edits, the v1 of the document lives here. The crux of it is that we would like to measure the quantity \kappa = \frac{P_L}{P_M}, where P_{L(M)} is the power reflected from the resonant cavity (just the ITM). This quantity can then be used to back out the round-trip loss in the resonant cavity, with further model parameters which are:

  1. ITM and ETM power transmissivities
  2. Modulation depths and mode-matching efficiency into the cavity
  3. The statistical uncertainty on the measurement of the quantity \kappa, call it \sigma_{\kappa}

If we ignore the 3rd for a start, we can calculate the "expected" value of \kappa as a function of the round-trip loss, for some assumed uncertainties on the above-mentioned model parameters. This is shown in the top plot in Attachment #1, and while this was generated using emcee, is consistent with the first order uncertainty propagation based result I posted in my previous elog on this subject. The actual samples of the model parameters used to generate these curves are shown in the bottom. What this is telling us is that even if we have no measurement uncertainty on \kappa, the systematic uncertainties are of the order of 5 ppm, for the assumed variation in model parameters.

The same machinery can be run backwards - assuming we have multiple measurements of \kappa, we then also have a sample variance, \sigma_{\kappa}. The uncertainty on the sample variance estimator is also known, and serves to quantify the prior distribution on the parameter \sigma_{\kappa} for our Monte-Carlo sampling. The parameter \sigma_{\kappa} itself is required to quantify the likelihood of a given set of model parameters, given our measurement. For the measurements I did this week, my best estimate of \kappa \pm \sigma_{\kappa} = 0.995 \pm 0.005. Plugging this in, and assuming uncorrelated gaussian uncertainties on the model parameters, I can back out the posterior distributions.

For convenience, I separate the parameters into two groups - (i) All the model parameters excluding the RT loss, and (ii) the RT loss. Attachment #2 and Attachment #3 show the priors (orange) and posteriors (black) of these quantities. 


  1. This particular technique only gives us information about the RT loss - much less so about the other model parameters. This can be seen by the fact that the posteriors for the loss is significantly different from the prior for the loss, but not for the other parameters. Potentially, the power of the technique is improved if we throw other measurements at it, like ringdowns.
  2. If we want to reach the 5 ppm uncertainty target, we need to do better both on the measurement of the DC reflection signals, and also narrow down the uncertainties on the other model parameters.

Some assumptions:

So that the experts on MC analysis can correct me wheere I'm wrong.

  1. The prior distributions are truncated independent Gaussians - truncated to avoid sampling from unphysical regions (e.g. negative ITM transmission). I've not enforced the truncation analytically - i.e. I just assume a -infinity probability to samples drawn from the unphysical parts, but to be completely sure, the actual cavity equations enforce physicality independently (i.e. the MC generates a set of parameters which is input to another function, which checks for the feasibility before making an evaluation). One could argue that the priors on some of these should be different - e.g. uniform PDF for loss between some bounds? Jeffrey's prior for \sigma_{\kappa}?
  2. How reasonable is it to assume the model parameter uncertainties are uncorrelated? For exaple, \eta, \beta_1, \beta_2 are all determined from the ALS-controlled cavity scan
  14455   Thu Feb 14 23:14:12 2019 gautamUpdateCDSc1rfm errors

The pressure is still 2e-4 torr according to CC1 so I thought I'd give ASS debugging a go tonight. But the arm transmission signal isn't coming through to the LSC model from the end PDs - so a resurfacing of this problem. Rebooting the sender model, c1scy, did not fix the problem. Moreover, c1susaux is dead. The last time I rebooted it, ITMY got stuck so I'm not going to attempt a revival tonight.

  14457   Fri Feb 15 15:22:08 2019 gautamUpdateCDSc1rfm errors persist

I restarted c1scyc1rfm (so both sender and receiver models were cycled) and power-cycled the c1iscey and c1sus machines. The TRY PD is certainly seeing light - it is just not getting piped over to c1rfm. dmesg doesn't give any clues. I'm out of ideas.

P.S. The new reality seems to be that getting ITMY stuck in the event of a c1susaux reboot is inevitable. As is the practise for ITMX, I tried slowly ramping the PIT and YAW biases to 0 slowly - but in the process of ramping YAW to 0, the optic got stuck. I am ramping in steps of 0.1 (in units of the PIT/YAW sliders, waiting ~3 seconds between steps), I guess I can try ramping even more slowly.

Update: I power cycled the physical RFM switch. This necessitated reboot of all vertex FEs. But seems like things are back to normal now...

Note: to unstick ITMY, seems like the best approach is:

  1. Jiggle bias until SIDE shadow sensor is on average above it's half-light level. This is the critical step. A bias of +20000 cts on the fast SIDE output seems to help.
  2. Set YAW bias to -10, ramp down the BIAS in steps of 0.1, watching shadow sensor levels to ensure optic doesn't get stuck again.
  3. Hope for the best. Iterate if necessary.

The pressure is still 2e-4 torr according to CC1 so I thought I'd give ASS debugging a go tonight. But the arm transmission signal isn't coming through to the LSC model from the end PDs - so a resurfacing of this problem. Rebooting the sender model, c1scy, did not fix the problem. Moreover, c1susaux is dead. The last time I rebooted it, ITMY got stuck so I'm not going to attempt a revival tonight.

  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.

  14462   Fri Feb 15 21:15:42 2019 gautamUpdateVACdd backup of c1vac made
  1. Connected one of the solid-state drives to c1vac. It was /dev/sdb.
  2. Formatted the drive using sudo mkfs -t ext4 /dev/sdb
  3.  Mounted it as /mnt/backup using sudo mount /dev/sdb /mnt/backup
  4. Started a tmux session for the dd, called DDbackup
  5. Started the dd backup using  sudo dd if=/dev/sda of=/dev/sdb bs=64K conv=noerror,sync
  6. Backup completed in 719 seconds: need to test if it works...
controls@c1vac:~$ sudo dd if=/dev/sda of=/dev/sdb bs=64K conv=noerror,sync
[sudo] password for controls: 
^C283422+0 records in
283422+0 records out
18574344192 bytes (19 GB) copied, 719.699 s, 25.8 MB/s
  • Generate a bootable backup hard drive for c1vac, which could be swapped in on a short time scale after a failure.
  14463   Sun Feb 17 17:35:04 2019 gautamSummaryLoss MeasurementInferred X arm loss


To complete the story before moving on to ALS, I decided to measure the X arm loss. It is estimated to be 20 +/- 5 ppm. This is surprising to say the least, so I'm skeptical - the camera image of the ETMX spot when locked almost certainly looks brighter than in Oct 2016, but I don't have numerical proof. But I don't see any obvious red flags in the data quality/analysis yet. If true, this suggests that the "cleaning" of the Yarm optics actually did more harm than good, and if that's true, we should attempt to identify where in the procedure the problem lies - was it in my usage of non-optical grade solvents?


  1. Unlike the Y arm, the ratio \kappa = 1.006 \pm 0.002 is quite unambiguously greater than 1, which is already indicative of the loss being lower than for the Y arm. This is reliably repeatable over 15 datapoints at least.
  2. Attachment #1 shows the spectrum of the single-bounce off ITMX beam and compares it to ITMY - there is clearly a difference, and my intuition is to suspect some scatter / clipping, but I confirmed that on the AS table, in air, there is no clipping. So maybe it's something in vacuum? But I'm not sure how to explain its absence for the ITMX reflection. I didn't check the Michelson alignment since I misaligned ITMY before locking the XARM - so maybe there's a small shift in the axis of the X arm reflection relative to the Yarm because of the BS alignment. The other possibility is clipping at the BS?
  3. Attachment #2 shows the filtered time series for a short segment of the measurement. The X arm ASS is mostly well behaved, but the main thing preventing me from getting more statistics in is the familiar ETMX glitching problem, which while doesn't directly break the lock causes large swings in TRX. Given the recent experience with ETMY satellite box, I'm leaning towards blaming flaky electronics for this. If this weren't a problem, I'd run a spatial scan of ETMX, but I'm not going to attack this problem today.
  4. Attachments #3 and #4 show the posterior distributions for model parameters and loss respectively. 
  5. Data quality checks done so far (suggestions welcome):
    • Confirmed that there is no fringing from other ITM (in this case ITMY) / PRM / SRM / ETM in the single-bounce off ITMX config, by first macroscopically misaligning all these optics (the spots could be seen to move on the AS port PD, until they vanished, at some point presumably getting clipped in-vac), and then moving the optics around in PIT/YAW and looking for any effect in the fast time-series using NDScope.
    • Checked for slow drifts in locked / misaligned states - looks okay.
    • Checked centering on PDA520 using both o'scope plateau method and IR viewer - I believe the beam to be well centered.

Provisional conclusions:

  1. The actual act of venting / pumping down doesn't have nearly as large an effect on the round-trip loss as does working in chamber - the IX and EX chambers have not been opened since the 2016 vent.
  2. The solvent marks visible with the green flashlight on ETMY possibly signals the larger loss for the Y arm. 
  14466   Tue Feb 19 22:52:17 2019 gautamUpdateASSY arm clipping doubtful

In an earlier elog, I had claimed that the suspected clipping of the cavity axis in the Y arm was not solved even after shifting the heater. I now think that it is extremely unlikely that there is still clipping due to the heater. Nevertheless, the ASS system is not working well. Some notes:

  1. The heater has been shifted nearly 1-inch relative to the cavity axis compared to its old position - see Attachment #1 which compares the overhead shot of the suspension cage before and after the Jan 2019 vent.
  2. On Sunday, I was able to recover TRY ~ 1.0 (but not as high as I was able to get by intentionally setting a yaw offset to the ASS) by hand alignment with the spot on ETMY much closer to the center of the optic, judging by the camera. There are offsets on the dither alignment error signals which depend on the dither frequency, so the A2L signals are not good judges of how well centered we are on the optic.
  3. By calculating the power lost by clipping a Gaussian beam cross-section with a rectangular block from one side (an admittedly naive model of clipping), I find that we'd have to be within 15 mm of the line connecting the centers of ITMY and ETMY to even see ~10 ppm loss, see Attachment #2. So it is hard to believe that this is still a problem. Also, see  Attachment #3 which compares side-by-side the view of ETMY as seen through the EY optical table viewport before and after the Jan 2019 vent.

We have to systematically re-commission the ASS system to get to the bottom of this.

  14467   Wed Feb 20 18:26:05 2019 gautamUpdateIOOIPPOS recommissioned

I've suspected that the TTs are drifting significantly over the course of the last couple of days, because despite repeated alignment efforts, the AS beam spot has drifted off the center of the camera view. I tried looking at IPPOS, but found that there was no data. Looking at the table, the QPD was turned backwards, and the DAQ cable wasn't connected (neither at the PD end, nor at 1Y2, where instead, a cable labelled "Spare QPD" was plugged in). Fortunately, the beam was making it out of the vacuum. So as to have a quantitative diagnostic, I reconnected the QPD, turned it the right way round, and adjusted the steering onto it such that with the AS spot on the center of the CCD monitor, the beam is also centered on the QPD. The calibration is uncertain, but at least we will be able to see how much the spot drifts on the QPD over some days. Also, we only have 16 Hz readback of this stuff.

I leave it to Chub to take the high-res photo and update the wiki, which was last done in 2012.

Already, in the last ~1 hour, there has been considerable drift - see Attachment #2. The spot, which started at the center of the CCD monitor, has now nearly drifted off the top end. The ITMX and BS Oplev spots have been pretty constant over the same timescale, so it has to be the TTs?

  14468   Wed Feb 20 23:55:51 2019 gautamUpdateALSALS delay line electronics


Last year, I worked on the ALS delay line electronics, thinking that we were in danger of saturation. The analysis was incorrect. I find that for RF signal levels between -10 dBm and +15 dBm, assuming 3dB insertion loss due to components and 5 dB conversion loss in the mixer, there is no danger of saturation in the I/F part of the circuit.


The key is that the MOSFET mixer used in the demodulation circuit drives an I/F current and not voltage. The I-to-V conversion is done by a transimpedance amplifier and not a voltage amplifier. The confusion arose from interpreting the gain of the first stage of the I/F amplifier as 1 kohm/10 ohm = 100. The real figures of merit we have to look at are the current through, and voltage across, the transimpedance resistor.  So I think we should revert to the old setup. This analysis is consistent with an actual test I did on the board, details of which may be found here.

We may still benefit from some whitening of the signal before digitization between 10-100 Hz, need to check what is an appropriate place in the signal chain to put in some whitening, there are some constraints to the circuit topology because of the MOSFET mixer.

One part of the circuit topology I'm still confused by is the choice of impedance-matching transformer at the RF-input of this demod board - why is a 75 ohm part used instead of a 50 ohm part? Isn't this going to actually result in an impedance mismatch given our RG405 cabling?

Update: Having pulled out the board, it looks like the input transformer is an ADT-1-1, and NOT an ADT1-1WT as labelled on the schematic. The former is indeed a 50ohm part. So it makes sense to me now.

Since we have the NF1611 fiber coupled PDs, I'm going to try reviving the X arm ALS to check out what the noise is after bypassing the suspect Menlo PDs we were using thus far. My re-analysis can be found in the attached zip of my ipynb (in PDF form).

  14469   Fri Feb 22 12:19:46 2019 gautamUpdateIOOTT coil driver Vmon

To debug the issue of the suspected drifting TTs further, I temporarily hijacked CH0-CH8 of ADC1 in the c1lsc expansion chassis, and connected the "MON" outputs of the coil drivers (D010001) to them via some DB9 breakouts. The idea is to see if the problem is electrical. We should see some  slow drift in the voltage to the TTs correlated with the spot walking off the IPPOS QPD. From the wiring diagram, it doesn't look like there is any monitoring (slow or fast) of the control voltages to the TT coils, this should be factored into the Acromag upgrade of c1iscaux/c1iscaux2. EPICS monitoring should be sufficient for this purpose so I didn't setup any new DQ channels, I'll just look at the EPICS from the IOP model.

Already, in the last ~1 hour, there has been considerable drift - see Attachment #2. The spot, which started at the center of the CCD monitor, has now nearly drifted off the top end. The ITMX and BS Oplev spots have been pretty constant over the same timescale, so it has to be the TTs?
  14471   Wed Feb 27 21:34:21 2019 gautamUpdateGeneralSuspension diagnosis

In my effort to understand what's going on with the suspensions, I've kicked all the suspensions and shutdown the watchdogs at 1235366912. PSL shutter is closed to avoid trying to lock to the swinging cavity. The primary aims are

  1. To see how much the resonant peaks have shifted w.r.t. the database, if at all - I claim that the ETMY resonances have shifted by a large amount and also has lost one of the resonant peaks.
  2. To check the status of the existing diagonalization.

All the tests I have done so far (looking at free swinging data, resonant frequencies in the Oplev error signals etc) seem to suggest that the problem is mechanical rather than electrical. I'll do a quick check of the OSEM PD whitening unit in 1Y4 to be sure.But the fact that the same three peaks appear in the OSEM and Oplev spectra suggests to me that the problem is not electrical.

Watchdogs restored at 10 AM PST

  14472   Sat Mar 2 14:19:35 2019 gautamUpdateCDSFSS Slow servo gains not burt-ed

PSL NPRO PZT voltage showed large low frequency (hour timescale) excursions on the control room StripTool trace, leading me to suspect the slow servo wasn't working as expected. Yesterday evening, I keyed the unresponsive c1psl crate at ~9 PM PST, and had to run the burtrestore to get the PMC locking working. I must have pressed the wrong button on burtgooey or something, because all the FSS_SLOW channels were reset to 0. What's more, their values were not being saved by the hourly burt-snap script, so I don't have any lookback on what these values were. There isn't any detailed record on the elog about what the optimal values for these are, and the most recent reference I could find was Ki=0.1, Kp=Kd=0, which is what I've set it now to. The servo isn't running away, so I'm leaving things in this state, PID tuning can be done later.

I also added the FSS Slow servo channels to the burt snapshot requirement file at /cvs/cds/caltech/target/c1psl/autoBurt.req, and confirmed that the snapshots are getting the channels from now onwards.

While looking at the req file, I saw a bunch of *_MOPA* channels and also several other currently unused channels. Probably would benefit from going through these and commenting out all the legacy channels, to minimize disk space wastage (though we compress the snapshot files every few years anyways I guess).

Reminder that this (unrelated) issue still needs to be looked into... Note also that the new vacuum system does not have burt snapshot set up (i.e. it is still trying to get the old channels from the c1vac1 and c1vac2 databases, which while has significant overlap with the new system, should probably be setup correctly).

  14473   Sun Mar 3 14:16:31 2019 gautamUpdateIOOMegatron hard-rebooted

IMC was not locked for the past several hours. Turned out MC autolocker was stuck, and I could not ssh into megatron because it was in some unresponsive state. I had to hard-reboot megatron, and once it came back up, I restarted the MCautolocker, FSS slow servo and nds2 processes. IMC re-locked immediately.

I was pulling long stretches of OSEM data from the NDS2 server (megatron) last night, I wonder if this flakiness is connected. Megatron is still running Ubuntu12.

  14474   Tue Mar 5 15:56:27 2019 gautamSummaryTip-TIltDiscussion points about TT re-design

Chub, Koji and I have been talking about Udit's re-design. Here are a few points that were raised. Chub/Koji can add to/correct where necessary. Summary is that this needs considerable work before we can order the parts for a prototype and characterize it. I think the requirements may be stated as:

  1. The overall pendulum length should be similar to that of the SOS, i.e. ~0.3m (current length is more like 0.1m) such that the eigenfrequencies are lowered to more like ~1 Hz. Mainly we wan't to avoid any overlap with the stack eigenmodes. This may require an additional stiffening piece near the top of the tower as we have for the SOS. What is a numerical way to spec this?
  2. The center of the 2" optic should be 6" from the table.
  3. The mass of the optic + holder should be similar to the current design so we may use the same suspension wires (I believe they are a different thickness than that used for the SOS).
  4. Ensure we can extract any transmitted beams without clipping.
  5. Fine pitch adjustment capablity should be yyy mrad (20mrad?).
  6. We should preserve the footprint of the existing TTs, given the space constraints in vacuum. Moreover, we should be able to use dog-clamps to fix the tower in place, so the base plate should be designed accordingly.
  7. Keep the machining requirements as simple as possible while achieving the above requirements- i.e. do we really need rounded optic holder? Why not just rectangular? Similarly for other complicated features in the current design.

Some problems with Udit's design as it stands:

  1. I noticed that the base of the TT and the center of the 2" optic are 4" separated. The SOS cage base and center of 3" optic are separated by 6". Currently, there is an adaptor piece that raises the TT height to match that of the SOS. If we are doing a re-design, shouldn't we just aim for the correct height in the first place?
  2. Udit doesn't seem to have taken into account the torque due to the optic+holder in the pitch balancing calculations he did. Since this is expected to be >> that of any rod/screw we use for fine pitch balancing, we need to factor that into the calculation.
  3. For the coarse pitch adjustment, we'd need to slide the wire clamping piece relative to the optic holding piece. Rather than do this stochastically and hope for the best, the idea was to use a threaded screw to realize this operation in a controlled way. However, Udit's design doesn't include the threaded hole.
  4. There are many complicated machining features which are un-necessary.
  14475   Thu Mar 7 01:06:38 2019 gautamUpdateALSALS delay line electronics


The restoration of the delay-line electronics is complete. The chassis has not been re-installed yet, I will put it back in tomorrow. I think the calculations and measurements are in good agreement.


Apart from restoring the transimpedance of the I/F amplifier, I also had to replace the two differential-sending AD8672s in the RF Log detector circuit for both LO and RF paths in the ALS-X board. I performed the same tests as I did the last time on the electronics bench, results will be uploaded to the DCC page for the 40m version of the board. I think the board is performing as advertised, although there is some variation in the noise of the two pairs of I/Q readouts. Sticking with the notation of the HP Application Note for delay line frequency discriminators, here are some numebrs for our delay line system:

  • K_{\phi} = 3.7 \ \mathrm{V/rad}  - measured by driving the LO/RF inputs with Fluke/Marconi at 7dBm/0dBm (which are the expected signal levels accounting for losses between the BeatMouth and the demodulator) and looking at the Vpp of the resulting I/F beat signal on a scope. This is assuming we use the differential output of the demodulator (divide by 2 if we use the single-ended output instead).
  • \tau_d = \frac{45 \ \mathrm{m}}{0.75c} \approx 0.2 \mu s [see measurement]
  • K_{d} = K_{\phi}2 \pi \tau_{d} \approx 4 \mu \mathrm{V/Hz} (to be confirmed by measurement by driving a known FM signal with the Marconi)
  • Assuming 1mW of light on our beat PDs and perfect contrast, the phase noise due to shot noise is \pi \sqrt{2\bar{P}\frac{hc}{\lambda}} / 1 \ \mathrm{mW} \approx 60 \ \mathrm{nrad /}\sqrt{\mathrm{Hz}}which is ~ 5 orders of magnitude lower than the electronics noise in equivalent frequency noise at 100 Hz.
  • The noise due to the FET mixer seems quite complicated to calculate - but as a lower bound, the Johnson current noise due to the 182 ohms at each RF input is ~ 10 pA/rtHz. With a transimpedance gain of 1 kohm, this corresponds to ~10 nV/rtHz. 

In conclusion: the ALS noise is very likely limited by ADC noise (~1 Hz/rtHz frequency noise for 5uV/rtHz ADC noise). We need some whiteningWhy whiten the demodulated signal instead of directly incorporating the whitening into the I/F amplifier input stage? Because I couldn't find a design that satisfies all the following criteria (this was why my previous design was flawed):

  1. The commutating part of the FET mixer must be close to ground potential always.
  2. The loading of the FET mixer is mostly capacitive.
  3. The DC gain of the I/F amplifier is low, with 20-30dB gain at 100 Hz, and then rolled off again at high frequencies for stability and sum-frequency rejection. In fact, it's not even obvious to me that we want a low DC gain - the quantity K_{\phi} is directly proportional to the DC transimpedance gain, and we want that to be large for more sensitive frequency discriminating.

So Rich suggested separating the transimpedance and whitening operations. The output noise of the differential outputs of the demodulator unit is <100 nV/rtHz at 100 Hz, so we should be able to saturate that noise level with a whitening unit whose input referred noise level is < 100 nV/rtHz. I'm going to see if there are any aLIGO whitening board spares - the existing whitening boards are not a good candidate I think because of the large DC signal level.

  14477   Tue Mar 12 22:51:25 2019 gautamUpdateALSALS delay line electronics

This Hanford alog may be of relevance as we are using the aLIGO AA chassis for the IR ALS channels. We aren't expecting any large amplitude high frequency signals for this application, but putting this here in case it's useful someday.

  14478   Wed Mar 13 01:27:30 2019 gautamUpdateALSALS delay line electronics

This test was done, and I determine the frequency discriminant to be \approx 5 \mu \mathrm{V}/\mathrm{Hz} (for an RF signal level of ~2 dBm). 

Attachment #1: Measured and predicted value of the DFD discriminant for a few RF signal levels.

  • Methodology was to drive an FM (deviation = 25 Hz, fMod = 221 Hz, fCarrier ~ 40 MHz) with the Marconi, and look at the IF spectrum peak height on a SR785
  • The "Design" curve is calculated using the circuit parameters, assuming 4dB conversion loss in the mixer itself, and 3dB insertion loss due to various impedance matching transformers and couplers in the RF signal chain. I fudged the insertion/convertion loss numbers to get this curve to line up with the measurements (by eye).
  • For the measurement, I assume the value for FM deviation displayed on the Marconi is an RMS value (this is the best I can gather from the manual). I'll double checking by looking at the RFmon spectrum directly on the Agilent NA.
  • X axis calibrated by reading off from the RF power monitor using a DMM and using the calibration data from the bench.
  • I could never get the ratio of peak heights in Ichan/Qchan (or the other way around) to better than ~ 1/8 (by moving the carrier frequency around). Not sure I can explain that - small non-orthogonality between I and Q channels cannot explain this level of leakage.

Attachment #2: Measured noise spectrum in the 1Y2 (LSC) electronics rack, calibrated to Hz/rtHz using the discriminant from Attachment #1.

  • Something funky with the I channel for X, I'll re-take that spectrum.

I'm still waiting on some parts for the new BeatMouth before giving the whole system a whirl. In the meantime, I'll work on the EX and EY green setups, to try and improve the mode-matching and better characterize the expected suppressed frequency noise of the end NPROs - the goal here is to rule out the excess low-frequency noise that was seen in the ALS signals coming from unsuppressed frequency noise.

Bottom lines: 

  1. The DFD noise is at the level of ~ 10mHz/rtHz above 10 Hz. This justifies the need for whitening before ADC-ing.
  2. The measured signal/noise levels in the DFD chain are in good agreement with the "expected" levels from circuit component values and typical insertion/conversion loss values.
  3. Why are there so many 60 Hz harmonics???
  14480   Sun Mar 17 00:42:20 2019 gautamUpdateALSNF1611 cannot be shot-noise limited?


Per the manual (pg12) of the NF 1611 photodiode, the "Input Noise Current" is 16 pA/rtHz. It also specifies that for "Linear Operation", the max input power is 1 mW, which at 1um corresponds to a current shot noise of ~14 pA/rtHz. Therefore,

  1. This photodiode cannot be shot-noise limited if we also want to stay in the spec-ed linear regime.
  2. We don't need to worry so much about the noise figure of the RF amplifier that follows the photodiode. In fact, I think we can use a higher gain RF amplifier with a slightly worse noise figure (e.g. ZHL-3A) as we will benefit from having a larger frequency discriminant with more RF power reaching the delay line.


Attachment #1: Here, I plot the expected voltage noise due to shot noise of the incident light, assuming 0.75 A/W for InGaAs and 700V/A transimpedance gain. 

  • For convenience, I've calibrated on the twin axes the current shot noise (X) and equivalent amplifier noise figure at a given voltage noise, assuming a 50 ohm system (Y).
  • The 16 pA/rtHz input current noise exceeds the shot noise contribution for powers as high as 1 mW.
  • Even at 0.5 mW power on the PD, we can use the ZHL-3A rather than the Teledyne:
    • This calculation was motivated by some suspicious features in the Teledyne amplifier gain, I will write a separate elog about that. 
    • For the light levels we have, I expect ~3dBm RF signal from the photodiode. With the 24dB of gain from the ZHL-3A, the signal becomes 27dBm, which is smaller (but close to) the spec-ed max output of the ZHL-3A, which is 29.5 dBm. Is this too close to the edge?
    • I will measure the gain/noise of the ZHL-3A to get a better answer to these questions.
  • If in the future we get a better photodiode setup that reaches sub-1nV/rtHz (dark/electronics) voltage noise, we may have to re-evaluate what is an appropriate RF amplifier.
  14483   Mon Mar 18 12:27:42 2019 gautamUpdateGeneralIFO status
  1. c1iscaux2 VME crate is damaged - see Attachment #1. 
    • It is not generating the 12V supply voltage, and so nothing in the crate works.
    • Tried resetting via front panel button, power cycling by removing power cable on rear, all to no effect.
    • Tried pulling out all cards and checking if there was an internal short that was causing the failure - looks like the problem is with the crate itself.
    • Not sure how long this machine has been unresponsive as we don't have any readback of the status of the eurocrate machines.
    • Not a showstopper, mainly we can't control the whitening settings for AS55, REFL55, REFL165 and ALSY. 
    • Acromag installation schedule should be accelerated.
    • * Koji reminded me that \text{VME crate} \ \neq \ \text{eurocrate}. The former is what is used for the slow machines, the latter is what is used for holding the iLIGO style electronics boards.
  2. ITMX oplev is dead - see Attachment #2.
    • Lasted ~3 years (installed March 2016).
    • I confirmed that no light is coming out of the laser head on the optical table.
    • I'll ask Chub to replace it this afternoon.
  3. c1susaux is unresponsive
    • I didn't reboot it as I didn't want to spend some hours freeing ITMY. 
    • At some point we will have to bite the bullet and do it.
  4. Input pointing is still not stable
    • I aligned the input pointing using TT1/TT2 to maximize TRX/TRY before lunch, but in 1 hour, the pointing has already drifted.
  5. POX/POY locking is working okay. TRX has large low-frequency fluctuations because of ITMX not having an Oplev servo, should be rectified once we swap out the HeNe.

The goal for this week is to test out the ALS system, so this is kind of a workable state since POX/POY locking is working. But the number of broken things is accumulating fast.

  14484   Mon Mar 18 17:06:12 2019 gautamUpdateOptical LeversITMY HeNe replaced

Oplev HeNe was replaced this afternoon. We did some HeNe shuffling:

  1. A new HeNe was being used for the fiber illumination demo at EX. We took that out and decided to use it as the new ITMX HeNe. It had 2.6mW output at 632nm (measured with the Ophir power meter)
  2. Old ETMY HeNe was used for fiber illumination demo.
  3. Old ITMX HeNe was putting out no light - it will be disposed.

Attachment #1 shows the RIN and Attachment #2 and #3 show the PIT and YAW TFs with the new HeNe.

The ITMX Oplev path is still not great - the ingoing beam is within 2mm of clipping on a 2" lens used in the POX path, and there is a bunch of scattered red light everywhere. We should take the opportunity when the chamber is open to try and have a better layout (it may be tricky to optize without touching the two in-vacuum steering optics).


I'll ask Chub to replace it this afternoon.

  14486   Mon Mar 18 20:22:28 2019 gautamUpdateALSALS stability test

I'm running a test to see how stable the EX green lock is. For this purpose, I've left the slow temperature tuning servo on (there is a 100 count limiter enabled, so nothing crazy should happen).

  14498   Thu Mar 28 19:40:02 2019 gautamUpdateALSBeatMouth with NF1611s assembled


The parts I was waiting for arrived. I finished the beat mouth assembly, and did some characterization. Everything looks to be working as expected.


Attachment #1: Photo of the front panel. I am short of two fiber mating sleeves that are compatible with PM fibers, but those are just for monitoring, so not critical to the assembly at this stage. I'll ask Chub to procure these.

Attachment #2: Photo of the inside of the BeatMouth. I opted to use the flexible RG-316 cables for all the RF interconnects. Rana said these aren't the best option, remains to be seen if interference between cables is an issue. If so, we can replace them with RG-58. I took the opportunity to give each fiber beam splitter its own spool, and cleaned all the fiber tips.

Attachment #3: Transfer function measurement. The PDFR setup behind 1X5/1X6 was used. I set the DC current to the laser to 30.0 mA (as read off the display of the current source), which produced ~400uW of light at the fiber coupled output of the diode laser. This was injected into the "PSL" input coupler of the BeatMouth, and so gets divided down to ~100 uW by the time it reaches the PDs. From the DC monitor values (~430mV), the light hitting the PDs is actually more consistent with 60uW, which is in agreement with the insertion loss of the fiber beamsplitters, and the mating sleeves.

The two responses seem reasonably well balanced (to within 20% - do we expect this to be better?). Even though judging by the DC monitor, there was more light incident on the Y PD than on the X PD, the X response was actually stronger than the Y. 

I also took the chance to do some other tests:

  • Inject light into the "X(Y)-ARM" input coupler of the Beat Mouth - confirmed that only the X(Y) NF1611's DC monitor output showed any change. The DC light level was ~1V in this condition, which again is consistent with expected insertion losses as compared to the "PSL" input case, there is 1 less fiber beamsplitter and mating sleeve.
  • Injected light into each of the input couplers, looked at the interior of the BeatMouth with an IR viewer for evidence of fiber damage, and saw none. Note that we are not doing anything special to dump the light at the unused leg of the fiber beamsplitter (which will eventually be a monitor port). Perhaps, nominally, this port should be dumped in some appropriate way.

Attachment #4: Dark Noise analysis. I used a ZHL-500-HLN+ to boost the PD's dark noise above the AG4395's measurement noise floor. The measured noise level seems to suggest either (i) the input-referred current noise of the PD circuitry is a little lower than the spec of 16 pA/rtHz (more like 13 pA/rtHz) or (ii) the transimpedance is lower than the spec of 700 V/A (more like 600 V/A). Probably some combination of the two. Seems reasonable to me.

Next steps:

The optical part of the ALS detection setup is now complete. The next step is to measure the ALS noise with this sysytem. I will use the X arm for this purpose (I'd like to make the minor change of switching the existing resistive power splitter at the delay line to the newly acquired splitters which have 3dB lower insertion loss). 

  14501   Fri Mar 29 15:47:58 2019 gautamUpdateAUXAUX laser fiber moved from AS table to PSL table

[anjali, gautam]

To facilitate the 1um MZ frequency stabilization project, I decided that the AUX laser was a better candidate than any of the other 3 active NPROs in the lab as (i) it is already coupled into a ~60m long fiber, (ii) the PSL table has the most room available to set up the readout optics for the delayed/non-delayed beams and (iii) this way I can keep working on the IR ALS system in parallel. So we moved the end of the fiber from the AS table to the SE corner of the PSL table. None of the optics mode-matching the AUX beam to the interferometer were touched, and we do not anticipate disturbing the input coupling into the fiber either, so it should be possible to recover the AUX beam injection into the IFO relatively easily.

Anjali is going to post detailed photos, beam layout, and her proposed layout/MM solutions later today. The plan is to use free space components for everything except the fiber delay line, as we have these available readily. It is not necessarily the most low-noise option, but for a first pass, maybe this is sufficient and we can start building up a noise budget and identify possible improvements.

The AUX laser remians in STANDBY mode for now. HEPA was turned up while working at the PSL table, and remains on high while Anjali works on the layout.

  14502   Fri Mar 29 21:00:06 2019 gautamUpdateALSBeatMouth with NF1611s installed
  • Newfocus 15V current limited supply was taken from bottom NE corner of the ITMY Oplev table to power the BeatMouth on the PSL table
  • BeatMouth was installed on top shelf on PSL table [Attachment #1].
  • Light levels in fibers were checked:
    • PSL: initially, only ~200uW / 4mW was coupled in. This was improved to 2.6mW/4mW (~65% MM) which was deemed sufficient for a first test), by tweaking the alignment of, and into the collimator.
    • EX: ~900uW measured at the PSL table. I remember the incident power being ~1mW. So this is pretty good.
  • Fibers hooked up to BeatMouth:
    • EX light only, DC mon of X PD reads -2.1V.
    • With PSL light, I get -4.6 V.
    • For these numbers, with the DC transimpedance of 10kohm and the RF transimpedance of 700 ohm, I expect a beat of ~0dBm
  • DC light level stability is being monitored by a temporarily hijacked PSL NPRO diagnostic Acromag channel. Main motivation is to confirm that the alignment to the special axes of the PM fibers is still good and we aren't seeing large tempreature-driven waveplate effects.
  • RF part of the circuit is terminated into 50ohms for now -
    • there is still a quesiton as to what is the correct RF amplifier to use in sending the signal to the 1Y3 rack.
    • An initial RF beat power level measurement yielded -5dBm, which is inconsistent with the DC monitor voltages, but I'm not sure what frequency the beat was at, will make a more careful measurement with a scope or the network analyzer.
    • We want the RF pre-amp to be:
      • Low noise, keeping this in mind
      • High enough gain to boost the V/Hz discriminant of the electronic delay line
      • Not too high gain that we run into compression / saturate some of the delay line electronics - specifically, the LO input of the LSC demod board has a Teledyne amp in the signal chain, and so we need to ensure the signal level there is <16dBm (nominal level is 10dBm).
      • I'm evaluating options...
  • At 1Y3:
    • I pulled out the delay-line enclosure, and removed the (superglued) resistive power splitters with the help of some acetone
    • The newly acquired power splitters (ZAPD-2-252-S+) were affixed to the front panel, in which I made some mounting holes.
    • The new look setup, re-installed at 1Y3, is shown in Attachment #2.
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