We experimentally and numerically investigate the modulation instability of incoherent optical beams in photorefractive media. Due to material anisotropy the modulation instability occurs successively from formation of stripes at a first threshold to full two dimensional filaments at a second threshold. We study the dependence of these onsets on the coherence properties of the beam. We derive experimental thresholds for modulation instability from contrast measurements and compare experimental and numerical results.

© 2005 Optical Society of America

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