jenne.driggers@LIGO.ORG - posted 20:35, Friday 12 February 2016 - last comment - 11:09, Monday 15 February 2016(25525)
New ISS servo oscillates
The new ISS digital AC coupling caused the diffracted power to oscillate by ~1% with a period much less than 1Hz. I tried turning the gain down on the servo, but we lost lock (we were in NomLowNoise).
Later, I locked the interferometer to nominal low noise in order to check the behavior of the ISS. The 2nd loop was able to automatically engage the servo. I did not see a fault behavior with the ISS at least after twenty minutes in full lock.
Perhaps what Jenne saw is a type of oscillation that reported in alog 24665 ? Since the interferometer is locked, I am leaving it undistrubed. By the way OMC DCPDs are seemingly with low transimpedance. Noise in DARM below 50 Hz seems higher than the reference curve. I did not pay attention to the calibration at all.
The attached show an overnight trend (for 12 hours) of the relevant channels. As Jenne pointed out, the diffraction power indeed fluctuated (below 1 Hz) by 1 % or so all the time, but this is a known behavior (alog 24665) -- the 2nd loop changes the operating point of the first loop since the 2nd loop still has some gain at low frequencies even though it is digitally AC-coupled. As Gabriele pointed out, we think this fluctuation is added by the angular motion of the input mode cleaner (alog 24677). If we want to improve fluctuations in the diffraction power, the next attacking point would be the input mode cleaner.