What I did

Today, I tried to measure the finesse of the cavity by locking the laser.
However, I could not lock and found the servo had some problems at low-pass parts.
I tried to remedy the circuit but could not solve the problem.
So I decided to make another circuit.
Actually this work has not finished yet...

Besides it, I did some measurements without locking the laser, i.e. doppler method which depends on the dynamical response of the cavity.
Also I measured the frequency split between transmitted p- or s- polarized beam.
As shown in the attached picture, there is a frequency shift between the s- and p- polarization.
The injected beam is pure s-polarized beam, but by playing with a HWP one can inject p-polarized beam. 
This frequency spilt measurement corresponds to a cavity enhanced ellipsometry.

Results

To be honest, the analysis has not yet done...
The frequency spit was about 150 usec by 100 Hz 3 Vpp triangle wave.

Notes

The temperature of the cavity was about 120 K.

At first, the reflected (and transmitted) power was about 70 % less than I expected.
I doubted that the misalignment, but it seemed not.
Though I optimized the alignment, the reflected power did not improve.
After one or two minuites illumination, suddenly both of them recovered.
I could not figure out the reason, but the desorption of moleuclar layer is one of the possible reason.
Further investigations are needed.