"Polarization dynamics are considered in the presence of an anisotropic Kerr non-lin-
earity in the most common semiconductor waveguide geometry. The equations of
motion are formulated in terms of Stokes polarization parameters and their Hamiltonian
form is derived. Stationary solutions and their stability are found for plane-wave prop-
agation. It is found that the non-integrable problem of mixed-polarization spatial soliton
dynamics can be largely explained in terms of the equivalent plane-wave solutions."
It sounds like the solutions- either solitons or plane waves- are mixtures of pure polarization states. If the polarization state changes in time (i.e. linear to circular and back again), the state can be written as a time-dependent mixture of two orthogonal polarization states.
Can you provide a proper attribution to the article?