Laser emission under resonant pump in the emitting level of highly doped Nd materials

Abstract

Summary form only given. Heating effects in laser active media under pumping constitute a major limitation in the construction of highly efficient or high-power solid state lasers. These effects could induce optical distortion of the resonator or even the physical destruction of the active component. In the case of neodymium-doped crystals two major sources exist: the quantum defect between the pump and laser radiation and the non-radiative processes (multiphonon relaxation, down-conversion cross-relaxation on intermediate levels, upconversion from the emitting level /sup 4/F/sub 3/2/). While the non-radiative processes constitute a physically inherent loss, the quantum defect can be controlled by the pump and emission wavelengths. In an actual diode pumped Nd laser pumping is accomplished to the /sup 4/F/sub 5/2/ level (808.7 nm) and the excitation relaxes by electron-phonon interaction to the components R/sub 1/ and R/sub 2/ of the emitting level, placed at /spl sim/945 and 860 cm/sup -1/ below the pump level. In the absence of laser emission about 39% from the absorbed pump radiation in /sup 4/F/sub 5/2/ is transformed into heat in a 1.04 at.% Nd:YAG crystal, in accordance with theoretical modeling that accounts for the non-radiative effects on the quantum efficiency. In the case of efficient laser emission, when the effect of the non-radiative processes is very small and the heat generation is dominated by the quantum defect, the fractional thermal load in this sample is reduced up to about 0.25. However, this figure is much larger than the value of about 0.09 in the case of Yb lasers, where pumping is performed to an upper crystal field component of the emitting level.