Investigation of an integrated fiber laser sensor system in ultrasonic structural health monitoring

Abstract

Various optical fiber sensors have been utilized in structural health monitoring (SHM) to detect strain, temperature, and the ultrasonic guided wave, which is the main measurand discussed in this study. By leveraging existing ultrasonic optical fiber sensors based on fiber Bragg grating (FBG) technique, we proposed a novel sensing concept in which the ultrasonic-induced Bragg wavelength shift was demodulated by an erbium fiber laser (EFL). The ultrasonic sensor and light source, i.e., an FBG and EFL, were seamlessly integrated into a fiber laser sensor system, in which the output power change directly represents the ultrasonic signal. In addition to discussing the theory behind the sensor, we focused on the applications and actual performance of the sensor, including sensitivity, bandwidth, and robustness, in practical ultrasonic SHM. We were able to constantly detect ultrasonic signals in the range of 25 °C–85 °C or simultaneously detect static measurands (temperature or strain) and ultrasonic signals by changing the position of the FBG where it is attached. Because of the diverse structures of the EFL and FBG, we created, investigated, and compared three sensing systems with different properties and advantages to find their most suitable applications and fully explore their maximum potential. In addition, the experimental results from these novel optical fiber sensors were compared to those from a conventional broadband PZT sensor. The results showed that the new sensor has some unique advantages, such as self-adjustable ultrasonic detection.