We're still aligning the arm, it's difficult because we cannot see the beam at all at this stage on the mirror.

Anyway, the beam goes to the ITM, and then to the ETM, then to the ITM and comes back to the ETM. We're seeing some signal from the demod board, which goes zero when the ITM is misaligned. So this is the real interference signal!

Unfortunately it's not like the arm is flashing or anything, I suspect that our alignment is off mainly in yaw because I can tilt the ETM in yaw and see the beam on both righ and left side of the ETM at about the same height. 

Tomorrow we'll look at the interference pattern in the reflection from the arm, DC level of the reflection, and the demod signal amplitude to help our alignment effort. We'll also optimize the demod phase from Beckhoff world.