High-Sensitivity Fiber-Optic Multiparameter Sensor Based on Femtosecond Laser-Inscribed Helical Waveguide

A fiber optic multi-parameter sensor is proposed based on a Mach-Zehnder interferometer (MZI). The in-line MZI is constructed by inscribing a linear waveguide and a helical waveguide within a segment of no-core fiber (NCF) via a femtosecond laser, with the entire structure positioned between two segments of multi-mode fiber (MMF). Owing to its unique helical structure, the sensor demonstrates high sensitivity to torsion and curvature, and possesses the capability to distinguish torsion and curvature directions. Experimental results demonstrate that it achieves a maximum torsional sensitivity of -1.382 nm/rad/m within a counterclockwise (CCW) torsional range of 0-6.108 rad/m, and a maximum sensitivity of 1.274 nm/rad/m within a clockwise (CW) torsional range of 0-6.981 rad/m. In the curvature range of 0-2 m-1, the sensor exhibits a maximum curvature sensitivity of 13.468 nm/m-1in the 180° bending direction. Over the temperature range of 40-80 °C, it presents a temperature sensitivity of -35.33 pm/°C. Moreover, by building a sensitivity matrix, it is possible to measure the curvature, torsion, and temperature parameters simultaneously. Featuring high sensitivity, compact design, and robust performance, the proposed sensor is promising for applications in flexible tactile sensing for intelligent robots, adaptive intelligent sensing systems, and structural health monitoring fields.