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Entry  Tue Apr 26 01:28:09 2011, Zach, Laser, GYRO, Aha! (?) dc_trans_vs_gyro_noise.png
    Reply  Tue Apr 26 10:34:29 2011, Alastair, Laser, GYRO, Aha! (?) 
       Reply  Tue Apr 26 15:25:24 2011, Zach, Laser, GYRO, Aha! (?) 
    Reply  Tue Apr 26 17:24:46 2011, Zach, Laser, GYRO, not mode rotation from pointing 
Message ID: 1400     Entry time: Tue Apr 26 10:34:29 2011     In reply to: 1399     Reply to this: 1401
Author: Alastair 
Type: Laser 
Category: GYRO 
Subject: Aha! (?) 

In terms of a coupling mechanism, I'm wondering about this:  If the pointing of the carrier as it goes into the cavity is moving around at low frequency, then that's going to modulate the coupling into the cavity and give us some AM on the reflection PD  (I'm not talking about RFAM here, but low frequency AM as the coupling to the cavity wanders around).  This then causes the AOM to try to act, so we see noise superimposed on the reflection and transmitted beam.

It seems that whatever the cause, if we are seeing AM at low frequency in the transmitted beam then it is likely that we will also be seeing the exact same signal on our REFL PDs.

EDIT:  Zach pointed out to me that there needs to also be some phase shift between the carrier and sidebands for us to get an errors signal.  I think that I'm not clear now how we ever expected the RFAM to show up as noise.  Where is the phase shift there?



Since it looks like AM at the input side is not the root of our current low-frequency problem, we have been trying yet again to come up with new sources. Alastair had the idea that pointing noise of the beam going into the cavity might be causing us a problem in some not-yet-well-understood way (this was first discussed in the context of 19 MHz jitter from the EOM being converted to RFAM as it could be in the AOM, but the cavity should have very little response at 19 MHz with which to do this).

I decided to look at the low-frequency spectrum of the DC_TRANS signal to see if there was the same type of behavior as in the gyro spectrum. As it turns out, there is. Below is a plot of the two compared side-by-side (though I had to scale the DC_TRANS plot up by a factor of ~10 to get it to coincide). There is extremely good agreement in the shape of the curves, and I think we'll see that the HF floor of the DC_TRANS plot is just the noise floor of the (broadband wide-area Thorlabs) PD.


I can't say that I can think of any obvious coupling mechanism here. I think it's feasible that pointing drift of the input beam is causing the spatial eigenmodes of the cavity to wander differentially (i.e., the points at which the supported mode in one direction touches the mirrors all move horizontally across the mirrors a bit, making the whole square 'rotate'---this is dependent on the input beam and can thus happen independently in each direction). In this case, the cavity length would differ in the two directions, resulting in a gyro signal.

We need to do some more work to figure out what exactly is going on, but this is another data point that helps with the diagnosis. I think the next step is to look at the power at some pickoff point close to the laser to see if this is an input power drift or something caused by varying degree of coupling into the cavity (as from misalignment).


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