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Message ID: 1417     Entry time: Tue May 17 12:23:44 2011
Author: Zach 
Type: Laser 
Category: GYRO 
Subject: electronics noise 

[Alastair, Zach]

Yesterday we did some loop characterization to see if the increased LF noise we are seeing could be from electronics noise. First we measured the OLTFs:


We also verified that the servo transfer functions are what they are supposed to be. They were. Here are the LISO TFs:


Using the above two pieces of information, along with:

  • PZT gain: ~4 MHz/V
  • AOM VCO gain: ~2 x 70,000 Hz/V (double-pass)
  • 3 dB attenuator between CCW PD and demod electronics

We can compute the optical gains ( [V/Hz] ) to be:

  • CCW: 6 x 10-8 V/Hz
  • CW: 7 x 10-6 V/Hz

There is an equal power of about 3 mW in each direction, and the REFL PDs are identical, so it is very confusing that the optical gains are different by more than a factor of 100!! This needs to be investigated immediately.

Taking the above as true, we can still calculate the current gyro noise contributions by taking the output noise level (with the input terminated), referring to the input, dividing by the optical gain (and 3 dB attenuation for CCW) and multiplying by the standard Hz->rad/s factor. The result of this is below:


Comparing to the red curve in the plot here, we see that the electronics noise is not high enough to explain what we see in this configuration. We still need to check the dark PD noise and demod noise in this configuration, though.

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