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 1.55 μ m , 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 1 0 − 11 and flat dispersion. The proposed fiber can be used in the MDM system and realize the stable transmission of the OAM modes. • We propose a GRC-PCF to improve the OAM mode purity and quality. • The proposed GRC-PCF is better than SRC-PCF in terms of mode purity and quality. • The OAM mode purity and quality are higher than 97.05% and 98.42% in C+L bands, respectively. [ABSTRACT FROM AUTHOR]