Sheila, Jennie W
At around 21:49 UTC we used the step_45MHz.py in userapps/isc/h1/scripts to step the modulation depth in steps of 1dB down from 21dBm to 18dBm. This script adjusts the loop gains to compensate for a drop in power on the RF45 PDs.
Its also important to open the POP beam diverter to monitor what is happening to the RF 45 PD signals.
This is to check that the noise does not decrease with decreasing modulation depth which would imply that the modulator was imposing noise on the carrier through the ISS loop.
At around 22:18 UTC we put the modulation depth down by 3 dB we saw a 4% increase in the DARM broadband noise (measured the level at 2kHz with cursors) which is the green trace in the second image. The level of KAPPA C (shown in first image on bottom plot) only decreased by around 1% over this time, so we suspect some other cause other than a change in optical gain being responsible for this.
When we stepped the modulation depth up by 3dB (third image) we saw no change in the noise from the nominal state (purple trace on second image). Kappa C (bottom plot) looks almost the same as the nominal also.
In the fourth image it can be seen that f_C does not change much either.
After we did this test Daniel commented that we should check what the PCAL lines did as we stepped down and up the 45 MHz modulation depth.
To do this I used the spectra we took and measured the height of the 410.3 Hz line (PCAL Y) in DARM in m.
Our nominal level was 1.35370 e-19 m
The line height decreased by 1.1% when we decreased the modulation depth by 3dB. I think this is consistent with our optical gain decreasing by 1%. I am therefore not sure why lowering the mod depth causes a 4% increase in DARM.
The line height increased by 0.04 % when we increased the mnodulation depth by 3dB. I think this is consistent with us not seeing much decrease in the DARM noise when we increase the modulation depth.
