High efficiency holmium-doped triple-clad fiber laser at 2120 nm

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 $\mu$m by a fiber Bragg grating, and the active fiber is pumped in the clad by a 1.94 $\mu$m thulium-doped fiber laser. The slope efficiency reaches 73% with respect to absorbed power (60% considering coupled power), 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 $\mu$m. 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.