paper

Extraction of Bend-Resolved Modal Basis in Deformed Multimode Fiber

arXiv:2503.16303 · doi:10.1063/5.0262067

Abstract

Mode mixing in optical fibers caused by mechanical bending induces perturbations that distort the spatial field profile of coherent beams as they propagate through few-mode or multimode fibers. The observed output from a bent fiber commonly appears as complex speckle, which is challenging to relate directly to the underlying deformation, particularly in continuously varying systems such as aerially deployed fibers or fiber-integrated sensors in mechanical structures. We introduce a novel method for constructing a complete deformation-resolved orthonormal modal basis that captures the optical response of a multimode fiber across a range of controlled mechanical deformations. The basis is derived via a two-stage singular value decomposition framework that initially constructs deformation-specific orthonormal mode sets from speckle pattern correlation matrices and subsequently decomposes the aggregated sets to produce a unified functional basis that comprehensively spans the deformation-induced modal subspace supported by the fiber. This hierarchical framework yields an energy-balanced representation that isolates statistically dominant field components across all deformation states, approximates superpositions of the fiber's propagation-invariant modes, systematically encodes deformation-induced perturbations, and supports robust decomposition of output fields across varying mechanical conditions. Such a basis enables tracking of mechanically induced modal evolution in deployed fibers, supporting distributed sensing, network resilience, and predictive fault diagnostics, with potential for integration into mode-division multiplexing systems.

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