I turned on the refrigerator on Friday to see the temperature the cavity can reach.
Actually, the mirror temperature was below 10 K.
Therefore, we can measure the mirror properties from room temp. to around 10 K where the ET's test mass target temperature.
Fig. 1 shows the measured temperature, A represents the temperature of the mirror and B represents that of the table.

In addition, I could lock the laser to the cavity.
The red line in Fig. 2 shows the transmitted beam power though this picture is taken when the cavity was not locked.

I will make the servo to add the offset to feedback signal.
After that I will implement it to stabilize the lock.