Yaochin and Yuhang

As we reported in elog1867 and elog1857, we improved the matching between LO/BAB into AMC. However, we didn't care about the residual p-pol component after the optimization. And actually, it is really necessary to optimize them and remove the p-pol component.

LO part (The AMC spectrum before the change is attached as picture 2) (s-pol 11V, p-pol 14mV)

     Actually, I am confused that how can we have a p-pol component from LO. Because LO is provided by IRMC TEM00, TEM00 should provide a quite clean s-pol light. 

     1. We replaced the first mirror after IRMC with a PBS plate.

             After this, the p-pol peak is reduced from 14mV to 9mV(attached picture 3). (We also double-checked that total power doesn't change after the replacement of PBS, so this p-pol reduction is a real      reduction effect from PBS)

     2. We put an HWP just before homodyne BS.

            By rotating HWP, we could almost remove the p-pol component totally(attached picture 4).

BAB part (The AMC spectrum before the change is attached as picture 5) (s-pol 4V, p-pol 43.2mV)

     For the p-pol component inside BAB, it comes from the not optimized HWP. So we just rotate HWP. And then the p-pol is basically removed totally. (attached picture 6)

We measured the squeezing spectrum before and after this change, but there is no obvious change.