Fields and Modes in Thin Film Coated Optical Waveguides

Rigorous, computationally efficient method of mode-field calculation in multilayer optical fibers based on a new algorithm for building up matrix eigenvalue equation is presented. For an arbitrary number of dielectric, metallic, semiconductor, or topological insulator layers, the cylindrical wave equations are solvable exactly both for guided and lossy modes. Lossy mode resonance in a fiber taper prepared by etching out of a significant part of silica cladding and coated with an ultrathin ZnTe film is described as an example. It is shown that in this case the lossy mode resonance is a consequence of two mode coupling in a transient wavelength region in which these modes transform form guided into lossy type and vice versa.