Based on TJ's log 72857 about recent FC alignment drifts, I wanted to check out the situation with possible scattered light from the filter cavity since alignments can drift around (worth investigating more), and I wanted to check the CLF fiber polarization (this was basically fine).

Summary screenshot shows some measurements from today. I'm wondering if we might be closer than I realized to filter cavity backscatter. With an excitation that is ~5-10x greater than the ambient lsc control to FC2, it created scatter about 2x above darm at 100 Hz. Based on previous (higher-freq) measurements (LHO:68022) and estimates (LHO:67586), I had thought ambient LSC control was about 10-fold below DARM; this suggests we are within a factor of ~5? Though, there is a lot of uncertainty (on my end) of how hard we are driving FC2 given the suspension/loop roll-offs/etc, so I need to think more about the scaling between measured scatter and the excitation.

Strange line in FC-LSC error signal -- it seems to wander; I've seen it (or things like it) sometimes in other SQZ-related signals; but I haven't figured out where it comes from yet. It can be easily seen on SQZ summary pages (sqz > subsystems tab) since Derek and Iara helped us it up (thank you Detchar!!). I don't see it in CLF_ISS but sometimes in other error signals. Not clear to me that this is an issue if the peak is >100 Hz, but if it drifts to lower frequencies and this line is real/physical (not some random artifact), it could be problematic. The peak amplitude seems large enough that if it were in-band of the FC2 suspension and controls, it could plausibly get injected as real FC length noise and drive some measurable backscatter.

For the excitation -- I used the template from LHO:68022 but ran it to lower frequencies, in-band of FC-LSC. Compared to the FC error signal (specifcally H1:SQZ-FC_WFS_A_I_SUM_OUTPUT which is the input sensor to FC_LSC_DOF2_IN1), this DTT injection of 30,000 counts increased the integrated RMS of the in-loop error signal at 10 Hz by about 9-fold (= 3375 (w/excitation) / 387 (ambient), measured from dtt rms cursors). I injected into the fc-lsc loop at H1:SQZ-FC_LSC_DOF2_EXC, with various amplitudes (like 30k), and filter = butter("BandPass",4,10,300); this should then go to FC2_L for suspension feedback. I'm not sure that I'm using the best witness sensors for actual length noise driven in this excitation, but wasn't able to totally figure it out in time.

With this excitation going, I tried to walk the alignment to see if there was an alignment that minimizes backscatter, but I didn't figure this out in time. I tried to walk ZM2 with beam spot control off, and then set the QPD offsets where it landed. This was probably the wrong approach, since I wasn't able to then set the QPD offsets in time; maybe I should have walked the FC-QPD offsets with full ASC running at higher gain, since this loop is so slow. Might be worth trying this again for a bit; with the injection running, I wasn't sure if I was able to minimize scatter by walking ZM2 (p/y/ maybe psams?), but there were a couple directions in both pitch and yaw that looked promising. 

In the end, SDF diffs for ZM2 were accepted (ZM2 is not under asc-control), and I accepted the beam spot position change in the FC QPD Pitch offset (from 0.07 --> 0.08) while reverting the yaw change that I didn't figure out in time. I don't anticipate much overall change in the squeezer after these tests.