Continuous-Wave, Pair-Pumped Laser

Abstract

We report room-temperature operation of what we believe is the first continuous-wave laser that relies exclusively on cooperative upconversion by coupled ion pairs to achieve population inversion.Conventional excitation of laser media raises individual atoms or ions directly to energy levels above the emitting level.' However, as we show here, energy states lying well above those populated by incident light can be reached by spontaneous, cooperative upconversion processes at rates sufficient to sustain stable, steady-state population inversions. Here we demonstrate cw laser action from upconverted states without significant contributions from multiphoton absorption. This novel operating principle holds promise as a means of pumping short-wavelength solid-state lasers without the need for radiation sources of still shorter wavelength, without the need for multiphoton absorption coincidences, and without the usual problems of color-center formation and short extinction length for pump light near the band edge of a solid.While the cooperative upconversion pair process was previously found to account for modified kinetics in erbium lasers, 2 it has not been shown to be capable on its own of sustaining continuous inversion, as we show here. A variety of pulsed 3 and cw upconversion 4 ' 5 lasers pumped by sequential multiphoton absorption processes at low temperatures have been reported. Also, earlier studies of upconversion lasers included those of pulsed operation of pair-pumped solid-state erbium lasers by Pollack et al. 6 and Rand et al. 7 In addition, conventional excitation was used to achieve cw operation on the 2 . 8 -gm transition of erbium in CaF 2 (Ref. 6) and other hosts. 8 Here, however, steady-state inversion of the upper laser level is achieved exclusively by upconversion due to spontaneous Er-Er interactions in the excited state. We also discuss why earlier studies 2 ' 6 ' 7 or traditional laser analysis could not have predicted that true cw pairpumped laser operation is even possible.The energy-level diagram of the Er 3 + ion is shown in Fig. 1. The crosses on level 4I13/2 represent erbium ions initially prepared in this level by resonant absorption of light from a cw NaCl color-center laser operating at 1.51 gm. Pair interactions of near-neighbor ions can induce cooperative upconversion as shown in the figure, and similar As a result, even when the lower laser level is pumped directly, as in the study reported here, the lower-state lifetime decreases with increasing occupation, and it becomes possible to achieve steady-state inversion from upconverted states without the assistance of multiphoton excitation. This surprising result is in agreement with an exact analysis 12 in which the comparative rates of two-photon and two-atom pumping of the high-lying 4I9/2 state may be varied. In our model each manifold is represented as a single level to furnish a simple but realistic picture of the basic dynamics. Within the rate-equation approximation, diagonal density matrix elements for the nonlinear four-le...