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Entry  Mon Nov 7 17:24:12 2016, gautam, Update, Green Locking, Green beat setup on PSL table CurrentX.pdfCurrentY.pdfProposedShift_copy.pdf
    Reply  Wed Nov 9 23:21:44 2016, gautam, Update, Green Locking, Green beat setup on PSL table 
Message ID: 12609     Entry time: Wed Nov 9 23:21:44 2016     In reply to: 12603
Author: gautam 
Type: Update 
Category: Green Locking 
Subject: Green beat setup on PSL table 

I tried to realize an improvement in the mode matching onto the BBPDs by moving the lens mentioned in the previous elog in this thread. My best efforts today yielded X and Y beats at amplitudes -15.9dBm (@37MHz) and -25.9dBm (@25MHz) respectively. The procedure I followed was roughly:

  1. Do the near-field far-field alignment of the arm and PSL green beams
  2. Steer beam onto BBPD, center as best as possible using the usual technique of walking the beam across the photodiode
  3. Hook up the output of the scope to the Agilent network analyzer. Tweak the arm and PSL green alignments to maximize the beat amplitude. Then move the lens to maximize the beat amplitude.

As per my earlier power budget, these numbers translate to a mode matching efficiency of ~53% for the X arm beat and ~58% for the Y arm beat, which is a far cry from the numbers promised by the a la mode simulation (~90% at the optimal point, I could not achieve this for either arm scanning the lens through a maximum of the beat amplitude). Looks like this is the best we can do without putting in any extra lenses. Still a marginal improvement from the previous state though...

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