Since we are using the AOM actuation signal readout for the time being, I thought it would be a good time to see what effect AOM/VCO noise currently has on our gyro signal in this readout. My hypothesis was that if VCO noise was currently limiting us in some frequency band, then replacing the Tektronix with a Marconi should reduce the noise there. Alastair did this a while back, but at that point we were reading out in transmission, where the AOM/VCO noise is suppressed by the CW loop gain. When using the AOM actuation signal as the gyro readout, however, we see the full effect of noise in the AOM and VCO. Attached is a noise spectrum with the Marconi in place. Comparing it to the last spectrum, it is clear that there is no change.
You may be wondering why, if there will be no AOM actuator noise in the final transmission readout, it is that I am even worrying about this at the moment. The answer is that given the recent insight into displacement noise coupling, we may want to reconsider the possibility of using the AOM actuation signal as the final gyro readout after all. Some things to consider:
The main reason we were pursuing the transmission PLL readout was to avoid differential-mode noise incurred in the AOM path, including noise in the AOM itself, noise in the VCO driving the AOM, and noise in the optics encountered only by the CW beam. Analysis showed that this noise is absent from the light emerging from the transmission port of the cavity, so we sought to do the readout at this relatively clean point. The fact is, however, that we will rely on another low-noise oscillator for the PLL, and noise in this oscillator will couple directly into the gyro signal anyway.
What remains is noise in the AOM and from the shaking of the pre-cavity CW mirrors. It may be that noise in the AOM itself is so terrible that the PLL still wins out, but as for phase noise from mechanical displacements, the choice between the two gyro readouts is likely to be a toss-up, as the transmission readout will see the cavity phase noise as described in the recent document on the elog, while (I think) the AOM actuation readout will not. If this effect (not to mention the ADDITIONAL noise contribution of the turning mirrors in the transmission optical demod setup) is of the same order as the mechanical noise in the pre-cavity AOM path, then it may not be worth the hassle to do the PLL at all.
We need to think about this some more, and we should also begin to think in earnest about how we plan to stabilize the cavity via PZT; the conclusion of the displacement noise coupling analysis was that---even with perfect feedback loops---the length of the gyro cavity would need to be stabilized to ~10-12 m/rHz over a wide band in order to reach the sensitivity requirement.