Last night I used the good alignment of the OMC from the dither loops to sweep the cavity with a single-bounce beam from ITMX and ITMY.  The result is that I think the modulation depth mystery is gone (Gamma2 = 0.077, for the 45MHz sidebands; in hindsight my earlier calculation was wrong) and we see a difference in the mode-matching to the OMC between ITMX and ITMY that matches what we expect from the as-built optical parameters.  So, maybe life makes sense again...

===== Modulation Depth =====

The short story is that I mis-identified the peaks from the 45MHz sidebands in the measurement from Sep 19th.  Probably I was looking at the 1st and 4th order carrier modes; with the good alignment the 1st order mode is much smaller, and there's no way to mistake that the 4th order peak doesn't have a matching peak at the other end of the FSR.  The two plots attached show the cavity sweeps of ITMX and ITMY; the peaks used to calculate the modulation depth are marked with black crosses.  There are five sweeps for each optic. The peak heights are averaged and used to calculate J1(Gamma), and the value of Gamma that returns J1(Gamma) is calculated using the scipy.special.jn() function.  The result is:

Gamma1 (9MHz) = 0.211

Gamma2 (45MHz) = 0.077

These results are consistent for the upper and lower sidebands of each modulation frequency, and are the same for ITMX and ITMY.  The variation from one sweep to the next is small (usually less than 0.001).  Also, this measurement of the modulation depth for the 45MHz sidebands agrees with a measurement from last December.  No modulation mystery after all?

(I hope that Koji will check these results with his more-sophisticated peak-calibration code.  If you are comparing my plots to his, note that my data use up-going sweeps of the PZT, while his are down-going (for example here), and so the order of the peaks is flipped between the two.  The latest (Sep 28th) sweep data are here.)

===== Mode Matching =====

Using the mode scans we can make an estimate of the mode-matching of the single bounce beam into OMC.  Following Koji's calculation I use the peak height of the 2nd-order carrier mode to make a rough estimate of the mode overlap, using the expression MM = 1 - (CR_TEM20/CR_TEM00).  This calculation is rough, in that it ignores the power in the higher order modes, and just uses the peak height; I will try to include the HOMs and integrate the peaks later, but the answer should be the same to within a percent or two.  For each ITM we get:

ITMX = 0.884

ITMY = 0.913

The magnitude of the mode mismatch, and the difference between the ITMs, is in agreement with predictions from the beam-propagation.  Good alignment matters!  I suppose the next step will be to check that we can improve things with TCS.  (Again, someone should make an independent calculation using this sweep data...)

===== Other Stuff =====

• Last week I plugged in the picomotor controller for the in-vacuum HAM6 picos.  It was an easy job to figure out which pico driver was which: channels 1-3 are AS_A, B, and C; channels 5 and 6 are the picos in front of the OMC REFL QPDs.  I centered the beam on AS_A,B,C with the OMC in its good-alignment state, and unless something dramatically changes with the SRC alignment I think they will stay centered.  Now that there is beam on AS_A and AS_B we can use them for initial alignment.  (OMCR_A and B are not centered, this is kind of tricky since the picos for that beam path are upstream of both QPDs, and there are two lenses between the picos and the diodes, so there's a lot of degeneracy in the actuation.  I'll need to spent an hour or two pico-ing this week.  Heh, maybe the ISS team wants to help?)
• I fixed the channel swap in the AC and DC monitor channels for OMC PZT1 and PZT2, now all the readbacks make sense.
• We need to commission a shutter for the OMC.  For the moment all we can do is use PZT1 to kick the cavity length when the trigger diode (AS_C? how to get this signal out of the analog electronics?) goes over the threshold.
• I remeasured the OMC QPD and dither alignment loops.  The QPD loops all have a UGF around ~12Hz, the dither is around 0.1Hz.  Both have plenty of phase.
• With the beam aligned to the OMC and the AS WFS, I measured the {OM1,2,3} --> {OMC QPD, AS WFS} sensing matrix.  This is in preparation for setting up a combined OMC/WFS centering servo.  The matrix needs to be inverted and the control topolgy chosen (tricky, with four sensors and three actuators), but before we can do anything we need to add the inputs from the ASC model to the SUSHTTS model.  Right now the OMs get their ISC inputs from the OMC model.  For comparison, Livingston is sending the AS_B signal to OM1 (with low bandwidth), and the OMC QPD signal to OM2 and OM3.