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Random distributed feedback fibre lasers are well known type of fiber lasers where the optical feedback is organized via amplified Rayleigh scattering on random in space sub-micron refractive index inhomogenities1. Random distributed feedback fiber lasers found their applications in telecommunications and distributed sensing systems, as well as attracted considerable amount of interest from researches2. It is well-known that the generation spectrum of random distributed feedback fiber laser is a wide spectrum of typical width of 1 nm. It can be specifically tailored to demonstrate multiwavelength, tunable operation etc. However, the main features of the generation spectrum should be defined by the nature of the feedback itself. Usually the smooth bell-shaped spectrum is attributed to the incoherent nature of the feedback3. It is well known however that the Rayleigh scattering is an elastic scattering and should be resulted in the coherent feedback, which in turn leads to narrow features in the generation spectrum. Recently, narrow modes have been observed in the generation of the random distributed feedback fiber laser by means of scanning Fabry-Perot interferometer4. The spectral width of modes was about tens of picometers and was limited by the spectral resolution of the scanning interferometer. Those modes were attributed to the stimulated Brillouin scattering.
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