It is shown that counterpropagation of two light beams in a photorefractive medium can be unstable against spontaneous emission of light waves of the same frequency and polarization traveling at small angles. The mechanism of the instability is based on the buildup of a set of spatial reflection gratings. The threshold thickness and the corresponding emission angle are found. Results of the theory are applied to interpretation of existing experimental data.
© 1995 Optical Society of America
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