Last Friday we wanted to do this characterization again because it seems that our previous points were still inside the Rayleigh range of the beam (which should be few meters)

The solution is to add a lens and recover the beam parameters without the lens.

We faced few issues during this measurement :

Faraday Isolator

1. The Faraday Isolator output (facing the FC) is clipping probably higher order mode which are quite large w.r.t. the FI aperture
2. The FI PBS which reflects the beam toward the locking photodiode is a bit broken ( it is the cause of the "strange" behavior of the beam we could observe. However, with proper alignment it is possible to avoid hitting the broken part of the PBS and have a good beam ( meaning we can avoid to have black line on the beam)

For now it seems we can keep going on with this FI at least for the beam characterization.

It may be useful in the future to change this FI for a larger aperture one or at least change the broken PBS.

BS

1. We found out that the beam was shaking a lot ( ~40 urad rms ) while performing this measurement. This is due to the fact that the BS pitch correction is close to saturation (-10V) and some peaks lead to correction saturation.

The solution is to move the BS pitch picomotor. Matteo also recommends to follow the GEO procedure : Every ~2,4 weeks, use picomotors to put mirrors on good positions in order to avoid this kind of saturation.