[en] Dispersion relations for TM polarized non-linear waves propagating in a symmetric single film optical waveguide are derived. The system consists of a layer of thickness d with dielectric constant epsilon₁ bounded at two sides by a non-linear medium characterized by the diagonal dielectric tensor epsilon₁₁=epsilon₂₂=epsilon₀, epsilon₃₃=epsilon₀+α|E₃|², where E₃ is the normal electric field component. For sufficiently large d/lambda (lambda is the wavelength) we predict bistable states of both symmetric and antisymmetric modes provided that the power flow is the control parameter. (author)