Summary:

There is poor separation of the PRCL and MICH length error signals as sensed in the 3f photodiodes. I don't know why this is so - one possibility is that the MICH-->PRM matrix element in the LSC output matrix needs to be tuned to minimize the MICH -->PRCL coupling.

Details:

Over the last few days, I've been trying to make the 3f locking of the PRMI more reliable. Turns out that while I was able to lock the PRMI on 3f error signals, it was just a fluke. So I set about trying to be more systematic. Here are the steps I followed:

1. Lock the PRMI (i.e. ETMs misaligned) using REFL11 for PRCL, AS55 for MICH.
   • This is the so-called 1f scheme.
   • The servo signs are chosen such that the carrier field is resonant in the PRC.
   • Run the dither alignment to maximize POPDC, minimize ASDC. This is the main purpose of locking in this config.
   • Measure some loop TFs, make sure the servo gains are giving us ~100 Hz UGF on these loops.
2. Change the sign of the servo loops to make the sidebands resonant in the PRC.
   • The error signals are still sourced from the 1f photodiodes.
   • Measure loop TFs, and also the TF between the 1f and 3f error signals. 
   • This allowed me to determine how the servo gains (and signs) that would be appropriate when using the 3f signals in place of the 1f.
   • Determine the offsets in the 3f error signals when the PRMI is locked on 1f error signals. This allows me to set the error point offsets for the PRCL_B and MICH_B paths, which are what is used for the 3f locking.
3. Change the error signals from 1f to 3f. 
   • After much trial and error, I finally managed to get a stable (>10 mins) lock going.
   • Measured some loop TFs.
   • Turned on the notch filters in the control filter bank at the sensMat oscillator frequencies, and ran some sensing lines.

Attachment #1 is the result. I don't know what is the reason for such poor separation of the MICH and PRCL error signals in REFL165. The situation seems very different from when I had the DRMI locked in Nov last year.

After this exercise, I tried for some hours to get the 3f PRMI locking going with the arm cavities held off resonance under ALS control, but had no success. The angular motion of the PRC isn't helping, but I feel this shouldn't be a show stopper.