8 W thulium-doped gallosilicate fiber laser at 2 µm.

New glass compositions and fiber designs are sought to improve thulium fiber laser efficiency, simplify fiber structure, and increase output power/energy. Herein, a gallosilicate fiber laser operating at moderate output power levels is demonstrated for proof-of-concept purposes. The fiber has a core composition of 3.6 wt% Ga2O3 and 2.0 wt% Tm2O3, yielding an NA of 0.18. A simple fiber laser cavity is constructed with a high reflector and a flat fiber cleave. 8 W output at 2030 nm is achieved in a 7 m long fiber, with an uncorrected launched power slope efficiency of 34%. By accounting for uncoupled and residual pump, the estimated slope efficiency with respect to absorbed power is 44%, exceeding the Stokes efficiency and indicating that the gallosilicate supports Tm-Tm cross-relaxation. The work demonstrates that gallosilicate fibers have great potential for efficient, and possibly high-power, 2-µm class fiber lasers.