Various optical isolators are disclosed. One embodiment provides an optical isolator comprising a waveguide that includes polymer magneto-optical media. In a particular embodiment, the waveguide is dimensioned for single mode operation in the selected isolation range. A cross-section of the waveguide is inhomogeneous in terms of magneto-optical materials. Polymer magneto-optical material is a part of the optical waveguide structure. The inhomogeneity induces the propagation constant shift, which is propagation-direction-dependent. An embodiment is characterized by a cutoff frequency for forward propagating waves that is different than the cutoff frequency for reverse waves; the dimensions and direction of magnetization of the waveguide can be tailored so that, in a particular embodiment, the cutoff frequency for forward propagating waves is lower than the cutoff frequency for reverse waves.