Fiber Bragg Grating Displacement Sensor With High Measurement Accuracy for Crack Monitoring

This paper presents a fiber Bragg grating (FBG) displacement sensor for crack monitoring in high-speed railway tunnels. Two FBGs placed in a single-mode optical fiber around two cylindrical rods are employed and configured in a prestretched and suspended status with the two ends fixed to two beams. This design can effectively avoid the pasting process of the grating area and the concomitant problem of chirp phenomenon in the reflection spectrum and promises a high sensitivity and resolution. The mechanical structure, operation principle, and corresponding theoretical model are presented in detail in sequence. Full-scale tests including the accuracy of this sensing system have been performed. The experimental results show that the sensor possesses a high sensitivity of 3304.7 pm/mm, a high accuracy of 20 μm within a range of 0-2 mm, and a low repeatability error of 2.71%. Furthermore, excellent creep resistance (fluctuant value is within 2 pm) and good temperature compensation capacity (maximum value is 27 pm) have also been verified. The proposed sensor shows good performance that satisfies the actual requirements of long-term crack monitoring.