Broadband degenerate mode selective coupler based on an inner cladding elliptical core fiber structure

The mode-selective coupler (MSC) enables the conversion from the fundamental mode to higher-order modes, being important for mode-division multiplexing communication. However, the current MSC is limited by the narrow bandwidth and it hardly distinguishes the degenerate modes. This paper designs a broadband MSC with an inner-cladding elliptical-core (ICEC) optical fiber structure. The ICEC realizes the conversion of degenerate modes by breaking the circular symmetry of the fiber and optimizes the phase-matching condition by adding an inner cladding. The coupling efficiency of the MSC is higher than 90% over a 100 nm bandwidth (from LP₀₁ to LP₀₁, LP_11a, LP_11b, LP_21a, and LP_21b). At the center wavelength, the conversion efficiency is more than 94.6%. Over the wavelength of 1400-1700 nm, the lowest efficiency in the band is 54.6%.