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Message ID: 13985     Entry time: Tue Jun 19 00:19:00 2018
Author: gautam 
Type: Update 
Category: ASC 
Subject: POP status check 

Motivation:

  1. I want to use the QPD at POP, calibrate it into physical units, and quantify the amount of angular jitter in the PRC (which I claim is what limits DRMI stability atm).
  2. I want to revive the PRC angular feedforward to try and mitigate this a bit. But is feedforward even the best approach? Can we use feedback using the POP QPD?

POP QPD checkout:

  • The POP QPD sits on the ITMX optical table. 
  • It is interfaced to the CDS system via an OT301 and then a Pentek whitening stage (z:p = 15:150). 
  • The OT301 claims to have a switchable offset nulling capability - but despite my best efforts tonight, I couldn't use the knobs on the front to null the offset (even with the PRC locked on carrier and a strong POP beam on the QPD).
    • We don't have readbacks of the individual quadrants available.
    •  
  • So I moved the QPD with the PRC locked, to center the CDS readback of the spot position at (0,0).
  • Next step is to calibrate the POP QPD readback into physical units.
    • I'm thinking of using the EricG diode laser for this purpose.
    • I can calibrate counts to mm of displacement on the QPD active area.
    • After which I can use the estimated position to PR2 (from which POP is extracted) to convert this to angular motion.
  • I guess I should check for coherence between the POP QPD signal and all angular sensors of PRM/BS/MC1/MC2/MC3 to try and confirm the hypothesis that the folding mirrors are dominating the angular noise of the cavity. Unfortunately we don't have readbacks of the angular positions of TT1 and TT2.
  • I moved the POP camera a bit in YAW so that the POP spot is now better centered on the CCD monitor.
  • I also wanted to check the centering on the other POP QPD (POP22/POP110/POPDC?) but I think the POPDC signal, used for triggering the PRCL LSC servo, is derived from that PD, so everytime I blocked it, the lock was lost. Need to think of another strategy.
  • MC3 has been rather glitchy tonight.
    • So I will wait for a quieter time when I can collect some data to train the WF for angular FF.
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