We wanted to lock both the arms using ALS and get IR to resonate while arms are held using ALS. The X arm was locked using ALS and offsetter2 was used to scan the arm and find IR resonance. The Y arm was locked using ALS. But as the Y arm was brought closer to IR resonance, the X arm ALS loses lock. (attachment 1)
We believe that this comes from the X and Y transmission not being well separated at the PSL table. The PBS is not sufficient to decouple them (A strong beatnote ~35dB between the X and the Y arm green lasers can be seen on the spectrum analyzer).
Decouple the X and Y arm transmitted beams at the PSL table. I am trying to find a wedged mirror/window that can separate the 2 beams at the PSL table before the beat PD (sadly the laseroptik HR532nm optics have no wedge)