How To Measure the Length of the Input Mode Cleaner.
- Make sure that the PRM is aligned, and other IFO optics are misaligned. There should thus be a REFL beam which is only a direct bounce off the PRM, going back to the REFL diode (we'll take REFL166).
- Make sure that the mode cleaner is locked and well aligned.
- Inject a few ~mV signal at ~2kHz into the AO path of the mode cleaner servo (IN2 input on the front panel).
Look at a spectrum of REFL166 (I&Q). You should see a peak in the power spectrum at the frequency of the signal you're injecting into the mode cleaner.
- Change the 166MHz modulation frequency using the settings on the RF signal generator until the peak in the power spectrum of REFL166 is minimized. This is your new 166MHz modulation frequency.
- Do the appropriate arithmetic to adjust the other modulation frequencies (33MHz, 133MHz, 199MHz).
Once you are done measuring the length, and have appropriately set the RF modulation/demodulation frequencies, you should ../Set_the_double_demod_phases_for_the_Short_DOFs. Those phases can change not only because the resonant conditions of the RF sidebands in the mode cleaner can change, but because the Marconi RF signal generators will randomly reset the phase when you change the frequency. This normally isn't a problem if you're using the same Marconi for modulation as demodulation, but we use different Marconis for these two processes, and they can get a relative phase shift from this process.