Sudarshan, Gabriele
Today when the IFO was locked in low noise, we could see the 300-400 Hz peaks in the sensitivity again. Those are know to be due beam jitter caused by the PSL periscope.
We looked into the performance of the ISS second loop, which was closed at the time. The first strange thing that we can't understand is that both the in-loop and out-of-loop signals are dominated by a flat noise background at a level of 1e-8 /rHz. We would expect the out-of-loop to be at this level, but the in-loop should be squeezed down by the larg loop gain.
We changed the ISS gain, anmd discovered that if we turned down the gain at the minimun (-26-20 dB with respect to nominal), and disengage both boost and integrator, the in and out of loop ISS signals got worse, but the sensitivity got better at the 300-400 Hz peaks. So out guess is that beam jitter on the ISS array is actually causing an excess sensing noise on the ISS diodes, which is then re-injected into the laser beam as real intensity noise when the ISS second loop is closed with high gain.
We tried briefly to optimize the centering of the beam on the ISS array. We couldn't find any spot better than the initial one in terms of power on the diodes. One possible strategy would be to move the beam into the ISS array with the IFO in full lock, and use the coupling of a PZT jitter line to the sensitivity as a figure of merit.
I saw the same strange noise floor that were at a level of 1x10-8 RIN/sqrtHz a week ago. See the spectra from this alog:
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=20148
In light of this strange noise floor, I looked at some of the RIN spectrum I had taken peridocally. This change in In-loop PD noise seem to have happened after we started using the actual DC signal from the PD to obtain RIN ( we were using a hardcoded offset that was an estimate of DC signal). So we implemented a low pass filter with two poles at 10 mHz in the DC signal this morning and the problem is gone. Attached is a spectrum before and after the low pass filter was implemented. SDF table is appropriately updated.