Ultrasonic optical fiber sensing for composite structural health monitoring

Fiber Bragg grating has been widely used as a sensor for detecting static quantities such as strain and temperature, but it is still difficult to detect ultrasonic guided waves due to the limitation of sensitivity and bandwidth. By comparing the traditional fiber Bragg grating and the phase-shifted fiber Bragg grating (PSFBG), it was found that the latter has a shorter and effective grating length and steeper slope. Thus, the PSFBG can greatly improve bandwidth and sensitivity, which is suitable for the detection of ultrasonic Lamb waves. Furthermore, a high-speed demodulator and a feedback controller were designed and integrated with the PSFBG to reduce the noise and enhance the robustness of the sensing system. The excellent performance of the PSFBG-based system is demonstrated in linear and nonlinear acousto-ultrasonic detection, and acoustic emission detection, by comparing with the PZT sensors.