Distributed ultrasonic sensor system based on fiber ring lasers for acoustic emission detection

Acoustic emission (AE) testing is a powerful tool for structural health monitoring, which detects, identifies, locates, and displays damage-related changes, such as crack initiation and growth, fiber breakage, or corrosion, in structures without causing damages. Compared with traditional piezoelectric AE sensors, fiber-optic AE sensors present many advantages. They are small, lightweight, immune to electromagnetic interference, and capable of multiplexing. Therefore, fiber-optic sensors, particularly fiber Bragg grating (FBG) based sensors have been extensively investigated for acoustic emission detection. Those FBG ultrasonic sensors rely on detecting the ultrasonic-induced spectral shift of the Bragg wavelength, employing narrow bandwidth tunable lasers. However, in practical applications such as on-aircraft monitoring or subsea monitoring, operating point can easily drift out of linear range due to quasi-static strains or ambient temperature variations. To address this challenge, we propose an intensity-demodulated fiber-optic ultrasonic sensor system based on fiber ring laser (FRL). Ultrasonic signals are received by a FBG in the FRL cavity, and are demodulated directly from the laser intensity variations. Theoretical and experimental results both reveal that the FRL ultrasonic sensor can faithfully detect ultrasonic waves. With a tandem design where two FBGs are installed side-by-side at the same location on a structure, we demonstrate that the FRL sensor can self-adapt to large quasi-static background strains or ambient temperature changes. Additionally, we propose a novel multiplexing approach for FRL sensors to avoid mode competition in a single span of erbium-doped fiber. The multiplexing is achieved by utilizing add-drop filters to route light signals, according to their wavelength, into different optical paths, each of which contains a separate span of erbium-doped fiber as the gain medium. The proposed method is experimentally demonstrated using a three-sensor distributed ultrasonic sensor system where each sensor can independently operate. The simple intensity demodulation, self-adaptive operation as well as all-fiber configuration make the distributed ultrasonic sensor system particularly attractive for practical applications in structural health monitoring