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Message ID: 149     Entry time: Wed May 18 18:52:12 2011
Author: Aidan 
Type: Computing 
Category: Hartmann sensor 
Subject: Test of position modulation algorithm 

 I measured the prism and displacement of the Gaussian beam on the Hartmann sensor. The beam pointing was modulated at 10mHz using a galvo mirror as illustrated in Attachment 1. The galvo was around 680mm from the Hartmann sensor. The amplitude of the prism modulation was approximately 1E-5 radians. The displacement of the beam was measured using a new algorithm that tries to fit a parabola to the logarithm of the intensity of each Hartmann spot. The amplitude of the displacement modulation was measured at around 42 microns: corresponding to around 6E-5 radians (=42um/680mm).

To resolve the discrepancy between the prism and displacement measurements, I removed the Hartmann plate to simply get a Gaussian beam on the CCD (bottom right image in Figures 2 & 3 - the beam is slightly clipped and there is a ghost beam in the center - I'm not yet certain where this is coming from). I measured the Gaussian beam displacement directly by fitting a Gaussian to the mean horizontal cross-section of the intensity distribution (top right plot in Figures 2 & 3). Using this technique the measured displacement on the CCD had an amplitude of around 0.7 +/- 0.05 pixels = 8.4 +/- 0.6 microns, corresponding to a prism of 12.5E-6 radians (seen in top left plot in Figures 2 and 3). This indicates that there is an error in the Gaussian fitting algorithm using the Hartmann sensor data.

 The second plot simply shows the position modulation of the beam as I increased the amplitude of the signal going to the galvo. 

 

Voltage (Vpp) Displacement on CCD (pixels)
0.01 0.325
0.012 0.400
0.014 0.487
0.015 0.541
0.016 0.576
0.019 0.708
0.02 0.763
0.023 0.887
0.028 1.194
0.034 1.803
0.041 3.053
0.049 7.107

 

Attachment 1: galvo_mirror_experiment.jpg  345 kB  | Hide | Hide all
galvo_mirror_experiment.jpg
Attachment 2: 2011-05-18_gaussian_beam_with_galvo_constant.pdf  176 kB  | Hide | Hide all
2011-05-18_gaussian_beam_with_galvo_constant.pdf
Attachment 3: 2011-05-18_gaussian_beam_with_galvo.pdf  163 kB  | Hide | Hide all
2011-05-18_gaussian_beam_with_galvo.pdf
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