Biomedical Engineering Reference
In-Depth Information
4.6 Experimental System for Aberration Measurement
For the measurement of the specimen-induced aberration, a Mach-Zehnder interferometer type
setup was chosen. It essentially compares two wavefronts: a lat reference wavefront and an aber-
rated wavefront that results from the propagation through the specimen. he interference of the two
coherent beams from the reference and the object path delivers an intensity pattern. A phase-shiting
technique is used to extract the phase information of the wavefront through the measurement of
three interferograms. A schematic drawing of the setup is shown in Figure 4.4. he expanded He-Ne
(632 nm) laser beam illuminating the setup was split into a reference path and an object path. A rota-
tion of the λ/2-plate in front of the polarizing beam splitter allows us to adjust the relative intensities
of the two paths. he Zeiss LD-Achroplan, 40×, 0.6 numerical aperture (NA) lens, which was used
as a condenser, has a correction ring to adjust for the thickness of the microscope slide minimizing
spherical aberration.
We used a Zeiss 20×, Plan-Neoluar, 0.5 NA (dry lens without cover glass correction) and an
Olympus 20×, UPlanApo, 0.8 NA, oil immersion as objective lenses. When the object beam has tra-
versed the specimen, it is made to interfere with the reference beam and the resulting interference
pattern is recorded on a CCD camera. he image plane of the CCD is conjugate to the pupil plane
of the objective lens. he phase-stepping unit placed in the reference-beam path takes the form of a
lat mirror shited by a calibrated piezo drive. his allows us to change the relative phase between
reference and object beams in well-deined steps. his phase stepping is synchronized with the CCD
camera and permits the recording of digital interferogram images at the video rate of 25 Hz using
a frame grabber itted to a Linux PC. he specimen is attached to a piezo-driven stage that can be
positioned in three dimensions and feedback control prevents long-term drit of the sample (P-611
NanoCube; Physik Instrumente, Germany). Furthermore, the object path can be switched to operate
as a conventional transmission microscope using the additional camera and illumination elements as
shown in
Figure 4.5
.
his bright ield illumination mode is used to ind regions of interest within the sample prior to the
data recording in the interferometer mode and the alignment is such that the focal plane of the trans-
mitted light image coincides with the focal spot of the interferometer.
Phase stepper
Reference
beam
Specimen
Analyzer
λ
/2 - Plate
Object
beam
CCD
camera
Laser
PBS
Condenser
Objective
Figure 4.4
Coniguration of the setup in interferometer mode for wavefront measurements. he pattern result-
ing from the interference of the two beam paths is recorded on a CCD camera. he phase-stepping unit allows
shiting of the relative phase between the paths.
