Peter, Hang

One brute-force way of reducing the LSC MICH noise is to reduce its loop gain, and then we can move the LP filters to lower freqs. To achieve so we need to know the LSC MICH noise budget at low freq (which dominates the rms) and see if it is possible to reduce the input noise to the loop.

In the first plot we show the noise projection to MICH ctrl output based on f-domain MISO coherence analysis. The low-freq rms is largely dominated by the BS local damping noise (mostly from L, with a non-negligible contribution from P and Y). As a comparison, the largest ISI contribution (from BS ISI itself) is almost a factor of 10 below the BOSEM noise.

Since LSC MICH is already controlled with a clean, interferometric signal, it might not be necessary to have high gains on the local damping loops anymore. Thus we might want to reduce the local damping gain and thus reduce the amount of input noise to MICH. This should allow us to further reduce MICH noise to DARM by lowering MICH BW.