As discussed at today's meeting, we would like to (re)measure the Arm cavity lengths to ~mm precision, and their g-factors. Any arm length mismatch affects the reflection phase of the sidebands in the PRMI, which might be one source of our woes. Also, as I mentioned in a previous elog, the g-factors influence whether our 2f sidebands are getting pulled into the interferometer or not.

These both can be done by scanning the arm on ALS and measuring the green beat frequency at each IR resonance. (Misaligning the input beam will enhance the TM10 Mode content, and let us measure its guoy phase shift)

I started working on this today, but I have measurements to do, since at the time of today's measurements, I was fooled by the limits of the ALS offset sliders that I could only scan through two FSRs. Looking back at Manasa's previous measurment (ELOG 9804), I see now that more FSRs are possible.

Ways I will try to improve the measurement:

• Jenne claims that the main limitation on ALS scanning range is the length to pitch coupling of the ETMs. If so, I should be able to get even more FSRs by scanning with MC2, as I did today, since the IMC cavity length is shorter, meaning more arm FSRs/unit length. More FSRs mean better statistics on the FSR slope fitting.
• FSR error:
  • I am measuring the out-of-loop PDH signal of the arm at the same time as the beat spectrum is being measured, to know the magnitude of displacement fluctuations and any overall offset from the PDH zero crossing.
• Beat frequency error:
  • I updated the HP8591E gpib scripts to be able to set the bandwidth and averaging settings in order to really nail down observed beat frequency.
  • I've written some code to fit the spectrum to a lorentzian profile, for evaluation of the linewidth/frequency uncertainty
  • I am also considering beating the analyzer with a rubidium clock to compensate for systematic errors, since ELOG 9837 says the analyzer is off by 140Hz/10MHz, i.e. 10ppm. Since we're trying to measure 1mm/40m~25ppm, this can matter.

Just for kicks, here are scans from today.