In this paper, we report a novel semiconductor waveguide autocorrelator that employs two-photon absorption (TPA), rather than the more commonly used second-harmonic generation, as its nonlinear mechanism. An introduction to TPA is given, and the reasons for its use in the autocorrelator are explained with reference to nonlinear absorption experiments. We then describe the fabrication of the p-i-n GaAs/AlGaAs waveguide. The initial experiments carried out with a single beam in the waveguide are described and fitted to theory. Autocorrelation traces for both a CW modelocked Nd/sup 3+/:YAG laser at 1.06 /spl mu/m and a Q-switched InGaAsP/InP laser at 1.3 /spl mu/m are presented and their pulsewidths obtained by means of a theoretical model. The sensitivity and resolution of the TPA waveguide autocorrelator are then discussed, as is a proposal for a multiple-contact TPA waveguide autocorrelator, designed so that the two pulse trains are counter-propagating. This would result in an autocorrelator from which the laser pulsewidth could be obtained in a single measurement, with no moving parts, and which could be integrated with a semiconductor laser.< >