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Message ID: 878     Entry time: Thu Jul 29 19:19:42 2010
Author: Alastair 
Type: Laser 
Category: GYRO 
Subject: CW beam alignment 

 This afternoon I tried getting the clockwise beam (AOM beam) into the cavity.  We didn't have much success the last time, mainly because it was so difficult aligning the beam from waaaaaayy back before the faraday.  So this time I tried a different approach.  The plan this time was to align the CW beam to the cavity, and then to put the faraday back in place after the alignment.

I took out the faraday altogether, and the mirrors after the faraday.  I then aligned the incoming CW beam along a line of holes on the bench and checked the 4" height along the length.  This step was just so that we had a well defined beam path to put the faraday back in place later.

After some fiddling with the mirrors that go after the faraday I got the CW beam aligned to the cavity and resonating.  I made sure it was overlapping the CCW beam in the cavity, and checked that the reflected CW beam was overlapping along the input path.  Then I put the faraday back in place and moved it around till I got the beam through it and the CCW reflected beam back out of it.  I put the 1/2 wave plate back in, checked the polarization and then setup the mirror, lens and PD for the reflection locking of the CCW beam.  I got the cavity locked in the CCW direction again after changing the power back up to 5mW in that direction (somewhere between 5 and 10mW seems to give a nice error signal.  It may lock below this, but we were using 5mW before so I'm continuing with that just now).

With the CCW locked on the 00 mode I then swept through the frequency on the AOM till I got the 00 for the CW direction.  Then I played with the alignment a little till this looked good (I've not tried maximizing the transmitted power in this direction yet like we did for the CCW direction, so that's on the 'to do' list).  Then I got the PD for the reflection locking of this direction aligned and put the signal into the second PDH box.  I checked that we were getting an error signal by sweeping the cavity and we are.  Then I hooked the PDH box up to the frequency modulation input of the VCO for the AOM.  

The CW direction was already close to being resonant on the 00 mode so it was difficult to tell if this was really locking.  As a sanity check I t-ed the output of the PDH box to a scope and then changed the center frequency on the VCO for the AOM.  As I changed the frequency you could see the output of the 2nd PDH box tracking this and the CW direction of the cavity stayed locked to the 00 mode. 

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