Compact Optical Fiber Directional Curvature Sensor With Large Curvature Range Based on Michelson Interferometer

Curvature sensing is essential in robotics, aerospace, and structural health monitoring, where precise deformation measurements are critical. However, existing curvature sensing technologies—such as strain gauges, inertial measurement units, and various fiber-optic approaches—suffer from limitations including electromagnetic interference, bulky form factors, low spatial resolution, temperature drift, complex fabrication, and restricted measurement ranges, especially for large-curvature and full-direction recognition. Here, we present a Michelson interferometer (MI)-based fiber curvature sensor composed of single-mode fiber, multimode fiber, and three-core fiber (SMF–MMF–TCF). The spatially distributed cores of the TCF inherently provide a directional reference, eliminating the need for inscribed gratings or asymmetric cladding, and enabling ultra-wide curvature measurement from 0 to 5.49 m⁻¹ with a maximum directional sensitivity of 1.07 dB/m⁻¹. Experimental results reveal a sinusoidal curvature sensitivity pattern, allowing precise bending angle recognition with excellent linearity (R² > 0.98) even at maximum curvature. The temperature sensitivity is −0.073 dB/°C within the temperature range of 25–95°C, and the corresponding temperature cross-sensitivity is −0.068 m⁻¹/°C, indicating that the proposed sensor exhibits reliable curvature sensing performance in the presence of moderate temperature fluctuations. This plug-and-play design, requiring only simple fusion splicing, offers a low-cost, high-performance, and robust solution for full 360° vector curvature sensing, paving the way for practical deployment in demanding industrial applications.