Fiber Vector Torsion Sensor with Low Crosstalk for Temperature and Strain Based on Mach-Zehnder Interference

A Mach-Zehnder interferometer (MZI) based on a polarization-maintaining fiber-few-mode fiber (PMF-FMF) structure is proposed to achieve high-sensitivity vector torsion sensing. The effects of fiber lengths, core offset directions, and offset magnitudes of both PMF and FMF on the interference spectra are investigated. Leveraging the properties of PMF, when the sensor is twisted, changes in the polarization state of the transmitted light alter the intensity of interference dips, causing the wavelengths of the envelope valleys to shift in opposite directions, which enables the discrimination of torsion direction. Additionally, the influence of PMF and FMF lengths on the interference spectra is studied, and experimental measurements of torsion, temperature, and strain sensing are conducted. Experimental results show that the maximum torsion sensitivity for clockwise (CW) rotation is 0.91 nm/(rad/m), and that for counterclockwise (CCW) rotation is -0.61 nm/(rad/m). Additionally, the sensor features simple fabrication, low sensitivity to strain and temperature interference, and a measurement error of less than 4%. These characteristics make it highly suitable for structural health monitoring applications in civil engineering and aerospace industries.