Liquid Crystal Optical Phase Modulators for Beam Steering
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Liquid Crystal Optical Phase Modulators for Beam Steering Jay Stockley, Xiaowei Xia, Teresa Ewing, and Steve Serati Boulder Nonlinear Systems Incorporated, 450 Courtney Way, Unit 107 Lafayette, CO 80026, U.S.A. ABSTRACT Beamsteering using liquid crystals can be achieved with refractive or diffractive implementations. The common thread in these various structures is that the liquid crystal is employed as an optical phase modulator. Either nematic or smectic liquid crystal phases can be used to shift the phase of light and steer an optical beam. Various liquid crystal optical phase modulating schemes will be described. Examples include polarization independent and quasiachromatic modulators. Model predictions and experimental results demonstrating the optical phase modulation and beamsteering made possible using different liquid crystal based designs will be presented. INTRODUCTION Liquid crystals are frequently used as optical modulators. These materials offer several advantages including large modulation depth, no moving parts, low power dissipation, potential for large aperture operation, and low cost. Beamsteerers that employ liquid crystal modulators can be categorized according to the physical mechanism used to redirect light: refraction and diffraction. McManamon et. al. [1] provide an excellent review of liquid crystal beamsteering technology. An example of refractive beam deflectors are liquid crystal wedges. In general, refractive beamsteerers offer high efficiency but small angular deflection. The deflection angle for a wedge is proportional to the optical path difference induced by electrically changing the effective refractive index of the liquid crystal. Consequently this approach yields deflection angles on the order of milli-radians. Cascading multiple elements in series can improve the deflection angle without drastically reducing the efficiency. Optical phased arrays can be used in refractive mode if no resets are used and the phase ramps continuously across the aperture. Since there are no phase resets, grating dispersion is not present and broadband radiation can be steered. Diffractive beamsteerers can be implemented with an optical phased array analogous to some radar systems. Alternatively, the diffractive optical phased array can be thought of as a quantized multiple level phase grating. The more phase levels used in the array, the higher the diffraction efficiency. For example, a binary phase grating ideally provides a diffraction efficiency of 40.5% in each of the two first order diffracted beams. For a quantized phase grating using three phase levels the ideal first order diffraction efficiency is 68.4%, while for 4 phase levels, it increases to 81%. For more than four levels the improvement in diffraction efficiency with increasing number of phase levels slows. At 5 levels the percentage of light
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diffracted into the first order is ideally 87.5% and for 8 phase levels the ideal first order diffraction efficiency is 94.9%. Due to the effects of fringing fields between electrode lin
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