Biomedical Engineering Reference
In-Depth Information
Before
reflection
After
reflection
d
d/
2
Incoming
wavefront with
aberration
Deformable
mirror
Corrected
wavefront
Deformable
mirror
FIGuRE 8.1
Wavefront.correction.using.a.deformable.mirror..Before.relection,.the.leading.edge.of.the.wavefront.
is.at.a.distance.
d
.ahead.of.the.trailing.edge..If.the.deformable.mirror.has.an.indentation.that.is.
d
/2.deep,.the.lead-
ing.edge.will.have.to.travel.a.distance.
d
.further.than.the.trailing.edge.ater.relection,.allowing.the.trailing.edge.to.
catch.up.to.the.leading.edge..(Courtesy.of.Lawrence.Livermore.National.Laboratory.and.NSF.Center.for.Adaptive.
Optics.)
Before
transmission
After
transmission
L
d
n
2
n
1
n
2
Incoming
wavefront with
aberration
Liquid
crystal
Corrected
wavefront
FIGuRE 8.2
Wavefront.correction.using.a.liquid.crystal.spatial.light.modulator..he.index.of.refraction.in.the.
center.of.the.spatial.light.modulator.is.
n
1
.and.
n
2
.at.the.edges..If.
n
1
.>.
n
2
,.the.leading.edge.of.the.wavefront.will.be.
slowed.down.more.than.the.trailing.edge,.enabling.the.trailing.edge.to.catch.up.ater.passing.through.the.liquid.
crystal.spatial.light.modulator.
is.lagging.is.
n
2
,.so.that.the.wavefront.velocity.is.
c
/
n
2
..If.the.spatial.light.modulator.has.a.length.
L
,.then.
the.trailing.edge.will.be.able.to.catch.up.with.the.leading.edge.when.the.index.diference.Δ
n
.has.been.
adjusted.to.be.Δ
n
.=.
n
2
.−.
n
1
.=.
d
/
L
.
8.2 Liquid crystal Spatial Light Modulators
Liquid.crystal.spatial.light.modulators.(LC-SLM).operate.by.changing.the.orientation.of.liquid.crys-
tals.using.electrostatic.forces..By.changing.the.orientation.of.the.liquid.crystal,.the.index.of.refraction.
can.be.varied.spatially.across.the.modulator..Either.the.phase.or.amplitude.of.the.wavefront,.or.both,.
can.be.varied.in.transmission.through,.or.relection.from,.the.SLM..he.liquid-crystal.pattern.can.be.
controlled. directly. from. a. computer-graphics. card.. Since. the. index. is. wavelength. and. polarization.
dependent,.the.use.of.an.SLM.for.wavefront.correction.is.limited.to.monochromatic.polarized.light..
