Participants: Tomura, Eleonora

As reported in entry 654, we had to move the yaw of SM2 dicroic mirror in PR chamber to avoid the IR reflection to touch the viewport side. Since after the last 1000 steps done, we were not able to get a good IR alignent anymore, yesterday we moved back SM2, hoping to find a good postion where we could have both a good alignement and the maximum of the power in reflection. 

The input power is about 16 mW.

The situation is such that in the original position (before last thurday) the aligment can be very good but the beam hits the viewport side and the reflected power is very low (less than 1 mW). In order to have the maximum reflected power possible (about 12.5 mW) SM2 has to be moved so much and it is not possibile to recover a good alignent. (We remarked that in this case the cavity seems very mismatched, but we where not able to improve the situation by moving the last IR lens on the bench).

Today we tried to put back SM2 towards the original position and to fine-tune its position in order to have the best compromise between good alignement and high reflected power.

We did the followoing series of steps (velocity 500)

-300, -300, -300, -1000, -1000, -1000, +1000, +1000, +500, +300, -300, -300, +200, -100

In the end, we stopped in a position where we can have quite a good alignement (but not the best possible) and about 11 mW of reflected power. By looking with the camera it is not evident that the reflection is touching the viewport side.

We also moved the last IR lens back, from 20 mm to 17 mm.

In the attached files we recoreded the new reference for the green beam: the white cross corresponds to the orginal position and the black point corresponds to the maximum displacement done by SM2.

Considering the maximum reflection we could get (12.5 mW over 16 mW), we estimated the injection losses in vacuum to be about 22% (11% single pass) which is not much higher than we expected.