40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log  Not logged in ELOG logo
Entry  Wed Jan 8 13:01:48 2014, Gabriele, Summary, LSC, Effect of PRC length mismatch on error signals REFL_vs_PRClength.pdf
    Reply  Wed Jan 8 16:08:52 2014, ericq, Summary, LSC, Effect of PRC length mismatch on error signals phaseVlength.pdf
       Reply  Wed Jan 8 19:05:30 2014, Gabriele, Summary, LSC, Effect of PRC length mismatch on error signals as11.pngas55.png
          Reply  Thu Jan 9 17:58:31 2014, ericq, Summary, LSC, Effect of PRC length mismatch on error signals 
             Reply  Fri Jan 10 15:31:07 2014, rana, Summary, LSC, Effect of PRC length mismatch on error signals 
                Reply  Sun Jan 12 09:57:24 2014, Gabriele, Summary, LSC, Effect of PRC length mismatch on error signals 
Message ID: 9539     Entry time: Wed Jan 8 16:08:52 2014     In reply to: 9537     Reply to this: 9541
Author: ericq 
Type: Summary 
Category: LSC 
Subject: Effect of PRC length mismatch on error signals 

 [ericq,Gabriele]

So, we want an relatively quick measurement of the PRC length error (with sign!) at the order of .5 centimeter or so. Rana suggested the "demodulation phase method," i.e. lock the simple Michelson, measure what demodulation phase brings the 1F signal entirely within the phase quadrature, then lock the PRMI and measure the demodulation phase again. This tells you something about the length of the PRC. 

Gabriele and I worked through a simulation using MIST to determine how to actually do this. We simulated the case of injecting a line at 1kHz in the laser frequency via the laser's PZT and looking at the transfer function of the 1kHz signal to the I and Q at the 1F AS demodulated signal when locked. (Michelson locked on the dark fringe, PRC locked on 11MHz sideband) With the I and Q in hand, we can measure some demodulation phase angle that would bring everything into I. 

When the PRC length is in the ideal location, the demodulation phases in the two cases are the just about the same. Sweeping the length of the PRC around the ideal length gives us a monotonic function in the difference in the demodulation phases:

phaseVlength.pdf

So, with this simulation, we should be able to calibrate a measured difference in demod phase into the length error of the cavity! We will proceed and report...

ELOG V3.1.3-