Cross relaxation and energy transfer upconversion processes relevant to the functioning of 2 μm Tm~(3+)-doped silica fibre lasers

摘要

We identify the cross relaxation (CR) and energy transfer upconversion (ETU) processes that are important to the operation of Tm~(3+)-doped silica fibre lasers using the slope efficiency measured from a number of diode-pumped Tm~(3+)-doped silica fibre lasers displayed as a function of Tm~(3+) concentration and Al~(3+):Tm~(3+) concentration ratio. These assessments are correlated with minimum energy mismatch between the initial and final states of each energy transfer process determined from the Stark level assignments relevant to a number of Tm~(3+)-doped crystals. The CR process ~(3)H_(4), ~(3)H_(6)→~(3)F_(4), ~(3)F_(4) is highly probable in silica because of the strong spectral overlap between the ~(3)H_(4)→~(3)F_(4) fluorescence spectrum and ~(3)H_(6)→~(3)F_(4) absorption spectrum. The endothermic phonon-assisted CR process ~(3)H_(4), ~(3)H_(6)→~(3)H_(5), ~(3)F_(4), however, may have a non-negligible contribution to the total cross relaxation. The exothermic phonon-assisted ETU process ~(3)F_(4), ~(3)F_(4)→~(3)H_(5), ~(3)H_(6) may be the major contributor to quenching of the lifetime of the ~(3)F_(4) multiplet particularly when the degree of clustering is large even though the ~(3)F_(4)→~(3)H_(5) excited absorption band is outside the transparency range for silica glass. The endothermic phonon-assisted ETU process ~(3)F_(4), ~(3)F_(4)→~(3)H_(4), ~(3)H_(6) is also shown to have a non-negligible contribution to the quenching of the ~(3)F_(4) multiplet. These results have been combined to produce a Tm~(3+)-doped silica fibre laser that produces a slope efficiency of ～74%; the highest yet reported for a ～2 μm fibre laser.