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Entry  Mon May 10 20:03:22 2010, Kevin, Update, Green Locking, Green Laser Beam Profile vbp.jpgvbp_residuals.jpghbp.jpghbp_residuals.jpg
    Reply  Mon May 10 22:25:03 2010, Koji, Update, Green Locking, Green Laser Beam Profile 
       Reply  Tue May 11 14:39:17 2010, Aidan, Update, Green Locking, Green Laser Beam Profile P5110057_beams.jpg
       Reply  Tue May 11 17:02:43 2010, Kevin, Update, Green Locking, Green Laser Beam Profile vbp.jpgvbp_residuals.jpghbp.jpghbp_residuals.jpg
          Reply  Wed May 12 03:42:38 2010, Koji, Update, Green Locking, Green Laser Beam Profile 
             Reply  Fri May 14 16:14:37 2010, Kevin, Update, Green Locking, Green Laser Beam Profile profile.pngresiduals.png
Message ID: 2907     Entry time: Mon May 10 20:03:22 2010     Reply to this: 2909
Author: Kevin 
Type: Update 
Category: Green Locking 
Subject: Green Laser Beam Profile 

Kiwamu and Kevin measured the beam profile of the green laser by the south arm ETM.

The following measurements were made with 1.984A injection current and 39.65°C laser crystal temperature.

 

Two vertical scans (one up and one down) were taken with a razor blocking light entering a photodiode with the razor 7.2cm from the center of the lens. This data was fit to

b + a*erf(sqrt(2)*(x-x0)/w) with the following results:

scan down: w = (0.908 ± 0.030)mm  chi^2 = 3.8

scan up:      w = (0.853 ± 0.025)mm   chi^2 = 2.9

giving a weighted value of w = (0.876 ± 0.019)mm at this distance.

 

The beam widths for the profile fits were measured with the beam scanner. The widths are measured as the full width at 13.5% of the maximum. Each measurement was averaged over 100 samples. The distance is measured from the back of the lens mount to the front face of the beam scanner.

distance (cm) vertical w (µm) horizontal w (µm)
3.2 ± 0.1 1231 ± 8 1186 ± 7
4.7 ± 0.1 1400 ± 4 1363 ± 6
7.4 ± 0.1 1656 ± 5 1625 ± 9
9.6 ± 0.1 1910 ± 10 1863 ± 9
12.5 ± 0.1 2197 ± 8 2176 ± 8
14.6 ± 0.1 2450 ± 12 2416 ± 10
17.5 ± 0.1 2717 ± 12 2694 ± 14
20.0 ± 0.1 2973 ± 16 2959 ± 8
22.4 ± 0.1 3234 ± 12 3193 ± 14

This data was fit to w = sqrt(w0^2+lambda^2*(x-x0)^2/(pi*w0)^2) with lambda = 532nm with the following results:

For the vertical beam profile:

reduced chi^2 = 3.29

x0 = (-87   ± 1)    mm

w0 = (16.30 ± 0.14) µm

For the horizontal beam profile:

reduced chi^2 = 2.01

x0 = (-82   ± 1)    mm

w0 = (16.12 ± 0.10) µm

Note: These fits were done with the beam diameter instead of the beam radius. The correct fits to the beam radius are here: http://nodus.ligo.caltech.edu:8080/40m/2912

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