Propagation of a spatially-modulated incoherent beam in a photorefractive medium

Summary form only given. Although modulation instability (MI) of a coherent wave propagating through a nonlinear medium is well known, modulation instability of an incoherent wave was brought into attention only recently, soon followed by a series of experimental demonstrations unique to incoherent waves, including incoherent anti-dark solitons, incoherent pattern formation, and soliton clustering on weakly-correlated wavefronts. Here we provide direct experimental evidence that modulation instability of a partially spatially incoherent optical beam driven by saturable photorefractive nonlinearity and seeding spatial noise depends strongly on the perturbation wavelengths, as well as on the strength of the nonlinearity and the degree of spatial coherence. After a threshold value of the nonlinearity, patterns and soliton pixels arising from induced incoherent MI are observed at some preferred spatial frequencies of perturbation.