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Entry  Tue May 10 18:46:22 2011, Zach, Laser, GYRO, Sideband trouble again TEK00011.PNGTEK00012.PNG
    Reply  Wed May 11 11:46:45 2011, Alastair, Laser, GYRO, Sideband trouble again polarization_into_EOM.epsRFAM_change.eps
Message ID: 1415     Entry time: Wed May 11 11:46:45 2011     In reply to: 1414
Author: Alastair 
Type: Laser 
Category: GYRO 
Subject: Sideband trouble again 

A couple of things I notice:

1)  If you scan the cavity and look at the relative sizes of carrier and sidebands in transmission, then as you move away from having a zero error signal in the region far from resonance, you can increase the relative size of the sidebands.  In other words, the point where we have zero error siganl far from resonance, is also a place where we are not maximizing the modulation depth through the EOM.  This is another way of saying we have some offset.

2) If you close the shutter on the laser there is still a 19MHz signal coming from the PD.  There is some sort of coupling happnening, and this will give us some offset. I measure roughtly 4mV offset in the error signal with the laser shutter closed.

3) Rotating the QWP also changes the power going into this loop of the gyro since there is a PBS after it.  This is a total pain because when you put a PD in and try to reduce the RFAM you are also measuring the reduction in power caused by the PBS.  The only way to measure this is to rotate the QWP, measure the power, adjust the power to the same level, then measure the RFAM, and then to iterate.

Attached is a plot of measuring RFAM as a function of polarizer position.  Although we moved the EOM, it seems that the position of minimum RFAM is still 100 degrees (actually just a fraction before 100 degrees) where is was possible to get the signal down to approx 450uV (blocking the cavity and using our REFL PD to make the measurement).

Again I saw that this minimum value started to vary a bit, so I set it to the minimum and locked the actuator on the wave plate.  Then I recorded the RFAM for a while to see how far it would change.  The graph is attached.  It varied up to just below 5mV and then started coming back down again.



Attachment 1: polarization_into_EOM.eps  9 kB  Uploaded Wed May 11 14:33:21 2011  | Hide | Hide all
Attachment 2: RFAM_change.eps  9 kB  Uploaded Wed May 11 14:44:52 2011  | Hide | Hide all
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