ACHROMATIC WAVEPLATES

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. it's dependence on wavelength, there is a variation of retardation with wavelength. In an achromatic retarder, this variation is minimized by fabricating a double plate retarder using two different birefringent crystals of magnesium fluoride and crystalline quartz. Achromatic quarter and half wave retarders are optimized for three standard wavelength regions of 400 to 700 nm, 700 to 1000 nm and 1000 to 1600 nm. Standard waveplates are cemented or air-spaced type and are available in clear aperture diameters of 12 mm and 22 mm. Non standard waveplates optimized for different wavelength regions and clear apertures are available on special order.