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For
a beam of light incident on a surface at normal incidence, polarization
properties of the reflected and transmitted beams are described by
Fresnel's equations. According to these, if angle of incidence is equal to
Brewster's angle, the degree of polarization of reflected -s beam (
perpendicular to plane of incidence ) is unity. The transmitted -p (
parallel to plane of incidence ) beam is partially polarized. Polarization
of the transmitted beam can be further purified by successive films each
of suitable thickness and refractive index so that Brewster's condition at
each interface is met. In actual practice this does not happen which
limits the performance of thin film polarizing cubes.
The standard available cube sizes are 5, 12, 25 and 50 mm. Standard
central wavelengths are 550, 830, 1100 and 1400 nm. These are all made of
SF2 substrate. Other sizes and wavelengths are available on special order. All
outside faces of the cubes are broad band single layer magnesium fluoride anti-reflection coated (
1.5 % average reflectivity per surface in respective wavelength ranges as
shown in following plots ). Please visit our
Anti-Reflection Coating page
on this web site for various reflectivity plots.
The
extinction of transmitted -p component is at least 1 part in 500. As shown
in the following plots, some of the transmitted -p polarization is
reflected which degrades the extinction of -s polarization. Almost all -s
polarization is reflected. Wavefront deformation of the transmitted -p
beam is at least quarter wave at 633 nm.
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