What I did

I replaced the PD at transmitted port in order for precise measurement of decay time.
Current PD has the bandwidth of 150 MHz.

Then I measured the ringdown of transmitted beam around 8 K.

Results

The PD has fast response such that the measurement can be done well.

The finesse was obtained by fitting the measured data, and it was about 1.65*10^4  though 1.67*10^4 at room temperature.

Next step

As the UGF of PDH servo is about 3 kHz and cannot achieve stable lock, I have to modify the servo to set higher UGF.
In addition, I will monitor the finesse behavior for a while.