-Hi Yoichi,

A good point. We don't have a measured number for the polarization contents of the forward propagation beam at POP.

We actually looked at the brightness of the S and P-pol. of the forward propagating beam at POP by eyes with a sensor card. The P-pol. light was so dim that we were not able to see it by eyes and therefore assumed that the incoming beam was purely S-pol.  We will try to measure it shortly.

The measurement was done by inserting a PBS plate, PBSW-1064 1 inch from Thorlabs, and measuring the amount light in the transmission and reflection sides at a time using a PDA100A. Since we didn't like to have an extra polarization sensitive optic in the measurement, such as a harmonic separator and S-pol BSs, we inserted the PBSW-1064 before the beam arrives at the periscope. The dark offset was carefully subtracted every time when we made the power measurement. In addition, we paid attention such that the incident angle of the beam on PBSW-1064 to be close to 45 deg although this is done by our eyeball. Perhaps a precision of 1 deg or smaller.

-Hi Shinji,

It is possible that the incident beam was already messed up in the polarization. You are totally right that the BS can introduce asymmetry in between the optical power in reflection and transmission if the incident beam already contained a P-pol content. As stated above, we will be assessing the polarization content of the incident beam shortly.

-Hi Masayuki,

Aha, sorry for the confusion. The trick is that we have assumed the transmissivity of PRM to be 10.35% rather than 10.00%. This 10.35% was a value that Yutaro obtained from Hirose-san through the email-exchange at one point in the past. We don't know how accurate this number is. In fact, this illuminates another issue that our estimation is quite vulnerable against uncertainties in the PRM transmissivity.