I got a quarter wave plate from Greg Ogin this afternoon. The attached plots show:

black-> RC noise when there are no quarter wave plates. It's the beat of circularly pol beams.

green-> when one wave plate is intalled.(from last week)

Blue-> two wave plates are in used. I'm surprised that nothing changed much from green.

Red-> I use the Buzby box to amplifie the demodulated signal and connect it to SR560 for filtering. The SR560 complains about overloading signal when I

set gain at 1, but it's ok when gain is 2.

It seems still far from noise budget Rana gave me. The peak around 1000 Hz might to be investigated.