Measurement

Excited the broadband EOM for the FSS phase actuation and measured the error signal of the PMC LSC. Since the OLG of the PMC LSC loop is smaller than 1 at the frequencies around the PMC cavity pole (the UGF is around 1 kHz for this measurement), the transfer function from the broadband EOM to the error signal is a production of the cavity response and the EOM response. Therefore, from this measurement, we can estimate the cavity pole of the PMC.

Result

Figure shows the transfer function with the compensation of the EOM actuator efficiency, that is, the transfer function was divided by the frequencies.  There seem to be four poles. One is the PMC cavity pole and the two poles at the same frequency might be the poles of the mixer. We are not sure where the pole at the highest frequency comes from. The cavity pole and the mixer poles were estimated by the fitting as follows:
f_pmc = 695 (55) kHz
f_mixer = 2.5 (0.4) MHz
f_unkown = 5.1(0.1) MHz

Soon we will measure the FSR, and then, we will know the finesse and the losses of the PMC.