Upconversion laser processes

Abstract

Pump processes that produce upconversion laser emission in trivalent rare earth ion-doped gain media are described, and the performance of upconversion lasers pumped by these processes is reviewed. Sequential two-photon absorption upconversion, cooperative energy transfer upconversion and photon avalanche upconversion are discussed in relation to the production of visible laser emission. Using Er:YALO as an example, the specific energy flow pathways for each of these pump processes are described in detail. Experimental results are presented for laser emission in Er:YALO pumped by sequential two-photon absorption, cooperative energy transfer and photon avalanche upconversion. The spectral and temporal dependence of upconversion emission is reviewed and experimental evidence for photon avalanche upconversion in Er:YALO is presented. The laser oscillation conditions including gain and loss are related to upconversion lasers using a three-level ion kinetics model. Rate equations are presented that illustrate the impact of various competing pump processes on the upconversion laser output. Upconversion laser emission under Q-switched operation is discussed and equations for optimum output coupling are presented. The design considerations for efficient upconversion laser resonators and pump optics are addressed. Upconversion laser results obtained over the past three decades are reviewed. The laser performance obtained for crystals doped with trivalent Pr, Nd, Er and Tm ions is discussed. Sensitized upconversion using Yb ion co-doping is described and results obtained for Yb-Er and Yb-Ho upconversion lasers are summarized. Upconversion fiber laser development is also reviewed and details are presented for room temperature upconversion laser emission using ZBLAN fibers doped with trivalent Pr, Nd, Ho, Er and Tm ions. The effect of upconversion processes on the efficiency and operation of conventional lasers is discussed. Upconversion pumping involving the upper laser level is shown to increase the laser threshold power and lower the optical conversion efficiency. Upconversion pumping involving the lower laser level of a self-terminating transition lowers the terminal level lifetime. The conditions are described under which upconversion pumping can enable cw laser operation on a self-terminating transition.