Biomedical Engineering Reference
In-Depth Information
optical.component.is.of-center,.additional.aberrations.such.as.astigmatism.and.coma.may.be.created.
and.the.system.performance.will.be.degraded.
Centering.optical.components.using.the.line-of-sight.concept.is.straightforward..Establish.the.line.
of.sight.using.an.alignment.laser.and.a.target.without.any.optical.components.in.between.and.record-
ing.where.the.line.of.sight.intersects.the.target..Put.the.transmission.lens.in.the.optical.beam.path.and.
adjust. the. lens. transversely. until. the. beam. hits. the. same. mark. on. the. target.. Typically. based. on. the.
scattered.light.from.the.relection.of.the.lens.surface,.one.can.position.the.lens.roughly.in.the.center.of.
the.optical.path.before.starting.ine.adjustment..Place.the.target.as.far.as.practically.possible.from.the.
alignment.laser.so.that.the.measurement.is.more.sensitive.and.the.placement.of.the.optical.component.
is.more.accurate.
9.5.5 Align the Science camera Last
he.science.camera.should.be.aligned.and.integrated.into.the.system.last..his.approach.is.important,.
because.it.provides.a.inal.opportunity.to.optimize.the.alignment.using.performance.feedback.that.is.
measured.through.the.complete.optical.system..his.is.particularly.important.if.multiple.wavelengths.
are. involved. in. the. optical. system.. he. position. of. the. CCD. camera. should. be. adjusted. to. the. opti-
mized.position.for.all.wavelengths..Whenever.possible,.an.efort.should.be.made.to.avoid.allowing.all.
six.degrees.of.freedom.in.the.inal.alignment,.as.this.can.greatly.complicate.the.mount.and.increase.
material.and.labor.costs.
9.5.6 Multiple Wavelengths
Multiple.wavelengths.are.typically.built.in.the.biological.imaging.tools.for.lexibility.in.imaging.dif-
ferent.types.of.biological.samples.for.best.resolution.and.contrast..It.is.best.to.start.optical.alignment.
using.a.single.source.and.inish.the.entire.system.alignment.from.end.to.end..If.multiple.sources.need.
to. operate. simultaneously,. separate. sources. can. be. inserted. in. to. the. system. by. using. beam. splitters,.
gratings,. or. folding. mirrors.. If. necessary,. more. adjustment. freedom. is. given. to. the. mounts. of. beam.
splitters,. gratings,. or. folding. mirrors. to. avoid. the. situation. where. the. alignment. of. shared. optical.
paths.needs.to.be.readjusted..In.imaging.tools.where.an.optical.source.with.a.single.wavelength.is.used.
one. at. a. time. instead. of. being. used. simultaneously,. laser. sources. can. be. coupled. into. optical. ibers..
Wavelength. switching. can. be. easily. achieved. by. switching. the. iber. connector. attached. to. the. iber-
coupled.laser.sources..As.discussed.in.Section.9.5.5,.the.CCD.camera.should.be.placed.on.a.translation.
stage.with.freedom.of.motion.along.the.optical.axis.to.compensate.the.defocusing.error.caused.by.dif-
ferent.wavelengths.
9.6 Alignment tools
9.6.1 Point Source with a Single-Mode optical Fiber
he.core.diameter.of.a.single-mode.iber.is.typically.about.3-5.µm,.making.it.an.ideal.point.source.for.
illumination,.characterization,.and.alignment..An.optical.iber.is.a.very.versatile.tool.in.alignment..he.
through-hole.in.the.iber.optics.connector.can.be.used.to.provide.a.reference.for.quick.and.coarse.align-
ment.of.the.line.of.sight.in.an.AO.system..he.relection.coupled.back.to.the.single-mode.iber.source.
is.a.perfect.indicator.that.the.optical.surface.measured.is.at.the.conjugate.plane.of.the.iber.source..For.
example,.if.the.AO.system.is.well.aligned,.the.back.relection.from.the.CCD.camera.of.the.wavefront.
sensor.should.be.coupled.back.to.the.iber.source.
Figure.9.9
.shows.a.compact.point.source.with.a.single-mode.iber..he.iber.connector.is.inserted.to.
a. cage. plate. mounted. on. a. stand-alone. post.. he. compact. design. makes. it. easy. to. be. placed. virtually.
anywhere.in.the.optical.system.where.a.point.source.is.needed.for.illumination,.characterization,.and.
alignment.
