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Tue Aug 25 03:30:23 2009
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.
The result plot is attached.
(Probably the seismic noise I used is an over estimate.)