A high-sensitivity ultrasonic sensing system is proposed and demonstrated. In this system, a phase-shifted fiber Bragg grating (PS-FBG) is used as a sensor to achieve broadband and highly sensitive detection. The PS-FBG modulates the output of a tunable laser to detect the ultrasonic strain directly. Balanced photo-detector (BPD) is used for eliminating the DC component and further amplifying the AC component in the detected signal. Another major function of the BPD is to reject laser intensity noise. As a result, the minimum detectable strain is limited by the BPD’s noise and laser frequency noise. The sensitivity of the system is 9 nε/Hz^1/2. Because of its high sensitivity, this system has the potential to be used in acousto-ultrasonic testing without amplifying the input signal and in practical acoustic emission detection.

The fiber Bragg grating (FBG) sensor, which was developed over recent decades, has been widely used to measure manifold static measurands in a variety of industrial sectors. Multiple experiments have demonstrated its ability in ultrasonic detection and its potential in ultrasonic structural health monitoring. Unlike static measurements, ultrasonic detection requires a higher sensitivity and broader bandwidth to ensure the fidelity of the ultrasonic Lamb wave that propagates in a plate-like structure for the subsequent waveform analysis. Thus, the FBG sensor head and its corresponding demodulation system need to be carefully designed, and other practical issues, such as the installation methods and data process methods, should also be properly addressed. In this review, the mature techniques of FBG-based ultrasonic sensors and their practical applications in ultrasonic structural health monitoring are discussed. In addition, state-of-the-art techniques are introduced to fully present the current developments.