We developed a new holmium-doped triple-clad fiber (Ho-3CF), reducing the clad diameter to improve the overlap between pump and doped-core, and the holmium concentration to reduce the influence of ion clustering on the laser efficiency. We illustrate here the performance of this fiber in a laser oscillator configuration. The laser emission is centered at 2.12 µm by a fiber Bragg grating, and the active fiber is pumped in the clad by a 1.94 µm thulium-doped fiber laser. The slope efficiency reaches 60 %, for a maximum signal power of 62 W delivered on a quasi single spatial mode (M 2 =1.2). To the best of our knowledge, this slope efficiency is the highest reported for a high power clad-pumped fiber laser emitting at a wavelength higher than 2.1 µm. We also analyze experimentally the impact of the pump Numerical Aperture (NA), at the input of the Ho-3CF, on the laser efficiency. Finally, we use our numerical simulation to comment on the best choice of pump wavelength.

We recently developed a holmium-doped triple-clad fiber (Ho-3CF) for laser emission beyond 2.1 μm. In a clad-pumped fiber laser oscillator emitting at 2.12 μm, we obtained an optical efficiency of 73% with respect to the absorbed pump power at 1.94 μm, and a maximum signal power of 62 W. We present here the comparison between the laser measurements and a numerical simulation, together with the measurements of the required physical parameters (crosssections, attenuations…). The alumino-silicate core composition of our initial Ho-3CF samples required the introduction of a pedestal to preserve the single spatial-mode guiding. We also present our preliminary results on a new aluminophospho-silicate core composition, in order to suppress the initial pedestal and simplify the fabrication process. Both samples were also analyzed in core-pumped laser configuration.