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

Abstract

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 3H 4, 3H 6 → 3F 4, 3F 4 is highly probable in silica because of the strong spectral overlap between the 3H 4 → 3F 4 fluorescence spectrum and 3H 6 → 3F 4 absorption spectrum. The endothermic phonon-assisted CR process 3H 4, 3H 6 → 3H 5, 3F 4, however, may have a non-negligible contribution to the total cross relaxation. The exothermic phonon-assisted ETU process 3F 4, 3F 4 → 3H 5, 3H 6 may be the major contributor to quenching of the lifetime of the 3F 4 multiplet particularly when the degree of clustering is large even though the 3F 4 → 3H 5 excited absorption band is outside the transparency range for silica glass. The endothermic phonon-assisted ETU process 3F 4, 3F 4 → 3H 4, 3H 6 is also shown to have a non-negligible contribution to the quenching of the 3F 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.