Abstract:

After increasing the laser power, I adjusted the gain of the MICH and PRCL so that the UGFs of these loops would be the same as those with 1 W.
Thanks to this adjustment, the oscillation at 20 Hz disappeared.

Detail:

Since 20Hz oscillation was reported in klog35193, I adjusted the gain of MICH and PRCL after increasing laser power.
Figure 1 and 2 shows the OLTF measurement results after gain adjustment.
Both UGFs became nominal values (MICH:40Hz, PRCL:100Hz).

To adjust the UGFs, I reduced the gain of MICH and PRCL by 9dB and 4.5dB at FM8 of MICH1 and PRCL1 filter banks, respectively.
New compensation gains are shown in fig3 and fig4.

After these adjustment, oscillation at 20Hz disappeared and IFO can be locked with low noise PRMI controls.