Participants: Yuhang, Yuefan, Tomura, Raffaele, Eleonora

In the past days we have worked in order to improve the IR alignment.

As a first thing we placed a camera on the optical bench to look at the IR reflected beam and we tried to maximized the trasmitted power while monitoring the shape of the reflected beam. According to our understanding, in reflection we should see the superposition of the resonant TEM 00 (dephased of 180 deg after it is reflected by the cavity) and the HOM due to misalignement/mismatching which are promplty reflected. 

The procedure to align the cavity both for green and IR is the following:

1) Adjust BS position to center the beam on the end mirror (reference on the end camera screen)

2) Align the cavity for the green beam by moving input and end mirror to maximize the transmitted power

3) Move the last two steering mirror for the IR on the bench to maximize IR trasmitted power 

4) While aligning the IR we take care that it is always centered on the resonance by looking at the error signal and adjustig the AOM frequency to null its offset.

During in this activity  we realized that the alignement improvement was limited by the position of the last IR steering mirror on the bench. So we have shifted it after carefully taking some references in order not to loose the alignment. After this change we were able to improve the IR transimitted power from about 2.5 up to more than 3.5 V 

Currently in the best alignement condition we have about 1.8 V of transmitted power for the green and 3.8 for the IR.

The attached video shows the reflected and the trasmitted IR beam when we change the alignment condition in pitch and yaw by moving the steering mirror on the bench. In the case of strong misalignement the presence of first order modes becomes evident. Anyway also in the best aligment condition (about 95 %) there is still a small black dot in reflection. 

The oscilloscope in the video shows the transmitted power (yellow line) and the IR error signal (blue line).

After this change in the alignement we have verified that the IR beam in refection was not touching a side of the viewport. (See entry #659 related to this issue )