Biomedical Engineering Reference
In-Depth Information
throughout. the. volume,. they. would. act. as. a. mirror,. where. the. incoming. wavefront. is. relected. back.
with. all. its. aberrations.. Because. of. the. relection,. odd. wavefront. aberrations. for. the. irst. and. second.
pass.through.the.tissue.would.cancel.each.other.out,.even.though.aberrations.would.double.(Artal.et.al..
1995)..So.both.large.scatterers.and.nonrandom.arrangements.of.scatterers,.such.as.relective.surfaces.
inside.the.sample,.will.hinder.the.correct.measurement.of.single-pass.aberrations.*
14.4.2 Virtual Shack-Hartmann Sensor
As.stated.earlier,.a.vSHS.is.a.convenient.way.to.perform.phase.unwrapping.in.CGWS,.which.is.stable.
against.phase.singularities.caused.by.the.speckelization.of.the.coherence-gated.phase.measurements.
Shack-Hartmann. sensors. (
Figure. 14.3
)
. have. been. commercially. available. for. quite. some. time..
Unfortunately,.such.of-the-shelf.instruments.are.not.compatible.with.the.interferometric.detection.that.
is.necessary.in.CGWS..Imagine.one.replaced.the.camera.in.our.setup.with.an.SHS.to.perform.wave-
front.sensing.and.phase.unwrapping.at.the.same.time.(Figure.14.6a)..he.aberrated.light.from.the.CV,.
the.background.light.scattered.back.in.the.other.layers.of.the.sample,.and.the.lat.reference.wavefront.
would.all.pass.through.the.lenslet.array.on.their.way.to.the.camera..he.reference.light.would.be.focused.
exactly.on.the.optical.axis.of.each.lenslet,.while.the.aberrated.wavefront.of.the.light.from.the.CV.would.
be.more.or.less.tilted.on.diferent.lenslets..For.strong.enough.tilts,.the.reference.and.sample.spots.on.the.
camera.would.not.overlap.and.could.therefore.not.interfere..his.would.make.it.impossible.to.use.coher-
ence.gating.to.separate.the.in-focus.light.from.the.dominant.background.of.the.light.backscattered.in.
other.depths,.making.the.whole.scheme.unusable.
here. is. a. way. to. solve. this. problem. and. still. use. a. physical. lenslet. array. (Figure. 14.6b):. if. the. col-
limated.reference.beam.is.recombined.with.the.sample.light.behind.the.lenslet.array,.interference.takes.
place. no. matter. where. the. lenslets. focus. the. light. from. the. CV.. Although. this. is. in. principle. possible.
(Marcus. and. Feierabend. private. communication;. Tuohy. and. Podoleanu. 2010),. such. a. modiication.
is.not.compatible.with.commercially.available.SHSs,.which.removes.one.good.reason.for.using.a.real.
(a)
(b)
FIGuRE 14.6
(a). A. lat. reference. wavefront. (depicted. in. gray,. coupled. in. using. a. beam. splitter. cube). and. the.
sample.wavefront.both.pass.through.the.lenslet.array.of.an.of-the-shelf.SHS;.(b).a.custom-made.lenslet.array.with.
a.long.focal.length.allows.accommodation.of.the.beam.splitter.cube.between.lenslets.and.detection.plane.
*
.
In.a.closed-loop.adaptive.optics.system,.where.aberrations.are.measured,.then.corrected,.and.residual.aberrations.remea-
sured,.an.oscillatory.behavior.of.even.aberrations.modes.would.indicate.the.measurement.of.double-pass..aberrations..
he.solution.would.of.course.be.to.reduce.the.gain.of.wavefront.correction.to.0.5,.which.would.solve.the.issue.for.even.
aberrations.
