Theoretical and experimental analysis of inter-core crosstalk in strongly-coupled multicore fiber tapers

• This work presents a comprehensive theoretical model that describes the mode coupling in strongly-coupled multicore fiber because of the tapering technique. • This study introduces a detailed theoretical model that characterizes mode coupling in strongly-coupled multicore fibers resulting from the tapering process. • The model incorporates an analysis of coupling between both adjacent and nonadjacent cores. • The coupling theory was assessed under two conditions: when light is injected into the central core and when it is injected into the external cores. We present a comprehensive theoretical model based on the coupled-mode theory, capable of analyzing the crosstalk in a multicore optical fiber taper. The novelty of our developed model lies in its ability to describe the mode coupling in strongly-coupled multicore fiber tapers, a feature not covered by previously reported models that are only suitable for weakly-coupled multicore fibers. In our approach, we consider all possible coupling interactions among both adjacent and nonadjacent cores, along with the self-coupling within each core of a homogeneous seven-core optical fiber. Moreover, we also take into account that when a homogeneous multicore fiber undergoes tapering, a propagation constant mismatch appears between the central and external cores, due to the closer proximity of the external cores to the surrounding environment. To validate our model, we implemented fiber tapers of different waist lengths and diameters in a seven-core optical fiber and compared their inter-core crosstalk with the simulated results. Unlike previous studies that only injected light into the central core, we also explored the case where the input is launched into any external core of the fiber tapers. For both scenarios, we verified that our theoretical model can describe the dynamics of mode coupling occurring in a tapered multicore fiber for a large range of waist diameters. We believe that the findings from this study could be useful for designing sensing and signal-processing devices.