We present a study of the modulation instability of low-amplitude broad optical beams in biased photorefractive crystals under steady-state conditions. The modulation instability growth rate can be obtained provided that E0 is positive quantity, where E0 is the field constant correlated with terms in the space-charge field .Our analysis indicates that the growth rates depend on E0 and the ratio of the optical beam's intensity to that of the dark irradiance, and the maximum modulation instability gain as well as the corresponding spatial-frequency increase with increasing E0 and the ratio of the optical beam's intensity to that of the dark irradiance. Relevant examples are provided where the photorefractive crystal is assumed to be of strontium barium niobate type.
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