Biomedical Engineering Reference
In-Depth Information
PMT
PZT
SLED
OBJ
(a)
(b)
FIGuRE 14.1
(a).OCM.setup:.Michelson.interferometer.with.piezo-mounted.(PZT).reference.mirror.in.one.arm.
and.objective.plus.sample.in.the.other.arm;.the.PMT.is.conjugated.to.the.sample.focus;.(b).each.scatterer.in.the.
illumination.double.cone.produces.a.secondary.wave.
light,. allowing. OCT. to. provide. three-dimensional. imaging. with. decoupled. axial. and. lateral. resolu-
tion..A.multitude.of.diferent.variations.of.OCT.has.since.been.developed,.using.diferent.light.sources,.
detectors,.and.interferometer.layouts,.based.on.time-of-light.scanning.(time-domain.OCT).or.wave-
length.scanning.(frequency-domain.OCT);.for.a.review,.see.Schmitt.(1999).
Let.us.analyze.a.variant.of.OCT.further,.which.is.usually.referred.to.as.optical.coherence.microscopy.
(OCM);.see.Figure.14.1a..A.spatially.coherent.but.spectrally.broad.light.source,.such.as.a.superlumines-
cent.diode.(SLED).or.femtosecond.laser,.is.used.to.illuminate.a.Michelson.interferometer..In.one.arm.of.
the.interferometer—that.is,.the.sample.arm—the.light.is.focused.by.an.objective.lens.into.the.sample..
he.other.arm,.called.the.reference.arm,.consists.of.a.mirror.mounted.on.a.piezo.element..he.refer-
ence.arm.length.is.chosen.to.match.the.optical.path.to.the.geometric.focus.of.the.objective.lens.in.the.
sample.arm..Such.a.system.can.be.used.to.image.a.single.point;.the.sample.needs.to.be.scanned.in.
x
-,.
y
-,.
and.
z
-axis.to.acquire.a.three-dimensional.image.stack..Using.this.simplest.possible.OCM.setup.as.an.
example,.we.will.develop.an.intuitive.picture.of.low-coherence.interferometry.
he.light.is.scattered.by.point-like.scatterers.everywhere.in.the.illuminated.double.cone.of.the.sample,.
producing. secondary. spherical. wavelets. at. the. position. of. each. scatterer. (Figure. 14.1b).. Some. of. this.
scattered.light.is.collected.again.by.the.objective.lens,.recombined.with.the.reference.arm.light.by.the.
beam.splitter,.and.focused.by.the.tube.lens.onto.the.detector.
For. a. single. scatterer. precisely. at. the. focus. (
Figure. 14.2a
),. the. optical. path. length. to. the. detector.
matches. exactly. the. optical. path. length. in. the. reference. arm. so. that. all. wavelengths. in. the. spectrum.
interfere.constructively.with.the.light.from.the.reference.arm.
For.single.scatterers.in.positions.that.cause.the.optical.path.to.be.longer.or.shorter.by.λ/2.(
Figure.14.2b
)
,.
we.have.the.opposite.situation:.nearly.all.the.light.is.relected.back.to.the.source;.hardly.any.reaches.the.
detector.*.By.changing.the.reference-arm.length.by.λ/2,.the.situation.for.each.of.the.mentioned.scatterers.
can.be.reversed.
In. contrast,. for. scatterers. where. the. path. diference. between. the. arms. is. long. with. respect. to. the.
coherence.length.of.the.source,.constructive.and.destructive.interference.alternate.rapidly.as.a.function.
of.wavelength..As.a.result,.the.total.light.intensity.from.this.scatterer.(integrated.over.all.wavelengths).
*
.
Because.of.the.inite.width.of.the.wavelength.spectrum,.the.condition.δ.=.λ/2.can.only.be.approximately.fulilled.for.all.
wavelengths.at.the.same.time..As.a.consequence,.wavelengths.at.the.edge.of.the.spectrum.still.cause.a.small.constructive.
interference.in.the.direction.of.the.detector.
