All-optical dynamic filtering using nematic liquid crystal

Abstract

Dynamic spatial filtering has attracted much interest in recent years because of demands for optical parallel processing. Dynamic spatial filtering makes use of nonlinear-optical material that changes the local transmittance by the application of external fields. Most notable among the nonlinear materials used are photorefractive crystals (PRC). The major disadvantage of these materials are their response times which are in the order of 10^-2 seconds. Meanwhile, a nematic liquid crystal (LC) has a response time an order faster than the photorefractive materials' response time. Moreover, LC doped with dye shows large nonlinearity comparable to that of PRCs. In the work of Inoue and Tomita (1996) and Tanaka, et al. (1996), they utilized the large nonlinearity of dye-doped liquid crystal for filtering applications. The large polarization change in the transmitted light was exploited to effect spatial filtering with self-alignment capability. They investigated the characteristics of the filtering in terms of response time and signal to noise (S/N) ratio.
Our work presents a novel set-up in which we control independently the filtering process through an external pump beam. The set-up is robust in terms of the polarization ofthe imaging beam. The optical transfer function for different pump intensities were obtained. Also, a model based on Bessel filters was used to study the filtering properties.