Yohei,

I started to built a new set up of the speed meter experiment. 

What I did:

I put a QWP and HWP in front of the source laser (Mephisto, Innolight), mazimizing p (holizontal) polarization.

Then, I put a Faraday Isolator (FI), optimizing its extinction ratio.

I measured the transmissivity of the FI:

Input = 10.3 mW,

Ouput = 9.3 mW,

Transmissivity = 90.3 %.

When measuring the beam power by a power meter, I attatched a ND filter, ND=2.0. From this value, the source lase power can be extimated as ~ 1W, keeping its original value.