Biomedical Engineering Reference
In-Depth Information
components.are.needed.and.no.complications.associated.with.polarization.are.introduced..At.smaller.
angles,.clearly,.these.efects.are.reduced.and.thus.consideration.should.be.given.to.this.alternative.con-
iguration.and.in.a.system.using,.for.example,.a.difractive.liquid-crystal.spatial.light.modulator.(SLM).
where.the.required.modiied.beam.naturally.emerges.at.an.angle.via.a.blaze.angle.being.applied.to.the.
device,.there.is.a.positive.advantage.
11.2.3 Adaptive optical element
We.now.need.to.consider.the.optical.device.that.is.going.to.actually.provide.the.aberration.correction:.
the.AO..As.with.any.design.parameter,.the.inal.choice.of.element.rests.on.a.large.number.of.factors..
he.vast.majority.of.AO.systems.have.used.relective.mirror.devices.and.they.will.form.the.focus.of.this.
section.but.brief.consideration.is.given.to.SLMs.
SLMs.work.by.altering.the.relative.phase.of.diferent.parts.of.the.incoming.wave.such.that.the.cor-
rect. wavefront. is. inally. achieved.. hey. have. been. used. in. microscopes. for. altering. the. wavefront. to.
modify.the.point.spread.function.at.the.focus.of.the.objective.(Neil.et.al..2000a)..Generally,.these.AO.
elements. work. as. difractive. optical. elements,. which. normally. means. that. they. are. less. eicient. than.
relective. mirror-based. devices.. Conventionally,. nematic. SLMs. are. frequently. slow. with. update. rates.
rarely.exceeding.100.Hz.at.present.and.are.harder.to.control..Other.liquid-crystal.devices.are.available.
with.higher.update.rates.but.these.have.other.drawbacks.including.the.need.for.additional.polarizing.
optics..In.the.case.of.a.deformable.membrane.mirror,.voltages.are.simply.applied.to.the.individual.actu-
ators.on.the.device.and.typically.there.are.at.most.around.120.such.actuators.on.a.mirror..In.the.case.
of.an.SLM,.they.are.driven.as.extension.monitors.with.VGA.or.XVGA.formats.and.thus.a.large.number.
of.pixel.values.need.to.be.determined.and.then.addressed—although.the.format.makes.the.addressing.
easy,.the.more.diicult.aspect.is.to.determine.what.values.should.be.placed.on.the.individual.elements..
As.will.be.discussed.later,.if.speciic.Zernike.(or.other.orthogonal.wavefront).terms.are.applied.then.the.
value.determination.is.easier,.for.a.more.randomized.search.method.the.combination.of.possible.terms.
is.very.large..Finally,.when.operating.in.a.difractive.mode,.the.SLMs.are.clearly.wavelength-dependent.
and. thus. with. the. broad. spectrum. of. a. femtosecond. laser. pulse,. signiicant. complications. can. occur.
and.in.a.descanned.coniguration,.the.returned.light.will.be.altered.in.a.totally.diferent.manner.to.the.
excitation.light.
With.these.obvious.disadvantages,.the.clear.question.is,.would.anyone.consider.using.such.a.device?.
he.answer.is.that.SLMs.have.the.potential.of.almost.limitless.correction.of.the.wavefront..As.will.be.
discussed.later,.in.the.case.of.an.AO.mirror,.there.is.a.limitation.to.the.movement.of.the.mirror.that.is.
possible.and.hence.on.the.level.of.aberration.that.can.be.corrected,.although.the.number.of.actuators.
sets.a.limit.on.the.order.of.aberration.terms.that.can.be.corrected..In.the.case.of.the.SLM,.an.almost.ini-
nite.correction.is.possible.by.phase-wrapping,.and.high-order.terms.can.also.be.easily.corrected..With.
such.devices.either.on.their.own.(Neil.et.al..2000b).or.in.combination.with.mirror.AO.elements.(Wright.
et.al..2006).aberration.correction.is.possible..hus,.such.devices.do.have.a.potential.role.in.nonlinear.
imaging.systems,.but.will.not.be.considered.further.here,.as.their.use.to.date.has.been.limited.
he.main.focus.in.this.chapter.is.on.the.use.of.mirror.devices.and.in.the.past.10.years.a.number.of.
manufacturers.of.membrane.mirror.devices.have.become.established..he.exact.method.of.actuation.
varies.with.some.being.electrostatically.driven.and.some.magnetically.operated..Most.devices.appear.
as.continuous.mirror.surfaces,.whereas.a.few.have.a.pixilated.appearance.because.of.their.method.of.
.manufacture,.although.the.actual.mirror.surfaces.are.continual..Some.level.of.print-through.has.been.
seen.with.such.devices,.but.it.has.not.been.reported.that.it.actually.afects.the.inal.performance,.in.par-
ticular.in.a.nonlinear.imaging.system..he.mirror.price.and.the.associated.drive.electronics.also.vary.
widely,.and.although.clearly.a.inal.purchasing.factor,.the.diferent.drive.methods.and.conigurations.
will.not.be.discussed.further—neither.will.the.detailed.performance.of.the.diferent.devices,.as.these.
are.changing.continually.and.the.reader.is.advised.to.search.for.the.most-recent.publications..he.actual.
quantiication. of. these. devices. will. also. not. be. considered,. although. again. publications. are. readily.
