Summary:

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

Details:

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

Next (for LSC activities):

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

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

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

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

Summary:

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

Details:

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

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

wonder if its possible to do variable finesse locking

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

Summary:

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

Details:

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

Update 820pm: 

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

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

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

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

TBC...

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

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

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

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

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

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

Summary:

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

Hardware issues that need addressing:

1. Both EX and EY Trans QPDs need a look. I believe the one at EY is simply blocked (on account of the mode spectroscopy project), while the one at EX shows a weird discontinuity between the Thorlabs PD and the QPD. Could be just a gain/normalization issue I guess. See Attachment #1.
2. While the PRMI stayed locked, I don't think I was using anywhere close to optimal settings. Need to run some sensing lines, measure transfer functions etc, to make the PRMI + arms lock more robust. The PRMI always lost lock when I brought the CARM offset to 0. Could also benefit from some finesse modeling I guess. I could not get a reliable estimate of what the PRG is tonight, because the PRMI didn't stay locked as I approached 0 CARM offset.
3. REFL 55 whitening board needs a checkup.

1. I removed the flip-mount that was installed on the EY in-air table for the mode-spectroscopy project (see Attachment #1). The Transmon QPD at EY sees IR light again.
2. Dark noise checkout - see Attachment #2.
3. Light-level expectations:
   • For the current config, let's say 0.8 W reaches the PRM, and we will have a PRG of 50. 
   • This implies ~5.5 kW circulating power in the arms.
   • This implies ~70mW will get transmitted through the ETM, of which at most half makes it to the QPD. 
   • In the nominal operating condition, we expect more like 6 W circulating in the arm cavity. So something like 30uW is expected to make it out onto the Trans QPDs.
   • But in this condition, we expect to run with the high-gain Thorlabs PD.
   • In reality the number is likely to be somewhat smaller. But we should set the transimpedance gain of this photodiode accordingly. Currently, there are a bunch of ND filters installed on this photodiode, which probably should be removed.
4. Angular control
   • The other purpose these QPDs are expected to serve is to stabilize the angular motion of the cavities when locked with high circulating power.
   • Need to calculate what the sensing noise requirement is.

Summary:

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

Details:

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

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

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

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

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

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

1. Transition of arms from POX/POY to CARM/DARM was much smoother today - a change was made at the EX PDH setup, see here.
2. Reliable settings for 3f locking with arms held off resonance seem to have been found.
3. Took sensing matrix in this condition, measured loop TFs.
4. Reduced CARM offset - reached arm powers ~50 at which point the PRMI lost lock. Reacquisition was quick though.
   • The POP22_I level seemed to decay as I reduced the CARM offset.
   • This would suggest that somehow the PRCL lock point is getting shifted as I reduce the CARM offset.
   • Tonight, I will investigate this more.

Summary:

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

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

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

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

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

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

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

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

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

Tried a bunch of things tonight.

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

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

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

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

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

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

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

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

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

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

Summary:

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

Details:

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

Summary:

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

Details:

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

Summary:

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

Details:

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

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

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

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

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

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

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

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

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

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

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

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

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

Other discussion points between Rana, Koji and Gautam:

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

Some ideas Koji suggested:

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

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

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

Summary:

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

Details:

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

Next steps:

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

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

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

{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):

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.

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.

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.

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.

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.

Summary:

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

Details:

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

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

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

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

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

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

[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).

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.

Summary:

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

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

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

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

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

Methodology:

The procedure used was the same as that outlined here.

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

Summary of DC actuator gains:

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.

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

Summary:

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

if the RP don't fit

u must acquit

sweep the laser amplitude

to divine the couplin w certitude

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).

Summary:

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

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

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

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

yes, reasonable

Summary:

1. While noisier, I was able to control the arm lengths to ~30pm RMS(!) using the green ALS beats as error signals (cf. ~10 pm RMS with the IR ALS system).
2. The PRMI could be locked with a CARM offset applied.
3. When lowering the CARM offset, I saw an increase in the in-loop ALS error signal, just as I had with the IR beat.
4. IR TRX / TRY unsurprisingly did not stabilize in any meaningful way.
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.

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.

• 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.

Summary:

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

Details:

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

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

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

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