REAL-TIME INTENSITY INVERSION USING FOUR-WAVE MIXING IN PHOTOREFRACTIVE CRYSTALS (NONLINEAR OPTICS)

In image processing applications for which speed is an important factor, optical systems can provide an attractive alternative to digital systems. However, one operation that has traditionally been very difficult to achieve optically is division. Since multiplication can be easily obtained by placing a transparency in an object beam path, an optical system is needed in which the output beam intensity is related inversely to the input object beam intensity. The purpose of the present work has been to examine the feasibility of using a photorefractive crystal in a four-wave mixing configuration to perform real-time intensity inversion of an input object beam. Photorefractive crystals exhibit a reversible change of refractive index in response to light and thus can be used as real-time holographic media. In a four-wave mixing system, two writing beams form a phase hologram within the crystal. A third beam, the probe or read-out beam, diffracts off the phase grating to produce the output beam. When the mechanics of photorefraction are examined, under approximations appropriate to the study of inversion, the resulting expression for diffraction efficiency of the hologram predicts the occurrence of inversion for certain conditions of writing beam intensities and values of applied electric field. These dependences are confirmed by experimental data obtained using a bismuth germanium oxide (BGO) crystal. The experimental set-up used in this work produced inversion over two orders of magnitude in intensity with a resolution of better than 100 lines/mm, mainly limited by imaging lenses. Using input intensities of 1 - 50 mW/cm('28), the time constant ranged from approximately 10 - 500 msec. Results of using a four-wave mixing system to perform real-time intensity inversion of gray-scale images are shown. An evaluation of the inversion process is given which describes the effects of crystal and experimental parameters on such measures of system performance as dynamic range, speed, resolution, and noise. The application of inversion to problems in image processing is considered; in particular, a bismuth silicon oxide (BSO) crystal is used to enhance defects in a periodic mask in real-time.