The non-linear upconversion processes in Nd-doped Ta2O5 waveguides have been investigated to determine the depopulation rate of the F-4(3/2) laser level due to energy transfer upconversion (ETU). A rate-equation analysis has been used to obtain analytical expressions of the population and average lifetime of the F-4(3/2) level under modulated cw excitation. The analysis of lifetime measurements as a function of excitation power yielded to an ETU rate of about 22 800 s(-1). Equations for the four level cw laser performance have been obtained using a rate-equation approximation that includes ETU processes. The effect of upconversion on the laser performance has been estimated using the parameters obtained for the Nd-doped Ta2O5 waveguide lasers. (c) 2006 Elsevier B.V. All rights reserved.