I reduced the gain to 2*10^2, and tried to lock.
The oscilation was improved than higher gain one, but the lock last only 0.1sec and DC REFL dip was ~0.1mW.

Anyway, it seems that it is possible to lock TEM00 mode.
Actually, feeding back to laser PZT induces laser power fluctuation.
Therfore, ISS is needed in this experiment.