%'e show how a collection of noninverted two-level systems, under the action of a quasiresonant pump field, can produce an exponential amplification of a probe signal through the mechanism of a dynamical instability with spatial self-bunching in a way very similar to that experienced by a high-gain freeelectron laser. We offer a detailed description of this phenomenon, specify the conditions under which it takes place, and sketch possible experimental parameters for its observation in a gaseous medium. PACS number(s): 42.55. -f, 42.50. -p, 32.80.Pj

We propose a description of the collective atomic-recoil laser based on a set of nonlinear differential equations for an appropriate set of collective variables. The main obvious advantage of this description is that, when applicable, it can describe systems having an arbitrary number of particles. A comparison between the predictions of the collective equations and those of their microscopic counterparts shows a satisfactory level of accuracy, even during the nonlinear stage of the evolution. The linearized version of the collective equations is especially useful for the study of the small-signal gain or absorption spectrum and the analysis of the start-up process.PACS number(s): 42.55.Ah, 32.80.Qk, 42.50.Vk