Biomedical Engineering Reference
In-Depth Information
Detector
Feedback loop
Light source
Lens
Planar
wavefront
Microscope objective
Biological
samples
Deformable
mirror
Distorted
wavefront
Dichroic filter
FIGuRE 12.4 he.deformable.mirror.is.used.to.manipulate.the.incoming.light.beam.wavefront.to..compensate.the.
aberrations.in.the.microscope.objective.and.biological.sample..A.planar.wavefront,.originated.from.the.light.source,.
is.bouncing.of.the.DM,.and.forms.a.focal.plane.on.the.biological.sample,.where.the.luorescent.light.is..emitted..he.
luorescent.signal,.generated.at.the.sample,.is.relected.by.the.dichroic.ilter.and.collected.by.a.detector.through.a.
lens..An.AO.feedback.loop.builds.the.compensating.DM.shape.based.on.maximizing.the.detected.luorescent.signal.
he. SPGD. controller. has. one. output—the. luorescence. signal. intensity—and. multiple-inputs:. the.
mirror.actuators..Such.a.structure.for.the.controller,.while.simple,.may.not.be.eicient..By.coordinating.
the.action.of.the.actuators,.one.can.establish.an.output.controller.basis.that.is.potentially.more.efective.
in.this.application.
In.this.section,.three.choices.for.the.input.control.basis.are.introduced..he.three.choices.are.com-
pared.based.on.the.closed-loop.performance.of.the.control.loop,.as.measured.by.the.inal.value.of.the.
luorescence.signal.(i.e.,.the.metric).
he. most. straightforward. control. basis. for. the. SPGD. controller. is. to. treat. individual. actuators. as.
independent,.local.control.channels..In.the.BU-Wellman.Center.for.Photomedicine.two-photon.luores-
cent.microscope.setup,.the.DM.has.about.100.actuators.in.the.pupil.plane,.and.each.actuator.essentially.
deines.a.sub-aperture.of.the.pupil..he.advantage.for.this.actuator.basis.is.that.it.can.produce.the.inest.
spatial.resolution.achievable.with.the.DM..On.the.other.hand,.this.actuator.basis.has.the.most.control.
channels.and.it.makes.no.use.of.the.information.about.relative.positions.of.the.actuators.in.the.array..
he.SPGD.algorithm.eiciency.decreases.with.the.number.of.control.channels,.so.this.basis.will.have.
lower.eiciency,.characterized.by.slower.optimization.of.the.metric.
Since.the.main.aberrations.requiring.corrections.are.expected.to.have.relatively.low.spatial.frequen-
cies. [28],. actuator. binning. can. be. used. to. decrease. the. number. of. control. channels.. Groups. of. adja-
cent.actuators.in.two-by-two.arrays,.controlled.in.common,.were.examined.as.an.alternative.basis..In.
this.case,.the.control.basis.is.reduced.to.~25.channels.in.the.pupil.plane..Higher.convergence.speed.is.
expected.using.this.basis.
Considering. that. the. main. aberrations. known. to. degrade. deep-tissue. imaging. performance. are.
spherical.aberrations,.a.third.approach.was.taken.as.well—namely,.to.use.a.Zernike.polynomial.control.
basis..Zernike.polynomials.are.oten.used.to.represent.optical.aberrations..hey.comprise.an.ininite.set.
of.orthonormal.equations.deined.over.a.circular.aperture..An.arbitrary.wavefront.can.be.represented.
by.a.truncated.set.of.Zernike.polynomials,.and.this.representation.will.closely.approximate.aberrations.
composed. of. low. spatial. frequencies.. Zernike. polynomials. have. been. widely. used. in. optics,. primar-
ily.because.low-order.Zernike.terms.deine.well-known.optical.defects.(e.g.,.defocus,.astigmatism,.and.
coma)..hese,.along.with.radially.symmetric.terms.(e.g.,.spherical.aberrations),.are.the.primary.aber-
rations. expected. in. deep-tissue-imaging. microscopy.. A. truncated. set. of. Zernike. polynomials. can. be.
 
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