Reports until 10:03, Tuesday 02 January 2018
H1 SQZ
sheila.dwyer@LIGO.ORG - posted 10:03, Tuesday 02 January 2018 - last comment - 11:20, Tuesday 09 January 2018(39897)
Squeezer Faraday TFP extinction ratios

The afternoon of December 22nd I went back into the optics lab and made a few measurements to try to understand why our Faraday isolation was only 20dB(see alog 39861).  It turned out that one of the TFPs had an extinction ratio that didn't meet the spec, and by switching the positions of the TFPs I was able to measure an isolation of -28dB. 

Thin Film Polarizer extinction ratio:

To measure the extinction ratio of each of the TFPs I used a set up very similar to the image in 39861, with the rotator removed (PD monitoring input power in position A, PD monitoring TFP transmission in position C which is after the Faraday path, used chopper to measure transfer function between PDs).  

After the fiber collimator, there was already in place a PBS mounted to clean up the input polarization by reflecting horizontally polarized light.  I also used whichever TFP I wasn't measuring to further clean up the polarization.  I rotated the half wave plate to measure the maximum and minimum power transmitted by the second TFP to get its extinction ratio.  For the TFP mounted with the backplate labeled SN9, I got a ratio of 2220:1 (coated side facing down in the mount, so that incident beam hit uncoated side first), for the one mounted on the backplate SN08, I got 336:1 with the coated side down and 577:1 with the coated side up (so that the incident beam first hits the coated side).  The spec for these TFPs is greater than 1000:1 (spec here)

Better Faraday Performance

In the original set up, SN08 was the first polarizer in the Faraday (the one closer to the OPO), and both TFPs were mounted coated side down.  Scattered light from the interferometer will be mostly in the polarization to be rejected by the TFP closer to the OPO, so swapped the two positions.  Now SN09 (2220:1) is mounted coated side down closer to the OPO, and SN08 is mounted coated side down further from the OPO.  

With this arrangement I repeated the measurements of isolation, transmission, and backscatter (I also increased the laser power compared to 39861).  For transmission measurements I got 95.6% and 97.6%, for isolation I got -27.9 dB and -27.8dB (0.16%), and for backscatter I got -40dB and -41dB.  

Comments related to this report
sheila.dwyer@LIGO.ORG - 11:20, Tuesday 09 January 2018 (40053)
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Sheila, Nutsinee, TJ

We made a couple of measurements to try to measure the Faraday rotation angle, but our measurements don't provide any better information than the constraint placed on the error by the isolation measurement.

  • The extinction ratio of the thin film polarizer that rejects the return beam is 1:2200, which places an upper limit of 33dB on the possible isolation.
  • If the half wave plate is set to maximize transmission, the polarization of the return beam before this TFP is going to be 90+2delta, where delta is the difference between the Faraday rotation angle and 45 degrees.  This will allow a fraction of sin^2(2delta) of the return beam to pass the first polarizer.  
  • We measured a return beam with 0.16% of the return beam transmitted through the Faraday.  If  0.04% is from the TFP transmitting the wrong polarization and the remained is from an error in the rotator, the rotator error is 0.98 degrees.  

We tried a few methods of measuring this, including setting the half wave plate to maximize transmission with the rotator both in place and removed and comparing the angles (47+/-3 degrees). We also used a polarizer in a rotation stage mounted after the Faraday and setting its angle to maximize transmission with vertically polarized light (with HWP and TFP after rotator left in place) and with the light directly out of the rotator.  This method gave us fairly good accuracy (about 1 degree) but we found that repeated measurements varied by up to 5 degrees, so there must be something mechanically unreliable about the rotation stage we were using.