Transition from narrowband generation to turbulent multimode spectrum in a random fiber laser

It is known, that well above the generation threshold random fiber laser radiation is characterized with wide spectrum, representing continuous set of spectral components. At the same time, just above the threshold generation possesses a higher degree of coherency, consisting of a discrete number of localized modes, each with a certain lifetime of about 1 ms. We study – both experimentally and by means of numerical simulation – in detail the statistical manifestation of the transition process between these two states. We demonstrate the clear analogy between exponential grown of energy in the spectral mode of a single-mode laser and increase of spectral modes number in the random fiber laser, and emphasize the role of nonlinear interactions in this process.