PARTICIPANTS: Yuhang, Matteo, Raffaele, Tomura, Eleonora

Finally today we managed to realign the IR beam.

First we aligned and locked the cavity with the green light and then we cut the green.

We checked the IR reflection from the invacuum faraday on the bench and we maximized it by moving the two last steering mirrors for the IR on the bench. 

The laser power is set to the standard value (current: 1.2 A),  the incoming IR beam just before the windows is about 16 mW and the maximum reflection we could find is 12.8 mW. Increasing the laser power (2 A) we have an incoming beam of about 33 mW and the maximum reflection is about 25.7 mW

We checked that the reflection was moving accordingly to the IM motion, so we were pretty sure that it was coming back from it.

After maximizing the reflected power the beam was good on the PR reference but still we couldn't see anything on the first target, except for a very dim scattered light.

Then we put down the first target and rised the second target where we found a very strange beam, showing some fringes. We observed that fringes were reduced and the shape changed when we misaligned the end mirror.

After that, we reinstall the IR camera in the end bench and we center it by using the residual transmission of the green (when it was locked). By changing the yaw of the BS of a small amount we were able to see IR flashes on the camera. Then we put the BS back in the initial position (the one that makes the green to flash) and we adjust the yaw of the two last IR steering mirrors on the bench in order to make the IR flashes appear again. In this condition we have both green and IR aligned in the cavity.

The IR aligment was not optimized: we were still able to see HOM flashing but it is not easy to fine tune the alignment by moving the steering mirror on the bench

We remarked that there is a very bright circle of IR light on the end camera around the flashes. It resembles the contour of the end mirror and it is not clear why we can see it. Since we suspect a mismatching we tried to move a bit the last IR lens on the bench but it didn't improve the situation. (We put it back to the previouse position.)

Lessons learned:

1) The reflected beam is a good reference for the alignment of the beam and it should be maximized as a good starting point.

2) The behaviour of the IR beam on the target when it is aligned is much diffent from that of the green

    - we can't see anything on the first target, so we should probably not consider it in the future

    - when the second target is up proobably same unpredictable reflections between the mirrors and the target produce a very strange beam. 

More work will be done to understand the origin of the bright circle and to improve the alignment.