Vector Bending Sensor Based on Long-Period Gratings in Linearly Arranged Three-Core Fiber

In this paper, a vector bending sensor based on long-period gratings (LPGs) in three-core fiber (TCF) is proposed. The three cores of TCF are linearly arranged, and the middle core is distributed in the center. LPGs with a period of 465 $\mu \mathrm{m}$ and period number of 35 were prepared by arc-discharge method. Due to the special spatial distribution of the cores in TCF, the proposed TCF-LPGs can realize vector bending measurement. The directional bending sensing characteristics of TCF-LPGs with curvature range from 0.2490 $\mathrm{m}^{-1}$ to 0.8731 $\mathrm{m}^{-1}$ are demonstrated. The bending responses in different directions all show good linearity, and the bending sensitivities are obviously different, which show the sine function behavior with bending directions from -180° to $180^{\circ}.$ And the maximum of bending sensitivity is -$8.83\mathrm{n}\mathrm{m}/\mathrm{m}^{-1},$ obtained at 0°. Therefore, the fiber structure has potential application prospects in the field of vector bending measurement.