Efficiency dependence on the Tm^3+ and Al^3+ concentrations for Tm^3+-doped silica double-clad fiber lasers

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Abstract
We present measurements of the slope efficiency and the pump power at threshold for a number of Tm3+-doped silica double-clad fiber lasers that incorporate fibers that have a range of Tm3+ concentrations. We obtain a slope efficiency for the ∼2-μm 3 H 4 → 3 H 6 laser transition that is greater than the Stokes efficiency limit for a Tm3+ concentration as low as 1.3 wt. %. These results indicate that the cross relaxation process, 3 F 4, 3 H 6 → 3 H 4, 3 H 4, has a significant effect on the efficiency of the laser despite the relatively short lifetime of the 3 F 4 energy level. Energy migration of the excitation at the 3 F 4 level through the process 3 F 4, 3 H 6 → 3 H 6, 3 F 4 may be enhancing the cross-relaxation mechanism. We also show the importance of reducing the level of clustering of the Tm3+ ion when it is doped into silica by use of appropriate amounts of Al3+ codopant. For Tm3+ concentrations of >1 wt. %, Al3+/Tm3+ concentration ratios of >10 are recommended for reducing scattering losses, quenching the 3 H 4 lifetime, or both.