Effect of the spin-orbit interaction on polarization conversion in coupled waveguides

A generalization of perturbation theory for weakly coupled optical waveguides beyond the scalar approximation is suggested that explicitly allows for the spin-orbit interaction embedded in the gradient term of the vector wave equation. The obtained perturbation theory is applied to study the problem of cross-talk in coupled ideal monomode fibres. We demonstrate the absence of cross-talk between orthogonal linear polarizations and the inversion of the circular polarization's sign caused by the spin-orbit interaction. The spin conversion length is determined.