Femtosecond Laser-Inscribed Double-Effect Off-Axis Ring Bragg Gratings for Curvature and Temperature Sensing

A novel curvature and temperature sensor based on a double-effect off-axis ring fiber Bragg grating (OAR-FBG) is proposed and experimentally demonstrated. This innovative device integrates both fiber Bragg grating (FBG) and Mach-Zehnder interferometer (MZI) characteristics, inscribed simultaneously in a single step using a Femtosecond laser in a coating-unstripped single-mode fiber. The curvature sensing mechanism relies on the wavelength shift observed in the MZI interference fringe dip. Experimental results reveal a strong correlation between the curvature response and the direction of bending. The curvature sensitivities measured in the 90° and 180° directions are 1.587 nm/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> and 1.155 nm/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>, respectively, in the range of 0–10 m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>. The temperature response of the FBG resonance is almost the same for different bending states. The temperature sensitivities of the MZI and FBG are −60 pm/°C and 18 pm/°C, respectively. The compact design, ease of fabrication, and the absence of any assembly requirements make this sensor exceptionally appealing for applications such as structural health monitoring and robotic posture detection.