[Paul, Kiwamu, Keita, Dick, Rodica]

Today we measured the mode cleaner cavity length by modulating the EOM at 45 MHz and doing a sweep around this frequency. We measured the spectrum using an HP 43968 Spectrum Analyzer. The maximum source power was 19 dBm from a Mini-Circuits RF amplifyier with gain 19 dB. The bandwidth for the highest zoom on the peak was 300 Hz.

The frequency of the dip in the middle should correspond to the 5th FSR away from the carrier resonance. The calculated length is 9.09918 MHz +/-  100 Hz, only 291 Hz away from the design value of 9,099,471 Hz. This is equivalent to 1.05 mm difference from the design length.

We tried a full sweep over one FSR but we realized that the modulation away from resonance would be too small to give us any useful results, so we focused on the 45 MHz resonant peak. Attached is a plot of the data over a full FSR, that also shows a peak corresponding to the non-resonant polarization, and few other peaks possibly due to higer order modes.

Similar measurements at LLO are described in alog 4702.