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.