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When
any form of polarized light is incident on a birefringent waveplate, it is
resolved into two linearly polarized components which vibrate
perpendicular to each other and travel with different velocities in the
crystal. The beams emerge with a phase delay. This phase delay or
retardation is dependent on birefringence of the crystal. Birefringence at
a specified wavelength is the difference in refractive indices of the
extraordinary and ordinary resolved components. Because of dispersion of
birefringence i.e. its dependence on wavelength, there is a variation of
retardation with wavelength. In a crystalline quartz zero-order double
plate waveplates, variation of retardation with wavelength is less than in
multi-order single plate type. This dependence becomes greater at shorter
wavelengths. A zero order double plate retarder is made of two plates of
crystal quartz with fast axes at 90 degrees to each other. The thickness
difference of the two gives the desired zero order retardation at a
desired wavelength. A multi order waveplate gives several full waves
and a desired fraction ( e.g. quarter or half ) of a wave of retardation
at a desired wavelength.
Zero
and multi-order quarter or half waveplates for a specified wavelength from
250 to 1600 nm are available in standard clear
aperture diameters of 12 mm and 22 mm. These are cemented, optically contacted or air-spaced. Other clear apertures including eighth wave retarders are
available on special order.
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