Since the secondary microseism has been very high this evening and preventing H1 from locking, we decided to leave just the PMC locked (no FSS, ISS, or IMC) for an extended time and watch for any glitches. At around 23:45 UTC, we unlocked the FSS, Richard turned off the high voltage supply for the FSS, and Jason and I unplugged the DB37 cable from the FSS fieldbox in the PSL-R1 rack in order to ensure no feedback from the FSS made it to the NPRO. Pictures of the DB37 cable's location are attached.
The first attachment shows the changes seen when the FSS was unlocked. Since then, I've seen several instances of groups of glitches come through, such as those shown in the second and third attachments. These glitches in the PMC_MIXER channel are smaller than ones seen previously that have unlocked the IMC (like in alog81228). There have also been times where the PMC_MIXER channel gets "fuzzier" for a bit and then calms down, shown in the fourth attachment; it's possible this is due to the NPRO frequency not being controlled so the PMC sees some frequency changes? Finally, I only recall one instance of the NPRO jumping in power like in the final attachment; the PMC doesn't seem to care much about this, only having one very small glitch at this time.
I'll leave the PSL in this configuration to collect more data overnight as the secondary microseism is still much too high for H1 to successfully lock.
A zoom-in on some of the glitches from the third figure above.
After Ryan's shift ended last night, there were some larger glitches, with a similar amplitude in the PMC mixer channel to the ones that we saw unlocking the reference cavity 81356 (and IMC in 81228)
The first plot shows one of these times with larger glitches, the second one zooms in for 60ms when the glitches were frequent, this looks fairly similar to Peter's plot above.
The period of large glitches started around 2 am (7:37 UTC on Nov 20th), and ended when a power glitch turned off the laser at 7 am (15 UTC) 81376. Some of the small glitches in that time frame time seem to be at the same time that the reference cavity was resonanting (with low transmission), but many of the large glitches do not line up with times when the reference cavity was resonanting.
I've zoomed in on most of the times when the PMC mixer glitches reached 0.1, and see that there are usually small jumps in NPRO power at the time of these glitches, although the times don't always line up well and the small power glitches are happening very often so this might be a coincidence.
Sheila, Jason, Vicky - Compared the PSL + PMC mixer glitches between last night (Nov 22, 2024, no FSS no ISS) and the emergency vent (Aug 2024, PSL+PMC+FSS+ISS), as in 81354.
- Previously 80999, the IMC could not stay locked for long periods of time: "IMC losses lock when it's locked by itself, with PMC, FSS, and ISS."
- Fixed a bunch of things last week, but then still had many fast "IMC"-tagged locklosses over the weekend. Hard to tell the cause.
- Start removing stuff again this week, to isolate problem upstream of the IMC.
- Laser + PMC + FSS (no ISS) = bad still glitchy 81356. Rules out ISS and likely IMC as cause of glitches.
- Laser + PMC (no FSS no ISS) = bad still glitchy 81371. FSS depowered and HV off to really turn FSS "off". Rules out FSS as cause of glitches.
As a reference, "before" during the emergency vent in August 2024, the Laser + PMC + FSS + ISS were all locked with no PMC mixer glitches for >1 month.
Updating our matrix of tests to isolate the problem, and thinking things through:
| Before (vent Aug 2024) | Now (Nov 2024) |
| laser + PMC + FSS + ISS good = no glitches | laser + PMC + FSS bad = glitches 81356 |
| laser + PMC ??? (presumably good) | laser + PMC bad = same PMC mixer glitches 81371 |
1) Are these +/-0.1 V PMC mixer glitches the problem? Yes, probably.
2) Are these big PMC mixer glitches caused or worsened by the FSS? No. PMC mixer glitches basically same with FSS on 81356 and off 81371.
3) Are the laser + PMC mixer glitches new? Yes, probably. If these PMC glitches always there, could it be that we were previously able to ride them out before, and not now? But this would imply that in addition to the new glitches, the FSS secondarily degraded. Seems very unlikely: already several problems (bad amp, new eom needed new notch, etc) have been fixed with FSS, and the FSS OLTFs and in-loop spectra look good. FSS on/off does not change the PMC mixer glitches, so the problem seems most likely laser or PMC.
Sheila, Daniel, Jason, Ryan S, many others
We think the problem is not the PMC, and likely the laser.
Daniel looked at PMC mixer gliches on the remote scope: 81390. If PMC mixer glitches are indicative of the problem, we can try to track down the origin of the glitches.
- What can cause PMC mixer glitches?
- NPRO Laser frequency glitches .... seems likely
- PMC cavity length glitches (e.g. servo electronics or PZT) .... most likely no 81390.
- Likely PDH is optically sensing real glitches, as we have already switched the 35.5 MHz LO to the Marconi 81277, that made no difference ...... rule out the 35.5 MHz LO.
- Unlikely PMC, because the PMC mixer glitches on the scope seem too fast for the PMC locking electronics.
- Daniel found that the "PMC servo drives the PZT with a 3.3K series resistor forming a ~1kHz low pass with the PZT capacitance of ~45nF. This yields a chracteristic time constant of ~150us. The glitches as seen by the PMC mixer are at least a few times faster than this." 81390
- Daniel's scope photo here shows "a train of PMC mixer glitches, going as high as ~70mVpk (or ~70kHz)", similar to the zoomed-in glitches Peter showed above.
- So, this means that glitches originating in the PMC locking servo or PZT electronics would change the PMC cavity length too slowly to create the fast PMC mixer glitches we observe. Seems unlikely that PMC servo locking electronics are the issue.
- Unlikely mechanical / PZT issues given the very nice PMC cavity scans - optically, the cavity scans don't show glitches or issues as a function of the PZT drive (remote scope traces + sheila pmc pzt scan photo).
- Daniel found that the "PMC servo drives the PZT with a 3.3K series resistor forming a ~1kHz low pass with the PZT capacitance of ~45nF. This yields a chracteristic time constant of ~150us. The glitches as seen by the PMC mixer are at least a few times faster than this." 81390
- Biggest change to the glitches came from swapping the original O4 laser to now the O3 laser. Glitches changed when lasers changed: see Camilla lho:81386. Seems like a clue that the glitches are related to the specific lasers themselves.
Talking with Jason and RyanS, some summary of the NPROs available:
- (S/N 1661) The O3 NPRO was swapped in recently. Potentially we now see the same problems with 1661 was seen in the end of O3b.
- Camilla's 81386 shows that the FSS oscillations with this laser in O4 look a lot like its issues at the end of O3b.
- In 2020, these FSS issues originally prompted Camilla to create the FSS_OSCILLATION tags. These FSS_OSCILLATION tags started to be flagged regularly again in mid-Sept 2024, but with the original O4 laser, the FSS oscillations looked totally different: see her log 81386.
- (S/N 7974) The original O4 NPRO was in use Sept 2017 - June 2018, when it was uninstalled (42652) seemingly due to similar fast FSS glitches, leading to a broken noise eater. Jason looked at alogs from time of removing this NPRO in 2018, and the 2018 glitches look like the original Sept 2024 glitches. 7974 was then sent back to Coherent for repair + refurbished. This is the newest NPRO bought in 2015, and the only one that Coherent will service. Then 7974 was installed for O4.
- (S/N 1639F) The "third" NPRO is from O1, and was in use for many years 2011-2017. It was wearing out its lifetime and running out of power, then removed and refurbished. Since, was only used a few times in the optics lab. Will try this 3rd NRPO.
