In this paper, we present a 10 kW-level output power fiber amplifier based on tandem-pumping. The characteristics of active fiber for amplifier stage is reported. To ensure adequate absorption of tandem pumping light in relatively short fiber length, a large-scale Yb3+ doped aluminophosphosilicate (Al2O3-P2O5-SiO2, APS) fiber with 50 mu m core and 400 mu m inner-clad in diameter, i.e., a 50/400 Yb-APS fiber, was theoretically designed and experimentally fabricated by using modified chemical vapor deposition system combined with chelate precursor doping technique. The measurements showed that the dopant concentration of Yb2O3, Al2O3 , and P2O5 was estimated to be 0.17 mol, 1.4 mol, and 1.5 mol, respectively, and the absorption coefficient of the fiber was about 0.77 dB/m at 1018 nm. The amplifier stage with this 40 m Yb-APS fiber tandem-pumped by 1018 nm fiber laser, delivered 9.82 kW laser output at 1080.08 nm with an optical-to optical efficiency of 86.8. The knowledge of the characteristics of the Yb3+ doped APS fiber can usefully guide future design of high output power fiber amplifier based on tandem pumping.
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