Structural Perturbations and Full Field Evolution in Tapered Spun Ring-Core Fiber Amplifiers

Abstract

We theoretically investigate the complex full-vector modal field evolution in tapered spun ring-core fiber amplifiers subject to structural perturbations, utilizing a combined coupled-mode theory and rate-equation model. To provide a comprehensive characterization, we introduce a normalized complex modal-state fidelity factor that simultaneously incorporates intensity, phase and polarization into a single quantitative measure. We demonstrate that the twist pitch of the structure can be purposely tuned to either suppress or leverage the effect of specific geometrical perturbations. These findings provide insight into the physical conditions governing modal stability, establishing a robust mechanism for highly efficient in-fiber OAM mode conversion.