Biomedical Engineering Reference
In-Depth Information
FIGuRE 9.9
A.compact,.versatile.point.source.with.a.single-mode.iber.
9.6.2
Alignment Sources
Whenever.possible,.start.the.optical.system.alignment.using.a.laser.with.visible.wavelength.from.400.
to.700.nm..Coarse.alignment.of.optical.subassemblies.and.the.entire.optical.system.can.be.done.much.
more.efectively.with.a.visible.laser.rather.than.an.infrared.laser..Fine.adjustment,.especially.along.the.
optical. axis,. must. be. performed. if. the. wavelength. used. for. biological. imaging. is. not. the. same. as. the.
alignment.laser.
A.helium-neon.(HeNe).laser.is.widely.used.as.an.alignment.laser.for.its.low.cost.and.ease.of.opera-
tion..It.is.primarily.used.at.a.wavelength.of.632.8.nm,.which.is.in.the.red.portion.of.the.visible.spectrum..
he.output.beam.quality.of.a.HeNe.laser.is.typically.not.good.enough.to.be.used.directly.for.alignment..
Additional.beam.cleaning.and.collimation.are.needed.
Light. coming. out. from. a. HeNe. laser. is. focused. down. by. a. spherical. lens. to. a. pinhole.. he. pinhole.
ilters.out.the.high-order.aberrations.in.the.beam.and.passes.the.good-quality.lower-order.modes.of.the.
beam.through..Adjust.the.pinhole.so.that.most.of.the.light.passes.through.to.avoid.light.loss..A.good.
collimation.spherical.optics.is.placed.behind.the.pinhole.to.re-collimate.the.beam.
With.the.recent.development.of.semiconductor.laser.diodes,.low-cost.laser.diodes.are.used.more.and.
more.in.optical.alignment.and.illumination.in.biological.imaging.instruments..he.light.that.comes.out.
of.the.laser.diode.is.highly.diverging.and.has.an.elliptical.cross.section..he.divergence.is.typically.speci-
ied.in.both.the.
x
.and.
y
.axes..he.beam.is.collimated.by.passing.it.through.a.pair.of.cylindrical.lenses.
that.are.perpendicular.to.each.other.
Collimated. light. can. be. focused. down. to. a. point. source. by. a. good. focusing. lens. or. coupled. into. a.
single-mode. iber.. Based. on. the. light-reversal. principle,. light. coming. out. of. a. single-mode. iber. can.
be. collimated. well. if. the. lens. is. placed. one. focal. length. away.. A. iber-coupled. laser. beam. is. exploited.
in.many.biological.instruments.for.its.signiicant. beneits.to.the.system..Once.the.iber-coupled.laser.
is. collimated,. minimum. optical. realignment. is. needed.. By. using. a. connectorized. iber. and. a. sturdy.
connector,. the. position. of. the. iber. can. be. easily. controlled. and. repeated. at. the. micrometer. level..
Optomechanical.alignment.considerations.of.the.laser.beam.to.the.application.are.thus.much.simpli-
ied..It.allows.the.laser.source.to.be.separated.from.the.microscopes.and.isolates.the.heat.and.vibration.
generated.by.laser.sources.and.power.supplies..he.small.footprint.also.makes.it.much.easier.to.insert.
