Highly Sensitive Torsion and Curvature Sensor in Helical Elliptical-Core Optical Fiber Based on Mach-Zehnder Interference

We propose a highly sensitive torsion and curvature sensor based on Mach-Zehnder interference using a helical elliptical-core polarization maintaining (HEPM) fiber embedded in a multimode fiber, and the sensitive region of the sensor is the HEPM fiber. The sensor can recognize the direction of torsion and maintain excellent stability under different torsions. Experimental results show that the free spectral range (FSR) of interference spectrum gradually increases as the length of the HEPM fiber is shortened, and the sensitivity of the torsion sensor increases accordingly. The maximum torsion sensitivity of 426.09 pm/(rad/m) was achieved in the torsion range from −90° to 150° when the length of the HEPM fiber was 0.7 cm, and the length of the multimode fiber was 1.0 cm. Similarly, as the length of the HEPM fiber decreases, the curvature sensing sensitivity is improved. The maximum curvature sensitivity of 2.33381 nm/m ^{-{1}} was achieved when the length of HEPM fiber was 0.7 cm, and the length of multimode fiber was 1.0 cm in the curvature range from 0 to 18 m ^{-{1}} . We also experimentally tested the temperature stability of the sensor by setting up a sensing matrix that can simultaneously measure torsion and temperature, eliminating the temperature-influenced error. The proposed sensor can measure torsion and curvature changes with good linear response. The sensor can not only recognize the direction of torsion in torsion sensing, but also has a wide measurement range in curvature sensing. Moreover, the sensor has simple linear structure, so it has great potential in practical applications.