PACS numbers: A recent demonstration of lasing in ultralong Raman fiber lasers has attracted the attention to their specific features. In this paper, we analyze modifications of the mode structure, shape, and width of the generated spectrum, fluctuations of the intensity and phase upon the lengthening of such a laser, and increasing its power. The mode structure is shown to be resolvable in an RF spectra up to 84-km long (with similar to 1 kHz spacing), and the RF peak width appears proportional to the intracavity power. At the same time, the optical spectra broaden with the power according to the square-root law and take the specific shape with exponential wings. The model based on the wave kinetic equations has been developed that describes the observed effects. It is shown that ultralong Raman lasers exhibit a new laser regime through the generation of multiple (up to similar to 10(8)) cavity modes interacting via FWM processes that lead to stochastic (turbulent) behavior of their phases and amplitudes.
Babin, S. A., & Podivilov, E. V. (2008). New physical effects in ultralong Raman fiber lasers. Laser Physics, 18(2), 122–128. https://doi.org/10.1007/s11490-008-2005-y