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Message ID: 301     Entry time: Thu Feb 9 11:35:27 2017
Author: Gabriele 
Type: Optics 
Category: Configuration 
Subject: QPD calibration  

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

g = \frac{4 L}{w} \sqrt{\frac{2}{\pi}}

  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

Attachment 1: qpd1.bmp  3.003 MB
Attachment 9: IN3.png  304 kB  | Hide | Hide all
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