Low temperature cross-sensitivity vector torsion sensor based on an in-fiber Mach-Zehnder interferometer with helical side-hole fiber

What we believe to be a novel in-fiber vector torsion sensor based on a Mach-Zehnder interferometer (MZI) constructed using helical side-hole fibers (HSHFs) is proposed. A segment of HSHF is spliced between two single-mode fibers to excite the LP₀₁ and LP₁₁ modes supported by the HSHF, generating intermodal interference and forming the MZI structure. Owing to the helical side-hole configuration, the interference patterns of the LP₀₁ and LP₁₁ modes rotate with the applied torsion angle. A theoretical model for torsion sensing reveals that the light intensity at the resonant dip of the interference patterns varies periodically with the increasing torsion angle. In contrast, the wavelength shift of the resonant dip exhibits a linear relationship with the torsion angle, enabling precise torsion measurement. Experimental results confirm the validation of the model, showing torsion sensitivities of 2.13 dB/(rad/m) and 0.282 nm/(rad/m) for intensity and wavelength responses, respectively. Furthermore, the proposed torsion sensor shows minimal crosstalk to temperature and axial strain thanks to the interference between the two core modes. Additionally, introducing the helical structure during fiber drawing simplifies processing and preserves strength, providing a wide measurement range. The proposed HSHF-MZI torsion sensor exhibits high sensitivity, low temperature and strain crosstalk, and the ability to detect the torsion direction, making it a promising candidate for applications in structural health monitoring and engineering measurements.