Torsion-Insensitive Curvature Sensing via Azimuthal-Order-Controlled Cladding Mode Coupling in Long-Period Fiber Gratings

We proposed a novel long-period fiber grating (LPFG) capable of simultaneously coupling cladding modes with different azimuthal orders. A theoretical model of an LPFG with asymmetric refractive index modulation (RIM) was developed in a single-mode fiber to investigate the effects of cross-sectional distribution and modulation depth on the coupling efficiency of cladding modes. Unlike conventional fabrication methods, a novel CO2 laser-based point-shaped exposure technique was introduced to induce asymmetric RIM, enabling the simultaneous coupling of the LP15 and LP06 mode within a single grating. The proposed LPFG leverages the resonance wavelength difference between the two cladding modes for sensing applications. Experimental results demonstrated effective suppression of torsion sensitivity, achieving a minimal value of 1.2 pm/(rad/m) over a twist rate range of −34.5 rad/m to 34.5 rad/m. Additionally, the LPFG reduced sensitivity to temperature and refractive index while exhibiting a high curvature sensitivity of 13.34 nm/m ${}^{-{1}}$ over a curvature range of 0–3.88 m ${}^{-{1}}$ . The combination of reduced cross-sensitivities, high curvature sensitivity, and compact structure highlights the potential of the proposed LPFG for practical sensing applications in complex environments.