Biomedical Engineering Reference
In-Depth Information
more.coupling.there.will.be.between.actuators..A.stifer.mirror.will.remain.latter.in.the.presence.
of. thin. ilm. stress. and. stress. gradients. from. thin. ilm. depositions. such. as. mirror. coatings. and.
adhesion.layers.for.mirror.coatings.
•.
he surface coating (relectivity at diferent wavelengths (visible [VIS]/infrared [IR]), maximum
optical power)
..he.surface.coating.of.the.mirror.must.be.chosen.to.have.high.relectivity.at.the.
wavelength.that.it.will.be.used.at..Some.of.the.common.mirror.coatings.include.the.following:
•.
Gold.for.operation.in.the.IR.region
•.
Protected.aluminum.for.the.VIS.wavelength.region
•.
Protected.silver.for.the.VIS.wavelength.region
•.
Dielectrics.(Bragg.mirror.for.high-power.laser)
•.
Protective window (transmittance at diferent wavelengths [VIS/IR]).
.If.a.protective.window.is.to.
be.included.in.the.packaging.of.the.DM,.it.must.allow.light.from.the.operating.spectrum.to.pass.
through. to. the. mirror.. It. must. include. an. antirelection. coating. to. eliminate. relections. at. the.
window.interface..Ideally.a.DM.would.have.no.window,.but.many.DMs.need.to.be.protected.from.
the.ambient.environment.and.dust.
•.
Mirror environment (exposed to atmosphere, hermetic seal, vacuum,
or
inert gas).
.he.mirror.will.
typically.be.operated.in.an.environment.that.can.damage.the.mirror,.such.as.water.that.can.con-
dense.on.the.mirror.below.the.dew.point.in.a.humid.environment.and.dirt.that.can.land.on.the.
mirror.causing.an.optical.defect..Moisture.can.also.lead.to.electrochemical.corrosion,.which.is.
accelerated. in. the. presence. of. high-operating. drive. voltages.. To. avoid. corrosion. and. dust,. the.
mirror.can.be.hermetically.sealed.in.vacuum.or.in.an.inert.gas.environment.behind.an.optically.
clear.window.
•.
Mirror type.
. here. are. a. number. of. diferent. types. of. DMs. that. are. best. suited. for. diferent.
applications.
•.
Continuous facesheet mirrors.
.hese.mirrors.are.the.typical.choice.for.astronomical.appli-
cations.to.avoid.light.difraction.that.can.occur.at.the.sharp.edges.of.a.segmented.mirror..
hey. are. also. the. typical. mirror. of. choice. for. microscopy.. he. downside. of. continuous.
facesheet. mirrors. is. the. inter-actuator. coupling. due. to. the. mechanical. linkage. through.
the.facesheet.
•.
Segmented mirrors.
.hese.mirrors.have.cuts.in.the.facesheet.between.actuators..his.elimi-
nates.coupling.between.the.mirror.segments,.but.it.also.has.an.impact.on.the.itting.error,.
since.the.wavefront.will.have.to.be.approximated.by.straight.line.segments.rather.than.a.
continuous. smooth. curve.. he. segment. size. and. shape. will. have. an. impact. on. the. opti-
cal.performance.of.the.mirror,.and.the.ill.factor.between.mirror.segments.will.have.an.
impact. on. optical. losses.. Typical. ill. factors. are. now. approaching. 99%,. so. optical. losses.
are. minimal.. he. mirror. segments. can. be. operated. in. piston-only. (i.e.,. straight. up. and.
down.in.the.surface.normal.direction).with.a.single.actuator/segment.or.in.a.piston.tip-tilt.
mode.with.three.actuators/segment..Segmented.MEMS.DMs.have.been.commonly.used.in.
vision..science.applications.since.they.have.been.able.to.attain.a.relatively.larger.stroke.than.
continuous.facesheet.mirrors..he.larger.the.stroke.in.vision.science,.the.wider.the.popu-
lation.that.can.be.corrected..Mirror.strokes.of.10-15.μm.are.desired.to.correct.the.larger.
.aberrations.at.the.tail.ends.of.the.population..One.potential.problem.with.segmented.mir-
rors. is. the. large. phase. jump. that. can. occur. between. segments. if. they. are. not. properly.
controlled.to.assure.continuity.between.segments.
•.
Mirror edge support:
.here.are.diferent.possible.boundary.conditions.for.the.support.of.
the.mirror.surface.
−
he.edge.of.the.membrane.can.be.supported.around.the.periphery.of.the.mirror,.either.
rigidly.attached.to.a.frame.or.connected.to.a.support.frame.through.springs..his.spring.
support.geometry.is.similar.to.a.trampoline.
