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Entry  Tue Feb 4 02:13:02 2020, gautam, Update, LSC, Locking updates modifiedOutMat.pngOLTFs.pdf
    Reply  Wed Feb 5 16:35:12 2020, gautam, Update, LSC, Diagnosis plan ALSnoise.pdf
       Reply  Thu Feb 6 01:16:58 2020, gautam, Update, LSC, Diagnosis results IMCspec_ALS.pdfIMCspec_ALS_smallSpan.pdf
    Reply  Thu Feb 6 01:25:50 2020, gautam, Update, LSC, Locking updates semiStableArms.pngarmAngStability.png
       Reply  Fri Feb 7 02:24:24 2020, gautam, Update, LSC, Some short notes 
Message ID: 15188     Entry time: Wed Feb 5 16:35:12 2020     In reply to: 15185     Reply to this: 15191
Author: gautam 
Type: Update 
Category: LSC 
Subject: Diagnosis plan 

The goal is to try and identify the source of the excess ALS noise as the CARM offset is reduced. The idea is to look at the MC_F spectrum (or the IMC error point) in a few conditions:

  1. Regular CARM --> MC2 actuation scheme, PRMI locked on 3f signals, CARM held off resonance.
  2. Regular CARM --> MC2 actuation scheme, PRMI locked on 3f signals, CARM held on resonance.
  3. Alternate CARM --> 7.5*ETMX + 1.5*ETMY, PRMI locked on 3f signals, CARM held on resonance.
  4. Control arms in X/Y basis, lock PRMI on 3f signals and bring the arms into resonance individually, look for excess ALS noise.

#1 vs #2 is like a control experiment, we expect to see the excess noise imprinted on the MC length and hence in MC_F (provided the sensing noise is low enough). #2 vs #3 will be informative of something like backscatter to the PSL increasing the frequency noise. #2/3 vs #4 will help isolate the problem to an individual arm's AUX PDH loop or some optomechanical effect.

I was looking back at some spectra from the last couple of nights but I don't really have an apple-to-apple comparison in the various actuation schemes (some ALS loops were engaged/disengaged), so I'll do a more systematic test tonight. Already, it looks like MC_F is not a good candidate to look for the excess frequency noise, I don't really see a big difference between conditions #1 and #2. According to this, we are looking for an increase at the level of a few 100Hz/rtHz @ ~40 Hz, wheras MC_F is much noisier.

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