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Entry  Fri Mar 23 05:10:46 2012, Suresh, Update, IOO, Beam Profile measurements: Errors too large to yield good fits. rawdataplot.pngPRM-to-REFL-profile.pdf
    Reply  Mon Mar 26 15:15:16 2012, kiwamu, Update, IOO, expected beam profile of PRM reflection expected_edit.png
       Reply  Mon Mar 26 16:25:44 2012, kiwamu, Update, IOO, expected v.s. measured beam profile of PRM reflection PRMreflection.pngDSC_4001_small.jpg
          Reply  Mon Mar 26 18:04:43 2012, kiwamu, Update, IOO, mode scan at the REFL port mode_profile.pngREFLmodescan.zip
Message ID: 6441     Entry time: Fri Mar 23 05:10:46 2012     Reply to this: 6444
Author: Suresh 
Type: Update 
Category: IOO 
Subject: Beam Profile measurements: Errors too large to yield good fits. 

[Kiwamu, Suresh]

   Today we attempted to measure the beam profile of the REFL beam under two conditions:

              (a) with PRM aligned and ITMs misaligned 

              (b) with PRM misaligned and ITMs aligned


The raw data is shown below.  In each of the above conditions we measured in both the vertical (v) and horizontal (h) directions.   The measurements in the vertical direction were better than the ones in the horizontal direction because the optics had a horizontal oscillation which gave larger errors in measurement.



Looking at the general trend of these lines it is clear that modes are not matched since the beam reflected by the PRM has a different divergence than that reflected from ITMs.  The beam is also astigmatic as the vertical and horizontal directions have different divergences.

I could find beam parameters only for the Blue line above (Profile in the vertical direction while PRM was aligned).   The fit is quite sensitive to the data points close to the waist, so we need to make better (lower St.Dev.) measurements near the AP table closer to the beam waist.  The intensity with only one ITM aligned is too low and also contributes to the errors.   The beam size is close to 6mm in the horizontal direction, this coupled with yaw oscillations give large errors in this measurement.

Here is the only reliable fit that could be obtained, which is for the prompt reflection from the PRM in the vertical direction


The fit function I used is  Beam Dia = Waist { Sqrt [ 1+  ((z + z0)/zr)^2). The fit parameters we get for this data are

z0 = 7.7 m 

Waist = 2.4 mm

zr = 6.9 m


Will make another attempt later today...





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