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Message ID: 1941     Entry time: Tue Aug 25 03:30:23 2009
Author: Yoichi 
Type: Summary 
Category: WIKI-40M Update 
Subject: Green lock and phase noise 
While Koji and I were discussing about the green laser lock, we wondered if the common motion of the cavity mirrors,
which won't be suppressed by the green laser servo, will cause any problem to the locking.

Since the common motion of the cavity mirrors is equivalent to the change of the path length from the laser to the
input mirror, it will show up as a phase noise in the error signal.
Unfortunately, since we inject the green laser from the end mirror, this phase noise has opposite sign for the
PSL and the green laser.

I calculated the magnitude of the phase noise using an extremely rough estimate of the common motion of the mirrors.
It is explained in the 40m wiki.
http://lhocds.ligo-wa.caltech.edu:8000/40m/Upgrade_09/GreenLock

The result plot is attached.
(Probably the seismic noise I used is an over estimate.)
Attachment 1: PhaseNoise.png  37 kB  | Hide | Hide all
PhaseNoise.png
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