We study orbital angular momentum (OAM) mode coupling in ring-core fibers (RCFs) due to elliptical shape deformation. We introduce a coupling model based on numerical mode solver outputs of perturbation. We show improved predictions in calculating coupling strength compared to the classical modeling approach. Our model captures and quantifies the disparate behaviors of coupling in lower and higher order degenerate OAM modes. The ideal orthogonality of modes is undermined by fiber imperfections. Our model predicts the OAM order at which the orthogonality within OAM mode pair is maintained despite elliptical deformation. We use our coupling model to simulate propagation effects and compare the performance of two fibers (thin and thick RCF) designed under the same constraints. Our numerical propagation results show different performance for the two fibers under the same level of elliptical deformation. This model uncovers distinct digital signal processing requirements for these two types of fiber, and predicts their signal-to-noise ratio penalty. For each fiber, we examine the large number of supported modes and find the optimal subset of mode groups, i.e., the groups with the lowest penalty. We show that this optimal subset is different from that predicted during the fiber design optimization.Index Terms-Orbital angular momentum (OAM), ring core fiber (RCF), coupled-mode equations (CMEs), elliptical deformation, mode coupling, crosstalk (XT), multiple-input multipleoutput (MIMO), digital signal processing (DSP)

Our numerical model of elliptical deformation of ring cores uncovers distinctly different behaviors of lower and higher order OAM modes. Degeneracy of modes, across topological charge and polarization are laid bare in simulations.

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We successfully demonstrate a novel enhanced asymmetrically clipped optical OFDM (eACO-OFDM) solution over a PON link. The proposed technique doubles the spectral efficiency of conventional ACO-OFDM at the expense of a slight increase in complexity.

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