Spiral photonic crystal fiber structure for supporting orbital angular momentum modes

We propose a spiral photonic crystal fiber structure for supporting orbital angular momentum modes. The structure has 12 arms, arranged in a spiral shape in the cladding region, and a big air hole at the center of the structure. It can support 14 well-separated OAM modes with a mode index difference of 10−4. Our numerical results show a nonlinear coefficient of 2.78 W−1 Km−1 for the HE21 mode at 1550-nm wavelength. The fiber shows a dispersion difference over a 600 nm bandwidth for HE21 mode is 12.1 ps/km-nm, and the structure shows a low confinement loss. We have also calculated the effect of ellipticity on the mode's effective index. Numerical results suggest that, this structure can be utilized in high-capacity communication systems for multiplexing fiber-based OAM.