Mode analysis of orbital angular momentum modes carrying multi-mode ring-core fibers

Abstract We discuss the mode properties for orbital angular momentum (OAM) modes supporting multi-mode ring-core fiber (MM-RCF). For strongly-guiding MM-RCF, by introducing one more layer to the ring-core, a two-layer ring-core fiber (TL-RCF) is proposed. It is proven that this TL-RCF can increase the separation degree between the vector eigenmodes and meanwhile weaken the spin–orbit coupling. For mode-group supporting MM-RCF, the maximum mode number is limited by the thickness of ring-core and the cut-off of radial higher-order mode. We also present the tradeoff relationship between the mode group effective index difference and inter-eigenmode differential mode delay in one mode group. The discussion in this paper can give guidance for the design of MM-RCF.