Abstract

In this work, a novel mid-infrared continuous wave laser, based on a fluoroindate fiber co-doped with holmium and neodymium, is designed to emit at ${\lambda }_s = \ 3.92\ \mu {\rm m}$ , when pumped at ${\lambda }_p = \ 808\ {\rm nm}$ . The laser is modeled considering a nine-level system, by taking into account experimental spectroscopical parameters. Since the energy transfer coefficients are unknown, they have been evaluated starting from the measured emission spectra of the bulk glass, reported in literature, and comparing their ratio with respect to the ratio between the simulated signal gain coefficients. The designed laser promises higher slope efficiency and power threshold lower than those obtainable with a holmium-heavily-doped fiber, having same fiber section geometry, same refractive indices and pumped at ${\lambda }_p = \ 888\ {\rm nm}$ . Slope efficiency $\eta \ = \ 16.67{\boldsymbol{\% }}$ and input power threshold ${P}_{th} = \ 0.2\ W$ are obtained for the fiber length ${L}_{fiber} = \ 0.4\ {\rm m}$ , dopants concentrations ${N}_{Ho} = \ 8 \times {10}^{26}\ {\rm ions}/{\rm m}^3$ and ${N}_{Nd} = \ 1 \times {10}^{26}\ {\rm ions}/{\rm m}^3$ , and output mirror reflectivity ${R}_{\rm out} = \ 60{\boldsymbol{\% }}$ . This result encourages the fabrication of a continuous wave laser based on a Ho:Nd-codoped fluoroindate fiber.

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