Random Lasing in Multicore and Multimode Fibers

Random lasers utilize multiple scattering in disordered amplifying medium which provides necessary feedback even in the absence of resonant cavity. 1D random lasing via Rayleigh backscattering in single-mode fibers (SMF) with pump-induced Raman gain enabled generation of stable high-quality narrow-line output beam, which can even outperform traditional Raman fiber lasers (RFLs) with cavity made of fiber Bragg grating (FBG) pairs. Use of multicore or multimode fibers complicates transverse mode structure of random fiber lasers making them closer to traditional bulk-media random lasers, but they offer new opportunities. Here we review our latest results on investigation of random lasing in multicore and multimode fibers. First, random lasing has been explored in a PM twin-core fiber (TCF) with half-open cavity formed by Rayleigh backscattering distributed along TCF and lumped reflector at one fiber end: either a broad-band fiber-loop mirror (FLM) or femtosecond-pulse inscribed in-core FBGs (in one core or in both cores). The inscription of two FBGs in different (but coupled) cores leads to the formation of a Michelson-type interferometer resulting in the multi-wavelength generation with single peak width of < 0.02\ \mathrm{nm}. Second, direct pumping of multimode graded-index (GRIN) fibers by high-power LDs at 915 nm provides Raman lasing at Stokes line near 950 nm selected by multimode fs-inscribed FBGs, whereas 2-order random lasing in half-open cavity with one fs FBG appears possible in a broad wavelength range (978 - 996 nm). At that, the cascaded Raman conversion is accompanied by sufficient improvement of the output beam quality in comparison with that for multimode LDs. As a result, high-efficiency generation of nearly diffraction limited beam in the all-fiber LD-pumped GRIN RFL has been demonstrated.