Abstract

Avalanche excitation of thulium in YLF using laser light at 1.04 microns produces blue fluorescence and even stronger emissions of ultraviolet. The pump frequency is resonant only with a transition (³F₄ → ³F₂) from an excited state, not the ground state. If the pump intensity exceeds a certain threshold value, sustaining an absorber population becomes possible through crossrelaxation feedback from an upper state. We have previously reported¹ on an avalanche model for Tm ions based on a presumed known two-ion cross-relaxation process (³H₄, ³H₆ → ³F₄,³F₄) and the assumption that Tm excitation is quasi-homogeneous in the pumped volume. Although the model correctly predicts important qualitative features of the avalanche, quantitative predictions are not in good agreement with experimental data. In this paper, we will report on several new directions taken in our theoretical modeling. Most successful is the idea of small clusters of Tm ions distinguishable in optical response from a more uniform background. The clusters account for both avalanche absorption of pump energy and upconversion.

© 1992 Optical Society of America