Participants: Yuhang, Tomura, Eleonora

Today we kept working on the alignemet of the infrared beam:

1) We have centered the green beam on the end mirror. To do this we have made a scan in pitch and yaw with the BS in order to reach the edge of the mirror and see the green beam on the coils. We recorded the corresponent offsets and pick the intereidiate value as the good one. 

2) We have investigated the origin of the bright halo visible on the camera for the IR trasmission (see pic1). We have looked insiede the end bench putting a camera in the viewport behind the end mirror (the one through which the cavity transmission passes) (See picture 2).  The bright halo seems to come from the contour of the end mirror. Raffaele suggested that it may be the light diffused by the input mirror at small angle, which is trasmitted by the most external part of the end mirror. In fact the HR coating has not bean applied there and it is almost trasparent. The fact that we see it very bright could be due to the comparison with the very small transmissivity (few ppm) of the HR coating anywhere else on the mirror surface.

3) We tried to maximize the alignement of the IR beam by moving the steering mirrors on the bench. As usualy it was done after having aligned the cavity for the green. We could obtain quite a good alignement but we observed that in this condition the reflection from the faraday was not maximized anymore and it became actually very small (less the 1 mW, with 16 mW in input). This suggest that the axis of the faraday is not well aligned with that of the cavity and so an additional work to align them will be required.  To my knowledge we have never payed attention to the faraday reflection while aligning the IR beam in the past, so it is likey that it was the case also before.