X,Y arms were stabilized using ALS and moved 5 nm from the resonance, PRMI was locked on sideband using REFL165 I&Q. POP angular servo was running; PRMI RIN was good (~2-3%)
During slow offset reduction I was sweeping MICH, PRCL and POP servos for instabilities due to possible optical gain variations, loops were fine.
I could reduce offset down to ~200 pm and then lost lock due to 60 Hz oscillations as shown on the attached plot "arm_offset"
Arms were stabilized with RMS comparable to the offset and power in arms was fluctuating from 3 to 45.
60 Hz line most probably comes from MICH. RMS is dominated by the power lines and is ~ 1 nm as seen on the plot "PRMI_CAL". I think this is too much but we need to do simulations.