The effects of energy transfer upconversion on the performance of Tm/sup 3+/, Ho/sup 3+/-doped silica fiber lasers

We report the performance characteristics of a diode-pumped Tm³⁺, Ho³⁺-doped silica fiber laser as a function of the dopant concentrations. We show, albeit indirectly, that the rates of energy transfer upconversion (ETU) are significantly increased when the Tm³⁺ concentration and hence rates of cross relaxation (CR) are increased. The net result of ETU is a reduction in the ³ F₄ population of Tm³⁺ and heating of the fiber as a result of the large rates of multiphonon emission from the higher lying energy levels of Ho³⁺. Fibers with low Tm³⁺ concentrations display the highest slope efficiencies and lowest thresholds, with 95% of the Stokes efficiency limit being achieved. CR, therefore, cannot be completely harnessed as a mechanism to enhance the rate of upper laser level excitation in Tm³⁺, Ho³⁺-doped silica fiber lasers