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.

Calculations are presented describing a novel optical isolator which works by complete TE(0)-TM(0) mode conversion in magnetic garnet films caused by stress-induced optical anisotropy (50%) and by Faraday rotation (50%). These conversions take place along two different, perpendicular light paths in the same crystal that are connected by an integrated mirror. Possible tolerances of the film parameters are given so that a 30-dB isolation is still guaranteed.

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Propagation of optical modes in (110) oriented garnet films is calculated. Due to the in‐plane anisotropy of these films the phase constants of the modes depend on the direction of propagation. It is shown that this effect can be applied for a fine adjustment of the phase matching between TE and TM modes which is necessary for mode conversion. The optical anisotropy of a (110) oriented garnet film is measured; it is sufficient to achieve phase matching.

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