Self-Assembling Vectorial Curvature Sensing System Based on a Single Helix of Polarization-Maintaining Fiber

This article presents a novel composite vector bending sensing system capable of switching freely between wavelength and intensity modulation. The fundamental sensing unit of the system depends on an offset rotation twisted Mach-Zehnder interferometer (TW-MZI) with unique refractive index (RI) distribution characteristics proven through experiments. The sensing unit delivers remarkable performance in intensity modulation, reaching a maximum of 18.74 ± 0.01 dB/m-1. The sensing unit features a nonuniform geometric distribution and utilizes polarization-maintaining optical fibers as its structural material, enabling it to detect the correlation of polarized light and recognize the bending direction. With these advantages, a quasi-parallel distributed curvature sensing system is constructed, integrating a Sagnac interferometer (SI) as the sensing reference arm combined with the TW-MZI sensing unit. Adjusting the interference length and optical power can obtain a first-order harmonic vernier spectrum. Additionally, the envelope differencing technique enhances the interference fringe contrast, reducing the reliance on the monitoring wavelength domain. This combination highlights the composite system's wavelength modulation-based vector curvature sensing capability, achieving a maximum value of 15.59 ± 0.01 nm/m-1. Moreover, this system exhibits characteristics such as low-temperature crosstalk, high stability, and rapid responsiveness, making it well suited for applications in medical device diagnostics, oil and gas pipeline inspection, and structural safety monitoring within the construction industry.