Bulk Crystal Upconversion Lasers

Recent advances in high-power, high-efficiency semiconductor laser diodes and diode arrays has revived interest in the identification and development of practical upconversion lasers (for which the output laser wavelength is shorter than the wavelength(s) of the optical pump(s)). It is hoped that upconversion lasers will compete favorably with alternative approaches, such as wavelength shifting of laser diode radiation using nonlinear processes, to perform commercial applications, such as the production of full color images using laser-based tri-stimulus sources. For upconversion laser action to occur, the energy carried by two or more pump photons must be accumulated in the upper laser level of the gain medium. In most general terms, this accumulation of pump energy occurs by the successive absorption of pump photons via intra-ion radiative transitions, by the summation of absorbed pump photon energy via ion-ion interactions, or by combinations of both. To compete favorably with nonlinear wavelength shifting devices, practical upconversion lasers will have to operate at room temperature, and with conversion efficiencies of pump energy to laser output that match or exceed those of nonlinear conversion-based devices.