Curvature sensor based on D-shape fiber long period fiber grating inscribed and polished by CO2 laser

In this paper, a D-shape fiber long period fiber grating (DLPFG) curvature sensor has been theoretically and experimentally investigated. Different from previous reported long period fiber grating (LPFG) curvature sensor based on D-shape single mode fiber (SMF), the fabrication of DLPFG is divided into two steps: LPFG with a low coupling efficiency is firstly written on a SMF by high-frequency CO2 laser, and then the grating area is subsequently polished to induce deeper resonance peaks and D-shape structure simultaneously. The polishing greatly enhances the fiber asymmetric modulation and thus results in the improvement of curvature sensitivity. Experiments show that the DLPFG has a high curvature sensitivity of 17.6 nm/m−1 and −11.9 nm/m−1 in two opposite directions within the plane perpendicular to CO2 laser-irradiated direction, which has exceeded that of the other reported LPFGs based on D-shape SMF. Besides, the temperature sensitivity is 45 pm/°C in range of −20 °C to 50 °C, and the temperature cross sensitivity is 2.58 × 10-3 m−1/°C. Therefore, this proposed DLPFG sensor with advantages of low-cost, easy-fabrication, high-reproducibility and high-sensitivity is promising for curvature measurement.