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 CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser-based point-shaped exposure technique was introduced to induce asymmetric RIM, enabling the simultaneous coupling of the LP<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sub> and LP<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">06</sub> 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<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> over a curvature range of 0 to 3.88 m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>. 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.