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Entry  Wed Sep 10 22:17:29 2014, Koji, Summary, LSC, X/Y green beat mode overlap measurement 
    Reply  Mon Jun 22 12:57:09 2015, ericq, Summary, LSC, X/Y green beat mode overlap measurement redone 
       Reply  Mon Jun 22 14:53:37 2015, rana, Summary, LSC, X/Y green beat mode overlap measurement redone 
          Reply  Fri Oct 28 15:46:29 2016, gautam, Summary, LSC, X/Y green beat mode overlap measurement redone 
Message ID: 11370     Entry time: Mon Jun 22 14:53:37 2015     In reply to: 11368     Reply to this: 12587
Author: rana 
Type: Summary 
Category: LSC 
Subject: X/Y green beat mode overlap measurement redone 
  • Why is there a factor of 20 power difference? Some of it is the IR laser power difference, but I thought that was just a factor of 4 in green.
  • Why is the mode overlap only 50% and not more like 75%?
  • IF we have enough PSL green power, we could do the Y-beat with a 80/20 instead of a 50/50 and get better SNR.
  • The FFD-100 response is more like 0.33 A/W at 532 nm, not 0.25 A/W.

In any case, this signal difference is not big, so we should not need a different amplifier chain for the two signals. The 20 dB of amplification in the BeatBox was a fine way, but not great in circuit layout.

The BBPD has an input referred current noise of 10 pA/rHz and a transimpedance of 2 kOhm, so an output voltage noise of 20 nV/rHz (into 50 Ohms). This would be matched by an Amp with NF = 26 dB, which is way worse than anything we could bur from mini-circuits, so we should definitely NOT use anything like the low-noise, low output power amps used currently (e.g. ZFL-1000LN....never, ever use these for anything). We should use a single ZHL-3A-S (G = 25 dB, NF < 6 dB, Max Out = 30 dBm) for each channel (and nothing else) before driving the cables over to the LSC rack into the aLIGO demod board. I just ordered two of these now.

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