I roughly investigated the transmittance of fused silica mirrors which will be used for input and output couplers.
I measured transmitted beam power for 2 of 4 mirrors.
The trasmitted power was 1.8uW for both of them with respect to 10mW incoming power.
This value corresponds to T=0.018% and R=99.982% assuming no loss in mirrors.
Then the finesse can be estimated as 1.7*10⁴.
This value is reasonable since the designed finesse was 1.5*10⁴.

I gonna construct FP cavity with these mirrors and try to lock TEM00.