Helical Long-Period Gratings in Three-Core Fiber for Directional Curvature and Torsion Measurements

We proposed a dual-parameter sensor designed for detecting directional curvature and torsion. The sensor was fabricated utilizing CO2 laser system to inscribe helical long-period gratings (HLPGs) in three-core fiber (TCF). These three cores of the TCF are arranged in a straight line, forming an array structure. We conducted experimental studies on the curvature, torsion, and temperature sensing characteristics of TCF-HLPGs with period of 430 μm and period number of 60. In the curvature range from 0.249 m-1 to 0.98 m-1, both dual-resonances (dip C and dip D) near 1550 nm achieve their maximum curvature sensitivity in the 0° curvature direction, where the bend is perpendicular to the axis connecting the three cores, with values of −3.715 nm/m-1 and −9.814 nm/m-1, respectively. In the torsion rate range from −7.98 (rad/m) to 7.98 (rad/m), the torsion sensitivities of dip C and dip D are 0.2 nm/(rad/m) and −0.28 nm/(rad/m), respectively. Enhanced torsional sensitivity of −0.48 nm/(rad/m) is achieved by tracking the wavelength difference between dip C and dip D. This type of sensor is easy to manufacture with high integration, capable of simultaneously achieving dual-parameter sensing of directional curvature and torsion, and effectively eliminating temperature-induced interference. Additionally, its sensitivity is also outstanding., rendering it highly applicable in structural health monitoring and sensing applications associated with vector curvature and torsion.