COLLISION OF LIGHT-BEAMS WITHIN A NEGATIV E KERR-TYPE MEDIUM

We study the collision of two light beams within a negative Kerr-type medium, and we find that the beams can cross each other only if a prop er combination of the physical nonlinear parameters, such as the inten sity of the beams and the collision angle, are given. Such a phenomeno n may be associated with some inherent instability of the nonlinear me dium, which appears due to the modulation of the collision pattern. We also give a numerical characterization of the modulation instability in terms of the involved physical parameters.