Biomedical Engineering Reference
In-Depth Information
axis..By.placing.a.charge-coupled.device.(CCD).camera.in.the.focal.plane.of.the.lenslets,.the.difraction.
pattern.from.each.lenslet.can.be.recorded.and.its.shit.with.respect.to.the.optical.axis.determined..his.
provides.the.local.wavefront.tilt.measurements.that.allow.reconstruction.of.the.full.wavefront.
14.4 Methods
14.4.1 Basic coherence-Gated Wavefront Sensing Setup
We.now.want.to.perform.wavefront.sensing.on.the.backscattered.light,.in.a.manner.similar.to.a.Shack-
Hartmann.sensor..Coherence.gating.will.be.used.for.
z
-selectivity.of.the.analyzed.light,.so.the.new.setup.
is. based. on. the. OCM. setup. (
Figure. 14.1a
)
.. Although. OCM. used. confocal. detection,. wavefront. sens-
ing. is. best. performed. in. a. plane. conjugated. to. the. BFP. of. the. objective.. herefore,. the. CGWS. setup.
(Figure 14.4).contains.a.telescope.that.is.used.to.image.the.BFP.of.the.objective.lens.onto.the.detector..
At.the.same.time,.the.point.detector.is.replaced.by.a.camera,.allowing.simultaneous.measurement.of.the.
spatial.intensity.distribution.in.the.BFP,.ater.interference.with.the.reference.light.
he.backscattered.light.from.the.sample.entering.the.objective.under.a.certain.angle.to.the.optical.
axis.will.reach.the.BFP.at.a.position.corresponding.directly.to.the.ray.angle,.so.that.the.camera.efec-
tively.provides.angular.resolution.of.the.backscattered.light..Phase-shiting.interferometry.(Malacara.
1992).works.in.the.same.way.as.in.OCM:.although.the.sample.arm.remains.ixed,.the.length.of.the.refer-
ence.arm.is.varied,.and.several.camera.images.are.acquired..For.simplicity,.the.sample.arm.is.changed.
in.ixed.steps.of.λ/4;.with.four.such.images.
I
1
,.
I
2
,.
I
3
,.and.
I
4
,.amplitude.and.phase.of.the.coherence-gated.
sample.light,.
E
smp
,.can.be.determined.for.each.pixel.of.the.camera.individually:
.
.
.(14.1)
E
smp
=
(
I
−
I
)
+
i
(
I
−
I
)
1
3
4
2
where.
i
.is.the.imaginary.unit.
To. understand. in. detail. the. information. content. of. this. amplitude. and. phase. measurement. in. the.
pupil,
*
.we.will.analyze.sequentially.diferent.samples,.as.depicted.in.
Figure.14.5
..For.an.aberration-free.
sample. containing. one. single. scatterer. positioned. precisely. at. the. focus,. the. backscattered. spherical.
CCD
PZT
SLED
OBJ
FIGuRE 14.4
he.CGWS.setup.
*
.
he. pupil. of. the. objective. lens. is. assumed. to. coincide. with. the. BFP. of. the. objective. lens..If. this. was. not. the. case,. the.
illumination.intensity.in.the.focal.plane.would.be.ield-dependent,.so.microscope.objective.manufacturers.will.always.
strive.to.put.the.pupil.as.close.to.the.BFP.as.possible.
