Biomedical Engineering Reference
In-Depth Information
available. (Dalimier. 2005).. he. consideration. of. the. devices. for. a. practical. nonlinear. system. needs. to.
cover.the.mirror.size,.actuator.travel,.actuator.number,.push.and.pull.operation,.speed.of.response,.and.
update.rate.of.the.mirror.and.drive.electronics.along.with.the.relectivity.
he. mirrors. suitable. for. use. in. microscopy. are. now. available. from. around. 3. mm. diameter. (or. the.
length.of.the.diagonal.for.noncircular.devices).up.to.50.mm..Larger.devices.are.available.but.they.are.
generally.not.used.for.beam-scanned.microscopy.applications.typically.on.the.grounds.of.cost..Clearly,.
the.larger.the.mirror,.the.greater.the.beam.expansion.required.to.cover.the.mirror.so.that.all.the.actu-
ators. are. used.. It. should. also. be. remembered. that. the. laser-beam. size. given. is. normally. for. the. 1/
e
2
.
value—or.full.width.half.maximum—thus.the.beam.expansion.should.take.this.into.consideration..In.a.
prescanned.coniguration,.the.smaller.mirror.size.may.be.of.interest,.as.it.may.be.possible.to.direct.the.
beam.straight.onto.the.mirror,.reducing.the.number.of.optics.needed..he.smaller-size.devices.generally.
have.fewer.actuators.and.thus.may.be.suitable.only.for.correcting.low-order.aberrations.
he.travel.of.each.actuator.sets.the.level.of.aberration.that.can.be.corrected,.but.this.is.linked.to.the.
mirror.size..In.the.simplest.case.with.a.central.actuator.driven.to.its.maximum.movement,.one.will.cre-
ate.a.dimple.in.the.mirror.surface.(for.a.pull.mirror),.which.will.appear.as.a.concave.surface.thus.provid-
ing.a.positive.focus.on.the.beam..he.level.of.curvature.will.be.determined.by.the.distance.of.travel.and.
the.diameter.of.the.mirror,.where.the.smaller.the.mirror.diameter,.the.steeper.the.curve.(smaller.radius.
of. curvature). hence. the. greater. level. of. focus,. which. is. considered. a. low-order. aberration.. By. simple.
geometry,.it.is.thus.possible.to.determine.the.maximum.level.of.aberration.that.can.be.corrected.for.any.
given.actuator.travel.and.mirror.diameter.
One.then.needs.to.consider.the.number.of.actuators.that.set.the.order.of.aberration.that.can.be.cor-
rected..hese.currently.range.from.32.up.to.1024,.although.for.most.applications.at.present.there.has.
been.no.demonstrated.advantage.in.using.more.than.144.actuators.and.most.work.has.been.undertaken.
with.between.32.and.52.actuators..As.the.number.of.actuators.rises.the.complexity.of.mirror.shapes.and.
thus.aberrations.that.can.be.corrected.clearly.increases.but.so.does.the.control.problem..If.an.optimized.
search. routine. is. being. used. to. determine. the. best. mirror. shape,. there. is. clearly. a. time. advantage. in.
reducing.the.possible.number.of.mirror.patterns.that.can.be.applied.
Recently,.a.further.parameter.has.been.introduced.into.the.decision-making.process:.that.of.having.
a.mirror.that.can.be.made.convex.and.concave.via.“push/pull”.actuators..In.the.early.work,.the.mirrors.
were.initially.biased.with.all.actuators.to.a.midpoint.in.their.travel.(which.may.not.be.at.midvoltage.
setting. because. of. the. nonlinear. nature. of. electrostatic. attraction).. From. this. position,. the. actuators.
could. be. moved. either. forward. or. backward. to. produce. the. required. mirror. shape—for. example,. to.
compensate. for. spherical. aberration.. he. mirrors. are. now. available. that. can. be. moved. backward. or.
forward. from. their. unbiased. position. (position. with. zero. drive. volts. present). and. in. recent,. as. yet.
unpublished.work,.in.our.laboratory.these.devices.appear.to.work.very.well,.although.the.optimization.
algorithms.require.some.modiication.to.account.for.a.wider.range.of.parameters.as.will.be.discussed.
later..he fact.that.a.mirror.does.not.need.to.be.placed.into.an.initial.biased.state.also.means.that.the.
efective.travel,.and.hence.level.of.correction.possible,.is.increased.as.one.should.always.consider.the.
total.travel.available.
Most.mirrors.available.have.very.similar.update.rates.and.impulse.response.times.and.are.typically.
capable.of.operating.at.around.1-3.kHz..When.selecting.a.mirror,.the.interface.between.the.drive.elec-
tronics.and.the.computer.should.also.be.included.in.any.speed.calculations,.and.this.includes.the.time.
that.the.computer.may.take.to.address.the.drive.electronics..In.this.chapter,.I.am.considering.algorithm,.
or.sensorless,.AO,.where.the.speed.of.mirror.control.does.not.set.the.overall.speed.performance.of.the.
system.. For. most. biological. imaging. applications,. the. aberrations. are. not. dynamic—in. contrast,. for.
example,.to.astronomical.AO—and.thus.speed.of.response.is.not.the.irst.consideration.in.mirror.selec-
tion..All.manufacturers.now.supply.their.own.drive.electronics.at.a.reasonable.price.and.thus.the.use.of.
homebuilt.electronics.is.no.longer.a.real.consideration..he.main.questions.to.consider.for.the.electron-
ics,.beyond.speed,.is.the.simple.practical.aspect.of.how.easy.it.is.to.use.the.computer.interface.to.the.
drive.electronics.and.how.the.actuator.pattern.is.sent.to.the.mirror.
