Diode pumped erbium cascade fiber lasers

Cascading the 4I11/2 -> 4I13/2 transition at 2.8 μm and 4I13/2 -> 4I15/2 transition at 1.6 μm offers a solution to the thermal management of high power Er3+-doped fluoride fiber lasers. We demonstrate an output power of 8.2 W at 2.8 μm from an Er3+-doped fluorozirconate fiber laser with 56 W of launched pump power at 975 nm by cascade laser operation with the 1.6-μm ground-state transition. By careful selection of the Er3+ concentration that prevents significant interaction between Er3+ ions, it is shown that cascade lasing can occur from simple resonator arrangements including Fresnel reflection from the ends of the fiber only. The unsaturated output power suggests that no competing transitions are oscillating which advocates that radiative quenching of the metastable 4I13/2 level provides an elegant way of reducing undesired heat generation. We show that core temperature increases at the tip of the pumped end of the fiber can be an order of magnitude lower than fiber lasers employing single transition oscillation. The potential of further power scaling is demonstrated and the experimental results are verified with the use of a numerical model.