Abstract

Pattern formation in nonlinear systems is a very active subject in many areas of physics. For example, in hydrodynamics the arrangement of the convective cells of the Rayleigh-Bernard instability^[1] constitutes a model problem for pattern formation. In optics, important advances have been recently obtained in the case of lasers.^[2] In contrast, transverse pattern formation in mirrorless optical passive systems^[3] is not so wellknown because of the intrinsic complexity of a system without cavity and of the fact that the first experiments were done with pulsed lasers.^[4] We present here results obtained with c.w. lasers and rubidium atoms. In particular, we show some interesting relationships with the hydrodynamics observations.

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