Influence of Quantum Efficiency and Core Propagation Loss on the Performance of Cladding-Pumped Thulium-Doped Fibre Lasers

Thulium (Tm)-doped silica fibre lasers continue to attract much interest as a wavelength flexible and power-scalable source in the two-micron band, benefiting a diverse range of applications from laser surgery to materials processing [1]. One of the key power scaling attractions of Tm-doped silica is the ‘two-for-one’ cross-relaxation process ( ³ H 4 + ³ H 6 → ³ F 4 + ³ F 4 ), which can yield a quantum efficiency approaching ‘2’ when doped with sufficiently high Tm concentrations (>3 wt.%) and pumped at ∼793 nm. This opens up the prospect of optical-to-optical efficiencies of >80%. Slope efficiencies of ∼70% and higher have been realised in low to moderate power <100 W) devices [2], [3], but scaling to the kilowatt regime has come at the expense of lower efficiency. One of the key challenges in designing Tm fibres for operation at high power levels is maintaining high Tm-doping to promote cross-relaxation whilst avoiding the detrimental impact of high thermal loading. This requires more complex core designs to spread heat loading over a longer fibre length, bringing into play the influence of core propagation loss as well as cross-relaxation on overall efficiency. Knowledge of both is essential to understand their relative impact on performance, and for further optimising fibre design; however, direct measurement has not so far been possible.