Cross relaxation and energy transfer upconversion processes relevant to the functioning of 2 μm Tm3+-doped silica fibre lasers

We identify the cross relaxation (CR) and energy transfer upconversion (ETU) processes that are important to the operation of Tm3+-doped silica fibre lasers using the slope efficiency measured from a number of diode-pumped Tm3+-doped silica fibre lasers displayed as a function of Tm3+ concentration and Al3+:Tm3+ 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 Tm3+-doped crystals. The CR process H-3(4), H-3(6)-->F-3(4), F-3(4) highly probable in silica because of the strong spectral overlap between the H-3(4)-->F-3(4) fluorescence spectrum and H-3(6)-->F-3(4) absorption spectrum. The endothermic phonon-assisted CR process H-3(4), H-3(6)-->H-3(5), F-3(4), however, may have a non-negligible contribution to the total cross relaxation. The exothermic phonon-assisted ETU process F-3(4), F-3(4)-->H-3(5), H-3(6) may be the major contributor to quenching of the lifetime of the F-3(4) multiplet particularly when the degree of clustering is large even though the F-3(4)-->H-3(5) excited absorption band is outside the transparency range for silica glass. The endothermic phonon-assisted ETU process F-3(4), F-3(4)-->H-3(4), H-3(6) is also shown to have a non-negligible contribution to the quenching of the F-3(4) multiplet. These results have been combined to produce a Tm3+-doped silica fibre laser that produces a slope efficiency of similar to74%; the highest yet reported for a similar to2 mum fibre laser. (C) 2003 Elsevier B.V. All rights reserved.