V.G. Medvedev, G.G. Telegin

Investigations of the non-stationary generation in lasers with a nonlinear absorbing gas in the resonator are carried out. The effective amplification on radiation intensity in the laser resonator has non monotonic character. Depending on a sign of the control parameter, in the laser either soft or rigid modes of the electromagnetic field excitation can be observed. In this work the soft mode of the laser operation under fluctuations action is considered. In this mode the generation dynamics to two much differing, from each other by time scales is characterized: in an initial time interval there is a slow increase of coherent radiation as a result of linear amplification, and in the subsequent time interval radiation intensity growth occurs in a nonlinear mode more quickly so the transition process gets sharp "step" character. The statistical analysis of the transient process by numerical methods on the basis of the Fokker-Planck equation is carried out. It is shown, that at small positive values of the operating parameter the initial unimodular probability density of a variable during evolution on some time interval gets bimodal kind. Thus the first probability distribution density maxima remains near to an initial condition of generation, and the second maximum is formed near to a condition of stationary generation. In a finishing stage of process of generation the first peak of distribution of density of probability disappears also it again gets a unimodular kind. The similar phenomenon was find by Prigozhin and Nicolis in chemical reactions kinetics of burning and is named "transitive bimodality"

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