Birefringence is commonly associated with anisotropic or chiral media, but optical nonlinearity can also induce effective symmetry breaking, enabling circular birefringence even in isotropic materials through helicity-dependent Kerr phase shifts. Extending this concept to the orbital angular momentum (OAM) degree of freedom, optical vortices provide a natural platform to explore handedness-dependent nonlinear effects. In homogeneous isotropic media, however, Kerr-induced coupling to higher-order vortex states leads to modal instability, rendering orbital-handedness-dependent phase shifts ill defined. Here, we show that this limitation is naturally lifted in optical fibers, where guided vortex modes with opposite orbital handedness are mutually orthogonal eigenmodes of the waveguide. Using a full-vectorial formulation, we demonstrate that modal orthogonality suppresses Kerr-induced OAM cascading, resulting in a well-defined and stable nonlinear phase splitting between vortex modes of opposite topological charge. Our results establish optical fibers as a robust platform for Kerr-induced orbital handedness birefringence and provide a unified framework for understanding nonlinear birefringence phenomena in structured light beyond polarization.

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• Angular momentum of light
• Nonlinear optics
• Optical vortices
• Third order nonlinear optical processes