Broad-band polarization-insensitive optical-path-length modulator

Abstract

We present an improved design for a polarization-insensitive liquid-crystal optical-path-length modulator that can be used with white light in an adaptive optics system. IntroductionLiquid-crystal spatial light modulators (LCSLMs) have been shown to be good candidates for use in adaptive optical systems as high-performance optical-pathlength modulating elements, even with unpolarized light [1,2].The problem with such devices, originally, was that they had to be used with polarized light, since only the extraordinary refractive index of a nematic liquid crystal (LC) could be varied by the application of an electric eld. This problem can be overcome by using the LCSLM in the re ection mode [3].If unpolarized light is incident on the LCSLM, two orthogonally polarized components emerge from each cell, which have experienced diå erent retardations, corresponding to the ordinary (n o ) and extraordinary (n e ) refractive indices of the LC. A combination of a quarter-wave plate (QWP) set at 45 8 and a mirror rotates the planes of polarization of both these components by 90 8 , so that the component that experienced the ordinary index on the outward pass experiences the extraordinary index on the return pass, and vice versa. Accordingly, at the design wavelength of the QWP, both orthogonal components of the incident unpolarized light experience the same change in the optical path length when a voltage is applied to the LCSLM.However, many applications, such as astronomical image sharpening, require an adaptive system that works with white light [4]. While the change in optical path length introduced by the LCSLM varies by only §5% over the range of wavelengths from 450 to 700 nm [4], a problem with a simple QWP is that, at wavelengths other than the design wavelength, light that is linearly polarized at 45 8 to the fast axis of the QWP is converted to elliptically polarized light, instead of circularly polarized light. The result is that each of the orthogonally polarized components (say V e and V o ) emerging from the LCSLM produces two orthogonally polarized output beams, so that we have four incoherent output beams (V eo , V oe , V ee , V oo ) contributing to the nal point-spread function. Two of these beams (V eo and V oe ) experience the required change in optical path length, but the optical path length of the third beam (V ee ) changes by twice the required amount, while that of the fourth beam (V oo ) undergoes no change. The presence of these