Graded Ring-Core Photonic Crystal Fiber for the Transmission of OAM Modes with High Mode Purity and Quality in C+L Bands

In order to enhance the OAM mode purity and quality, we propose a graded-index ring-core photonic crystal fiber (GRC-PCF) and its general design strategy by the finite element method (FEM). In addition, the numerical results turn out that the GRC-PCF is better than conventional step-index ring-core-photonic crystal fiber (SRC-PCF) in terms of mode purity and mode quality, when the two fibers support the same numbers of OAM modes at 1.55μm, respectively. After optimizing structural parameters, a GRC-PCF that can support 14 OAM modes without phase distortion and high-radial modes in C+L bands is designed. Particularly, the mode purity and mode quality of all the OAM modes are higher than 97.05% and 98.42%, respectively. The confinement losses are maintained at the order of 10−11 and flat dispersion. The proposed fiber can be used in the MDM system and realize the stable transmission of the OAM modes.