ELECTROOPTIC BROAD-BAND TUNABLE FILTERS USING LIQUID-CRYSTALS

Broadband electro-optically tunable filters have uses in applications such as wavelength de-multiplexing and laser light detection and filtering. The materials requirements for such devices are that they must have a very low absorption over the wavelength region of interest, have a large electro-optic coefficient and be easily engineered into optical components. Liquid crystals satisfy these criteria. This work describes studies carried out on two types of tunable filter incorporating a range of liquid crystal materials, a Fabry–Perot etalon and a variable birefringence filter (VBF). Results are presented on the response of such filters to a white light source and laser radiation spanning the visible, and it is shown that the larger the material birefringence, the larger is the tuning range. The merits of the Fabry–Perot versus the VBF geometry are discussed. It is demonstrated by computer modelling and experiment that it is easier to achieve high contrast ratios with the VBF geometry. Contrast ratios in excess of 1000: 1 are reported. It is noted that the FP geometry offers better wavelength selectivity than the VBF configuration. Finally, comments are made on the power handling capabilities of each device and a study of the nonlinear optical response of the Fabry–Perot etalon is described. © 1990 Taylor & Francis Ltd.