The relaxation of low-lying excited states of Tm ³⁺ ions doped in YAG, YAlO 3 , and Y 2 O 3 due to photon and phonon emission is studied theoretically. Stimulated emission cross sections (integrated over frequency), fluorescence lifetimes, and radiative quantum efficiencies are calculated and their implications for laser operation on the 2.3-μm ³ F 4 → ³ H 5 line of Tm ³⁺ are discussed. The calculations, based on a few phenomenological parameters which have been determined by others, are easily generalizable to other host materials and other rare-earth (RE) ions. Room-temperature pulsed laser emission from Tm ³⁺ ions near 2.3 μm was observed on one line in Tm:Cr:YAG, and on four lines in Tm:Cr:YAlO 3 . Lower oscillation thresholds were generally obtained in the YAlO3 rods, consistent with the theory presented. A threshold of 31 J was obtained with a Tm:Cr:YAlO 3 rod at 2.274 μm. In the free-running pulsed mode, peak power levels up to several hundred watts and total output energies up to 12 mJ/pulse were observed. Other general, observed operating characteristics are discussed.

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