About two days before this elog post (I am a bad grad student), Koji and I took some more measurements in a continuing effort to fully characterize the CCW (main) loop.

Some things we did:

• Open-loop transfer function
• Error signal spectra
• Calibration using a 1-kHz sine injection

The last two were also done for the CW (AOM) loop, with the second also done with only the CCW loop closed to see the "free-running differential-mode" noise. We noticed at least two interesting things:

• The CW noise suppression was far from stellar at low frequencies and non-existent above a relatively low frequency (which escapes me at the moment)
• The 1-kHz calibration line was partially visible in the CCW error spectrum when the excitation was on the AOM VCO. This indicates either electrical cross-coupling or more likely backscatter noise at the cavity input (which surely got worse after we put in windows so that theta = 0 deg). On that note, we may eventually find it necessary to use a different modulation frequency for each of the two directions, but it's certainly not clear that that will be necessary yet.

Since the new front end is not quite up to speed (and due to bandwidth limitations), all of this was done using the Agilent. Due to Koji's far-superior Agilent prowess, he did most of the button-pushing, and he will give me the data once he offloads it so that I can process it.