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Message ID: 1394     Entry time: Fri Apr 15 14:44:15 2011
Author: Zach 
Type: Laser 
Category: GYRO 
Subject: EOM AM/PM 

 I got around to doing the EOM AM/PM measurement, and the results aren't fantastic.


already measured this the other day, but we've done some switching stuff around since then so I did it again. 


  • Carrier: 780 mV
  • SB: 11.4 mV
  • Γ ~ 2*sqrt(11.4/780) = 0.24 rads (this agrees with the other day's measurement)



This is where it gets messy. I measured this in two places: after the BS in the CCW path, and after the AOM in the CW path. It was measured by putting the signal from a PDA255 directly into the RF analyzer and measuring the peak (in V) at 19 MHz. The DC level of the light for all measurements was 3.00 V.


The first thing I noticed was that the peak was not minimized at the same HWP angle that made the beam have exactly S polarization. Rather, it was best about 5 degrees off.

Even at this minimal point, the peak fluctuated in amplitude from ~10 uV to 180 uV with a period of 1-2 minutes. You could literally sit there without touching it and watch it go up and down before your eyes. WTF?

Rotating the beam to exactly S made the peak climb to 500 uV!


This was even worse than the other direction. I had to turn it 17 degrees away from S in order to minimize the peak at ~50 uV. Putting it at S made it ~1.3 mV!

It didn't seem to oscillate as wildly as the CCW beam at the minimal point.


I think the most alarming thing is that the AM in both beams cannot be simultaneously minimized. This explains why we always see junk at low frequencies, I think: minimizing the peak for the CCW beam just ensures that there's more junk in the CW direction.

Need to figure out what gives.



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