Biomedical Engineering Reference
In-Depth Information
Move focus in
sample for new
realizations of speckle
Move focus in
sample for new
realizations of speckle
Move focus in
sample for new
realizations of speckle
Phase shifting
interferometry
...
Image quadruplets
E
=
(I4
-
I2)
+
i (I3
-
I1)
Reconstruction
of the electro-
magnetic
�
eld
...
Amplitude
Amplitude
Amplitude
Phase
Phase
Phase
One FFT per
virtual lenslet
...
Di
raction pattern
for each lenslet
Virtual
Shack-
Hartmann
sensor
1.2
1
0.8
Ensemble averaging
of corresponding
di
raction patterns
0.6
0.4
Least-squares
t
yields Zernike
coe
cients
Centroid estimation
to determine tip/tilt
over each lenslet
0.2
0
-
0.2
0
5
10
15
20
25
30
Zernike mode
FIGuRE 14.7
Experimental.procedure.of.wavefront.measurement.with.CGWS.
14.4.4 implementation considerations
No. practical. implementation. of. CGWS. will. be. as. simple. as.
Figure. 14.4
,
. since. aberration. correction.
and. other. experimental. constraints. will. invariably. add. additional. components. to. the. system.. Back-
relections.from.optical.components.can.become.a.limiting.factor.by.saturating.the.CCD.camera..One.
solution. is. to. use. a. forward-only. reference. arm. where. the. beam. is. not. relected. back. on. itself. and. to.
minimize.the.length.of.the.sample.arm.that.is.relected.back.on.itself.(Feierabend.et.al..2004)..However,.
using.a.CCD.with.a.large.dynamic.range.marginalizes.the.problem.of.back-relections..his.opens.up.the.
possibility.of.using.completely.symmetric.sample.and.reference.arms.(Wang.et.al..2012),.most.notably.
the.integration.of.a.microscope.objective.in.the.reference.arm.(Linnik.interferometer)..he.advantages.
are.an.automatic.alignment.of.both.interferometer.arms.because.of.the.cat's-eye.efect.(objective.and.
reference.mirror.relect.the.beam.back.onto.itself),.as.well.as.the.perfect.dispersion.correction.because.
of.the.symmetry.between.the.arms.
14.5 Application
14.5.1 experimental implementation
In.principle,.CGWS.is.a.wavefront.sensing.technique.that.works.with.any.suiciently.scattering.sample.
where.scatterers.are.randomly.distributed..It.is.independent.of.the.imaging.modality.and.can,.in.princi-
ple,.be.combined.with.anything.from.classical.wide-ield.microscopy,.confocal.microscopy,.two-photon.
microscopy.(2PM),.structured.illumination.microscopy,.coherent.anti-Stokes.Raman.scattering.micros-
copy,.second/third-harmonic.generation.microscopy,.or.even.stimulated.emission.depletion.microscopy.
For.a.irst.experimental.demonstration.of.the.power.of.CGWS,.the.wavefront.sensing.was.integrated.
into. a. custom-built. two-photon. microscope,. in. combination. with. a. deformable. mirror. (DM). for.
.wavefront.correction.(Rueckel.et.al..2006);.see
.
Figure.14.8
.
