Co-Lasing at 1.2 μm for Enhanced Visible Emission in Ho³⁺:ZrF₄ Fiber Lasers

In this study, we demonstrate that the slope efficiency for emission at 54X/75X nm in a holmium-doped zirconium fluoride (Ho<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup>:ZrF<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub>) fiber is significantly enhanced by the efficient recycling of the pump excitation to the upper laser level of the visible transition through co-lasing the transition at 1.2 μm. A 0.1 mol.% Ho<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup>:ZrF<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> fiber was able to produce continuous-wave lasers at 54X and 75X nm with slope efficiencies of 68.7% and 27.7% when directly pumped by a blue diode laser at 450 nm in a linear cavity. Although co-lasing at 1.2 μm can only improve the slope efficiency of the visible laser emission at lower pump powers, these results illustrate the potential of diode pumping for the efficient operation of rare-earth (RE) doped fluoride fiber lasers when implemented in conjunction with co-lasing mechanisms. The fundamental principles discussed in this study should be applicable to other glass and crystalline hosts.