Integrated optical isolator based on nonreciprocal-mode cut-off

A concept of an integrated optical isolator is presented which uses the TM(0) mode propagating perpendicular to the in-plane magnetization of a planar magnetooptical waveguide. The cut-off thickness of the waveguide depends on the propagation direction. If the magnetic film has a strong Faraday rotation and is weakly guiding, the cut-off thicknesses for forward and backward propagation differ markedly. Operating close to cut-off yields a large difference between coupling efficiencies to the waveguide for forward and backward propagation.