Single-frequency low-threshold linearly polarized DFB Raman fiber lasers.

Distributed feedback (DFB) fiber lasers have been demonstrated to be excellent narrow-linewidth (kilohertz range) single-frequency laser sources. However, this type of laser is normally limited to rare-earth-doped fibers. Raman gain offers an alternative, operating at any arbitrary wavelength. We demonstrate here linearly polarized, single-frequency, DFB fiber Bragg grating Raman lasers with, to the best of our knowledge, the lowest pump threshold of 350 mW, an output power of up to 50 mW at 1120 nm, an 8.5% slope efficiency, and 300 mW of output power at 1178 nm. In the high-power regime, stimulated Brillouin scattering plays an important role in the laser dynamics. We also report the characterization of the power profile inside the fiber along its axis through a novel side-scatter technique.