Interactions of incoherent localized beams in a photorefractive medium

We investigate numerically interactions between two bright or dark incoherent localized beams in an strontium barium niobate photorefractive crystal in one dimension, using the coherent density method. For the case of bright beams, if the interacting beams are in-phase, they attract each other during propagation and form bound breathers; if out-of-phase, the beams repel each other and fly away. The bright incoherent beams do not radiate much and form long-lived well-defined breathers or quasi-stable solitons. If the phase difference is $\pi/2$, the interacting beams may both attract or repel each other, depending on the interval between the two beams, the beam widths, and the degree of coherence. For the case of dark incoherent beams, in addition to the above the interactions also depend on the symmetry of the incident beams. As already known, an even-symmetric incident beam tends to split into a doublet, whereas an odd-symmetric incident beam tends to split into a triplet. When launched in pairs, the dark beams display dynamics consistent with such a picture and in general obey soliton-like conservation laws, so that the collisions are mostly elastic, leading to little energy and momentum exchange. But they also radiate and breathe while propagating. In all the cases, the smaller the interval between the two interacting beams, the stronger the mutual interaction. On the other hand, the larger the degree of incoherence, the weaker the interaction.