MAGNESIUM FLUORIDE 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. The shorter the wavelength, the more is the variation of retardation with wavelength. A zero order double plate retarder is made of two plates of single crystal magnesium fluoride with fast axes at 90 degrees to each other. Thickness difference of the two gives the desired zero order retardation at a desired wavelength. Zero order quarter or half waveplates for a specified wavelength from 150 to 6000 nm are available in standard clear aperture diameters of 12 mm and 22 mm in air-spaced version only. These are supplied uncoated. Other clear apertures including eighth wave retarders are available on special order.

Quarter and half wave retarders made for wavelengths below 220 nm are made on a best effort basis. At present, we cannot make a direct measurement of retardation at these wavelengths. For these retarders, retardation is measured at 220 to 300 nm and the results are extrapolated.