Nature of the orbital angular momentum (OAM) fields in a multilayered fiber

We provide a theoretical analysis of the nature of the orbital angular momentum (OAM) modal fields in a multilayered fiber, such as the step-index fiber and the ring-core fiber. In a detailed study of the vector field solutions of the step-index fiber (in the exponential basis), we discover that the polarization-induced field component is a modified scalar OAM field (as opposed to a standard OAM scalar field) with a shifted intensity pattern in the weakly guiding approximation (WGA); the familiar intensity donut pattern is reduced or increased in radius depending upon whether it is a case of spin-alignment or anti-alignment with the OAM. Such a shift in the intensity pattern appears to be a general feature of the field of a multilayered fiber as seen from an extension to the ring-core fiber. Additionally, we derive a general expression for the polarization-correction to the scalar propagation constant, which includes, for the first time, the contribution of the polarization-induced field. All the analytic expressions are illustrated and validated numerically with application to a step-index fiber, whose analytic solutions are well-known.