Today I did alignment work to see the flash of transmitted beam using PD.
I though that PD detected transmitted beam, but it turned out that it was scattered beam...

Then I re-aligned so that the reflected beam reach to FI, and scanned laser frequency with monitoring reflected beam.
At that time, the reflcted beam power was fluctuated.
Actually, it was caused by vibration of cryostat chamber.
So far I have used long pedestal to hold silicon mirrors, and this may affect the vibration of cavity.
Using fixed spacer may reduce the vibration effect, but it might be better to consider about vibration isolation system.