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Entry  Fri Feb 16 22:19:07 2018, gautam, Update, General, Fibel ALS input polarization tuning PER_setup.JPG
    Reply  Mon Feb 19 14:27:25 2018, gautam, Update, General, Fibel ALS input polarization tuning IMG_6900.JPG
Message ID: 13640     Entry time: Fri Feb 16 22:19:07 2018     Reply to this: 13641
Author: gautam 
Type: Update 
Category: General 
Subject: Fibel ALS input polarization tuning 

After discussing with Koji, I decided to try and align the input beam polarization at the PSL fiber coupler to one of the special axes of the PM fiber. The motivation is to try and narrow down the source of the large RF beatnote amplitude drift I noticed and reported last night.

The setup for doing so is shown in Attachment #1 - essentially, I setup one of the newly purchased couplers in a mount, set up a PBS, and placed two photodiodes at the S and P ports of the PBS. The idea is to rotate the input coupler in its mount, thereby maximizing the PER (monitored on two Thorlabs PDA520s - I didn't check the gain balance of them). 

I spent ~30mins doing some preliminary trials just now, and, I was able to achieve a PER of ~1/20. But I think much better numbers were reported in this SURF project (although I'm not entirely sure I understand that measurement). I will spend a little more time tweaking the alignment. The procedure is tricky as at some point, simply rotating the mount reduces the mode-matching efficiency into the fiber so much that it is not possible to get a meaningful PER measurement from the photodiodes. I'm adjouring for now, more to follow...

Attachment 1: PER_setup.JPG  1.607 MB  | Hide | Hide all
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