The polarization of the IMC REFL beam on IOT2L was measured.
It was found to be about 80 degrees off from S-polarization while pure S-polarization is desired for the optics in the RFPD paths.
This explains the recent funny splitting ratio of the 50/50 BSs in the RFPD branches [1][2].


Background :
 We found that the 50/50 BSs were not splitting the beam with a correct splitting ratio. To understand the current situation we decided to measure the actual polarization.

 
No polarization correcter :
 Apparently there is no polarization correcter in the current setup. This is another reason why the splitting ratio was funny. We used to have a HWP to rotate the polarization to S (see for example [4]). However I found that this had been put aside out of the main path for some reason. I guess this is because the setup is still under the process of rebuilding. Anyway according to the latest diagram [3] there will a calcite polarizer in addition to the HWP which then spatially separates the S and P polarizations. A quick fix would be to simply place the HWP again in the path if necessary.


Measurement:
 I inserted a corner cube PBS (PBSC-1064-50) to measure the polarization in-situ on the IOT2L table. The measurement was done in two places : one right after the bottom periscope mirror and the other after the window (IO_MCR_AR1). I put a convex lens in front of the PBS to make the beam go through the PBS. Note that the measurement was done before I increased the incidnet power on the IMC.

  Measurement 1 : Right after the bottom periscope mirror
  transmitted light (P-pol) = 141.0 mW
  reflected light (S-pol) = 26.6 mW
  polarization angle from S-pol = atan(141 mW / 26.6 mW) = 79.3 deg

  Measurement 2 : After the window (PBS was placed after IO_MCR_M6)
  transmitted light (P-pol) = 4.72 mW
  reflected light (S-pol) = 0.75 mW
  polarization angle from S-pol = atan(4.72 mW / 7.5 mW) = 81.0 deg

So the beam is pretty far off from the desired polarization of S currently. The PBS and lens were removed from the table.


[1] LHO alog 6264 "DC calibration of IMC WFS"
[2] LHO alog 6297 "IMC incident increased to 1 W"
[3] DCC D0902284-v9
[4] DCC D0902284-v7