I re-installed a folded cavity inside the chamber with one apex silicon mirror.
Then I aligned the 2 steering mirrors to pick off the reflected beam and I could pick off the beam from the chamber.
A PD was put the transmitted side to check the transmitted power when the cavity is composed.
After that, I installed a fused silica mirror as the output mirror and tried to see the transmitted beam power.
Some amount of stray light, however, prevented me to check the transmittance of the fused silica mirror.

I will install an input fused silica mirror and try to see the flash by scanning the laser frequency.

[note]
There is an optical loss at the viewport, about 10%.