DIRECTIONAL COUPLING INSIDE A STEP-INDEX MULTI-LAYER FIBER

We propose the transfer of the directional coupler principle between channel waveguides well known in Integrated Optics to a multi-layer step-index optical fiber. The coupling action takes place between the fundamental core mode and one or several annular modes, guided by high refractive index cladding layers well separated from one another and the fiber core. Key features of the fiber coupler are the strong wavelength dependence of its coupling efficiency and its capability of almost lossless power transfer between different circular areas across the cross-section of the fiber. This numerical study includes the analysis of the dispersion relations of the participating guided modes and the corresponding supermodes, complimented by typical field pattern over the fiber cross-section unveiling the underlying coupling mechanism. The spectral transmission of the fiber coupler will be discussed too. Potential applications for this coupling concept are the almost lossless conversion of small Gaussian beams into large annular intensity distributions and several filter and sensor utilizations as well.