Radiation related polarization instability of fast Kerr spatial solitons in slab waveguides

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.