Summary of the morning shift:
- for the first half of the shift, we saw increased frequency noise as in the past days, so we had to try and overcome it by changing the SSFS crossing frequency and the SSFS gain during lock acquisition; when we managed to get to LOW_NOISE_1, things got better thanks to the SSFS, but the noise is still there; the shape of the loops did not change changing the crossing frequency, as opposed to the previous trials (Fig. 1, PRITF @ 0.7 Fringe);
- when using the base filter for SSFS a huge line, with structures and bump around it, shows up at 10 kHz; it immediately disappears once we switch to the boost filter; the issue was already there a few days ago;
- what is strange is the overall noise level increase, even in the range of the sensing noise floor and after the cut-off of the PD electronics/acquisition chain; see Figure 2, step LOW_NOISE_1, compared to a good lock of three days ago;
- in the second half of the shift the noise level decreased and we got good locks as at similar times in the last days; the 150 Hz-ish line is occasionally present in DARM but not at all times, and it can show up or disappear in lock; DARM is most affected by it, with a little cross-talk to MICH/PRCL; it does not show up in the SSFS signals;
- what killed most of the locks, which are quite stable and repeatable if not for the following, is a sudden oscillation/glitch at 24 Hz, clearly visible in DARM; it looks like a quick loop oscillation, but it happens also when MICH has a quite low UGF.
These three issues (24 Hz, "150" Hz and 10 kHz) are currently the focus of our investigations.