The temporal and spectral characteristics of a laser diode-pumped, additive-pulse mode-locked Nd:YLF laser have been investigated with respect to the cavity detuning between main and external cavities. In the case of a usual output through the beam splitter between main and external cavities, a strong temporal modulation was observed from an intensity autocorrelation trace which was also confirmed by a calculation based on intensity-dependent reflectance. To obtain mode-locked pulses without any temporal modulation, an output was directly extracted from the main cavity, and clean pulses with 1.7 ps duration were obtained at 1053 nm. (C) 1999 Published by Elsevier Science B.V. All rights reserved.