Optical fiber laser system for torsion measurement using a three-core fiber and a nanocrystal-based reflector

We report a novel and highly sensitive torsion sensor that integrates a multicore fiber (MCF) within a Sagnac interferometer so that the same MCF segment serves as both the reflective element and the sensing head of a fiber-ring laser. The laser architecture incorporates a distributed reflector implemented as a ZnGa₂O₄-nanocrystal-doped fiber section, which enhances overall system performance. The device exhibits high torsional responsivity over 0°-150°, with distinct behavior across sub-ranges: in 0°-50°, phase analysis yields a sensitivity of 0.08 rad/° with R² = 0.991; in 88°-150°, amplitude analysis-under the ring-laser configuration-shows an improvement in sensitivity from 0.2 to 0.5 dBm/° (R² = 0.995). In the intermediate interval (50°-88°) neither phase nor power varies monotonically, so a function-fitting neural network was employed to bridge this gap, achieving a root-mean-square error of 0.06°. The system attains an angular resolution of 0.8°, ensuring accurate torsion estimation across the entire measurement span.