Image Processing Reference
Figure 3.7 Mach-Zehnder interferometer modified to function as a wavefront sensor.
The reference beam is generated using a spatial filter. The transmission versus reflec-
tivity of the beamsplitter cubes must be chosen to maximize the fringe contrast.
3.3.2 Shearing interferometer
One of the simplest interferometers to construct is the shearing interferometer. The
basic approach is to take a single wavefront and split it into two beams, which can
then be displaced by some fraction of the wavefront diameter and overlapped, or
sheared. Interference occurs in the sheared beams with respect to the displacement,
and observable fringes are formed in the overlap area. The shearing interferometer
is an indirect wavefront sensor as it measures the difference in the aberrated wave-
front compared with it, but displaced by a fixed amount. Shearing interferometers
operate by making comparisons in the direction of shear only. A second shearing
interferometer is needed, rotated 90 deg, to get information in the orthogonal direc-
A simple shearing interferometer can be constructed using a single plate of
glass, such as a microscope slide, and reflecting a collimated laser beam off it at an
angle. The optical path difference between light reflecting from the front and back
surfaces of the lens is naturally displaced, or sheared, by the time it strikes a screen
some distance away. The reflections from the front and back surfaces, shifted and
overlapped on a screen show fringes, which reveal the aberrations in the micro-
scope slide. Figure 3.8 illustrates a simple shearing interferometer showing the aber-
ration in a thin glass plate such as a microscope slide.
Interferometers have many variations and are usually selected to support the
measurement of optical-path-length difference based on the specific application.