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Entry  Thu Aug 28 17:36:03 2014, Evan, DailyProgress, optic, Mode-matching solution for north cavity ctn_algaas_alm.pdfctn_algaas.zip
    Reply  Thu Aug 28 19:10:40 2014, Evan, DailyProgress, optic, Mode-matching solution for north cavity 
Message ID: 1489     Entry time: Thu Aug 28 19:10:40 2014     In reply to: 1488
Author: Evan 
Type: DailyProgress 
Category: optic 
Subject: Mode-matching solution for north cavity 

Quote:

Current configuration:

  • Target waist: 180 µm, z = 0 mm
  • Lens 1: 140 mm focal length, z = −711 mm (24″ from center of vacuum chamber + 4″ through periscope)
  • Lens 2: 84 mm focal length, z = −991 mm (11″ further behind lens 1)
  • Seed waist = ??

Since we know we were mode-matched fairly well into the 180 µm waist of the silica/tantala cavity (>93% visibility), I asked alm to propagate this waist backward through the lenses in order to find a seed waist. It reports a waist of 161 µm at z = −1373 mm.

I asked alm for a new configuration using the same two lenses. The best configuration (mode overlap = 1) is as follows:

  • Seed waist: 161 µm at z = −1373 mm
  • Lens 1: 140 mm focal length, z = −743 mm
  • Lens 2: 84 mm focal length, z = −1023 mm
  • Target waist: 215 µm, z = 0 mm

So we should move lens 1 back by 32 mm (=1.3″), and move lens 2 back by the same amount.

I moved both lens mounts back by 1″, then adjusted the Vernier knobs and periscope mirrors to try to maximize the visibility as seen on north REFL DC.

The best I am able to do so far is a visibility of v = 1 − 0.57(1) V / 1.74(1) V = 0.672(6).

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