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Entry  Tue Oct 25 15:56:11 2016, gautam, Update, General, PRFPMI locked, arms loss improved PRFPMIlock_25Oct2016.pdfITMYwoes.png
    Reply  Tue Oct 25 18:07:28 2016, Koji, Update, General, PRFPMI locked, arms loss improved 
       Reply  Thu Oct 27 12:06:39 2016, gautam, Update, General, PRFPMI locked, arms loss improved PRG.pdf
          Reply  Thu Oct 27 13:48:20 2016, Koji, Update, General, PRFPMI locked, arms loss improved 
             Reply  Thu Oct 27 23:29:47 2016, ericq, Update, General, PRFPMI locked, arms loss improved TRY_REFL_2d.pngAS.pngREFL.pngseis_sub.pdf
                Reply  Fri Oct 28 01:44:48 2016, gautam, Update, General, PRFPMI model vs data studies 6x
                   Reply  Fri Oct 28 19:13:57 2016, rana, Update, General, PR gain 
Message ID: 12583     Entry time: Thu Oct 27 12:06:39 2016     In reply to: 12580     Reply to this: 12584
Author: gautam 
Type: Update 
Category: General 
Subject: PRFPMI locked, arms loss improved 
Quote:

Great to hear that we have the PRG of ~16 now!

Is this 150ppm an avg loss per mirror, or per arm?

I realized that I did not have a Finesse model to reflect the current situation of flipped folding mirrors (I've been looking at 'ideal' RC cavity lengths with folding mirrors oriented with HR side inside the cavity so we didn't have to worry about the substrate/AR surface losses), and it took me a while to put together a model for the current configuration. Of course this calculation does not need a Finesse model but I thought it would be useful nevertheless. 

In summary - the model with which the attached plot was generated assumes the following:

  • Arm lengths of 37.79m, given our recent modification of the Y arm length
  • RC lengths are all taken from here, I have modelled the RC folding mirrors as flipped with the substrate and AR surface losses taken from the spec sheet
  • The X axis is the average arm loss - i.e. (LITMX+LITMY+LETMX+LETMY)/2. In the model, I have distributed the loss equally between the ITMs and ETMs.

This calculation agrees well with the analytic results Yutaro computed here - the slight difference is possibly due to assuming different losses in the RC folding mirrors. 

The conclusion from this study seems to be that the arm loss is now in the 100-150ppm range (so each mirror has 50-75ppm loss). But these numbers are only so reliable, we need an independent loss measurement to verify. In fact, during last night's locking efforts, the arm transmission sometimes touched 400 (=> PRG ~22), which according to these plots suggest total arm losses of ~50ppm, which would mean each mirror has only 25ppm loss, which seems a bit hard to believe.

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PRG.pdf
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