Coupling characteristics of twin-core few-mode all-solid photonic bandgap fiber and their application in mode separation

Abstract

We theoretically investigate the coupling characteristics of a few-mode twin-core all-solid photonic bandgap fiber. We show that extrema in the coupling length, reversal of the ordering of supermodes, and complete decoupling can occur in high-order modes. Using significant differences in the coupling characteristics between different modes, we can separate degenerate mode pairs such as LP11a and LP11b, and LP21a and LP21b, as well as modes with the same azimuthal quantum number, e.g., LP01 and LP02. By setting a suitable photonic-crystal period, we can realize an optical fiber coupler with mode separation properties in the C-band.