Diode Pumped Erbium Cascade Fiber Lasers

Cascading the ⁴I_11/2→⁴I_13/2 transition at 2.8 μm and ⁴I_13/2→⁴I_15/2 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 ⁴I_13/2 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.