I would think that we want to fix the I/Q orthog inside the demod board by trimming the splitter. Mixing the Q phase signal to the I would otherwise allow coupling of low frequency Q phase junk from HOMs into the MC lock point.

[valera, gautam]

Motivation: see this elog

I was fiddling around for a few days trying to implement the method outlined in this paper to null this offset - I will post a separate elog about my efforts but Valera pointed out that we could try injecting an AF modulation at the IN2 input of the MC Servo Board. Last night, we hooked up an SR function generator (f = 312Hz, A = 0.01Vpp, IN2 gain = -5dB) to the unused BNC IN2 input of the MC Servo board. To avoid any additional offsets from the AO path during this measurement, I disconnected the LEMO cable (it is labelled).

We looked at the spectrum of the MC transmission around 312Hz and also 2*f = 624Hz. As a result of this modulation, we expect in the transmitted power, dP/P, a 2f term with amplitude ~(X_mod/X_0)^2 and a term at f with amplitude ~(X_offset * X_mod / X_0^2) - I may have missed out some numerical factors of order 1. So the latter should vanish if the offset at the error point is truly zero and the lock-point is the center of the resonance. Last night, we found that an offset in the range of -0.25 V to -0.19 V nulled this peak in the DTT spectrum. Today, the number was -0.05V. So the true offset seems to vary from lock to lock. Here are spectra around f=312Hz for a few different values of the offset slider (the center of the resonance seems to be -0.05V on the MEDM slider at this time).

Do these numbers make sense? Some time ago, I had pulled out the MC Servo board to find out what exactly is going on at this offset summing point. The MEDM slider goes from -10V to 10V, and by measuring the voltage at TP5 (see schematic below), I found that there is a 1/40 scaling factor between what is actually applied and the number on the MEDM slider (so for example, the numbers in the legend in the above plot have to be divided by 40). I've modified the MC Servo Board MEDM screen to reflect this. When I had pulled the board out, I noticed that in addition to the offset voltage applied via the backplane connector, there was also a potentiometer (R50 in the schematic below). I had nulled the voltage at TP5 using this potentiometer, but I guess drifts of ~5mV are possible. 

Discussion on calibration of offset slider in Hz/V:

I've yet to do a rigorous calibration of this slider into Hz, but looking at the spectrum of the transmitted intensity at 2f, we estimated the coefficient (X_mod/X_0) ~ 3e-3 for an offset of 0.2V. dP/P ~1 when the applied modulation equals the linewidth of the cavity, which is 3.6kHz. So 0.2V of offset slider corresponds to ~ 10Hz frequency offset. In other words, I estimate the slider calibration to be 50Hz/V. So with the full range of +/- 10V, we should be able to scan ~1kHz of frequency offset. What does this imply about the variation of the offset slider value that removes the peak at 1f between locks? As mentioned above, this variation is ~0.2V over a day - with the calibration mentioned above, this corresponds to a change in cavity length of ~10um, which seems reasonable to me...

So how did all of this tie in with WFS SUM offsets? We did the following:

• After nulling the length offset using the procedure detailed above, we noticed non-zero offsets on both WFS1 and WFS2 "I" SUM outputs
• So we set the dark offsets and RF offsets for the WFS, with no light incident on the WFS (PSL shutter closed). 
• Re-locking the IMC and closing the WFS loops, we noticed that WFS2 SUM offset was still hovering around 0, but WFS1 SUM offset was ~ -2000cts.
• Looking at some trends on dataviewer, this offset seems to drift around over a few days timescale by a few thousand counts - for example, the WFS1 offset today was +2000cts. Moreover, the WFS1 offset seems to drift around by ~factor of 3 times as much as WFS2 offset in the 24 hour period I looked up (plot to follow)...
• Misaligned MC2 and looked at the sum offset with just the single bounce beam off MC1 onto the WFS

I neglected to screenshot the StripTool from the times we were doing these trials but I have the times, I will pull up some dataviewer plots and upload them here tomorrow...

Now that all the front end models are running, I re-aligned the IMC, locked it manually, and then tweaked the alignment some more. The IMC transmission now is hovering around 15300 counts. I re-enabled the Autolocker and FSS Slow loops on Megatron as well.

IMC is locking now. There was nothing wrong: just a careful alignment + proper gain adj

=== Primary Alignment ===

- I used WFS error signals as the indicator of the PDH error signals. Checked C1:IOO-WFS1_(I/Q)n_ERR and ended up C1:IOO-WFS1_I4_ERR as it showed the largest PDH error PP.

- Then used MC2 and MC3 to align the IMC by maximizing the PDH error and the MC trans (C1:IOO-MC_TRANS_SUM_ERR)

=== Locking procedure ===

Note that the MC REFL path is still configured for the full power input

- (Only at the beginning) Run scripts/MC/mcdown for initialization / Run scripts/MC/MC2tickleOFF just in case

- Enable IOO-MC-SW1 (MC SERVO switch right after "IN1 Gain (dB)").
- Disable 40:4000 boost
- Increase VCO Gain from -15 to 0
- Jiggle IN1 Gain from low to +31 until the lock is achieved

- As soon as the lock is acquired, enable 40:4000
- Increase VCO Gain to +10
- Turn up "SUPER BOOST" from 0 to 3

=== Lock loss procedure ===

Note that the MC REFL path is still configured for the full power input

- Disable IOO-MC-SW1
- Disable 40:4000 boost
- Reduce VCO Gain 0
- Turn down "SUPER BOOST" to 0

- Then jiggle IN1 Gain again to lock the IMC

=== MC2 spot ===

- It was obvious that the MC2F spot was not on the center of the optic.
- I tried to move the spot on the camera as much as possible, but this did not make the trans beam to the center of the MC end QPD
- I had the impression that the trans beam started to be clipped when the beam was moved towards the end QPD,

We need to reestablish the reasonable/consistent MC2 spot on the mirror, the MC end optics, and the QPD.
We will need to use MC2 dithering and A2L coupling to determine the center of the mirror

But as long as the transmission is maximized, the transmitted beam thru MC1 and MC3 follows the input beam. So we can continue the vent work

The current maximized transmission was ~1300. MC1 refl CCD view was largely off -> The camera path was adjusted.

=== MC2 alignment note ===

During the alignment, I noticed a sudden change of the MC2 alignment. There might be some hysteresis in the MC2 suspension. If you are locking the IMC and noticed significant misalignment, the first thing to try is to touch MC2 alignment.

See trend. This is NOT symptomatic of some frozen slow machine - if I disable the WFS servo inputs, the lock holds just fine.

Turns out that the beam was almost completely missing the WFS2 QPD. WTF 😤. I re-aligned the beam using the steering mirror immediately before the WFS2 QPD, and re-set the dark offsets for good measure. Now the IMC remains stably locked. 

Please - after you work on the interferometer, return it to the state it was in. Locking is hard enough without me having to hunt down randomly misaligned/blocked beams or unplugged cables.

I took this opportunity to do some WFS offset updates.

• First I let the WFS servo settle to some operating point, and then offloaded the DC offsets to the IMC suspensions.
• Then I disabled the WFS servo.
• I hand-tweaked MC1 and MC3 PIT/YAW (while leaving MC2 untouched) to minimize IMC REFL (a more sensitive indicator of the optimal cavity alignment than the transmission).
• Once I felt the IMC REFL was minimized (~1-2% improvement), I set the RF offsets for the WFS while the IMC remained locked. I chose this way of setting the RF offsets as opposed to unlocking the cavity and having the high-power TEM00 mode incident on the WFS QPDs.
• Overnight, I'm going to run the MC2 spot position scanning code (in a tmux session on pianosa, started ~945pm) to see if we can find a place where the transmission is higher, looking at Kruthi's code now to see it makes sense...
• The convergence time of the MC2 spot position loop is pretty slow, so the scan is expected to take a while... Should be done by tomorrow morning though, and I expect no work with the IFO tonight.
• Does this loop have to be so slow? Why can't the gain be higher?

I think this offset setting thing is not so good. People do this every few years, but putting offsets in servos means that you cannot maintain a stable alignment when there are changes in the laser power, PMC trans, etc. The better thing is to do the centering of the WFS spots with the unlcoked beam after the control offsets have been offloaded to the suspensions.

Motivation:

To test the hypothesis that the IMC lock duty cycle is affected by the PRM alignment. Rana pointed out today that the input faraday has not been tuned to maximize the output->input isolation in a while, so the idea is that perhaps when the PRM is aligned, some of the reflected light comes back towards the PSL through the Faraday and hence, messes with the IMC lock.

A script to test this hypothesis is running over the weekend (in case anyone was thinking of doing anything with the IFO over the weekend).

Methodology:

I've made a simple script - the pseudocode is the following:

• Align PRM
• For the next half hour, look for downward transitions in the EPICS record for MC TRANS > 5000 cts - this is a proxy for an MC lockloss
• At the end of 30 minutes, record number of locklosses in the last 30 minutes
• Misalign PRM, repeat the above 3 bullets

The idea is to keep looping the above over the weekend, so we can expect ~100 datapoints, 50 each for PRM misaligned/aligned. The times at which PRM was aligned/misaligned is also being logged, so we can make some spectrograms of PC drive RMS (for example) with PRM aligned/misaligned. The script lives at /opt/rtcds/caltech/c1/scripts/SUS/FaradayIsolationTest/FaradayIsolCheck.py. Script is being run inside a tmux session on pianosa, hopefully the machine doesn't crash over the weekend and MC1/CDS stays happy.

A more direct measurement of the input Faraday isolation can be made by putting a photodiode in place of the beam dump shown in Attachment #1 (borrowed from this elog). I measured ~100uW of power leaking through this mirror with the PRM misaligned (but IMC locked). I'm not sure what kind of SNR we can expect for a DC measurement, but if we have a chopper handy, we could put a chopper (in the leaked beam just before the PD so as to allow the IMC to be locked) and demodulate at that frequency for a cleaner measurement? This way, we could also measure the contribution from prompt reflections (up to the input side of the Faraday) by simply blocking the beam going into the vacuum. The window itself is wedged so that shouldn't be a big contributor.

I stopped the test earlier today morning around 11:30am. The log file is located at /opt/rtcds/caltech/c1/scripts/SUS/FaradayIsolationTest/PRM_stepping.txt. It contains the times at which the PRM was aligned/misaligned for lookback, and also the number of MC unlocks during every 30 minute period that the PRM alignment was toggled. This was computed by:

• continuously reading the current value of the EPICS record for MC Trans.
• comparing its current value to its values 3 seconds ago.
• If there is a downward step in this comparison greater than 5000 counts, increment a counter variable by 1.
• Reset counter at the end of 30 minute period.

I think this method is a pretty reliable proxy, because the MC autolocker certainly takes >3 seconds to re-acquire the lock (it has to run mcdown, wait for the next cavity flash, and run mcup in the meantime).

Preliminary analysis suggests no obvious correlation between MC lock duty cycle and PRM alignment.

I leave further analysis to those who are well versed in the science/art of PRM/IMC statistical correlations.

Summary:

There seems to be significant phase loss in the TTFSS path, which is limiting the IMC OLTF to <100 kHz. 

Details:

See Attachment #1 and #2. The former shows the phase loss, while the latter is just to confirm that the optical gain of the error point is roughly the same, since I noticed this after working on and replacing the RF frequency distribution unit. Unfortunately there have been many other changes also (e.g. the work that Rana and Koji did at the IMC rack, swapping of backplane controls etc etc - maybe they have an OLTF measurement from the time they were working?) so I don't know which is to blame. Off the top of my head, I don't see how the RF source can change the phase lag of the IMC servo at 100 kHz. The only part of the IMC RF chain that I touched was the short cable inside the unit that routes the output of the Wenzel source to the front panel SMA feedthrough. I confirmed with a power meter that the power level of the 29.5 MHz signal at that point is the same before and after my work.

The time domain demod monitor point signals appear somewhat noisier in todays measurement compared to some old data I had from 2018, but I think this isn't significant. Once the SR785 becomes available, I will measure the error point spectrum as well to confirm. One thing I noticed was that like many of our 1U/2U chassis units, the feedthrough returns are shorted to the chassis on the RF source box (and hence presumably also to the rack). The design doc for this box makes many statements about the precautions taken to avoid this, but stops short of saying if the desired behavior was realized, and I can't find anything about it in the elog. Can someone confirm that the shields of all the connectors on the box were ever properly isolated? My suspicion is that the shorting is happening where the all-metal N-feedthroughs touch the drilled surfaces on the front panel - while the front and back surfaces of the panel are insulating, the machined surfaces are not.

This is an unacceptable state but no clear ideas of how to troubleshoot quickly (without going piece by piece into the IMC servo chain) occur to me. I still don't understand how the freq source work could have resulted in this problem but I'm probably overlooking something basic. I'm also wondering why the differential receiving at the TTFSS error point did not require a gain adjustment of the IMC servo? Shouldn't the differential-receiving-single-ended-sending have resulted in an overall x0.5 gain?

Update 8 Dec 1200: To test the hypothesis, I bypassed the SR560 based differential receiving and restored the original config. I am then able to run with the original gain settings, and you see in Attachment #4 that the IMC OLTF UGF is back above 100 kHz. It is still a little lower than it was in June 2019, not sure why. There must be some saturation issues somewhere in the signal chain because I cannot preserve the differential receiving and retain 100 kHz UGF, either by raising the "VCO gain" on the MC servo board, setting the SR560 to G=2, or raising the "Common Gain Adjust" on the FSS box by 6 dB. I don't have a good explanation for why this worked for some weeks and failed now - maybe some issue with the SR560? We don't have many working units so I didn't try switching it.

So either there is a whole mess of lines or the frequency noise suppression is limited. Sigh.

[koji, gautam]

1. I began my investigations by measuring the voltage noise of the demod board outputs with an SR560 (G=100) and SR785 in the audio band.
   • Measurement made with PSL shutter closed, LO input of demod board driven with the nominal level of ~2.5dBm, RF input terminated.
   • Motivation was to look for any noise features.
   • Expected noise level is ~2nV/rtHz (Johnson noise of 50ohm) since there are no preamp electronics post SCLF-5 LP filter on this board.
   • Attachment #1 shows the results of the measurement for a few scenarios. Spectra only shown for the I channel but the Q channel was similar. The LO=+5dBm curve corresponds to driving the input at 5dBm with a marconi, to see if the label of the nominal level being +5dBm had anything to it.
   • The arches above 1kHz seemed suspicious to me, so I decided to investigate further.
2. Looking at the IMC Demod board schematic, I I saw that there were 2 ERA-5SMs in there which are responsible for amplifying the 29.5MHz signal which serves as the LO to the oscillators.
3. I pulled the demod board out and tested it on the electronics workbench. Koji and I couldn't make sense of the numbers we were seeing (all measurements made with Agilent analyzer and active FET probe with 100:1 attentuator).
4. We eventually concluded that the ERA-5SMs were not exhibiting the expected gain of ~20dB. So we decided to swap these out.
5. This sort of measurement is not ironclad as the output of the ERA-5SM goes to the mixer whose input impedance is dynamically varying as the diodes are switching. So even after replacing the suspect amplifiers with new ones, we couldn't make the numbers jive.
6. We suspected that the new amplifiers were getting saturated. The 3dB saturation point for the ERA-5SM is spec'ed as ~19dBm.
   • We "measured" this by varying the input signal level and looking for deviation from linearity.
   • We saw that there was ~1dB compression for ~13dBm output from the ERA-5SM (after correcting for all attenuators etc). But this number may not be accurate in the absolute sense because of the unknown input impedance of the mixer.
   • Moreover, looking at the spec sheet for the mixer, JMS-1H, we found that while it wasn't ideal to operate the mixer with the LO level a few dBm below the expected +17dBm, it probably wasn't a show stopper.
7. So we figured that we need 10dB of attenuation between the "nominal" LO input level of 2.7dBm and the input of the demod board in order to keep the ERA-5SM in the linear range. This has now been implemented in the form of an SMA attenuator.
8. IMC locked straight away. But I noticed that PC drive RMS level was unusually large.
9. I found that by increasing the "IN1" gain of the CM board to 12dB (from 2dB) and the "VCO gain" to 10dB (from 7dB), I could recover a transfer function with UGF ~140kHz and PM ~30degrees (need more systematic and wider span measurements of this, and also probably need to optimize the crossover gains). See Attachment #2 for my quick measurement tonight.
10. Updated mcup to reflect these new gains. Tested autolocker a few times, seems to work okay.
11. While it presumably was a good thing to replace the faulty amplifiers and prevent them from saturating, this work has not solved the primary problem of excess frequency noise on the PSL.

It is not clear to me why installing an attenuator to prevent amplifier saturation has necessitated a 10dB increase in the IN1 gain and 3dB increase in the VCO gain. Initially, I was trying to compensate for the gain by increasing the FSS "Common Gain" but in that setting, I found an OLTF measurement impossible. The moment I enabled the excitation input to the CM board, the lock was blown, even with excitation amplitudes as small as -60dBm (from the Agilent network analyzer).

This may also be a good opportunity to test out one of the aLIGO style FET mixer demod boards (recall we have 2 spare from the 4 that were inside the ALS demod box). I'm going to ask Steve to package these into a 1U chassis so that I can try that setup out sometime. From a noise point of view, the aLIGO boards have the advantage of having a x100 preamp stage straight after the mixer+LPF. We may need to replace the lowpass filter though, I'm not sure if the one installed is 1.9MHz or 5MHz.

I've left an SR785 and AG4395 near 1X2 in anticipation of continuing this work tomorrow.

Unrelated to this work - seems like the WFS DC and RF offsets had not been set in a while so I reset these yesterday. The frequent model restarts in recent times may mean that we have to reset these to avoid using dated offset values.

Re-measured the demod board noises after replacing the suspect ERA-5SMs, with LO driven by a marconi at the "nominal" level of 2.5dBm, and RF input terminated. Attachment #1 is the input referred voltage noise spectra. I used the FET low noise pre-amp box for this purpose. I cannot explain the shape of the spectra above 1kHz. I tried doing the measurement on a minicircuits mixer (non-surface mount) and found the shape to be flat throughout the SR785 span. Unclear what else could be going on in the demod board though, all the other components on it are passive (except the ERA-5SMs which were replaced). I considered adopting a PMC style demod setup where we do the demod using some separate Minicircuits Mixer+LowPass filter combo. But the RF flashes for the IMC monitored at the RFmon port are ~0.2Vpp, and so the RF input to the mixer is expected to be ~2Vpp. The minicircuits mixer selection guide recommends choosing a diode mixer with LO level at least 10dBm above the expected RF input signal level, and we don't have any standalone mixers that are >Level 7. I've asked Steve to package the aLIGO demod board in the meantime, but even that might not be a plug and play replacement as the IF preamp stage has ~120degrees phase lag at 1MHz, which is significantly higher than the existing board which just has a SCLF5 low pass filter after the mixer and hence has <45degrees phase lag at 1MHz.

This elog by koji inspired me to consider power supply as a possible issue.

The demod board receives +/-24V DC (which is regulated down to +/-15V DC by 7815/7915), and also +15V DC via the backplane. The ERA-5SM receives DC power from the latter (unregulated) +15V DC. I can't think of why this is the case except perhaps the regulators can't source the current the amp wants? In any case, it doesn't look feasible to change this by cutting any traces on the PCB to me. While I had the board out, I decided to replace the JMS-1H mixers in a last ditch effort to improve the demod board noise. Unfortunately I'm having trouble de-soldering these MCL components from the board. So for now, I'm leaving the demod board out, IMC unlocked. Work will continue tomorrow. 

After some persistence, I managed to get the mixers off.

• Having gotten the mxiers off, I decided to temporarily solder on 50ohms between the LO pin pad and ground on the demod board and measure the RF signal levels in the LO chain with the active probe again.
• Today, with this change, I confirmed that the ERA-5SM begins to saturate closer to the +19dBm advertised on its datasheet. So we need only 2dB of attenuation at the input to have 17dBm at the LO pin of the mixer, assuming 50ohm input impedance.
• But this begs the question - what does minicircuits mean by a Level-YY mixer? Do they expect YY dBm delivered to a 50ohm load? Or do we need to supply YY dBm accounting for the dynamically changing input impedance of the mixer, as monitored by a high impedance probe?
• I soldered on some new mixers (JMS-1H) I procured from Downs earlier today.
• Re-installed the demod board in the eurocrate.

Unfortunately, the coherent noise between the arms persists so the sensing noise injection must be happening elsewhere. IMC seems to lock fine though so I'm leving the autolocker on

I took some transfer functions of the IMC loop and crossover, being careful that the PC drive never exceeding 1V during the measurements. 

I then did some algebra to try and back out the individual loop paths, without having to make assumptions/approximations about the loop gain being high enough. This only really works in the region where both the open loop and crossover measurements have coherence. 

It seems to me that the PZT path has pretty low phase margin on its own, but maybe this is ok, since its never really meant to run solo. The EOM path shape is harder to understand.

The data I took, and code that made the above plot is attached. This afternoon, I'll post an update comparing the measured OLG and crossover to earlier measurements. 

The promised historical comparisons follow. The crossover looks mostly the same as before. There is a new feature in the OLG at 50-60kHz; what could've changed about the EOM path in that time?

We want to measure the IMC round-trip loss using the Isogai et. al. ringdown technique. I spent some time looking at the various bits and pieces needed to make this measurement today, this elog is meant to be a summary of my thoughts.

1. Inventory
   • AOM (in its new mount to have the right polarization) has been installed upstream of the PMC by Johannes. He did a brief check to see that the beam is indeed diffracted, but a more thorough evaluation has to be done. There is currently no input to the AOM, the function generator on the PSL table is OFF.
   • The Isogai paper recommends 3 high BW PDs for the ringdown measurement. Souring through some old elogs, I gather that the QPDs aren't good for this kind of measurement, but the PDA255 (50MHz BW) is a suitable candidate. I found two in the lab today - one I used to diagnose the EX laser intensity noise and so I know it works, need to check the other one. We also have a working PDA10CF detector (150 MHz BW). In principle, we could get away with just two, as the ringdown in reflection and transmission do not have to be measured simultaneously, but it would be nice to have 3
   • DAQ - I think the way to go is to use a fast scope triggered on the signal sent to the AOM to cut the light to the IMC, need to figure out how to script this though judging by some 2007 elogs by rana, this shouldn't be too hard...
2. Layout plans
   • Where to put the various PDs? Keeping with the terminology of the Isogai paper, the "Trans diode" can go on the MC2 table - from past measurements, there is already a pickoff from the beam going to the MC TRANS QPD which is currently being dumped, so this should be straightforward...
     
   • For the "Incident Diode", we can use the beam that was used for the 3f cancellation trials - I checked that the beam still runs along the edge of the PSL table, we can put a fast PD in there...
   • For the "REFL diode" - I guess the MC REFL PD is high BW enough, but perhaps it is better to stick another PD in on the AS table, we can use one of the existing WFS paths? That way we avoid the complicated transfer function of the IMC REFL PD which is tuned to have a resonance at 29.4MHz, and keeps interfacing with the DAQ also easy, we can just use BNC cables...
   • We should be able to measure and calibrate the powers incident on these PDs relatively easily.
      
3. Other concerns
   • I have yet to do a thorough characterization of the AOM performance, there have been a number of elogs noting possible problems with the setup. For one, the RF driver datasheet recommends 28V supply voltage but we are currently giving it 24V. In the (not too distant) past, the AOM has been seen to not be very efficient at cutting the power, the datasheet suggests we should be able to diffract away 80% of the central beam but only 10-15% was realized, though this may have been due to sub-optimal alignment or that the AOM was receiving the wrong polarization...
4. Plan of action
   • Check RF driver, AOM performance, I have in mind following the methodology detailed here
   • Measure PMC ringdown - this elog says we want it to be faster than 1us
   • Put in the three high BW PDs required for the IMC ringdown, check that these PDs are working
   • Do the IMC ringdown

Does this sound like a sensible plan? Or do I need to do any further checks?

We expect that the MC sus are susceptible to the temperature change and the alignment drifts away with time.

Here is the proper alignment procedure.

0) Assume there is no TEM00 flash or locking, but the IMC is still flashing with higher-order modes.

1) Use the CCD camera and WFS DC spots to bring the beam to the nominal position.

2) Use only MC2 and MC3 to align the cavity to have low-order modes (TEM00,01,02 etc)

3) You should be able to lock the cavity on one of these modes. Minimize the reflection (maximize the transmission) for that mode.

4) This should allow you to jump to a better lower-order mode. Continue alignment optimization only with MC2/3 until you get TEM00.

5) Optimize the TEM00 alignment only with MC2/3

6) Look at the MC end QPD. use one of the scripts in scripts/MC/moveMC2 . Note that the spot moves opposite to the name of the scripts. i.e. MC2_spot_down moves the spot up, MC2_spot_right moved the spot left, etc...
These scripts move MC1/2/3 and try to keep the good MC transmission.

7) moveMC2 scripts are not perfect. As you use them, it makes the MC alignment gradually degraded. Use MC2 and MC3 to recover good transmission.

8) If MC2 spot is satisfactory, you are done.

-------------

Step 6-8 can be done with the WFS on. This way, you can skip step 7 as the WFS servo takes care of it. But if the spot move is too fast, the servo can't keep up with the change. If so, you have to wait for the settling of the servo. Once the spot position is satisfactory, MC servo relief should be run so that the servo offset (in actuation) can be offloaded to the bias slider.

I've added two curves to the NB. Both are measured (with FET preamp) at the output of the demod board, with the LO driven at the nominal level by the Wenzel RF source pickoff (as it would be when the IMC is locked) and the RF input connected to the IMC REFL PD. For one curve, I simply closed the PSL shutter, while for the other, I left the PSL shutter open, but macroscopically misaligned MC2 so that there was no IMC cavity. So barring RFAM, there should be no PDH signal on the REFL PD, but I wanted to have light on there. I'm not sure if I understand the difference between these two curves though, need to think on it. Perhaps the IMC REFL PD's optical/electrical response needs to be characterized?

Since ~ 2 hours ago, the IMC autolocker has not been able to keep the IMC locked. I don't see any obvious trends in the wall StripTool that may point to what's going on. For the brief periods in which a TEM00 mode is locked, the PC Drive RMS level is ~5x what the nominal level is, and while the autolocker is trying to lock the IMC, the PC drive RMS level is hovering around 4V DC, which is high. The PMC Error and Control signal spectra show huge 60 Hz (and harmonics) peaks, and indeed this is visible in the time domain signals as well (on ndscope or on the oscilloscope on the PSL table), but this is not a new feature in the last two hours. Usually, this kind of problem signals that either/both the c1psl or c1iool0 slow machines need to be power-cycled, but I confirmed that both machines are online and telnet-able. Possibilities: (i) some card in the c1psl / c1ioo crates have failed or (ii) something in the MC/FSS electronics chain has failed or (iii) there is a huge amount of excess high-frequency noise from the NPRO.

I am leaving the PSL shutter closed.

WFS loops were running for past 2 hours when I made the overall gain slider zero at:

PDT: 2022-10-18 20:42:53.505256 PDT
UTC: 2022-10-19 03:42:53.505256 UTC
GPS: 1350186191.505256

The output values are fixed to a good alignment. IMC transmission is about 14100 counts right now. I'll turn on the loop tomorrow morning. Data from tonight can be used for monitoing open loop noise.

Turning WFS loops back on at:

PDT: 2022-10-19 09:48:16.956979 PDT
UTC: 2022-10-19 16:48:16.956979 UTC
GPS: 1350233314.956979

Overtime the coil output filters on IMC optics have drifted into a bad configuration. Today at the meeting, Rana told us the correct configuration for these filters. I'll summarize this here and we have changes the filters on all IMC optics, MC1, MC2, and MC3 to match this configurations:

MC1 and MC3

Analog side:

Both MC1 and MC3 have a 28 Hz 5th order elliptical low pass filter as teh dewhitening filter in LIGO-D000316

Digital side:

At the coil output filters named as C1:SUS-MC1_ULCOIL, the filter module FM9 is connectd in RTCDS to the analog dewhitening filter such that only one of the two can remain ON. So For MC1 and MC3, we put a ellip("LowPass", 5, 1, 50, 28) filter on FM9 for all 5 coil output filters.

Note: We do not add a inverse dewhitening filter at FM10 like most other optics as inverting this filter will create resonant peaks at the dips of the elliptical filter which we want to avoid and we anyways do not use MC1 and MC3 optics for any kind of actuation above 20 Hz.

MC2

Analog side:

For MC2, the dewhitening filter is a 10 Hz pole, 30Hz zero like most other suspended optic.

Digital side:

At the coil output filters named as C1:SUS-MC2_ULCOIL, the filter module FM9 is connectd in RTCDS to the analog dewhitening filter such that only one of the two can remain ON. So For MC2 we put a SimDW filter which is matched to the anlog filter. We also put a InvDW filter on FM10, which is the analytical inverse of the SimDW filter. This filter does the anti-dewhitening required on the digital side and should be always ON.

To MC2 equivalent to other IMC optics in terms of overall transfer function for the local damping loops and ASC loops, we need additional 28 Hz elliptical low pass filter in these loops. But such a filter should not be in the path of LSC feedback when MC2 is used for locking CARM with a bandwidth of ~100 Hz. Thus, we put a ellip("LowPass", 5, 1, 50, 28) filter on FM6 of the following filters, which should be always ON as well:

• C1:SUS-MC2_ASCPIT
• C1:SUS-MC2_ASCYAW
• C1:SUS-MC2_SUSPOS
• C1:SUS-MC2_SUSPIT
• C1:SUS-MC2_SUSYAW
• C1:SUS-MC2_SUSSIDE

Effect on 60 Hz Noise

With the above changes, we see that the 60 Hz noise is same as the previous levels when we use the analog dewhitening filter (28 Hz elliptical filter) for MC1. We can move forward with our science experiments with that configuration but there is still something fishy about MC1 in comparison to MC3 which does not have this behavior. So this still needs to be looked at in future.

Wiki page for filter details and configurations

Information of this kind should be stored in a wiki page in my opinion. We should have a page where we list all common filter configurations for our suspensions and other loops, that can be generally classified and is useful for understanding legacy configuration for future folks who work here. I'm starting such a wiki page here, where I'll dump more information as I collect it and get time. Everyone is encouraged to update this in there free/procrastination times.

Also reply to: 40m/16125

I migrated the code used in 40m/16125 to our scripts git repo and used it to apply offsets to IMC optics and noticing the parabolic change in the transmission values. Fitting the data with parabola and using the calculations mentioned in the previous post, we get following angular actuation calibration at DC from the PIT/YAW alignment output channels (cts) to actual motion in (urad):

*Note that in the previous post, the radius of curvature of MC2 used was wrong and has been corrected in this calculation to 17.87 m taken from Gautam's thesis Table A.1

Due to lack of time, we ran test faster on MC2, hence more uncertainty in it's results. Also, during MC1 YAW test, lock for breifly lost which required me to manually throw away some data points, but it did not affect the quality of fit much. Please see attached the data plots and fit.

For calibration at AC, another test needs to be performed which I did not do right now. 40m/16125 also describes how to do that, so someone can repeat that in future.

[Paco, JC]

We came in this morning and noted the IMC was grossly misaligned, with MC3 still damped but with >= 100 rms motion in all coil monitors  (a lot but not enough to trip the WD)... Turning off the WFS didn't do much so it was obviously an issue with the recent f2A output filters, so we turned all off (though only MC3 had this excess motion). After this we aligned IMC, engaged the lock and turned WFS back on.

There was no elog about f2A beyond this test scheduled to run Friday, I guess the filters were meant to stay on long term?

Two days ago I opened the PSL shutter by switching the switch on the shutter driver. That caused the shutter's switch on the medm screen to work in reversed mode: open meant closed and closed meant open.

I fixed that. Now the medm screen switch state is correct.

The IMC has been misbehaving for the last 5 hours. Why? I turned the WFS servos off. afaik, aaron was the last person to work on the IFO, so i'm not taking any further debugging steps so as to not disturb his setup.

That was likely me. I had recentered the beam on the PD I'm using for the armloss measurements, and I probably moved the wrong steering mirror. The transmission from MC2 is sent to a steering mirror that directs it to the MC2 transmission QPD; the transmission from this steering mirror I direct to the armloss MC QPD (the second is what I was trying to adjust).

Note: The MC2 trans QPD goes out to a cable that is labelled MC2 op lev. This confusion should be fixed.

I realigned the MC and recentered the beam on the QPD. Indeed the beam on MC2 QPD was up and left, and the lock was lost pretty quickly, possibly because the beam wasn't centered. Lock was unstable for a while, and I rebooted C1PSL once during this process because the slow machine was unresponsive.

When tweaking the alignment near MC2, take care not to bump the table, as this also chang es the MC2 alignment.

Once the MC was stably locked, I was able to maximize MC transmission at ~15,400 counts. I then centered the spot on the MC2 trans QPD, and transmission dropped to ~14800 counts. After tweaking the alignment again, it was recovered to ~15,000 counts. Gautam then engaged the WFS servo and the beam was centered on MC2 trans QPD, transmission level dropped to ~14,900.

Gautam was doing some DRMI locking, so I replaced the photodiode at the AS port to begin loss measurements again.

I increased the resolution on the scope by selecting Average (512) mode. I was a bit confused by this, since Yuki was correct that I had only 4 digits recorded over ethernet, which made me think this was an i/o setting. However the sample acquisition setting was the only thing I could find on the tektronix scope or in its manual about improving vertical resolution. This didn't change the saved file, but I found the more extensive programming manual for the scope, which confirms that using average mode does increase the resolution... from 9 to 14 bits! I'm not even getting that many.

There's another setting for DATa:WIDth, that is the number of bytes per data point transferred from the scope.

I tried using the *.25 scope instead, no better results. Changing the vertical resolution directly doesn't change this either. I've also tried changing most of the ethernet settings. I don't think it's something on the scripts side, because I'm using the same scripts that apparently generated the most recent of Johannes' and Yuki's files; I did look through for eg tds3014b.py, and didn't see the resolution explicitly set. Indeed, I get 7 bits of resolution as that function specifies, but most of them aren't filled by the scope. This makes me think the problem is on the scope settings.

I ran a BNC from the PD on the AS table along the cable rack to a free ADC channel on the LSC whitening board. I lay the BNC on top of the other cables in the rack, so as not to disturb anything. I also was careful not to touch the other cables on the LSC whitening board when I plugged in my BNC. The PD now reads out to... a mystery channel. The mystery channel goes then to c1lsc ADC0 channels 9-16 (since the BNC goes to input 8, it should be #16). To find the channel, I opened the c1lsc model and found that adc0 channel 15 (0-indexed in the model) goes to a terminator.

Rather than mess with the LSC model, Gautam freed up C1:ALS-BEATY_FINE_I, and I'm reading out the AS signal there.

I misaligned the x-arm then re-installed the AS PO PD, using the scope to center the beam then connecting it to the BNC to (first the mystery channel, then BEATY). I turned off all the lights.

I went to misalign the x-arms, but the some of the control channels are white boxed. The only working screen is on pianosa.

The noise on the AS signal is much larger than that on the MC trans signal, and the DC difference for misaligned vs locked states is much less than the RMS (spectrum attached); the coherence between MC trans and AS is low. However, after estimating that for ~30ppm the locked vs misaligned states should only be ~0.3-0.4% different, and double checking that we are well above ADC and dark noise (blocked the beam, took another spectrum) and not saturating the PD, these observations started to make more sense.

To make the measurement in cds, I also made the following changes to a copy opf Johannes' assess_armloss_refl.py that I placed in /opt/rtcds/caltech/c1/scripts/lossmap_scripts/armloss_cds/   :

• function now takes as argument the number of averages, averaging time, channel of the AS PD, and YARM|XARM|DARK.
• made the data save to my directory, in /users/aaron/40m/data/armloss/

I started taking a measurement, but quickly realized that the mode cleaner has been locked to a higher order mode for about an hour, so I spend some time moving the MC. It would repeatedly lock on the 00 mode, but the alignment must be bad because the transmission fluctuates between 300 and 1400, and the lock only lasts about 5 minutes.

Back to loss measurements.

I replaced the PD I've been using for the AS beam.

I misaligned the x arm.

I tried to lock the y arm, but PRC was locked so I could was unable. Gautam reminded me where the config scripts are.

The armloss measurement script needed two additional modifications:

• It was setting the initial offset of the PIT and YAW demod signals to 0, but due to the clipping on the heater we are operating at an offset. I commented out these lines.
• When the script ran UNFREEZE_DITHER, it was running it using medmrun. The scope script hadn't been using this, and it seemed that when it ran UNFREEZE_DITHER in this way the YARM_ASS servo was passing only '0'. I don't really know why this was, but when I removed the call to medmrun it worked.

I ran successfully the loss measurement script for the x and y arms. I'm getting losses of ~100ppm from the first estimates.

I made the following changes to the lossmap script:

• make the averaging time an input to the script, so we can exceed 2 second averages
• remove anything about getting data from the scope, replace it with the correct analogues to save the averages for POX/POY refl, MC trans, op lev P/Y, and ASDC signal.
• record the GPS time in the file with the cds averages (this way I can grab the full data)
• Added a step in the lossmap script to misalign the optic, so we can continue getting data for the 'misaligned' state, both for the centered and not-centered measurements (that is, for every position on the lossmap).

When the optic aligns itself not at the ideal position, I'm noticing that it often locks on a 01. When the cavity is then misaligned and restored, it can no longer obtain lock. To fix this, I've moved my 'save' commands to just before the loop begins. This means the script may take longer to run, but as long as the cavity is initially locked and well aligned, this should make it more robust against wandering off and never reacquiring lock.

I left the lossmap script running for the x-arm. Next would be to run it for the y arm, but I see that after stepping to a few positions the lock is again lost. It's still trying to run, but if you want to stop it no data already taken will be lost. To stop it, go to the remaining terminal open on rossa and ctrl+c

the analysis needs:

• Windowing
• Filter, don't average
• detrend to get rid of the linear drifts in lock that we see.
  • Is this the right thing?

I made additional measurements on the x and y arms, at 5 offset positions for each arm (along with 6 measurements at the "zeroed" position).

While I stopped by the lab this morning to pick up some things, I took the opportunity to continue the recovery.

• IMC suspensions were sufficiently misaligned that the autolocker couldn't re-acqurie the lock. I manually recovered the alignment and now the IMC is locked again.
• ETMY illuminator was left ON, I turned it off. In the process, I modified the illuminator ON/OFF script to be compatible with python3, but unfortunately, it was written in a way that doesn't permit backward compatibility, so now the illuminators can't be turned ON/OFF via the MEDM screen on pianosa (since the default python is 2.7 on that machine). But it does work on rossa, which I'm using as my primary workstation now (hence the change).
• ITMX watchdog trip threshold was manually reset to the nominal value - the rampdown script was working, but the threshold was ~1400cts (normally ~200 cts) even at 1130am this morning (>12 hours after Koji's work yesterday evening), so I just accelerated the process.
• Suspension realignment - using a mix of green and IR beams and the various cameras/photodiodes as diagnostics, I roughly restored the alignment of all the suspensions, except ETMY. I can see IR resonances in the X arm now.

At some point, we should run the suspension eigenmode routine (kick optics, let them ringdown, measure peak locations and Qs) to confirm that the remaining suspensions are okay, will also help in actuation re-allocation efforts on ETMY. But I didn't do this today.

Leaving the lab at 1150.

The autolocker was struggling to lock the IMC. I disabled the autolocker and locked the IMC manually. It seems happy right now. 

With PMC trans at 0.717 counts, the IMC trans sum is ~15230.

Recently we wondered at the meeting what the IMC round trip loss was. I had done several ringdowns in the winter of 2017, but because the incident light on the cavity wasn't being extinguished completely (the AOM 0th order beam is used), the full Isogaio et. al. analysis could not be applied (there were FSS induced features in the reflection ringdown signal). Nevertheless, I fitted the transmission ringdowns. They looked like clean exponentials, and judging by the reflection signals (see previous elogs in this thread), the first ~20us of data is a clean exponential, so I figured we may get some rough value of the loss by just fitting the transmission data. 

The fitted storage time is .However, this number isn't commensurate with the 40m IMC spec of a critically coupled cavity with 2000ppm transmissivity for the input and output couplers.

Attachment #1: Expected storage time for a lossless cavity, with round-trip length ~27m. MC2 is assumed to be perfectly reflecting. The IMC length is known to better than 100 Hz uncertainty because the marconi RF modulation signal is set accordingly. For the 40m spec, I would expect storage times of ~40 usec, but I measure almost 30% longer, at ~60 usec.

Attachment #2: Fits and residuals from the 10 datasets I had collected. This isn't a super informative plot because there are 10 datasets and fits, but to eye, the fits are good, and the diagonal elements of the covariance matrix output by scipy's curve_fit back this up. The function used to fit the t > 0 portions of these signals (because the light was extinguished at t=0 by actuating on the AOM) is , where A and tau are the fitted parameters. In the residuals, the same artefacts visible in the reflection signal are seen.

Attachment #3: Scatter plot of the data. Width of circles are proportional to fit error on individual measurements (i just scaled the marker size arbitrarily to be able to visually see the difference in uncertainty, the width doesn't exactly indicate the error), while the dahsed lines are the global mean and +/- 1 sigma levels.

Attachment #4: Cavity pole measurement. Using this, I get an estimate of the loss that is a much more believable .

I started putting together some code to implement some ideas we discussed at the Tuesday meeting here. Pipeline isn't setup yet, but i think it's commented okay so if people want to play around with it, the code lives on the 40m gitlab. 

Model parameters:

• T+ --- average transmission of MC1 and MC3.
• T- --- difference in transmission between MC1 and MC3 (this basis is used rather than T1 and T3, because the assumption is that since they were coated in the same coating run, the difference in transmission should be small, even if there is considerable uncertainty in the actual average transmission number.
• T2 --- MC2 transmission.
• Lrt --- Round trip loss in the cavity.
• "sigma" --- a nuisance parameter quantifying the error in the time domain ringdown data.

Simulation:

• Using these model parameters, calculate some simulated time-domain ringdowns. Optionally, add some noise (assumed Gaussian).
• Try and back out the true values of the model parameters using emcee - priors were assumed to be uniformly distributed, with a +/- 20% uncertainty around the central value.
• For a first test, see if there is any improvement in the parameter estimation uncertainty using only transmission ringdown vs both transmission and reflection.

Initial results and conclusions:

• Attachment #1 - Simulated time series used for this study. The "fit" trace is computed using the median values from the monte-carlo.
• Attachment #2 - Corner plots showing the distribution of the estimated parameter values, using only transmission ringdown. The "true" values are indicated using the thick blue lines.
• Attachment #3 - Corner plots showing the distribution of the estimated parameter values, using both transmission and reflection ringdowns.
• The overall approach seems to work okay. There seems to be only marginal improvement in the uncertainty in estimated parameters using both ringdown signals, at least in the simulation.
• However, everything seems pretty sensitive to the way the likelihood and priors are coded up - need to explore this a bit more.

Next steps:

• Add more simulated measurements, see if we can constrain these parameters more tightly. 
• Use linear error analysis to see if that tells us which measurements we should do, without having to go through the emcee.

There still seems to be some data quality issues with the ringdown data I have, so I don't think we really gain anything from running this analysis on the data I have already collected - but in the future, we can do the ringdown with complete extinguishing of the input light, and repeat the analysis.

As for whether we should clean the IMC mirrors - I'm going to see how much power comes out at the REFL port (with PRM aligned) this afternoon, and compare to the input power. This technique suffers from uncertainty in the Faraday insertion loss, isolation and IMC parameters, but I am hoping we can at least set a bound on what the IMC loss is.

Over the weekend, I worked a bit on getting these ringdowns going. I will post a more detailed elog tomorrow but here is a quick summary of the changes I made hardware-wise in case anyone sees something unfamiliar in the lab...

• PDA10CF PD installed on PSL table in the beam path that was previously used for the 3f cancellation trials
• PDA255 installed on MC2 trans table, long BNC cable running from there to vertex via overhead cable tray
• PDA255 installed on AS table in front of one of the (currently unused) WFS

I spent a while in preparation for these trials (details tomorrow) like optimizing AOM alignment/diffracted power ratio, checking AOM and PMC switching times etc, but once the hardware is laid out, it is easy to do a bunch of ringdowns in quick succession with an ethernet scope. Tonight I did about 12 ringdowns - but stupidly, for the first 10, I was only saving 1 channel from the oscilloscope instead of the 3 we want to apply the MIT method.

Here is a representative plot of the ringdown - at the moment, I don't have an explanation for the funky oscillations in the reflected PD signal, need to think on this.. More details + analysis to follow...

Dec 5 2016, 130pm:

Actually the plot I meant to put up is this one, which has the time window acquired slightly longer. The feature I am referring to is the 100kHz oscillation in the REFL signal. Any ideas as to what could be causing this?

As promised, here is the more detailed elog.

Part 1: AOM alignment and diffraction efficiency optimization

I started out by plugging in the input to the AOM driver back to the DS345 on the PSL table, after which I re-inserted the 24V fuse that was removed. I first wanted to optimize the AOM alignment and see how well we could cut the input power by driving the AOM. In order to investigate this, I closed the PMC, unlocked the PSL shutter, and dialed the PSL power down to ~100mW using the waveplate in front of the laser. Power before touching anything just before the AOM was 1.36W as measured with the Coherent power meter. 

The photodiode (PDA255) for this experiment was placed downstream of the 1%(?) transmissive optic that steers the beam into the PMC (this PD would also be used in Part 2, but has since been removed)...

Then I tuned the AOM alignment till I maximized the DC power on this newly installed PD. It would have been nicer to have the AOM installed on the mount such that the alignment screws were more easily accessible, but I opted against doing any major re-organization for the time being. Even after optimizing the AOM alignment, the diffraction efficiency was only ~15%, for 1V to the AOM driver input. So I decided to play with the AOM driver a bit.

Note that the AOM driver is powered by 24V DC, even though the spec sheet says it wants 28V. Also, the "ALC" input is left unconnected, which should be fine for our purposes. I opted to not mess with this for the time being - rather, I decided to tweak the RF adjust potentiometer on the front of the unit, which the spec sheet says can adjust the RF power between 1W and 2W. By iteratively tuning this pot and the AOM alignment, I was able to achieve a diffraction efficiency of ~87% (spec sheet tells us to expect 80%), in a switching time of ~130ns (spec sheet tells us to expect 200ns, but this is presumably a function of the beam size in the AOM). These numbers seemed reasonable to me, so I decided to push on. Note that I did not do a thorough check of the linearity of the AOM driver after touching the RF adjust potentiometer as Koji did - this would be relevant if we want to use the AOM as an ISS servo actuator, but for the ringdown, all that matters is the diffraction efficiency and switching time, which seemed satisfactory. 

At this point, I turned the PSL power back up (measured 1.36W just before the AOM). Before this, I estimated the PD would have ~10mW power incident on it, and I wanted it to be more like 1mW, so I I put an ND 1.0 filter on to avoid saturation.

Part 2: PMC "ringdown"

As mentioned in my earlier elog, we want the PMC to cut the light to the IMC in less than 1us. While I was at it, I decided to see if I could do a ringdown measurement for the PMC. For this, I placed two more PDs in addition to the one mentioned in Part 1. One monitored the transmitted intensity (PDA10CF, installed in the old 3f cancellation trial beam path, ~1mW incident on it when PMC is locked and well aligned). I also split off half the light to the PMC REFL CCD (2mW, so after splitting, PMC CCD gets 1mW through some ND filters, and my newly installed PD (PDA255) receives ~1mW). Unfortunately, the PMC ringdown attempts were not successful - the PMC remains locked even if we cut the incident light by 85%. I guess this isn't entirely surprising, given that we aren't completely extinguishing the input light - this document deals with this issue.... But the PMC transmitted intensity does fall in <200ns (see plot in earlier elog), which is what is critical for the IMC ringdown anyways. So I moved on.

Part 3: IMC ringdown

The PDA10CF installed in part 2 was left where it was. The reflected and transmitted light monitors were PDA255. The former was installed in front of the WFS2 QPD on the AS table (needed an ND1.0 filter to avoid damage if the IMC unlocks not as part of the ringdown, in which case ~6mW of power would be incident on this PD), while the latter was installed on the MC2 transmission table. We may have to remove the former, but I don't see any reason to remove the latter PD. I also ran a long cable from the MC2 trans table to the vertex area, which is where I am monitoring the various signals.

The triggering arrangement is shown below.

To actually do the ringdown, here is the set of steps I followed.

1. Make sure settings on scope (X & Y scales, triggering) are optimized for data capture. All channels are set to 50ohm input impedance. The trigger comes from the "TTL" output of the DS345, whose "signal" output drives the AOM driver. Set the trigger to external, the mode should be "normal" and not "auto" (this keeps the data on the screen until the next trigger, allowing us to download the data via ethernet.
2. The DS345 is set to output a low frequency (0.005Hz) square wave, with 1Vpp amplitude, 0.5V offset (so the AOM driver input is driven between 0V and 1V DC, which is what we want). This gives us ~100 seconds to re-lock the IMC, and download the data, all while chilling in the control room
3. The autolocker was excellent yesterday, re-acquiring the IMC lock in ~30secs almost every time. But in the few instances it didn't work, turn the autolocker off (but make sure the MC2 tickle is on, it helps) and manually lock the IMC by twiddling the gain slider (basically manually do what the autolock script does). As mentioned above, you have ~100 secs to do this, if not just wait for 200secs and the next trigger...
4. In the meantime, download the data (script details to follow). I've made a little wrapper script (/users/gautam/2016_12_IMCloss/grabChans.sh) which uses Tobin's original python script, which unfortunately only grabs data one channel at a time. The shell script just calls the function thrice, and needs two command line arguments, namely the base name for the files to which the data will be written, and an IP address for the scope...

It is possible to do ~15 ringdowns in an hour, provided the seismic activity is low and the IMC is in a good mood. Unfortunately, I messed up my data acquisiton yesterday, so I only have data from 2 ringdowns, which I will work on fitting and extracting a loss number from. The ringing in the REFL signal is also a mystery to me. I will try using another PDA255 and see if this persists. But anyways, I think we can exclude the later part of the REFL signal, and fit the early exponential decay, in the worst case. The ringdown signal plots have been uploaded to my previous elog. Also, the triggering arrangement can be optimized further, for example by using the binary output from one of our FEs to trigger the actual waveform instead of leaving it in this low frequency oscillation, but given our recent experience with the Binary Output cards, I thought this is unnecessary for the time being...

Data analysis to follow.

I have left all the PDs I put in for this measurement. If anyone needs to remove the one in front of WFS2, go ahead, but I think we can leave the one on the MC2 trans table there...

The MC1 suspension troubles vanished as they came - but the IMC was remaining locked stably so I decided to do another round of ringdowns, and investigate this feature in the reflected light a bit more closely. Over 9 ringdowns, as seen in the below figure, the feature doesn't quite remain the same, but qualitatively the behaviour is similar.

Steve helped me find another PDA255 and so I will try switching out this detector and do another set of ringdowns later tonight. It just occurred to me that I should check the spectrum of the PD output out to high frequencies, but I doubt I will see anything interesting as the waveform looks clean (without oscillations) just before the trigger...

Following the discussion at the meeting today, I wanted to finish up the WFS tuning and then hand over the IFO to Johannes for his loss stuff. So I did the following:

1. First I set the dark offsets on the WFS (with PSL shutter closed). Then I hand aligned the MC to maximize transmission, centered the beam on the WFS, and set the RF offsets with the MC unlocked.
2. Given that the demod phase for the IMC PDH demodulation board changed by |45 degrees|, I tried changing the digital demod phases in each of the WFS quadrant signals by +/- 45 degrees. Turns out +45 degrees put all the error signal into the I Phase, which is what we use for the WFS loops.
3. Then I attempted to check the WFS loops. I estimated that we have ~25 times the modulation depth now, so I reduced the WFS1/2 P/Y gains by this factor (but left the MC2 TRANS P/Y gains as is). The loop gain seemed overall too low, so I upped the gain till I saw instability in the loop (error signals ringing up). Then I set the loop gains to 1/3 of this value - it was 0.01 before, and I found the loop behaved well (no oscillations, MC TRANS stabilized) at a gain of 0.002.

At this point, I figured I would leave the WFS in this state and observe its behaviour overnight. But abruptly, the IMC behaviour changed dramatically. I saw first that the IMC had trouble re-acquiring lock. Moreover, the PC Drive seemed saturated at 10.0V, even when there was no error signal to the MC Servo board. Looking at the MEDM screen, I noticed that the "C1-IOO_MC_SUM_MON" channel had picked up a large (~3V) DC offset, even with In1 and In2 disabled. Moreover, this phenomenon seemed completely correlated with opening/closing the PSL shutter. Johannes and I did some debugging to make sure that this wasn't a sticky button/slider issue, by disconnecting all the cables from the front panel of the servo board - but the behaviour persisted, there seemed to be some integration of the above-mentioned channel as soon as I opened the PSL shutter.

Next, I blocked first the MC REFL PD, and then each of the WFS - turns out, if the light to WFS2 was blocked and the PSL shutter opened, there was no integrating behaviour. But still, locking the MC was impossible. So I suspected that something was wrong with the LO inputs to the WFS Demod Boards. Sure enough, when I disconnected and terminated those outputs of the RF distribution box, I was able to re-lock the MC fine.

I can't explain this bizzare behaviour - why should an internal monitor channel of the MC Servo board integrate anything when the only input to it is the backplane connector (all front panel inputs physically disconnected, In1 and In2 MEDM switches off)? Also, I am not sure how my work on the WFS could have affected any hardware - I did not mess around at the 1X1 rack in the evening, and the light has been incident on the WFS heads for the past few days. The change in modulation depth shouldn't have resulted in the RF power in this chain crossing any sort of damage threshold since the measured power before the changes was at the level of -70dBm, and so should be at most -40dBm now (at the WFS demod board input). The only thing different today was that the digital inputs of the WFS servos were turned on...

So for tonight I am leaving the two outputs of the RF distribution box that serve as the LO for the WFS demod boards terminated, and have also blocked the light to both WFS with beam blocks. The IMC seems to be holding lock steady, PC drive levels look normal...

Unrelated to this work, but I have committed to the svn the updated versions of the mcup and mcdown scripts, to reflect the new gains for the autolocker...

Yesterday evening I inspected at IMC servo as a preparation of the CM servo recommissioning.

More details to come.

In order to accomplish CARM control with the PSL laser frequency, we use two actuators.

One is the longitudinal direction of one of the MC mirrors. The londitudinal motion of the MC induces
the laser frequency control via the MC servo. As we move the mirror, the range is sort of big,
but the BW is limited by the mechanical response.

The other is the additive offset path. We inject a signal to the additional input port of the MC.
The MC servo supresses this injection by giving the same amount but oppsite sign offset to
the error signal (before the addtion of the inputs). The bandwidth of this AO path is limited
by the bandwidth of the MC servo. Basically the BW of the AO path is about 1/10 of that of the MC servo.

In order to confirm the capability of the AO path as a frequency actuator, 1) OLTF of the MC servo
2) TF of the AO input to the servo error was measured.

Attachment 1 shows the openloop TF of the MC servo. The UGF seems just little bit higher than
100kHz. The OLTF is empirically modelled by LISO as seen in the figure.

Attachment 2 shows the TF from the additive input (In2) to the error monitor (MC Servo module Q error mon).
The gain setting of the MC servo box was: In1 +18dB, In2 0dB. The measured TF has arbitorary gain 
due to the gain setting, the measuemrent data was multiplied by 4 to mach the DC value to the unity.
This is to compare the measurement with the prediction from the OLTF.

The AO path TF is expected to show the character of -G/(1+G) where G is the OLTF. In my case,
G = 0.75*OLTF showed the best maching. There might have been some misalignment of the MC
upon the AO path measurement as I found after the measurement.

From the plot , we can see that the response is flat up to 20kHz. Above that it rapidly raises.
This should be dealt with the CM servo filter as the bump may hit the unity gain. Since we have to use
strong roll off to avoid the bump, this will eat the phase margin at low frequency.

In the case that we don't like this bump:
This bump is caused by low phase mergin of the OLTF at 30~40kHz. If the total gain
is increased, the bump is reduced. Or, we can decrease the PZT loop gain in order to
reduce the dip at the crossover ferquency between the PZT and PC loops. In both cases,
the PC path suffers more actuation. We may need to think about the HV actuation option
for the PC (Apex PA85).

Well, let's see how the CM servo can handle this.
The key point here is that we have enough data to start the design of the CM servo.

I wasn't sure how the IMC servo was optimized recently. We used to have the FSS over all gain (C1:PSL-FSS_MGAIN) of +6dB a few years back. It is not 0dB. So I decided to do a couple of measurements.

1) Default setting:

C1:IOO-MC_REFL_GAIN +4
C1:IOO-MC_BOOST2 +3
C1:IOO-MC_VCO_GAIN +13
C1:PSL-FSS_MGAIN +0
C1:PSL-FSS_FASTGAIN +19

2) Looked at the power spectrum at TEST1A output (error signal)
TEST1A is the signal right after the input gain stage (C1:IOO-MC_REFL_GAIN). Prior to the measurement, I've confirmed that the UGF is ~100Hz even at +0dB (see next section). It was not too bad even with the current default. Just wanted to check if we can increase the gain a bit more.
The input gain was fixed at +4dB and the FSS overall gain C1:PSL-FSS_MGAIN was swept from +0 to +6.
At +5dB and +6dB, the servo bump was very much visible (Attachment 1).
I decided to set the default to be +4dB (Attachment 3).

3) Took OLTF at 0dB and 4dB for the FSS overall gain.

Now the comparison of the opel loop transfer functions (OLTF) for C1:PSL-FSS_MGAIN at 0dB and 4dB. The OLTF were taken by injectiong the network analyzer signal into EXCA and measure the ratio between TEST1A and TEST1B (A/B).

C1:PSL-FSS_MGAIN +0 -> UGF 100kHz / Phase Margin ~50deg
C1:PSL-FSS_MGAIN +4 -> UGF 200kHz / Phase Margin 25~30deg

The phase margin was a bit less but it was acceptable.

4) IMC FSR

Took the opportunity to check the FSR of the IMC. Connected a cable to the RF MON of the IMC REFL demod board. Looked at the peak at 40.56MHz (29.5MHz + 11.066MHz). The peak was not so clear at 11.066195MHz (see 40m ELOG 15845). The peak was anyway minimized and the new modulation frequency was set to be 11.066081MHz (new FSR). The change is 10ppm level and it is within the range of the temp drift.

I want to work on the IFO this weekend, so I reverted the IMC suspension settings just now to what I know work (until the new settings are shown quantitatively to be superior). There isn't any instruction here on how to upload the new settings, so after my work, I will just restore from a burt-snapshot from before I changed settings.

In the process, I found something odd in the MC2 coil output filter banks. Attachment #1 shows what it it is today. This weird undetermined state of FM9 isn't great - I guess this flew under the radar because there isn't really any POS actuation on MC2. Where did the gain1 filter I installed go? Some foton filter file corruption? Eventually, we should migrate FM7,FM8-->FM9,FM10 but this isn't on my scope of things to do for today so I am just putting the gain1 filter back so as to have a clean FM9 switched on.

I wrote the values from the c1mcs burt snapshot from ~1400 Saturday May 15, at ~1600 Sunday May 16. I believe this undoes all my changes to the IMC suspension settings.

For future reference, the new settings can be upoaded from a script in the same directory. Run python /users/anchal/20210505_IMC_Tuned_SUS_with_Gains/uploadNewConfigIMC.py from allegra.

[Mirko, Kiwamu]

I tried to answer two questions regarding the IMC:

1. What is the coupling of fluctuations in the SB freq. to SB transmitted power?
2. What (if any) is the influence of the IMC on the AM?

I ran into some weird things regarding the corresponding optickle simulations:
1. There seems to be some artifact at the beginning of every simulation sweep.
2. The position of features depends on the parameters of the sweep.

I mailed Matt asking if he sees some error in the simulations

This is just my intuition but the IMC servo seems not so optimized. I can increase the servo gain by 6~10dB easily. And I couldn't see that the PC drive went mad (red) as I increase the gain (=UGF).
The IMC needs careful OLTF measurements as well as the high freq spectrum observation.

It seems that I have worked on the IMC servo tuning in 2014 July/Aug. Checking these elogs would be helpful.

Herewith, I describe an adventure

1. Balance the OSEM input matrix using the free swinging data (see prev elogs).
2. Balance the coil actuation by changing the digital coil gains. This should be done above 10 Hz using optical levers, or some IMC readout (like the WFS). At the end of this process, you should put a pringle vector into the column of the SUS output matrix that corresponds to one of the SUS OSC/LOCKIN screens. Verily, the pringle excitation should produce no signal in MC_F or da WFS.
3. use the Malik doc on the single suspension to design feed-forward filters for the SUS COIL filter banks. You can get the physical parameters using the design documents on DCC / 40m wiki and then modify them a bit based on the eigenfrequencies in the free swinging data.
4. Model the 2x2 system which includes longitudinal and pitch motion. Consider how accurate the filters must be to maintain a cross-coupling of < 3% in the 0.5-2 Hz band.
5. Is this decoupling forsooth still maintained when you close the SUS damping loops in the model? If not, why so?
6. Make step response measurements of the damping loops and record/plot data. Use physical units of um/urad for the y-axes. How much is the step response cross-coupling?
7. Consider the IMC noise budget: are the low pass filters in the damping loops low-passing enough? How much damping is demasiado (considering the CMRR of the concrete slab for seismic waves)?
8. Can we use Radhika's AAA representation to auto-tune the FF and damping filters? It would be very slick to be able to do this with one button click.

gautam: For those like me who don't know what the AAA representation is: the original algorithm is here, and Lee claims his implementation of it in IIRrational is better, see his slides.

[Paco, JC, Anchal]

We balanced the IMC table back again to point that got us 50% of nominal transmission from IMC. Then we tweaked the steering mirror for injection to IMC to get up to 90% of nominal transmission. Finally, we used WFS servo loop to get to the 100% nominal transmission from IMC. However, we found that the WFS loop has been compromised now. It eventually misaligns IMC if left running for a few minutes. This needs to be investigated and fixed.

Next steps:

• Align X-arm cavity and regain flashing.
• Fix the Oplev path for ITMX.
• Tune POX11 phase angle to get an error signal with which we can lock the cavity.
• Finish AS beam path setup.

The IMC table had to be leveled again, for two reasons: 1) It was un-leveled when Jenne and Kiwamu removed an extra beam dump when they took the beam profile measurements, and 2) the stack of weights I had put there before was too tall to allow the beam to pass (I didn't realize that the BS chamber is offset a bit to the north, so the beam passes right over the NE edge of the IMC table).

First of all, I was wrong before when I said that the stack of weights was 4 blocks tall; it was 6 blocks tall. I re-leveled the table this afternoon by removing the top three blocks and placing them immediately south of the bottom three in the original stack, while also moving the circular weight north of its previous position. The table is now balanced roughly to within the tolerance of the bubble level I was using.

After the leveling, I tried to re-lock the modecleaner. Upon removing the beam block on the PSL table, I got some sort of resonance flashes on the MC TRANS monitor. With some minor adjustments to MC2&3, I was able to get a decent TEM00 mode to hit. The cavity wouldn't lock, so I went to the AP table and checked to make sure that the REFL beam was hitting the PD. It was, but the beam was very close to the edge of the focusing lens, so I moved the steering mirror slightly to make the situation a little better.

I then went to the control room to finish the by-now-mundane task of fine-tuning the MC lock, but today's was a worthier opponent. For some reason, the thing didn't want to lock for more than a few seconds at a time. I saw that the spot on MC2 was quite a bit off-center, so I ran the MC2_spot_xxx scripts to get it visually in place, then revisited the AP table to ensure that the REFL beam was still on the PD. No dice.

I don't know what was different. I had Ameristat over the opening between the tanks, with posters on top and on the sides (as usual), and I checked to ensure that the servo gains were at the appropriate levels. Joe pointed out that IOO VME was not responding, but we didn't seem to think that this was the problem (based on nothing I can put in words or stick figure cartoons), and the "alive" indicator on the Auto-Lock control in the MEDM screen was not blinking, as it usually is, but I don't know what bearing this has on anything.

I will try to lock again tomorrow.

In order to address the issue of low MC1 OSEM voltages, Yuta and I looked at the IMC table levelling.  Looking with the bubble level, Yuta confirmed that the table was indeed out of level in the direction that would cause MC1 to move closer to it's cage, and therefore lower it's OSEM voltages.  Looking at the trends, it looks like the table was not well levelled after TT1 installation.  We should have been more careful, and we should have looked at the MC1/3 voltages after levelling.

Yuta moved weights around on the table to recover level with the bubble level.  Unfortunately this did not bring us back to good MC1 voltages.  We speculate that the table was maybe not perfectly level to begin with.  We decided to try to recover the MC1 OSEM voltages, rather than go solely with the bubble level, since we believe that the MC suspensions should be a good reference.  Yuta then moved weights around until we got the MC1/3 voltages back into an acceptable range.  The voltages are still not perfect, but I believe that they're acceptable.

The result is that, according to the bubble level, the IMC table is low towards MC2.  We are measuring spot positions now.  If the spot positions look ok, then I think we can live with this amount of skew.  Otherwise, we'll have to physically adjust the MC1 OSEMS.