EXCITED-STATE-ABSORPTION AND UP-CONVERSION STUDIES OF ND3+-DOPED-SINGLE CRYSTALS Y3AL5O12, YLIF4, AND LAMGAL11O19

In this paper we derive expressions for the laser slope efficiency and the threshold absorbed pump power which explicitly include excited-state absorption in the pump as well as in the laser-emission-wavelength domains. They are applied to the case of the diode-pumped Nd3+-doped laser systems and, along with expressions allowing the calculation of excited-state-absorption (ESA) cross sections within the framework of the Judd-Ofelt theory, they provide a theoretical framework for the second part in which we present both experimental measurements and theoretical calibrations and an analysis of excited-state-absorption spectra in the stimulated emission as well as in the pumping domains of Nd3+-doped Y3Al5O12, YLiF4, and LaMgAl11O19 laser crystals. The results indicate that ESA in the infrared metastable state of Nd3+ in these materials is negligible at the main laser wavelengths but can be more significant if it takes place, following some energy-transfer induced-excitation process, in higher-lying excited states. The analysis of these ESA within the Judd-Ofelt formalism shows a general agreement better than 50% between the experimental data and the theoretical predictions. This is used to estimate the influence of ESA in the pumping domain on the laser performance of the materials. It is found that it contributes but that it does not significantly perturb this laser performance, and that this effect is much less than some upconversion energy transfers that limit the fluorescence excitation efficiencies and reduce the energy-storage capacities by the reduction of the effective fluorescence lifetimes of the laser metastable level. © 1995 The American Physical Society.