Dhruva, Jenne, Craig, Naoki, Vicky

Summary: ZM4/5 PSAMS at 200V seems to continue improving SQZ-OMC mode-matching, when OMC is held at a spot near the "hot" IFO state. Some fuzziness in results as we saturated the OMC suspension when aligning high ZM4/5 PSAMS. Tentatively, the mode-matching (TEM20) peak seems to improve as we increase PSAMS, but the alignment was not perfect in these tests.

WIth PSAMS at 0V/0V, the beam out of ham7 is bigger; on AS AIR camera, the beam is dimmer and it looks clippy (same exposure), and the power levels on AS QPD's are all very sensitive to clipping (even with DC centering engaged). Maybe at 0/0 we are clipping on the OFI or something en route to the ham6? The 0/0V test is somewhat inconclusive. Also, at higher PSAMS voltages, we started saturating the OMC suspension, so we weren't able to resolve misalignment peaks-- but, the mode-misalignment 20 peak does seem to reduce at higher PSAMS.

Here are our mode-matchings, measured using cursors on ndscope:

• At ZM4/5 PSAMS = 0, 0 V, non-TEM00 ~ 40%,  TEM00~0.19, TEM01~0.127, TEM02~0.004.                 (prob clipping)
• At ZM4/5 PSAMS = 110, 110 V,   non-TEM00 ~ 23%,  TEM00~0.234,  TEM01~0.034,   TEM02~0.034
• At ZM4/5 PSAMS = 150, 150 V,  non-TEM00 ~ 16%,   TEM00~0.27,   TEM01~0.0245, TEM02~0.0245  (omc sus staurated) 
• At ZM4/5 PSAMS = 200, 200 V,  non-TEM00 ~ 15%,   TEM00~0.27,   TEM01~0.034,   TEM02~0.013    (omc sus staurated) 

------ Notes from the process of getting the omc mode scan:

To try and understand the SQZ-OMC matching, yesterday we took OMC mode scans today of the sqz/seed beam, with the OPO dither-locked on sqz laser carrier light. Jenne and Craig set us up for taking OMC mode scans.

• Seed powers: we launched 60mW on SQZT0, got 700uW of OPO IR TRANS when dither-locked, with 2.1mW in OPO IR REFL. 
• IFO had SRM aligned, SR2 misaligned
• AS_A/B_DC_NSUM_OUT_DQ ~ 36.6. ASC-AS_C_NSUM_OUT_DQ ~ 400e-6. Consistent with SRM reflectivity.
• We engaged OMC's AS DC centering - we had enough light on OM1+2 that this just worked.
• Jenne moved OMC alignment from its hot to cold alignment (mostly moving OMC YAW).
• At first, we couldn't find sqz beam on OMC QPD's. So, Jenne aligned the OMC to the IFO beam in single-bounce to get IFO light onto omc qpd's.
  • She recovered this single-bounce IFO situation by locking imc_trans at 5W PSL power, misaligning SRM, then aligning OMC --> IFO single bounce using OM3 + OMC P/Y sliders.
  • The idea here being that with DC centering on OM1/2, we will then align OM3 + OMC to IFO in single bounce so that OM1-2-3+OMC are consistently aligned. Then, we will OM3+OMC outputs to a nice ifo aligned state, and switch the input to the sqz/seed beam. To hold OMC-ASC outputs, Jenne paused OMC_GRD after having it at ASC_QPD_ON earlier. 
  • After going back to sqz beam, even with SR2 misaligned there was too much interference at the AS-port QPD's to align the sqz beam. So, Dhruva unlocked the IMC to totally take away the ifo beam, and we got much quieter AS_A/B QPD signals, and then successfully engaged DC centering on the sqz beam. 
• Craig has good notes for this process in https://git.ligo.org/aligo_commissioning/labutils/-/tree/master/omc_scan
• Basically, after getting OM3+OMC aligned in single-bounce at 5W, we switched back to sqz/seed beam, unlocked IMC, and then engaged DC centering loops for OM1/2.