Critical frequencies of anisotropic dielectric waveguides

Simple analytic expressions are obtained for critical frequencies (wavelengths) of all the modes in anisotropic dielectric waveguides. A comparison of the formulas obtained for symmetric modes of a circular waveguide exhibiting an axial anisotropy with the exact values shows that the agreement is good within a wide range of values of the ratio ε_z/ε_x of the permittivity tensor components.