Randomly‐Coupled Multicore Fibers for Ultra‐Sensitive Curvature Sensing

ABSTRACT Curvature sensing is a critical function of optical fiber sensors for applications in structural monitoring, biomedicine, and photonic integration, where multicore fibers (MCFs) provide distinctive opportunities. This work exploits intrinsic inter‐core coupling in randomly‐coupled MCFs (RC‐MCFs) to realise ultra‐sensitive curvature sensing. A coupled‐mode theory establishes the relationship between structural parameters and inter‐core coupling. Numerical simulations reveal that bending‐induced propagation constant mismatches dynamically reconfigure inter‐core coupling, leading to optical path difference amplification. This mechanism enables curvature sensing in centimeter‐scale RC‐MCFs and differs fundamentally from cladding‐mode coupling in weakly coupled MCFs and supermode coupling in strongly coupled MCFs. Broadband spectral interrogation experiments reveal pitch‐dependent and wavelength‐dependent interference patterns. A record curvature sensitivity of −65 nm/m −1 is achieved in 4CF over the range of 1.6–2.8 m −1 , corresponding to sub‐millimeter bending radius resolution. Using a 16CF with a larger core pitch and a two‐layer structure, sensitivities up to −190 nm·m −1 are achieved over a narrower curvature range, demonstrating the extendability of inter‐core‐coupling‐based curvature sensing to other RC‐MCF structures. Multi‐directional bending experiments further demonstrate orientation‐dependent responses, indicating the potential of RC‐MCFs for vector curvature sensing, underscoring their potential for structural monitoring and advanced photonic integration.