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Message ID: 1408     Entry time: Fri Apr 29 15:43:53 2011
Author: Zach 
Type: Laser 
Category: GYRO 
Subject: mode matching needs a touch-up 

[Alastair, Zach]

Our current idea is that clipping on the faraday isolator is resulting in disturbance of the error signal. The precise mechanism isn't quite understood yet, but there are several candidate theories. We have moved the FIs so that they are at the waists between the lenses of the mode-matching telescopes, and the beams going through them from the input sides (from the laser) are small enough that no clipping is evident as measured with a power meter.

The same can't be said of the beams returning from the cavity mirror. Moving the FIs to the MMT waists has highlighted the fact that the REFL beams do not seem to be of the same size as the input beams at this point, which indicates a problem with mode-matching.

Small changes to the MMT lens locations don't seem to do the trick, and since this is such a crucial aspect of our experiment (given the isolation method), it will probably pay to just redo the thing. Alastair had the idea of profiling the beam closer to where the lenses actually are/will be---instead of as farther upstream where we have done it in the past---and that way we can rule out anything weird going on in between. Since we have a fair amount of space at the right place on the table, we can also profile the beam after the lenses are in place by steering the beam to the unused area. This way we can actually verify that the beams have the right shape on the way into the cavity.

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