25 September 1989 Diode Pumped Holmium, Thulium And Erbium Lasers Between 2 And 3μm Operating CW At Room Temperature
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Diode pumped CW lasers operating between 2 and 3μm in the heavy rare earth activator ions are reviewed. In Ho,Tm:YAG we demonstrated high efficiency using Tm as the sensitizer ion which absorbed the pump radiation. This is followed by a cross relaxation process which allows nearly two excited Tm ions to be produced from one absorbed photon. There is rapid energy migration among the Tm ions followed by energy transfer to the Ho ion. The 2μm laser action is to a level 460 cm-1 above the ground state. In Tm,Ho:YLF we demonstrated CW cascade laser emission at 2.31 and 2.08μm. Above threshold for both transitions, two infrared photons are produced for each absorbed pump photon. The theoretical slope efficiency of this system is 72.3% for pumping at 0.791μm. In Er:YLF CW laser emission at 2.8μm with a 10% slope efficiency was demonstrated. The laser transition is normally self-terminating, but an upconversion process which depopulates the lower laser level and populates the upper laser level allows this transition to operate in a CW mode.
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L. Esterowitz "Diode Pumped Holmium, Thulium And Erbium Lasers Between 2 And 3μm Operating CW At Room Temperature", Proc. SPIE 1104, Growth, Characterization, and Applications of Laser Host and Nonlinear Crystals, (25 September 1989); doi: 10.1117/12.960597; https://doi.org/10.1117/12.960597

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