Tube waveguide for optical transmission

The dielectric optical waveguide described in this paper has an annular cross section, the refractive index of which is higher than the indices of the material inside and outside of the ring. The solution of the eigenvalue problem is an approximation which is valid for small refractive index differences of the three media. The resulting approximate eigenvalue equation is far simpler than its exact counterpart. The cutoff conditions of the first three modes and the eigenvalue of the lowest-order mode are presented graphically.