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After some navel gazing, this all no longer seems so surprising. A block diagram of the system with the noise terms labeled should reveal why there is almost no beat signal with the CW servo turned off. I believe that there is no gyro signal in the modulation off case.
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Indeed there is no gyro signal. With only one path locked, any rotation will cause the laser frequency to shift identically in both directions and there is no signal.
We do however still have a beat signal since the beam is still coming through in transmission.
There are a few noise sources that should still show up in this. Any non-common mode mechanical noise in the cavity for example. Also any mechanical noise in the M-Z readout, any noise in the transmission readout and electronics, and any phase noise in the PLL. We should also see noise from driving the AOM since it is still on.
There are also a lot of noise sources we shouldn't see. Modulation of the FSR by mechanical noise for example.
I guess the point I was trying to make is that this excess noise can't be a real rotation signal, so perhaps it points us towards what the main source of noise is. |