Wavefront Correctors - Adaptive Optics for Biological Imaging - page 125

MEMS. fabricated. using. surface. micromachining. processing.. To. protect. mirrors. from. environ-
mental.efects,.they.are.typically.packaged.in.a.hermetically.sealed.container,.with.optical.access.
to.the.mirror.surface.through.an.appropriate.window.that.is.optically.clear.and.treated.with.anti-
relection.coatings.for.the.operating.wavelengths.of.interest.

•. Lifetime (cycles). .he.lifetime.of.the.mirror.can.be.limited.by.environmental.efects.if.the.mirror.

is.exposed.to.a.harsh.environment.such.as.high.temperature.or.humidity..It.can.also.be.limited.
by. failure. mechanisms. such. as. stiction,. where. an. actuator. has. been. displaced. suiciently. far.
that. it. touches. down. to. the. substrate. and. becomes. stuck. to. it. because. of. surface. forces. such.
as. van. der. Waals. attraction.. DMs. fabricated. using. MEMS. processes. are. particularly. vulner-
able.to.this.failure.mechanism.since.surface.forces.can.dominate.spring.restoring.forces.in.the.
microdomain.

8.5 conventional Deformable Mirrors Using
Piezoelectric and electrostrictive Actuators

here.are.a.number.of.diferent.options.for.DMs.that.can.be.used.for.wavefront.correction..hey.can.
be.broadly.categorized.as.segmented.and.continuous.facesheet.mirrors.and.in.terms.of.the.amplitude.
(stroke). and. spatial. frequency. (order). of. wavefront. correction. they. are. able. to. make.. he. facesheet. is.
the.front.surface.mirror.that.relects.the.wavefront..It.can.have.various.metal.surface.coatings.(e.g.,.alu-
minum,.gold,.protected.silver).or.dielectric.coatings.to.optimize.relection.in.the.wavelength.range.of.
interest..Aluminum.coatings.have.high.relectivity.in.the.range.of.400-700.nm.( R .>.85%)..Gold.coatings.
have.high.relectivity.from.700-800.nm.( R .>.94%).to.800-10,000.nm.( R .>.97%),.and.protected.silver.
has.high.relectivity.in.the.range.from.500-800.nm.( R .>.98%).to.2,000-10,000.nm.( R .>.98%)..Mirrors.
for.high-power.laser.applications.typically.use.a.Bragg.stack.mirror.coating.that.consists.of.alternating.
layers.of.high.and.low.index.of.refraction.dielectric.layers.that.are.each.a.quarter.wavelength.thick.
Although.the.method.of.wavefront.correction.using.an.Eidophor,.as.described.earlier,.works.in.prin-
ciple,.the.astronomical.community.initially.used.adaptive.mirrors.rather.than.spatial.light.modulators.
for.making.wavefront.corrections..he.irst.mirrors.used.piezoelectric.actuators.to.deform.a.thin.glass.
mirror.facesheet..he.actuators.were.large,.resulting.in.a.large.pitch.(5-7.mm).between.the.actuators.
when.they.were.assembled.into.an.array,.and.the.resulting.mirrors.were.expensive.since.they.were.hand.
assembled.into.an.array..An.example.of.the.DM.that.was.used.in.the.AO.system.for.the.Keck.10.m.tele-
scope.is.shown.in.Figure.8.15..he.mirror,.manufactured.by.Xinetics.Inc.,.has.349.individual.actuators.

146mm clear aperture 349 actuators on 7mm spacing

FIGuRE 8.15 Deformable. mirror. used. in. the. Keck. Observatory. AO. system.. (Credit:. Peter. Wizinowich,. W.M..
Keck.Observatory.)