peter.king@LIGO.ORG - posted 13:33, Saturday 07 January 2017 - last comment - 13:53, Saturday 07 January 2017(33076)
FSS_PC_MON_OUTPUT and the noise eater
Cheryl called and said she found something interesting in the H1:PSL-FSS_PC_MON_OUTPUT that may have coincided
with either a lockloss or drop in range (I've forgotten which exactly). She pointed to a peak in the Pockels
cell monitor output. Ordinarily a peak or spike in that signal is caused by a fast frequency transient. However
this time the "peak" lasted at least a minute. It also did not coincide with a drop in range for the interferometer.
Attached is a 20-minute plot of the NPRO PZT and the Pockels cell. A couple of things to note:
- The average value of the Pockels cell output is not close to 0 (zero) as it typically is. By "close to" I
mean with an absolute value of less than either 0.5 or 1.
- The noise in the PZT output is a little larger than typical.
- The reference cavity transmission dropped even further.
After resetting the noise eater, things returned to normal.
Probably the fastest way to spot the status of the noise eater is to pull up the FSS MEDM screen. On the
left hand side, around the middle is an indicator labelled "NPRO RRO:NOISE EATER". The box to the right of the
text should be green if things are dandy.
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Attached is a plot of the NPRO output power when the Pockels cell output change was detected. Assuming
the NPRO signal is somewhat calibrated, the jump of ~5 mW is probably enough of a light level change
to upset the detector used by the noise eater circuit to cause an oscillation.
Thus far I haven't found the root cause of why the NPRO power would change or why the noise eater
would start oscillating unless it was related to the NPRO power. All the other signals look reasonable.
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