Minute of KAGRA AOS Meeting on Optical Levers
[Wiki page]
http://gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/LCGT/subgroup/AOS/Meetings#m9th
This page is written by Japanese,
but you can find Michimura-kun's slide and Sekiguchi-kun's
simulation/calculation)

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Date: Apr 27, 2012 10:00-12:00 (JST)
Place: Tsubono Lab + EVO
Participants: T. Akutsu, Y. Aso, R. DeSalvo, U. Iwasaki, Y. Saito,
E. Majorana, L. Naticchioni, Y.Michimura, T. Sekiguchi, R. Takahashi,
and M. Ando (EVO)
[Most of the minute by Michimura]

(1) THE FUNCTION OF OPLEVS
 - sensing angular motion of mirrors for damping the resonant peaks
(local control)
 - angular drift monitor for mirrors and optical benches
 - coil balancing


(2) WHERE DO OPLEVS GO?
 - TMs
  for iKAGRA: viewports above and below, lever length ~ 1.5 m, incident
angle ~ 45 deg
  for bKAGRA: viewports at AO chamber, use pick off mirror (or using a
crank position where vacuum tube radius would change), lever length ~ 30
m, incident angle ~ 1 deg

 - BS
  one viewport below, lever length ~ 1m, incident angle few deg

 - RC mirrors
  viewports above and below, lever length ~ 1.5 m, incident angle ~ 45 deg

 - MC mirrors, MMT mirrors, etc.
  same as RC mirrors? (TBD)

 - optical benches
  viewport below? (TBD)


(3) DESIGN
 - folding or 2-QPDs?
  OpLevs are for the angular sensing, not the longitudinal sensing
  so, let's set the folding method as a default


(4) REQUIREMENTS
 - power ~ 5 mW, RIN ~ 1e-7 /rtHz (preliminary)
  are there any S-LED which satisfy this requirement?

 - wavelength TBD
  decisive factor is the life time

 - pointing drift
  smaller than 0.4 urad/h
  we have to care about the drift due to the temperature drift of the pylon
  for bKAGRA TMs, we also have to care about the relative drift between
the base inside the chamber and the ground

 - sensing range
  we need two kind; low noise but small range + noisy but larger range
  for TMs we need ~ ± 100 mm at most (considering mirror motion ± 1
mrad)
  for other mirrors, ~ ± 10mm at most


(5) MISCS
 - we have to measure the reflectivity of the HR coating for OpLev light
 - the size of the viewports and also the holes on the optical bench
must be larger than the beam size of the OpLev beam and the beam motion
(10cm should be enough?)
 - QPDs should be rotatably mounted for pitch/yaw diagonalization
 - local control should be done at the higher stages of the suspention,
using steep servo filters specially designed for each suspention

(6) ACTION ITEMS
 - selection of the light source and sensors (TBD.)
 - design of the pylon for launcher/sensor (TBD.)
 - calculation of the local damping of the resonant peaks using OpLevs
and other local sensors (Sekiguchi, Michimura)