We are characterizing the PMC loop TF. We needed to measure the cavity's FWHM in order to know the cutoff frequency of the cavity.
This number will be used in the Simulink model to simulate the TF of PMC.
The length of the PZT was scanned, while the PDH error signal was also recorded.
The time span between the maximum and minimum peaks of the error signal was measured in order to obtain the width of the cavity resonance. The time-to-frequency conversion [second to Hz] will give us the FWHM. The conversion between the time and the frequency was obtained by looking at the zero crossings of the error signal which are separated by the modulation frequency.
The time span between the carrier and the sideband: 768 us.
The sideband frequency: 21.5 MHz
==> the conversion factor: 21.5 MHz/ 768 us = 28 GHz/s
1) The cavity pole frequency is obtained from the measured FWHM. It is FWHM/2 = 2 MHz.
FSR is c/Lroundtrip = c/(0.42m) = 714 MHz.
Thus we can compute the finesse F = FSR/FWHM = 188.
2) Another way to measure the FWHM is by measuring the transmission peak of the transmitted light while scanning the cavity. I’ll try this and see if two results agree.
Many thanks to Koji for useful discussion on the measurement and how to improve elog quality.