Composite Ring Core Based Antiresonant Fiber for Efficient Transmission of OAM Modes

We present a new type of composite ring core based antiresonant fiber that enhances orbital angular momentum (OAM) modes, preserving superior mode purity and quality across $\mathbf{O}, \mathbf{E}, \mathbf{S}, \mathbf{C}, \mathbf{L}$ and $\mathbf{U}$ bands. The proposed fiber consists of a unique core arrangement having a ring with a higher refractive index, which is encircled by exterior and interior rings. The refractive indices of these rings vary in a specific order and they are connected with the outer capillary of the fiber by negative curvature tubes having antiresonant properties. The finite element method (FEM) simulation shows that the proposed core design can accommodate a maximum of 206 OAM modes throughout a broad wavelength range of $1.28 \mu \mathrm{~m}$ to $1.70 \mu \mathrm{~m}$ without resulting in any higher-order radial modes. It maintains very high mode quality of over 97% and excellent mode purity, typically around $98 \%$. Moreover, the fiber maintains an extremely low confinement loss between $10^{-11}$ to $10^{-9} \mathbf{~ d B} / \mathbf{m}$. The suggested fiber demonstrates a great potential for efficient transmission of high-capacity OAM modes.