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Entry  Fri Dec 15 19:09:49 2017, gautam, Update, IOO, IMC lockloss correlated with PRM alignment? PSL_layout.JPG
    Reply  Mon Dec 18 16:45:44 2017, gautam, Update, IOO, IMC lockloss correlated with PRM alignment? 
Message ID: 13485     Entry time: Fri Dec 15 19:09:49 2017     Reply to this: 13486
Author: gautam 
Type: Update 
Category: IOO 
Subject: IMC lockloss correlated with PRM alignment? 

Motivation:

To test the hypothesis that the IMC lock duty cycle is affected by the PRM alignment. Rana pointed out today that the input faraday has not been tuned to maximize the output->input isolation in a while, so the idea is that perhaps when the PRM is aligned, some of the reflected light comes back towards the PSL through the Faraday and hence, messes with the IMC lock.

A script to test this hypothesis is running over the weekend (in case anyone was thinking of doing anything with the IFO over the weekend).

Methodology:

I've made a simple script - the pseudocode is the following:

  • Align PRM
  • For the next half hour, look for downward transitions in the EPICS record for MC TRANS > 5000 cts - this is a proxy for an MC lockloss
  • At the end of 30 minutes, record number of locklosses in the last 30 minutes
  • Misalign PRM, repeat the above 3 bullets

The idea is to keep looping the above over the weekend, so we can expect ~100 datapoints, 50 each for PRM misaligned/aligned. The times at which PRM was aligned/misaligned is also being logged, so we can make some spectrograms of PC drive RMS (for example) with PRM aligned/misaligned. The script lives at /opt/rtcds/caltech/c1/scripts/SUS/FaradayIsolationTest/FaradayIsolCheck.py. Script is being run inside a tmux session on pianosa, hopefully the machine doesn't crash over the weekend and MC1/CDS stays happy.

A more direct measurement of the input Faraday isolation can be made by putting a photodiode in place of the beam dump shown in Attachment #1 (borrowed from this elog). I measured ~100uW of power leaking through this mirror with the PRM misaligned (but IMC locked). I'm not sure what kind of SNR we can expect for a DC measurement, but if we have a chopper handy, we could put a chopper (in the leaked beam just before the PD so as to allow the IMC to be locked) and demodulate at that frequency for a cleaner measurement? This way, we could also measure the contribution from prompt reflections (up to the input side of the Faraday) by simply blocking the beam going into the vacuum. The window itself is wedged so that shouldn't be a big contributor.

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