  We introduce a novel mode-locking scheme based on polariton-polariton
interactions and we demonstrate its operation in a $60 \ \mu m-$long GaN-based
waveguide cavity. The exciton-polariton active layer acts both as a gain medium
and as a self-focusing medium with a large Kerr-like nonlinearity. The
monitoring of the cavity free spectral range across the lasing threshold
evidences the linearization of the strongly nonlinear mode dispersion due to
self-phase modulation. The Gross-Pitaevskii equation for the coupled exciton
and photon fields taking into account the exact cavity geometry, predicts the
appearance of optical pulses within the cavity that correspond to optical
solitons. Pulses widths are about 100 fs, both in theory and experiment.
