Polarization-Independent Tunable Spectral Filter by the Use of Liquid Crystal

Abstract

Liquid crystals are useful for creating spectral filters, since they exhibit a large birefringence and an efficient electro-optic effect. Liquid crystal devices, however, usually require a polarizer that halves light intensity. This polarization dependence is unfavorable particularly in the long-wavelength infrared region in which neither efficient polarizers nor strong light sources is available due to opaqueness of ordinary optical materials. Polarizer-free devices are therefore desired for extending the application fields of liquid crystals. A solution is to split a light signal into two beams with orthogonal polarization directions and superpose them after transmission through a liquid crystal cell. This optical system, however, has to be constructed with multiple optical components, which require a complicated alignment process and induce high insertion losses for infrared light. It is therefore beneficial to integrate multiple components in a single device. A compact optical device that integrates a polarization beam splitter and two retarders can be constructed by using silicon pentaprisms. The silicon prisms act as a substrate and an electrode for a nematic liquid crystal that achieves both polarization division and retardation. A function of a tunable spectral filter (Lyot filter) is attainable in the 2–8 μm wavelength range. Another solution for creating a polarizer-free device is to eliminate an optical anisotropy of the liquid crystal. Unique optical properties of cholesteric liquid crystals realize a polarization-independent index change. Whereas visible light changes its polarization direction as it passes through a chiral structure of the cholesteric liquid crystal, infrared light keeps its polarization state since their wavelength is too long to recognize the micro chiral structure. Consequently, the cholesteric liquid crystal exhibits an isotropic index of refraction that is tunable by voltage application. This isotropic property is useful to create a polarizer-free liquid crystal device. Interference filters (Fabry-Perot filters) for the infrared measurements can be constructed by enclosing a cholesteric liquid crystal in a narrow gap of two silicon plates. A polarization-independent index change between 1.52 and 1.61 is attainable by application of ~10 V.