Abstract

Photon avalanche pumping has been shown to be a practical way to achieve upconversion lasing.¹,² In this scheme, a single pump frequency, resonant only with a transition from an excited state, is used. Good pumping efficiency is possible because a cross-relaxation from an upper state multiplicatively repopulates the absorbing level. The specific steps involved are not always known with certainty, making it difficult to predict the degree of excitation achieved. We have studied avalanche upconversion in a variety of Tm-doped crystals in which the cross-relaxation process (³H₄, ³H₆ → ³F₄, ³F₄) is presumed known. Tm-doped Y₂O₃, YAG, and YLF crystals pumped at 1.03 μm (³F₄ → ³F₂) all show emission at IR and visible wavelengths. A model specific to the Tm:YAG system, consisting of six coupled, non-linear differential equations, has been developed. It predicts that upconversion will occur for pumping intensity in excess of a threshold, in agreement with experiments. If the aforementioned cross-relaxation goes in the forward direction only, as is expected at low temperatures, it further predicts that the ground-state population will be substantially depleted at pump intensities only slightly above threshold.

© 1991 Optical Society of America