【 Jenne, David, Matt, Evan 】

We spent some time trying out 2 W interferometer operation with lower modulation depths for the 9 MHz and 45 MHz sidebands. Nominally, the modulation depths are 0.22 rad for 9 MHz and 0.27 rad for 45 MHz.

I have attached scripts which can be used to step the modulation depths up or down using the Siggtronics EOM drivers in the PSL. These will adjust all digital photodiode gains appropriately, so that the interferometer stays locked. The only thing needing manual adjustment is the CARM gain.

In this way we were able to reduce the 9 MHz modulation depth by 6 dB without issue.

The interferometer became noticeably less stable with 4 dB of reduction of the 45 MHz modulation depth. In particular, the dHard loop gains seemed to increase, causing a loss of gain in the DARM loop around 20 Hz. The MICH and SRCL loop shapes did not change. Lowering the dHard loop gains by a factor of 2 enabled us to reduce the modulation depth by 6 dB compared to the nominal. There was also noticeable wobbling in the PRM yaw pointing loop control signal (maybe motion impressed by the 45 MHz PR3 loop?). If we want to run with lower 45 MHz modulation depth, a campaign of ASC loop measurement is probably a prerequisite.