High-Sensitivity Torsion Sensor Based on Er³⁺-Doped Multicore Fiber-Ring Laser

This work reports a torsion sensor based on a fiber-ring laser (FRL), whose active element is a 58 cm segment of three inline core erbium-doped fiber (EDF-3C). In this architecture, the multicore fiber simultaneously provides optical gain and torsion transduction. The applied twist alters the inter-core coupling, producing a gain splitting that appears as two laser peaks. The wavelength separation between the peaks encodes the torsion magnitude, whereas their relative powers reveal the twist direction. By fine-tuning the polarization controller, bidirectional operation remained stable over a ±70° range without mode extinction. The differential wavelength shift exhibited linear sensitivities of 0.013 nm/° for negative torsion and 0.019 nm/° for positive torsion. Direction discrimination is further enhanced by analyzing the peak-power difference and is additionally optimized with a function-fitting neural network trained on the measured spectra. The machine learning model achieved R² = 0.9999 and an angular detection limit of 1.28 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup>°.