Nishino,

This is a result on 2024/10/11.

I measured the open loop transfer function (OLTF, Fig.1) of the GR lock loop. The filter funtions are measured independently (Fig. 2 and 3).

Setting of SR560 (Fig. 5) and Moku (Fig. 6):

SR560: 0.3 Hz, first order low pass, gain=10000

Moku PID:  I=2.87 kHz, DS=25.0 dB

The inferred optical gain (not optical gain itself but A*H) is plotted in Fig. 4. From this result, I reduced the overall gain just by -2 dB, then the modified OLTF looks like Fig. 7, with UGF=2 kHz and phase margin=20 degrees.