sheila.dwyer@LIGO.ORG - posted 20:00, Tuesday 06 February 2018 - last comment - 09:11, Wednesday 07 February 2018(40443)
FSS oscillating
Since the HPO has been off, the FSS has been oscillating at 238Hz according to the PZT fastmon. THe attached screenshot shows some examples.
This caused us some problems during the arm peak, and it would be difficult/futile to do DRMI with this happening. It would be great if someone could look at this loop in the morning.
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This morning I looked at the noise spectra for the Pockels cell path and the PZT path. There is a broad hump
around the region Sheila mentioned, along with a large number of what I presume are mains related peaks.
Adjusting either the common gain or the fast gain did not appear to have any affect on the hump. In the
Pockels cell path, there was a large peak at ~55 kHz, which might be a harmonic of the PZT/Pockels cell
cross over.
Attached is the transfer function of the TTFSS measured this morning. It is noticeably different from
ones I remember. Not least of which the unity gain frequency is down a fair amount. Increasing the
common gain increased the unity gain and phase margin but the PZT monitor became noisier. Not 100% sure
of the reason. My suspicion is that it may be related to the alignment of the Pockels cell after the
NPRO. That would be one thing to look at when the laser gets overhauled.
Also attached are the measured transfer functions of the pre-modecleaner servo for slider gains of
16 dB and 19 dB.
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