We report the observation of a power-dependent polarization conversion from a fast spatial soliton-like beam into the orthogonal polarization in an AlGaAs slab waveguide at power levels below which the nonlinear index change becomes comparable to the birefringence. The slow soliton, however, shows no polarization conversion. We interpret these results in terms of the previously predicted polarization instability of a fast soliton into orthogonally polarized radiation. The experimental results are in qualitative agreement with theory.

This research was supported at CREOL by the National Science Foundation and an Army Research Office MURI, at the University of Glasgow by the Engineering and Physical Science Research Council, at the Australian National University by Australian Photonics Cooperative Research Centre and at the Instituto de Óptica of the Consejo Superior de Investigaciones Cientificas in Madrid, Spain by the DGES under contract PB96-0819. LF also wishes to acknowledge a number of helpful discussions with David C. Hutchings.

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