Biomedical Engineering Reference
In-Depth Information
8
Wavefront Correctors
8.1
Introduction ......................................................................................109
8.2
Liquid Crystal Spatial Light Modulators ...................................... 110
8.3
History of Deformable Mirrors in Adaptive Optics.................... 113
8.4
Speciications for Deformable Mirrors.......................................... 116
Mirror Requirements
8.5
Conventional Deformable Mirrors Using Piezoelectric and
Electrostrictive Actuators................................................................125
8.6
Microelectromechanical System Deformable Mirrors ...............127
Actuated Segmented Facesheet Microelectromechanical System
Mirrors
8.7
High-Stroke, High-Order “Woofer-Tweeter” Two-
Mirror System ...................................................................................140
8.8
High-Stroke, High-Order Microelectromechanical
System Mirrors..................................................................................140
Joel A. Kubby
University of California
at Santa Cruz
8.9
Comparison of Microelectromechanical System Mirrors .........145
8.10
Microelectromechanical System Mirror Solutions .....................146
8.1 Introduction
As described in Chapter 4, an initially planar wavefront will become aberrated ater passing through a
media with an inhomogeneous index of refraction. By correcting the wavefront using adaptive optics
(AO), the spatial resolution and contrast of an image can be increased. Wavefront correction is accom-
plished by delaying the leading parts of the wavefront so that the trailing parts have a chance to catch
up. As shown in
Figure 8.1
, if the wavefront is relected from a deformable mirror (DM), the optical
path length can be varied across the mirror surface by deforming it into a shape that is conjugate to the
wavefront aberration, so that the wavefront is corrected ater relection.
Another way to correct the wavefront is to use a liquid crystal spatial light modulator to slow down
the leading part of the wavefront by changing its velocity relative to the lagging part of the wavefront.
he lead edge is ahead by a distance
d
, so that it precedes the trailing edge in time by
d
/
c
, where
c
is the
speed of light. he velocity of the wavefront can be changed by changing the index of refraction of the
liquid crystal media through which the wavefront passes (transmission) or in relection. As shown in
Figure 8.2
, the index of refraction for the liquid crystal in the central region where the wavefront is lead-
ing is
n
1
so that the wavefront velocity is
c
/
n
1
, and the index in the outer regions where the wavefront
109
