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 similar to2-mum H-3(4) --> H-3(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, F-3(4), H-3(6) --> H-3(4), H-3(4), has a significant effect on the efficiency of the laser despite the relatively short lifetime of the F-3(4) energy level. Energy migration of the excitation at the F-3(4) level through the process F-3(4), H-3(6) --> H-3(6), F-3(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 H-3(4) lifetime, or both. (C) 2003 Optical Society of America.