Infrared Liquid Crystal Tunable Filters
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LIQUID CRYSTAL TUNABLE FILTERS
SHIN-TSON WU, CHIUNG-SHENG WU Hughes Research Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265 ABSTRACT Computer simulations on Fabry-Perot liquid crystal tunable filters are performed in the 3-5 and 8-12 gtm bands and some discrete laser lines. Effects of transmittance, contrast ratio and response time on liquid crystal birefringence, layer thickness, absorption coefficient and reflectivity of mirrors are analyzed. Absorption spectra and responsible mechanisms of liquid crystals in the 2.520 gim region are studied. The fluorinated tolane and diphenyl-diacetylenes and dialkyl diphenyldiacetylenes are found to possess low absorption in the mid-IR range. Mixtures of these compounds will find useful applications for the proposed IR tunable filters. INTRODUCTION Both active electro-optic switches and passive optical power limiters have been developed to protect sensors from being damaged or jammed by strong laser pulses. An ideal optical power limiter should transmit low-intensity light with high efficiency and truncate high-intensity light with high extinction. In addition, it should also exhibit a wide dynamic range and fast response time. Unfortunately, the performance of current passive limiters is not yet completely satisfactory. In particular, the activation threshold is still too high to protect against cw laser threats. On the other hand, active optical switches are being used for laser protection purposes in conjunction with advanced laser warning systems. Once the laser warning system detects the existence of laser threats, it can either send signals to alert the personnel to turn on the optical switches or transmit signals to activate the optical switch directly. Once the optical switch is activated, ideally, it will reject the laser threats while transmitting the background scenes. Several types of tunable filters, such as Fabry-Perot etalon [I] and tunable birefringent filter [2,3] have been investigated extensively. These filters can be made to selectively reject certain laser wavelengths by properly tuning the applied voltage. In this paper, we demonstrate the design parameters and performance of Fabry-Perot liquid crystal (LC) tunable filters in the 3-5 and 8-12 gtm bands and at three discrete laser lines. FABRY-PEROT FILTERS Fabry-Perot LC filters have been investigated in the visible [4,5], near IR [6] and mid-IR [7] regions. Here we use computer modeling to optimize the design parameters for mid-IR and long IR applications. The basic operation principle of a Fabry-Perot tunable filter is illustrated in Fig. 1.
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V Fig. I Operation principle of a Fabry-Perot LC filter Let us assume the incident light is a broad-band linearly polarized light with polarization axis parallel to the LC directors. The inner sides of the LC cell are coated with mirror, such as copper. Due to the multiple interference within the cavity, only discrete wavelengths are transmitted 215 Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research
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