Frequency Agile Optical Filters using Nanoscopic Polymer Dispersed Liquid Crystals
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FREQUENCY AGILE OPTICAL FILTERS USING NANOSCOPIC POLYMER DISPERSED LIQUID CRYSTALS KEITH LEWIS, GILBERT SMITH, IAN MASON AND KATE ROCHESTER QinetiQ Ltd Malvern Technology Centre Great Malvern Worcestershire, WR14 3PS, UK ABSTRACT The ability to realise frequency agile filters has always been a major goal of materials and device scientists. Polymer dispersed liquid crystal materials (PDLCs), in which the droplets are deliberately produced with dimensions smaller than the Rayleigh limit for optical scatter, provide a means of realising this goal. At visible wavelengths the material has the appearance of a phase-homogeneous medium both with and without applied electric field. Refractive indices are an average of those of the liquid crystal and those of its host and can be described on the basis of effective medium approximations using orientationally averaged values for the randomly aligned LC material for the ambient state. On application of electric field, the individual liquid crystal molecules align with a resulting reduction in the effective index of the medium. Extensive studies of the variation of process parameters and their effect on the morphology of the composite have enabled field induced changes in refractive index in excess of 0.05 to be achieved without recourse to the use of polarisers and with little or no hysteresis effects. At wavelengths associated with WDM devices for telecommunication applications, the degree of scatter loss is negligible. Design trade-offs have been assessed for candidate tunable WDM filters based on resonant cavity devices.
INTRODUCTION Current techniques for wavelength division multiplexing (WDM) exploit architectures in which the essential elements of wavelength discrimination are performed using fixed line optical filters, which are fabricated with pass-bands corresponding to the frequencies of the different laser wavelengths propagating through the network. The manufacture of interference filters for such applications is demanding [1], largely because of the bandwidth constraints imposed by the ITU grid. Arguably the architecture of the system could be simplified considerably if the wavelength discriminating components within the system could be provided as frequency agile elements [2]. The ability to realise tunable filters has always been a major goal of materials and device scientists. A number of materials have been explored to form the basis of such devices, ranging from electro-optic polar dielectrics such as lithium niobate, PLZT etc, to thermo-optic materials whose refractive indices are a sensitive function of temperature. More recently, attention has also been focussing on the potential afforded by combining the optical performance levels achievable in fixed line filters with the inherent flexibility of microelectromechanical systems (MEMS) [3]. CC4.3.1
Ultimately there are a number of factors determining the choice of materials for such devices. In the case of polar dielectrics it has proved difficult to realise refractive index changes of more than 1%, without
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