Abstract:

I reduced MCL filter gain by 5.5 dB so that the crossover frequency between frequency feedback and MCE feedback around 100 Hz.

Detail:

I noticed IFO went down several times when CARM fast gain was reduced by 3dB at INCREASING_LAS_POWER state.
This implies that fast gain reduction might be introduced instability, so I checked it.

When fast gain was reduced by 3dB, following things happen:
1. UGF of CARM loop was reduces by 3dB.
2. Crossover frequency between frequency feedback and MCE feedback increases by 3dB.

Figure 1 shows the OLTF of CARM loop and it seems fine even 3dB gain reduction happens.
Figure 2 shows the crossover frequency measurement result (red:today, green:reference).
According to fig2, gain is roughly 5.5dB high and if the relative gain becomes 3dB higher, phase margin is just 5 degrees, which is too small.
So, to adjust the relative gain, I added -5.5dB at FM1 of LSC-MCL filter bank (fig3).

I will check the stability during increasing laser power later.