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RESEARCH PRODUCT

Poincaré sphere analysis of a ferroelectric liquid crystal optical modulator: application to optimize the contrast ratio

subject

description

The Poincare sphere representation is used to analyze the polarization transformation achieved with a ferroelectric liquid crystal (FLC) optical modulator. This device acts as a switchable wave-plate, in which the orientation of the principal axes rotates under the action of an applied bipolar voltage. In the standard operational mode for intensity switching, the rotation angle of the principal axes is �θ = π/4 and the phase shift is φ = π (half-wave-plate). However, for wavelengths different from the design one, the FLC deviates from the half-wave-plate performance and the optical contrast is diminished. We use the Poincare sphere representation to perform a theoretical analysis of the intensity switch performance of the FLC modulator as a function of the phase shift φ. Using spherical trigonometric relations we derive analytical expressions for the intensity contrast as a function of φ and we show how to compensate its decrease when φ � π by using appropriate elliptically polarized illumination. These situations are experimentally demonstrated using a commercially available FLC cell.