980-nm highly GeO2-doped fiber Raman laser directly pumped by a 940-nm multimode laser diode

We report the first, to our knowledge, demonstration of a 980-nm highly GeO2-doped fiber Raman laser directly pumped by a multimode laser diode (LD). This laser consists of a 940-nm multimode LD pump, a double-clad highly GeO2-doped fiber, and wavelength-selective dielectric mirrors deposited on fiber end facets. The highly GeO2-doped fiber is self-designed with a 25/65/125-μm (core/inner/outer-cladding) structure and 42 mol.% GeO2 doping concentration: 1) enhancing stimulated Raman scattering and reducing the Raman lasing threshold, 2) optimizing the tailored cladding geometry to increase pump absorption and suppress cascaded Stokes generation. The dielectric fiber mirrors coated by Ta2O5/SiO2 multilayer films play three roles: 1) highly efficient pump injection at ∼940 nm; 2) forming a 980-nm resonant cavity; 3) suppressing ∼1026 nm second-order Raman lasing. This integrated design achieves 10 W continuous-wave output power at 982 nm with 53.4% slope efficiency and good stability (<2.02% fluctuation).