Measurement of fluorescence lifetime in thulium-doped laser active media: minimizing reabsorption effects with the confocal method

Radiation trapping due to reabsorption can significantly prolong the measured upper-state lifetime in optical materials with strong spectral overlap between absorption and emission. This paper presents an analysis of the impact of reabsorption on the measurement of the ³F₄ upper-state lifetime of thulium (Tm³⁺)-doped laser active media. The lifetimes of seven different crystalline matrices – YLF, YVO₄, YAP, YAG, GdVO₄, SrF₂, and LuVO₄ – with varying Tm³⁺ ion concentrations were measured using a confocal method. The intrinsic, reabsorption-free lifetime was obtained by linearly fitting the measured values. These intrinsic lifetimes were found to be 5% to 27% lower than the uncorrected measurements. For example, the Tm:GdVO₄ (4 at.% Tm) sample exhibited a measured fluorescence lifetime of approximately 1.52 ms with the largest pinhole, while the fitted intrinsic lifetime was 1.20 ms. The results underscore the importance of mitigating reabsorption effects, as accurate upper-state lifetime measurements are essential for characterizing laser dynamics and efficiency in active media, which is critical for modeling the performance of high-power thulium-doped lasers and amplifiers.