Nanoparticle doping method for highly thulium-doped optical fibers for efficient, eye-safe fiber lasers – A fluorescence lifetime study

Thulium-doped fiber lasers (TDFL) emitting around 2 µm receive growing attention. The Tm3+ ions may be pumped around 0.8 µm into the 3H4 level, but the maximum achievable efficiency in theory reaches only 40 %. Much higher efficiencies are achievable in practice thanks to the cross-relaxation (CR) effect, also called „2-for-1“ process. To trigger the CR effect, very high contents of Tm3+ ions are required, which places significant requirements on the material design; high concentrations of Al2O3 are typically needed to prevent concentration quenching in highly-doped fibers. MCVD method combined with nanoparticle doping is one of the most perspective methods for the preparation of highly doped alumino-silicate fibers. In this contribution, a large set of thulium-doped fibers was prepared by various methods. The fibers were analyzed with emphasis on the fluorescence lifetime and laser performance, and the nanoparticle doping method was evaluated in comparison with conventional fabrication methods.