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R&D (FilterCavity)
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NaokiAritomi - 23:23, Wednesday 22 December 2021 (2764)Get code to link to this report
FC lock problem

We found that FC green error signal is very noisy. Fig 1,2 show the FC green error signal before/after lock. Fig 3 shows the spectrum of the FC green error signal after lock. There are two peaks around 20 kHz in the FC green error signal. It seems that the two peaks come from SHG. The SHG error signal is also shown in Fig 3. The SHG gain was 1.4. We decreased the SHG gain from 1.4 to 1, but the peaks around 20 kHz did not change.

During this investigation, we checked green power and OLTF of FC green lock. The measured green power is as follows. The green power seems reasonable. The UGF was 14kHz, which is also reasonable. Note that FC gain was 1.6.

green power before AOM 48.6 mW
green power before FC 24.3 mW

We also found that the Z correction is not very stable. We quickly checked the filters in filter bank of Z correction as follows.

not bad filters damp, damp2
bad filters DCdamp, dc_damp2, dc_damp3 

The bad filters immediately caused FC unlock when they were engaged. The not bad filters didn't cause the immediate unlock, but FC was not very stable for long term. We need to investigate the Z correction.

Images attached to this report
2764_20211222152336_img9082.jpg 2764_20211222152343_img9081.jpg 2764_20211222152353_fcshgerrorsignal20211222.png