What I did

As the temperature became enough low, I tried to lock the laser to the cryogenic cavity and measure the finesse.
First, I scanned the laser frequency to measure the linewidth.
In addition to the linewidth, I could see the splitting peak probably due to the difference of polarization.
Then I did ringdown measurement several times.

Next step

I will analyze the obtained data.

Notes

As there will be a power outage work on next Sunday, I turned off the refrigerator to raise the temperature.
I will cool down to 10 K next time after improving the alignment.