This morning I measured the beam profiles at the QPD positions. As expected, the beams are not gaussian there, since we are seeing the interference of the reflections from the two faces of the disks. Nevertheless, the measured sizes are
| |
Beam radius [microns] |
| QPD1 |
302 |
| QPD2 |
338 |
| QPD3 |
270 |
| QPD4 |
401 |
Here are the images of the beams:
 
 
The arm lever lengths (distance from the disk surface to the QPD) can be estimated from the optical drawing:
| |
Distance disk to QPD [mm] |
| QPD1 |
1540 |
| QPD2 |
1813 |
| QPD3 |
1587 |
| QPD4 |
1652 |
The optical gain (normalized QPD signal over disk surface angle) is given by
| |
Gain 1/rad |
| QPD1 |
16300 |
| QPD2 |
17100 |
| QPD3 |
18800 |
| QPD4 |
13150 |
The uncertaint in those numbers is quite large, both from the beam size and from the arm lever. Anyhow, I'm using the average to have a rough calibration of the QPD signals in terms of disk surface angular motion. The value I plugged in into the X_NORM filter banks is 16300 /rad. Therefore now the signals *NORM_OUT_DQ are calibrated in radians (deflection of the disk surface).
Finally, I also measured the profiles of the beams going into the chamber, somewhere before the 2" folding mirror. They have nice gaussian shapes
 
 
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