Diode pumping Nd-laser efficiency limitations due to up-conversion processes in Nd : YLF and Nd : GLF

The visible up-conversion fluorescences mainly from the (4)G(7/2) level in Nd-doped LiYF4 and GdLiF4 crystals have been studied at 300 K under c.w. di ode laser pumping at 797 nm. These emissions originate from either two-step excitations, involving an excited-state absorption (ESA) from the F-4(3/2) metastable level or from energy transfer up-conversion (ETU) processes. A model to estimate the probability rates of the ETU processes based on the F orster-Dexter method was proposed. By solving the rate equations for these systems under continuous diode pumping, it was possible to calculate the pu mping efficiencies in order to attain the threshold conditions for the 1.04 7 mu m laser action, which showed a non-linear behavior. The theoretical ca lculations clearly indicate that the maximum possible inverted population ( at the F-4(3/2) level) is around one-tenth of the: total Nd concentration, for both crystals. The maximum gain is thus limited by the decrease of pump ing efficiencies due to the up-conversion processes at the laser threshold. (C) 2000 Elsevier Science B.V. All rights reserved.