Spatial coherence control for illumination by use of liquid-crystal spatial phase-modulator

Abstract

This paper presents a novel method for controlling the spatial coherence of illumination in an image-forming system. A liquid-crystal spatial phase-modulator is used for controlling it in the object plane. Spatial coherence of the light illuminating an object plays a critical role in determining the intensity distribution of an image or a diffraction pattern. From an experimental point of view, the development of excellent techniques for controlling the spatial coherence of illumination has been searched in the past. In this context, this paper reports on two-dimensional coherence control implemented by use of a liquid-crystal spatial phase- modulator (LCSPM). A scheme illustrating the working principle is shown in Fig. 1. A collimated beam of light from a laser source impinges upon LCSPM. A pair of arrayed transparent electrodes attached to its input and output faces are orthogonally arranged with each other, and the electrical potential across the electrodes are operated by the shift registers SR1 and SR2, respectively. Time-varying binary signals Vx(t) and Vy(t) are applied to SR1 and SR2, so that LCSPM can work with the space- and time-dependent electric signals. The applied electric signals cause a time-varying change in refractive index over LCSPM. As a result, the beam of laser light transmitted by LCSPM is phase-modulated in space and time. Since the output face of LCSPM is imaged over the object plane, the optical wave fields illuminating the object are controllable in spatial coherence. A twist-nematic liquid-crystal cell was available for LCSPM in the experiment. The two-dimensional spatial coherence control was examined with the help of a wave-front folded interferometer (see Fig. 2). This interferometer allows us to superimpose at the location of CCD TV camera the two optical waves that come from paired points