Pure Directional Bending Measurement With a Fiber Bragg Grating at the Connection Joint of Eccentric-Core and Single-Mode Fibers

A novel fiber Bragg grating (FBG) structure based on an eccentric core fiber (ECF) and a single-mode fiber (SMF) was proposed and experimentally demonstrated for distinguishing the bending effect from the axial strain effect and measuring the pure directional bending. The structure is fabricated by writing a FBG on the fusion splice junction between an ECF and a SMF. The FBG formed on the ECF-SMF junction has two different resonant peaks because of the different refractive indices of the two fibers. The peak relating to the ECF is sensitive to both directional bending and axial strain, while the peak corresponding to the SMF is only sensitive to strain. By using the unique FBG structure, one can directly know whether a bending is accompanied by an axial strain, and can obtain both the direction and curvature of the bending. Experimental results show that the bending sensitivities of the FBG in the ECF part are 49.3 and -50.3 pm/m ^-1 at directions of 0° and 180°, respectively, and the strain sensitivities of the FBG in the ECF and SMF are 0.74 and 0.70 pm/με, respectively.