Abstract

Tm doped fluoride crystals sensitized with Yb ions were early recognized as very efficient upconversion laser materials for visible room temperature operation under infrared excitation. Recently, the advantage of ytterbium as codopant was pointed out to enhance the 1.5µm emission ³H₄→³F₄ and to avoid the bottlenecking effect on this laser transition^[1]. In order to determine several energy-transfer parameters involved in the excitation processes of Yb-Tm doped laser materials, samples of Tm:Yb:LiYF₄ and Tm:Yb:KY₃F₁₀ with different dopant concentrations were grown by Czochralski technique, oriented and cut. Energy transfer processes and upconversion effects in Tm:Yb codoped fluoride crystals were quantitatively studied with classical energy transfer models (direct dipole-dipole interaction - Förster-Dexter model extended by Inokuti-Hirayama and migration assisted energy transfer - Yokota-Tanimoto or Burshtein models), using absorption, emission and excited-state absorption cross sections experimental data as input parameters. The results were systematically compared, in the case of fast energy migration, with those deduced from the adjustement between the rate equation model and the measured kinetics of the excited state levels of the Yb (²F_5/2) and Tm (³F₄,³H₄, ¹G₄ and ¹D₂) ions. Both decay times after short pulse excitation and rise times after square-pulse shape excitation on the Yb ions were investigated to obtain a complete description of the excitation scheme for the Tm upper energy levels.

© 1998 IEEE