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Entry  Tue May 4 12:06:37 2010, Zach, Laser, GYRO, 6-m mirrors are OK--back to the drawing board  
    Reply  Wed May 5 15:09:35 2010, Zach, Laser, GYRO, 6-m mirrors are OK--back to the drawing board  
Message ID: 749     Entry time: Wed May 5 15:09:35 2010     In reply to: 748
Author: Zach 
Type: Laser 
Category: GYRO 
Subject: 6-m mirrors are OK--back to the drawing board  

Haven't found anything in the literature supporting a polarization rotation in a planar cavity, with the exception of one article discussing the effect in rotating ring cavities(!). I highly doubt that the earth's rotation would cause such a major effect via this pathway (if it did, then we could build a pretty sensitive gyro just by measuring the polarization angle shift!).

It also seems unlikely that the plane of the high-extinction PBS we used to isolate the S beam for the input should be misaligned with respect to the cavity plane by enough to account for this effect. I was going to crunch some numbers (i.e., calculate the P-finesse of the cavity and thereby determine the expected transmission through the cavity mirrors of some residual P input light), but it is difficult to find data for Y1(S)-S transmission of P. Not to fear, as we can just measure this.

It appears there was a momentary lapse of experimental reason when we all first measured the excess transmission, as there is no record of the measured output in the elog, nor did we measure the polarization of the escaping light. I suppose it was just easier to say "damn those CVI bastards and their crappy crap".

Troubleshooting scheme:

  • Measure TP for Y1, Y1S cavity mirrors, and calculate expected cavity P-belching for a conceivable residual P in the input beam.

If the result seems consistent with observation, then we simply use the cavity itself as a polarization reference for the input beam (as we perhaps should have done in the first place). If this doesn't account for the effect:

  • When we get the cavity set up again (after moving it down the table), and inevitably observe the same problem, GET NUMBERS FOR THE TRANSMISSION AND DETERMINE THE POLARIZATION
  • Use the aforementioned information to figure out WTF is going on.


 Before doing the full T vs. angle test on the 6-m mirrors we have been using (so that we could send them back to CVI), I figured I would just test them at 45°, outside of the cavity, using the same simple configuration I used to measure the new 9-m mirror. What I found was that---for both 6-m mirrors---the transmission was in the range of 50 - 100 ppm, roughly independent of angular tuning away from 45° on the order of +/- 5°. This is pretty much consistent with the part specifications from CVI for Y1S, especially considering that they have been on our table for a couple months.

So.. why did we observe a disproportionate amount of light escaping these mirrors with the cavity locked? Good question. We suspect that it might have something to do with polarization issues, as a macroscopic polarization shift in the cavity would certainly account for the spurious transmission.

I don't know much about what could cause this, but I am reading. The good news is that our mirrors are all what they are supposed to be (yippidy doo da).


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