Today I tried to lock TEM00 mode with 10^3 gain.
The attached pictures show DC REFL (blue), error signal (green), and feedback signal (yellow).

Actually, the lock was not stable; it last only ~10msec.
This seemes due to the fluctuation of feedback siganl which oscillated about 20Hz.
Also the error signal oscillates about this frequency, and this hinders me to lock.
I will investigate from where this oscillation comes.