Cutoff Conditions in Three-Layer Cylindrical Dielectric Waveguides

Exact cutoff expressions for hybrid and circularly symmetric modes in three-layer cylindrical dielectric waveguides are derived. It analytically established that whenever the refractive index of the enter medium (n²) is higher than either the refractive index of the core (n₁) or of the inner cladding (n₂), i.e., n₁ >n₃ >n₂ or n₂>n₃ >n₁, the dominant HE₁₁ mode can have a nonzero cutoff frequency. Inequalities relating the permittivities to the ratio of the cladding radius to the core radius, as conditions for the nonzero cutoff of the HE₁₁ mode, are determined. The cutoff conditions presented in this paper are also applicable to similar structures used io millimeter-wave communications.