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Entry  Wed Feb 13 19:09:32 2013, yuta, Summary, General, possible explanations to oval REFL beam REFL_1044844506.bmp
    Reply  Wed Feb 13 19:30:45 2013, Koji, Summary, General, possible explanations to oval REFL beam 
       Reply  Wed Feb 13 19:41:07 2013, yuta, Summary, General, possible explanations to oval REFL beam 
    Reply  Wed Feb 13 22:09:26 2013, Jenne, Update, Alignment, REFL is not clipped 
       Reply  Thu Feb 14 00:10:12 2013, yuta, Summary, Alignment, REFL is not clipped 
          Reply  Thu Feb 14 10:42:41 2013, Jamie, Summary, Alignment, MMT, curved TTs does not explain beam ellipticity at Faraday T.pdfS.pdf
Message ID: 8078     Entry time: Wed Feb 13 19:09:32 2013     Reply to this: 8079   8081
Author: yuta 
Type: Summary 
Category: General 
Subject: possible explanations to oval REFL beam 

[Jenne, Manasa, Jamie, Yuta]

The shape of the REFL beam reflected from PRM is oval after the Faraday.
We tried to fix it by MC spot position centering and by tweaking input TT1/TT2/PRM. But REFL still looks bad (below).

REFL_1044844506.bmp

What has changed since:
  REFL looks OK in mid-Dec 2012. Possibly related things changed are;

  1. New active input TTs with new mirrors installed
  2. Leveling of IMC stack changed a little (although leveling was done after installing TTs)

Possible explanations to oval REFL:
  A. Angled input beam:
    Input beam is angled compared with the Faraday apertures. So, beam coming back from PRM is angled, and clipped by the Faraday aperture at the rejection port.

  B. Mode mis-match to PRM:
    New input TTs have different curvatures compared with before. Input mode matching to PRM is not good and beam reflected from PRM is expanding. So, there's clipping at the Faraday.

  C. Not clipping, but astigmatism:
    New input TTs are not flat. Incident angle to TT2 is ~ 45 deg. So, it is natural to have different tangential/sagittal waist sizes at REFL.

How to check:
  A. Angled input beam:
    Look beam position at the Faraday apertures. If it doesn't look centered, the incident beam may be angled.
   (But MC centering didn't help much......)

  B. Mode mis-match to PRM:
    Calculate how much the beam size will be at the Faraday when the beam is reflected back from PRM. Put some real numbers to curvatures of input TTs for calculation.

  C. Not clipping, but astigmatism:
    Same calculation as B. Let's see if REFL is with in our expectation or not by calculating the ratio of tangential/sagittal waist sizes at REFL.

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