Abstract

Any digital optical processing (DOP) system requires basic computational modules such as logic, memory, and shifting. Logic modules which utilize photorefractive crystals have been demonstrated recently.¹ In this paper we report experimental evaluations of the potential of photorefractive crystals for image shifting and memory. An image shifting module can be constructed by employing a photorefractive crystal in a four-wave mixing configuration with plane waves 1 and 4 as the writing beams and the Fourier transform of the image u(x, y) to be shifted as reading beam 2. The direction of output beam 3 is then controlled by the direction of plane wave 4, which is in turn determined by the location of a pixel (x_SLM, y_SLM) on a microcomputer-controlled spatial light modulator. A memory module can be constructed using a photorefractive crystal which behaves as a thick holographic medium. Thus it is possible to store many binary bit planes either by encoding the reference beam differently or by rotating the photorefractive crystal to different angular positions. Experimental evaluations indicate that photorefractive crystals can perform the tasks of various computational modules for DOP. They have the capacity of handling images of 10⁶ pixels and are reasonably priced. However their speed is dependent on the intensities of the optical beams and is generally quite slow at present. We plan to construct a DOP to study system and algorithm issues in anticipation that faster materials will be available in the near future (e.g., organic materials²).

© 1986 Optical Society of America