We measured the beat signal after we fixed the wiring problem. We are close to what we had before, but we still have a scattering light problem.
Just after we fixed the cable wiring between the table and the frame on 3/30, we measured the beat signal (beat 03/30, blue). There was a hughe scattering noise bump from 100-200 Hz, and it was not stable. We did not see this before (beat 03/12, pink). We found that there were plenty of dust on the periscope mirrors, lenses in front of RFPD. These optics are in the sensing path, and are more sensitive to scattering light. After cleaning them, the beat signal is better now (beat 04/03). The bump at low frequency is still higher than before, but now it is stable.
Note that the acoustic peaks around 1kHz are not reduced from fixing the cable, but the bump at low frequency is slightly lower. This is reasonable, the cables should be very soft at high frequency, and do not transfer any vibration from the frame to the table at acoustic bandwidth.
Other changes we did:
It is very likely that the when we removed all the cables on the table, dust was stirred up and stuck on the optics, because they are dirtier than usual. We might need to clean more optics on the table. However, from today measurement, it seems that the bump at low frequency might be the effect from the seismic as well (from the feature of the peaks @35 and 55 Hz). Noise budget from seismic will be revisited, also the flat noise at high frequency will be investigated next.