Wavelength tuning of LD pumped multimode fiber Raman laser with thin-film output mirror

Multimode (MM) fibers have recently attracted much interest for generating high-power laser radiation with a controlled transverse mode composition. Here, we demonstrate a Raman laser built on a 1 km long 100/140 µ m MM graded-index fiber with a resonator formed by an in-fiber highly-reflective fiber Bragg grating (FBG) and a dielectric thin-film mirror micro-fabricated on the central region of the fiber output end face. Due to selective reflection of low-order modes by the mirror, the generated Stokes beam quality ( M 2 ∼ 2.5) is greatly improved against the highly-MM ( M 2 ∼ 34) 936 nm diode pump. Due to broadband reflection of the mirror, stretching of the narrowband FBG enables tuning of laser wavelength from 972.7 to 979.1 nm (6.4 nm range) with output power of ∼6 W at the maximum Raman gain near 976 nm. The laser beam quality and spectrum shape are only slightly varying at the wavelength tuning, whereas the laser power decreases at the edges of the tuning range. Further tuning range and power scaling as well as potential applications of the proposed tunable fiber laser are discussed.