Simultaneous Measurement of Curvature Vector and Temperature Based on Composite Gratings Inscribed on D-Shaped Fiber

A novel two-parameter fiber sensor has been demonstrated, consisting of D-shaped composite fiber gratings (DCFG) which includes long period fiber grating (LPFG) and fiber Bragg grating (FBG) inscribed on D-shaped fiber. LPFG is inscribed on the side of D-shaped fiber by a high-frequency CO2 laser. On account of the asymmetry of the structure in 90° and 270° and refractive index modulation (RIM) in 0° and 180°, the sensor can recognize the bending directions. In the theory, it is proved that by adjusting the thickness of D-shaped fiber, the bending sensitivity can be enhanced in 90° and 270° directions. In the experiments, the fiber sensor achieves the sensitivity of curvature up to 13.70 nm/m−1, which is nearly two times as the similar structure without D-shaped fiber. The curvature and temperature sensitivities of LPFG and FBG are different in DCFG, so the proposed sensor can realize simultaneously measurement of curvature vector and temperature.