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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: 6445     Entry time: Mon Mar 26 16:25:44 2012     In reply to: 6444     Reply to this: 6446
Author: kiwamu 
Type: Update 
Category: IOO 
Subject: expected v.s. measured beam profile of PRM reflection 

[Suresh / Kiwamu]

 We did the 2nd round of the PRM reflection mode scan on Friday.

It seems that the PRM curvature maybe correct if we look at the vertical mode, however but the horizontal mode doesn't seem to agree with any of the expected lines.

In order to increase the reliability of the measurement, we need to confirm the beam profile of the incident beam by looking at the IP-POS beam.

Right now Suresh and Keiko are mode-scanning the IP-POS beam.

 

 


The plot below shows both the expected beam profiles (see the detail in #6444) and the actual data. 

PRMreflection.png

This plot is the same as one shown in the previous entry (#6444) with newly added actual data.
The errorbar in each data point is the standard deviation obtained by 100 times of averaging.
In this plot I made the error bars 10 times bigger in order to let them visible in the plot, so the actual deviation is much lesser than they appear.
 

(Discussion)

 The vertical profile (shown in red) seems to be close to the curve for the correct PRM case.
However the horizontal profile has a bigger waist size of about  2 mm.
While measuring the waist size Suresh and I have noticed that the rotational angle of the scan head affects the measurement by 10% or so.
Of course in each data point we tried making the incident beam normal to the scan head by rotating the scan head.
But this 10% is not big enough to explain the discrepancy in the horizontal mode.
There are some possible scenario which can distort the beam shape in the horizontal direction:
  • Clipping at some optics. (Since the beam shape looked very Gaussian, the amount of the clipping could be very slight ?)
  • Astigmatism at some optics. (Possibly in the telescope ?)

(Some distances)

DSC_4001_small.jpg
 

(Some notes)

We did the following things prior to the measurement.

  • Put a boost filter in the PRM_OLYAW to suppress the beam jitter below 1 Hz.
  • Checked the MC WFS servo loop although it looked healthy.

Quote from #6444

I have estimated how the mode profile of the PRM reflection should be, as shown in the plot blow.

A conclusion here is :

   we should be able to constrain the PRM curvature situation if measurements are precise and accurate enough with a level of less than ~ 100 um 

 

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