This paper presents the experimental results of constructing high-power high-efficiency Raman fiber lasers (RFLs) using phosphosilicate fibers as the gain medium and fiber Bragg gratings (FBGs) as the laser cavity. It is found that the fiber length is more critical than reflection levels of the FBGs in determining the laser performance. The experimental results reveal that high-power lasers can only be realized with fiber lengths shorter than 500 m in fiber containing 13 mol % P2O5. Longer fiber can result in a reduction of power in the first-order Stokes line due to stimulated scattering of other Stokes lines. More than 10 W of output power has been achieved at 1248 nm from lasers with fiber lengths of 200-500 m, pumped with a 20-W Yb double-clad fiber laser at 1070.75 nm. The maximum slope efficiency obtained is 84.2% in a 200-m RFL, which is close to the quantum limit of 85.8% for the wavelength conversion from 1070.75 to 1248 nm.
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