various optical isolator embodiments are disclosed . embodiments comprise a waveguide section utilizing materials that induce a propagation constant shift that is propagation - direction - dependent . embodiments are characterized by a cutoff frequency for forward propagating waves that is different than the cutoff frequency for reverse waves . a particular embodiment is constructed as a single - mode waveguide on a substrate . the cross - section of the waveguide is inhomogeneous in terms of materials . this inhomogeneity induces a propagation constant shift , which is propagation - direction - dependent . this device works as an optical isolator from the cut - off frequency of the lowest forward wave to one for the lowest reverse wave . various configurations consistent with the principles of the invention are disclosed .