Abstract
We report on the achieving and investigating random Raman lasing based on femtosecond (fs) pulse written randomly spaced scattering points (Rayleigh reflector) and random array of fiber Bragg gratings (FBGs) in graded-index (GRIN) fiber directly pumped by multimode laser, with much better output characteristics in the last case. The fabricated 1D-3D FBG arrays are used as a complex output coupling mirror together with input highly-reflective FBG fabricated by a phase mask technology. The 1-km GRIN fiber (100/140um) line was pumped by 940-nm laser diodes with a total power of up to 180 W and beam quality M<sup>2</sup>~34. Above threshold pump power of ~100W, random lasing of the 1<sup>st</sup>-order Stokes beam was obtained with output power exceeding 25 W at maximum pumping. The beam quality parameter M<sup>2</sup> varies with FBG distribution in the fiber cross-section and its best value amounts to ~2, while the linewidth narrows to 0.1-0.2 nm.
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