Optical data processing systems require 2-D spatial light modulators (SLMs) that are capable of converting an input optical (or electrical) signal in real-time into a transparency suitable for spatial modulation of a collimated laser beam. These devices must operate in real-time and be reusable. A well-documented candidate SLM is the Prom. It uses a bismuth silicon oxide (BSO) crystal that is both photoconductive and electrooptic. In operation, the Prom is illumi nated with white light of wavelength λw (350–450 nm). The spatial intensity distribution of this λw light is converted to a spatial charge pattern in the BSO. When read out with light of wavelength λR (633 nm usually), the amplitude of the readout beam is spatially modulated corresponding to the intensity modulation present on the λw light pattern. In the Prom, modulation of the ΛR light occurs by the lon gitudinal electrooptic effect (the direction of light propagation and the applied electric field are colinear). A [100] cut BSO crystal is used to optimize the corresponding electrooptic tensor coefficients for the longitudinal electrooptic effect. The sensitivity of the Prom is low by comparison with other candidate SLMs. Furthermore its frequency response rapidly decreases with increasing spatial frequency f. The diffraction efficiency is proportional to l/f with low f and to l/f for high f with a 3-dB point at about 4-5 cycles/mm.This performance occurs because the spatial charge layer in duced in the Prom by the λw light lies within the volume of
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