craig.cahillane@LIGO.ORG - posted 20:51, Thursday 31 October 2019 - last comment - 22:41, Thursday 31 October 2019(52855)
Increasing FSS Common Gain by +3dB removes most of frequency noise from DARM
We got back to nominal low noise. The frequency noise in DARM as extremely bad right when we returned. I measured the CARM and IMC loops and found the IMC UGF was 45 kHz and look wonky above 20 kHz. Georgia increased the FSS common gain by +3dB from 20 to 23 dB after noticing the plummeting transmission and reflection on the FSS. She also found that the IMC REFL DC light was relatively high (up to 14 mW from the normal 9 mW). After +1 dB the frequency noise was replaced by intensity noise, likely imprinted by the ISS from the freq noise incident on the IMC. Georgia further increased by +2dB. This cleared up everything further, now the spectrum is more clear of laser noise. See first PDF. The IMC REFL DC light is now back to 9 mW. The new IMC UGF is 80 kHz. New CARM gain is 16 kHz. See PDF 2. PNG shows the original gain sliders, before we made any changes, and some PD DC powers.
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Note, we saw this exact problem before in February, and it was solved in the same way (FSS gain increase): alog 46968 We expect that the power glitches we saw when powering up will go away now that the IMC loop is back to normal.
Attaching a screen shot of some time series while I increased the FSS gain:
- FSS PZT monitor (fast_mon) is less noisy
- IMC power in is less noisy
- MC2_trans is higher
- IMC_refl is lower, back to ~9mW where it was before Tuesday.
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