Liquid crystal phase retarder with broad spectral range

Phase retarders normally show a strong wavelength dependence of retardance. For example even without taking birefringence dispersion into account a qu arterwave retarder designed for 600 nm acts as a halfwave retarder at 300 n m. Achromatic phase retarders with retardance changes of only +/-5% in appr oximate to 200 nm broad spectral regions can be assembled from two layers o f different birefringent materials the fast axes of which are crossed. Furt her improvement is possible by combining three achromatic phase retarders b ased on a recipe given by Pancharatnam. A quarterwave plate is realized wit h an inner halfwave plate the fast axis of which is positioned at a certain angle with respect to the fast axes of the outer retarders. With this desi gn phase retardance can be kept constant within +/-2% in the entire visible spectrum from 400 to 900 nm. We report on a first realization of such a br oad band phase retarder using liquid crystals instead of crystals as the bi refringent materials. Although the performance is not as good as calculated it is well comparable to practically realized crystalline 'superachromatic ' phase retarders. (C) 2000 Elsevier Science B.V. All rights reserved.