Extraction of Bend-Resolved Modal Basis in Deformed Multimode Fiber

Abstract: Mode-mixing in fiber from mechanical bending leads to optical perturbations that distort the spatial field profile of coherent beams as they propagate through few or multiple-mode fiber. The observed output from a bent fiber commonly induces complex speckle, which is challenging to directly relate to the bend condition in the fiber, especially for continuously moving systems, such as aerial-deployed fiber or fiber sensors in machinery. We introduce a novel method for constructing a complete mechanically defined modal basis that embodies all fiber deformations resulting from the controlled bending of a multi-mode fiber. This specific basis arises from a differential exchange of power between spatially asymmetric, deformation-dependent orthogonal sets, obtained through singular value decomposition of correlation matrices derived exclusively from speckle patterns at the fiber's distal end under specific deformation conditions. These spatially quasi-symmetric basis closely resembles a superposition of the fiber's propagation-invariant modes, with aberrations that result from mechanical deformations. This approach could be readily integrated into mode-division-multiplexing systems to monitor changes in the mechanical variation along deployed fibers for network resilience and fault prediction.