I measured the finesse of cavity by scanning the laser frequency through the temperature actuator. The result shows that the finesse is twice lower than the designed value.
Designed power transmissivity and loss of mirrors from Layertec are:
T_in = 4000 ppm, T_end = 35 ppm, L_in=L_end=30 ppm,
therefore the cavity finesse should be ~1500. With cavity length l=15 cm, cavity pole should be:
T_in*c/4/l/2/pi = 320 kHz.
However, the measured cavity pole was 635 kHz i.e., finesse is =750. You can see the Lorentzian plot here. Calibration factor from scanning time to frequency is derived from the RF sideband frequency of 15.24 MHz.
This result means that cavity loss is ~4000 ppm, which makes this cavity totally useless for speed measurement.
This was my mistake. The cavity length was 7.5 cm, which makes the cavity pole half. We are building the cavity again with the proper length, 15 cm.