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ID Date Author Type Category Subject
  15088   Mon Dec 9 21:22:46 2019 shrutiUpdateGeneralPLL / PM measurement of Xend NPRO PZT

In short:

Using the same setup as before with a LPF changed to have a cutoff of 5 MHz, the PLL was implemented and a TF measurement of the phase modulation was attempted. But, the beatnote drift was too high to get a prolonged phase lock (many times over 5MHz in <5 min).

Steps undertaken:

1. Normally I would unlock the IMC (Disabling the servo between the 'Filter' and 'Polarity' on the Mode Cleaner Servo Screen), but today I did not have to since Rana had kept it unlocked.

2. Misaligned the ITMX. This is to prevent cavity resonances from returning to the laser

3. Turned up the air on the HEPA at the PSL table to 100% during the measurement

4. Cables were connected as before (diagram shown in attachment of elog 15069)

5. The X end laser NPRO was actuated for the TF measurement using a long cable connected to TO AUX_X LASER PZT


Thoughts and observations:

- Reading out the error signal after amplification cannot distinguish between a locked loop or one out of its range. The error signal would be very small in both cases.

- Looking at the beat note on an oscilloscope, there also seemed to be an additional amplitude modulation that I had not noticed earlier. Rana suggested that it may have something to do with the pre-mode cleaner and the AOM being driven at 80 MHz

- Even though the TF was attempted, it seemed too noisy, suggesting that the PLL did not seem to work

- Rana also suggested that it may be a better idea to use the PZT of one of the lasers as the VCO for the PLL feedback instead of the Marconi.



I worked on the setup up for the phase modulation measurement of the X end NPRO PZT. A previous similar measurement can be found here (12077). The setup was assembled based on the schematic in Attachment1.

Mixer used: Level 7, Mini circuits ZP-3+
LPF: up to 1.9MHz

Cables exiting the PSL table:
1. LO (Marconi -> Mixer)
2. RF (PSL+X beat note -> Mixer) The cable for this was taken from the Beat Mouth (otherwise connected to the oscilloscope)
3. Ext modulator (SR560 -> Marconi)

The long cable labled 'X Green Beat' was used to connect to the PZT (from the network analyzer).

Observations: The beat note kept floating between 0 and ~100 MHz

The PLL part of the circuit was tested coarsely with the spectrum analyzer function of the Agilent, where the loop was seen to stabilize when the carrier frequency of the Marconi was close to the instantaneous beat frequency.




  15087   Mon Dec 9 19:19:04 2019 YehonathanUpdatePSLAOM first order beam alignment

{Yehonathan, Rana}

In order to setup a ringdown measurement with perfect extinction we need to align the first order beam from the AOM to the PMC instead of the zeroth order.

We connected a signal generator to the AOM driver and applied some offset voltage. We spot the first order mode and align it to the PMC. The achieved transmitted power is roughly as it was before this procedure.

Along the way few changes has been made in the PSL table:

1. Some dangling BNCs were removed.

2. Laser on the south east side of the PSL table was turned off.

3. DC power supplies were removed (Attachment 1 & 2). The rubber legs on the first one are sticky and leave black residue.

4. The beam block that orginally blocked the AOM high order modes was raised to block the zeroth order mode (Attachment 3).

5. The unterminated BNC T junction (Attachment 4 - before picture). from the PMC mixer to the PMC servo was removed.

However, we are currently unable to lock the PMC on high gain. When the gain is too high the PZT voltage goes straight to max and the lock is lost.

Attachment 1: 20191209_193112.jpg
Attachment 2: 20191209_193203_HDR.jpg
Attachment 3: imageedit_2_7551928142.gif
Attachment 4: imageedit_3_5863650538.gif
  15086   Mon Dec 9 13:08:24 2019 YehonathanSummaryPEMJump test of seismometers: EX needs recentering

I check the seismometers in the last 14 hours (Attached). Seems like the coherenece is restored in the x direction.



Attachment 1: seis_191208.pdf
  15085   Sun Dec 8 20:48:29 2019 ranaConfigurationComputersMegatron: starts up grade

I noticed recently that Megatron was running Ubuntu 12, so I've started its OS upgrade.

  1. Unlocked the IMC + disabled the autolocker from the LockMC screen + closed the PSL shutter (IMC REFL shutter doesn't seem to do anythin)
  2. Disabled the "FSS" slow servo on the FSS screen
  3. did sudo apt-get update, sudo apt-get upgrade, and then sudo apt-get do-release-upgrade which starts the actual thing
  4. According to the internet, the LTS upgrades will go in series rather than up to 18 in one shot, so its now doing 12 -> 14 (Trusty Tapir)

Megatron and IMC autolocking will be down for awhile, so we should use a different 'script' computer this week.

Mon Dec 9 14:52:58 2019

upgrade to Ubuntu 14 complete; now upgrading to 16

  15084   Sun Dec 8 20:27:11 2019 ranaUpdateComputersViviana upgrade to Ubuntu 16

The IBM laptop at EX was running Ubuntu 14, so I allowed it to start upgrading itself to Ubuntu16 as it desired. After it is done, I will upgrade it to 18.04 LTS. We should have them all run LTS.

  15083   Sun Dec 8 20:15:41 2019 ranaSummaryPEMJump test of seismometers: EX needs recentering

I have re-centered the EX (and EY) seismometers. They are Guralp CMG40-T, and have no special centering procedure except cycling the power a few times. I turned off the power on their interface box, then waited 10s before turning it back on.

The fist atm shows the comparison using data from 8-9 PM Saturday night:

  1. there seems to be a factor of 2 calibration diff between the T240 near the BS, and the Guralp seismometers at the end. Which one is right? surpriseWhen was the last time they were cross calibrated?
  2. The low coherence between BS_X and EX_X shows the problem. They should be very coherent (> 0.9) for 0.1-1 Hz.sad


Attachment 1: seis_all_191208.pdf
  15082   Fri Dec 6 17:49:46 2019 ranaSummaryPEMJump test of seismometers: EX needs recentering

Yehonathan, please center the EX seismometer.

The attached PDF shows the seismometer signals (I'm assuming that they're already calibrated into microns/s) during the lab tour for the art students on 11/1. The big spike which I've zoomed in on shows the time when we were in the control room and we all jumped up at the same time. There were approximately 15 students each with a mass of ~50-70 kg. I estimate that out landing times were all sync'd to within ~0.1 s.

Attachment 1: Seismometers.pdf
Attachment 2: src.tgz
  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.
  15080   Fri Dec 6 00:02:48 2019 gautamUpdateLSCWhat is the correct way to set the 3f offsets?


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


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?

  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.

  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
Attachment 2: CDSnormal.png
  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.

  15076   Thu Dec 5 08:44:44 2019 GavinUpdateLoss MeasurementQ Measurements of Test Masses

[Yehonathan, Gavin]

Measuring POX11_Q_MON and injecting a signal into the ITMX_UL_IN port a signal could not be seen on the function generator. After debugging the source of the issue was two fold:

  • By using the quadrant drives for coils (UL, UR etc) a signal has to pass through a switch before reaching the driver. To resolve this the signal input was switched to POS_IN (driving the entire coil at once rather than in quadrants) which has no switch to bypass.
  • The averaging on the Stanford SR785 was set too low. By increasing the averages from 10 to 25 the signal became more visible.

Unrelated to these issues the signal input was switched to POY11_Q_MON and ITMY_POS_IN as part of the debugging process. The function generator used was switched from the Stanford to the Siglant SDG 1032X.

An unrelated issue but note worthy was the Lenovo 40m laptop used to measure the IFO state (locked or unlocked) ran out of battery in a very short timespan.

To gauge where the resonance of the test masses are FEA model of a simple 40m test mass was computed to give an esitimate at what frequency the eigenmodes exist. For the first two modes the model gave resonances at 20.366 kHz (butterfly mode) and 28.820 kHz (drumhead mode). Then by measuring with an acquisition time of 1 s at they frequencies on the SR785 and injecting broad band white noise with a mean of 0 V and a stdev of 2 V, small peaks were seen above the noise at 20.260 kHz and 28.846 kHz. By then injecting a sine wave at those frequencies with 9 Vpp, the peak became clearly visible above the noise floor.

The last step is to measure the natural decay of these modes when the excitation is turned off. It is difficult to tell currently if these are indeed eigenmodes or just large cavity injections with an associated stabilisation time (what could appear as a ringdown decay). More investigation is required.


Attachment 1: 20191205_132158.jpg
  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
  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.

  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
  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.


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

  15071   Wed Dec 4 09:11:42 2019 YehonathanUpdateCDSReboot script

After the CDSs crashed we run the rebootC1LSC.sh script.

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

The resulting CDS screen is a bit different than what was reported before (attached). Also, not all watchdogs were restored.

We restore the remaining watchdogs and do XARM locking. Everything seems to be fine.

Attachment 1: medmScreen11.ps
  15070   Wed Dec 4 08:54:07 2019 YehonathanUpdateIMCMirror analog shaking

{Yehonathan, Gavin}

Yesterday we tried to shake ITMX with a function generator in order to observe the 28.8kHz drum mode.

We laid a long BNC cable that runs from the YARM to the XARM. This cable either needs to be collected back to the BNC big plastic cable box under the IMC or be labeled so that it could be found easily in the future.

First, we tried to shake it at a lower frequency (100's of Hz) where the shaking should be easily observed in the POSX channel. We try driving the POS channel on the ITMX servo but nothing happens. Most likely it is disconnected.

While setting up for shaking the individual OSEM channels 4 CDSs crashed (c1lsc, c1ass, c1oaf, c1cal).


  15069   Tue Dec 3 22:41:17 2019 shrutiUpdateGeneralPLL for PM measurement

I worked on the setup up for the phase modulation measurement of the X end NPRO PZT. A previous similar measurement can be found here (12077). The setup was assembled based on the schematic in Attachment1.

Mixer used: Level 7, Mini circuits ZP-3+
LPF: up to 1.9MHz

Cables exiting the PSL table:
1. LO (Marconi -> Mixer)
2. RF (PSL+X beat note -> Mixer) The cable for this was taken from the Beat Mouth (otherwise connected to the oscilloscope)
3. Ext modulator (SR560 -> Marconi)

The long cable labled 'X Green Beat' was used to connect to the PZT (from the network analyzer).

Observations: The beat note kept floating between 0 and ~100 MHz

The PLL part of the circuit was tested coarsely with the spectrum analyzer function of the Agilent, where the loop was seen to stabilize when the carrier frequency of the Marconi was close to the instantaneous beat frequency.



Attachment 1: PM_measurement.jpeg
  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. 


 I'll work on preparing a notebook.

Attachment 1: uPDH.pdf
  15067   Tue Dec 3 20:32:37 2019 ranaOmnistructureDAQNDS2 situation

Recently, accordian to Gautam, the NDS2 server has been dying on Megatron ~daily or weekly. The prescription is to restart the server.

  1. I could find no instructions (that work) in the elog or wiki. We must remove the misleading entries from the wiki and update it with whatever works as of today.
  2. There is a line (which has been commented out) in the Megatron crontab which is close to the right command, but it has the wrong path.
  3. Running the command from the CRON (/home/nds2mgr/nds2-megatron/test_restart), gives several errrors.
  4. when I run the init.d command which is in the script, it seems to run fine
  5. the server then takes several minutes to get itself together; i.e. just because it is running doesn't mean that you can get data. I recommend waiting 5-10 minutes.

Also, megatron is running Ubuntu 12 !! Let's decide on a day to upgrade it to a Debian 18ish....word from Rolf is that Scientific Linux is fading out everywhere, so Debian is the new operating system for all conformists.

Attachment 1: getData.py
#!/usr/bin/env python
# this function gets some data (from the 40m) and saves it as
# a .mat file for the matlabs
# Ex. python -O getData.py

from scipy.io import savemat,loadmat
import scipy.signal as sig
from astropy.time import Time
import nds2
... 116 more lines ...
Attachment 2: chanlist.txt
  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.

                   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

  15065   Tue Dec 3 14:52:13 2019 ranaUpdateIMCQ Measurement of Test Masses



[Yehonathan, Gavin]

  1. Lock IMC
  2. Lock one of the arms (only) using the IR PDH signal feeding back to an ETM.
  3. Excite the ITM using the SR785 near 28.8 kHz
  4. Look for the resulting peak using the SR785 spectra of the POX or POY error signal from the demod board
  5. Based on the calibrated noise level of the POX/POY, estimate what the SNR will be of the internal mode peak.
  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.


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

  15063   Tue Dec 3 00:10:15 2019 KojiUpdateALSEY uPDH post mixer LPF

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

  15062   Tue Dec 3 00:03:57 2019 gautamUpdateLSCGreen ALS also shows elevated noise with high arm buildup


  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
  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
  15060   Mon Dec 2 20:18:18 2019 ranaUpdateALSEY uPDH post mixer LPF

                   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

  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
Attachment 2: EY_uPDH_OLTF.pdf
  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
Attachment 2: ALS_TR_RIN.pdf
Attachment 3: GreemAM.pdf
  15057   Sun Dec 1 13:38:48 2019 ranaUpdateLSCNo shaking but no inspiration either

yes, reasonable

  15056   Wed Nov 27 23:24:01 2019 gautamUpdateLSCNo shaking but no inspiration either


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?

  15055   Wed Nov 27 18:51:22 2019 Gavin WallaceUpdateIMCQ Measurement of Test Masses

[Yehonathan, Gavin]

As the resonant modes of the 40m TMs are at high frequencies (starting at 28.8 kHz) we started background checks to understand if we would be able to see resonant frequency excitations in the DCPD output. We used the SR785 in the Q_OUT_DEMODULATOR port of the INPUT_MODE_CLEANER to measure around this frequency. Currently we could not see any natural excitation about the noise floor indicating it may not be possible to see such a small excitation. In any case we are conducting additional measurements in the I_MON port of 1Y2_POY11 to understand if this is a certainty.

  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
Attachment 2: PRC_FF.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).


if the RP don't fit

u must acquit

sweep the laser amplitude

to divine the couplin w certitude

  15052   Wed Nov 27 13:14:02 2019 ranaUpdateLSCITMX and ITMY OSEMs with low and high circulating power

if the RP don't fit

u must acquit

sweep the laser amplitude

to divine the couplin w certitude

  15051   Wed Nov 27 12:16:52 2019 gautamUpdateLSCITMX and ITMY OSEMs with low and high circulating power


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.


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

Attachment 1: ITMXshadowSensors.pdf
Attachment 2: ITMYshadowSensors.pdf
  15050   Tue Nov 26 18:16:08 2019 ranaUpdateLSCPOX / POY calibration

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


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.
  15049   Tue Nov 26 17:07:41 2019 gautamUpdateLSCPOX / POY calibration


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.


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
Attachment 2: ALSnoise_20191125.pdf
  15048   Tue Nov 26 13:33:33 2019 YehonathanUpdateCamerasMC2 Camera rotated by 90 degrees

MC2 analog camera was rotated by 90 degrees. Orientation correctness was verified by exciting the MC2 Yaw degree of freedom.

Attached before and after photos of the camera setup.

Attachment 1: MC2AnalogCameraAfter.jpg
Attachment 2: MC2AnalogCameraBefore.jpg
  15047   Mon Nov 25 22:10:26 2019 shrutiUpdateNoiseBudgetDiagnostics

This is to help troubleshoot the excess noise measured earlier.

The following channels were measured at GPS times 1258586880 s and 1258597457 s, corresponding to low and high Power Recycling Gain (PRG) respectively.

Excess noise was seen between 25-110 Hz in the high PRG case when compared to the low PRG case in the following channels:


C1:LSC-CARM-IN1_DQ (shown in Attachment 1 where the reference is low PRG)






Surprisingly, it was also seen to a smaller extent in (refer Attachment 3)



A different type of noise spectrum, attributed to known electronic effects, was observed for

C1:SUS-ITMY_SENSOR_{LL,UL}    (refer Attachment 2)


These did not show any significant change in the noise spectrum:

C1:LSC-DARM-IN1_DQ (shown in Attachment 1 where the reference is low PRG)




C1:SUS-ITMY_SENSOR_{LR,UR}  (refer Attachment 2)


Broadband noise in:



Attachment 1: LSC.pdf
Attachment 2: ITMY.pdf
Attachment 3: ITMX_L.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
  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
  15044   Thu Nov 21 19:08:58 2019 gautamUpdateLSCHigh BW lock of Y arm length to PSL frequency


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.


  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
Attachment 2: CM_UGF.pdf
Attachment 3: IN2_ACcoupling.pdf
  15043   Thu Nov 21 13:14:33 2019 KojiUpdateLSCCM board study

One of the differences between the direct POY and the CM_SLOW POY is the presence of the CM Servo gain stages. So this might mean that you need to move some of the whitening gain to the CM IN1 gain.

  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
  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
  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
Attachment 2: TRX_TRY_comparison.pdf
Attachment 3: IMG_8186.JPG
  15039   Wed Nov 20 17:20:24 2019 YehonathanUpdateLSCQPD Investigation

{Gautam, Yehonathan}

In search of the source of discrepancy between the QPD readings in the X and Y arms, we look into the schematics of the QPD amplifier - DCC #D990272.

We find that there are 4 gain switches with the following gain characteristics (The 40m QPD whitening board has an additional gain of 4.5):

S4 S3 S2 S1 V/A
0 0 0 0 2e4
0 0 0 1 2e5
0 0 1 0 4e4
0 0 1 1 4e5
0 1 0 0 1e5
0 1 0 1 1e6
0 1 1 0 2e5
0 1 1 1 2e6
1     0 5e2
1     1 5e3

Switch 4 bypasses the amps controlled by switch 2 and 3 when it is set to 1 so they don't matter in this state.

Note that according to elog-13965 the switches are controlled through the QPD whitening board by a XT1111a Acromag whose normal state is 1.

Also, according to the QPD amplifier schematics, the resistor on the transimpedance, controlled by switch 1, is 25kOhm. However, according to the EPICS it is actually 5kOhm. We verify this by shining the QPD with uniform light from a flashlight and switching switch1 on and off while measuring the voltages of the different segments. The schematics should be updated on the DCC.

Surprisingly, QPDX switches where 0,0,0,0 while QPDY switches where 1,0,0,1. This explains the difference in their responses.

We check by shining a laser pointer with a known power on the different segments of QPDX that we get the expected number of counts on the ADC and that the response of the different segments is equal.

gautam edits:

  1. Lest there be confusion, the states of the switches in the (S1, S2, S3, S4) order are (0,0,0,0) for QPDX and (0,1,0,1) for QPDY.
  2. The Acromag XT1111 is a sinking BIO unit - so when the EPICS channel is zero, the output impedance is low and the DUT (i.e. MAX333) is shorted to ground. So, the state of the MAX333 shown on the schematics corresponds to EPICS logic level 1, and the switched state corresponds to logic level 0.
  3. For the laser pointer test, we used a red laser pointer. Using a power meter, we measured ~100uW of 632nm power. However, we think this particular laser pointer had failing batteries or something because the spot looked sometimes brighter/dimmer to the eye. Anyways, we saw ~10,000 ADC counts when illuminating a single segment (with the QPD gain switches at the 0,0,0,0 setting, before we changed anything). We expect 100uW * 0.4 A/W * 500 V/A * 10 * 40 * 4.5 * 3267.8 cts/V = ~12000 cts. So everything seems to check out. We changed the gain to the 5kohm setting and bypassed the subsequent gain stages, and saw the expected response too. The segments were only balanced to ~10%, but presumably this can be adjusted by tweaking digital gains.
ELOG V3.1.3-