Holmium-doped silica fibers combining high doping and high efficiency

We present a 2.1 µm holmium-doped fiber laser, core pumped by a 1.94 µm thulium-doped fiber laser. The laser is based on an in-house drawn, highly holmium-doped silica fiber (3850 mol ppm). We achieved a high output power up to 22.5 W and at the same time high slope efficiency of 81% with respect to absorbed pump power with an efficiency optimized resonator. We later optimized the resonator for output power and achieved a holmium core-pumped record output power of 35.4 W with 79% slope efficiency. We simulate a pedestal-cladding-pumped fiber with the same core composition, which can be used to further scale-up the output power, while maintaining high efficiency.