Efficiency dependence on the Tm³⁺and Al³⁺concentrations for Tm³⁺-doped silica double-clad fiber lasers

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 ³H₄→ ³H₆laser 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, ³F₄, ³H₆→ ³H₄, ³H₄, has a significant effect on the efficiency of the laser despite the relatively short lifetime of the ³F₄energy level. Energy migration of the excitation at the ³F₄level through the process ³F₄, ³H₆→ ³H₆, ³F₄may 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 ³H₄lifetime, or both.