[Aritomi, Yuhang, Eleonora, Matteo]

Recently we had a problem that when BAB is injected into filter cavity, we cannot lock green phase with p pol leakage of BAB transmission. Today we found that back reflection of IR from filter cavity makes green phase error signal unstable. When we block IR injected into filter cavity, green phase error signal is stable, but when we open it and IR is aligned to filter cavity, green phase error signal fluctuates a lot. We guess that back reflection of IR from filter cavity goes back to OPO and amplified and becomes p pol leakage.

To reduce back reflection, we put faraday (IO-3-1064-VHP) after OPO on the bench (Pic 1,2). BAB transmission without green before faraday is 103mV and 100mV after faraday, which means transmissivity is 97%. 

After installation of faraday, IR is totally misaligned. We'll try to align faraday tomorrow.