Optimization of random FBG array in multimode graded-index fiber for Raman lasing with improved spatio-spectral characteristics

We report on the optimization of femtosecond-pulse inscribed arrays of short randomly spaced FBGs for Raman lasing in multimode GRIN fiber pumped by highly multimode (M2 ~34) 940-nm diodes. The fabricated 1D-3D FBG arrays used as a complex output mirror together with input highly reflective FBG provide random lasing of the Stokes beam at 976 nm with output power around 30 W. The optimization of array structure results in beam quality improvement to M2 ⪅2 and the linewidth narrowing to 0.1 nm to 0.2 nm that is better than that for long regular FBG of comparable reflection. Potential of such cavity structure for further parameter improvement and its practical applications are discussed.