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

Abstract

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&#x00A0;m by a fiber Bragg grating, and the active fiber is pumped in the clad by a 1.94&#x00A0;m thulium-doped fiber laser. The slope efficiency reaches 73&#x00A0;&#x0025; with respect to absorbed power (60&#x0025; considering coupled power), for a maximum signal power of 62&#x00A0;W delivered on a quasi single spatial mode (M<inline-formula><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula>=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&#x00A0;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.