[Masayuki, Manasa]

While trying to lock the arms using ALS we found that the locks were not very stable and the in-loop noise was higher than seen before.

I looked into things and checked the out-of loop noise for ALS and found that the Y arm ALS noise (rms) was higher than the X arm.

To troubleshoot, I measured the OLTF of the phase tracking loop. While X arm was healthy, things weren't looking good for the Y arm. Sadly, the Y phase tracking loop gain was set too high with a phase margin of -2 degrees. We brought down the gain from 300 to 150 and set the phase margin close to ~55 degrees.

X arm Phase tracker loop:
UGF = 1.8 K Hz
Phase margin = 50 degrees

Y arm Phase tracker loop:
UGF = 1.6 KHz
Phase margin = 55 degrees