Nearly single-mode Raman lasing at 954 nm in a graded-index fiber directly pumped by a multimode laser diode

High beam quality and narrow spectrum have been obtained in a Raman fiber laser based on a 1.1 km long graded-index fiber directly pumped by a multimode 915 nm laser diode. The generation of a near-diffraction-limited beam at 954 nm with M2≤1.27 and Δλ=0.42 nm at an output power above 10 W is enabled by a cavity mirror made of a special fiber Bragg grating inscribed by a femtosecond technique in the central part of the graded-index fiber core.