Ultra-long raman laser with a feedback based on the Rayleigh scattering

Abstract

Rayleigh scattering (RS) is a fundamental optical effect that determines losses in optical fibres induced by micro-inhomogeneities of refractive index. Though the effect of Rayleigh back scattering in conventional fibre lasers with high-Q cavity is weak, it might be important in very long fibre cavities. Recently, lasing in a 165-km long fibre span has been obtained utilizing distributed Raman gain and fibre Bragg gratings (FBGs) as cavity reflectors [1]. It was noted that the RS effects remain observable in such ultra-long Raman fibre laser (URFL), though the operating Raman laser wavelength (∼1.55 µm) was chosen to minimize fibre losses and to reduce the amount of the backscattered radiation, accordingly. In this paper we report on the detailed study of the RS effects in URFLs. It has been found that in a long fibre cavity (∼100 km) the distributed feedback due to Rayleigh back scattering at propagation of light between FBG reflectors may be comparable with the lumped feedback provided by the FBG itself. As a result, Raman lasing in the fibre span limited by lumped (FBG) reflector at one side only appears possible due to significant reflection from the RS-based “random” distributed mirror at the other side.