[Elenna, Gabriele]
We injected broadband DHARD_Y noise and measured the couplig to DARM with different values of the ITMY Y2L gain. As we saw in the past, there are two coupling regimes:
- below 20-30 Hz, steep frequency dependency, changes little with the Y2L gain
- above 30 Hz, flat-ish in frequency, changes signficantly with Y2L
The nominal value of Y2L gain is -1.65, that we found yesterday to be optimal for DHARD_Y to DARM coupling, using a line at 30 Hz.
Today the situation is different, a gain of -0.9 would reduce the coupling above 30 Hz by 20+ dB, at the price of a larger coupling below 30 Hz (where we care for DHARD_Y noise). Between yesterday and today: new OMC alignment, new DARM offloading, new input alignment, so probably it's not a big surprise that the optimal A2L is different.
We left the value to -1.65, since that is the best at frequencies below 30 Hz, where there is still coherence between DHARD_Y and DARM.
With the new OMC ASC offsets, the DHARD Y coherence has changed again. https://ldas-jobs.ligo-wa.caltech.edu/~elenna.capote/brucos/New_OMC_alignment/ASC-DHARD_Y_OUT_DQ.html
The coherence is stronger now from 30-50 Hz, which suggests that we should consider a new A2L that reduces some of that noise injection. To quote Gabriele "perhaps we could find an intermediate value that reduces some noise 30-50 Hz without making the 10-30 Hz noise much worse"