Multimode interference in circular step-index fibers studied with the mode expansion approach

The mode expansion approach in vectorial form, using a complete set of guided modes of a circular step-index fiber (SIF), is developed and applied to analyze multimode interference in multimode fibers (MMFs) for the first time, to the best of our knowledge. The complete set of guided modes of an SIF is defined based on its modal properties, and a suitable modal orthogonality relation is identified to evaluate the coefficients in a mode expansion. An algorithm, adaptive to incident fields, is then developed to systematically and efficiently perform mode expansion in highly MMFs. The mode expansion approach is successfully applied to investigate the mode-selection properties of coreless fiber segments incorporated in multicore fiber lasers and the self-imaging in MMFs.