Yuhang, Michael

We took the opportunity to measure the vacuum FI loss when the vacuum tanks were opened, using BAB. The vacuum Faraday isolator assembly can be seen in the photo (figure 1). The polarizers are two mirrors tilted at 55 degrees (Brewster polarizers). The black tube is the Faraday rotator. Attached to the end of the rotator is the half wave plate. From the reflection of the second polarizer, the IR beam hits the dichroic mirror, where it is recombined with green. Instead of propagating to the filter cavity, we used a mirror (furthest left in the photo) to reflect the beam back through the Faraday. Then, we measured the power in the following locations:

P1 in: Incident power at the first polarizer, coming from the squeezer table
P1 ref: Reflected power after P1 in
P1 tra: Power transmitted through the first polarizer, with respect to P1 in
FR/HW tra: Power after one pass through the Faraday Rotator and Half Wave plate
P2 ref: Power reflected from the second polarizer after coming from the Faraday
P2 tra: Power transmitted through the second polarizer, with respect to FR/HW tr
Di in: Incident power on the dichroic mirror
Di ref: Reflected power from the dichroic mirror
RT P1 tra: Power transmitted through the first polarizer after two trips through the Faraday.

There is some weirdness with the power meter measuring more power after reflection from certain components, but it consistently happens in certain positions (P1 ref, P2 ref, Di ref), which makes us think it's more to do with the power meter geometry. It makes precise measurement of losses to be difficult. Still, we can draw two conclusions: 1) the polarizers have low transmission of the order of 1%, and 2) the round trip loss through the FI assembly is about 14%. We took several measurements at the positions that were causing problems. Also, the p-pol PLL unlocked at a couple of points (between columns 4/5, 10/11 on the table below)

Power measurement (units of uW)

Avg. RT P1 tra/P1 in = 0.857
RT P2 transmission = 2 uW