today, with Daniele, we did a simple polarization measurement in transmission of the FP cavity beam.

1) we locked the FP cavity and measured 82.5kw (for 33% amplifier ratio) with the power-meter moved a little bit further (this is maybe the reason why the power is a little bit lower than usually measured).

2) then, we installed a high power PBS CCM1-PBS25-1064-HP(/M) with a transmission of 89% of the P-polarization @ 1030nm
We measured 66.5kW in transmission of the cube.

3) then we installed a half waveplate to find the optimum angle => 77kW measured at an angle of 342°
(almost no power has been observed in the vertical polarization state of the PBS).

Then we rotated the waveplate to find the previous measurement => 66.5kW at an angle of 348°
(we checked that adding the waveplate in the path almost does not change the measured power)

Conclusions :
- the FP cavity polarization is almost horizontal (P-polarization) and linear (77kW ~ 89%*82.5kW)
- the angle of the polarization is roughly 2*(348-342)° = 12°