Remote bonding of fiber Bragg grating sensors for Lamb wave measurements: a review

This article reviews the state-of-the-art in remote bonding of fiber Bragg grating sensors, primarily for Lamb wave measurements in structures. The presence of damage in a structure modifies Lamb waves through reflection and scattering, as well as the potential conversion between Lamb modes. While FBG sensors have been applied to capture ultrasonic waves for the past 30 years, the mounting configuration called remote bonding has recently come to attention as an alternative method to capture guided waves from structures. In this case, the FBG is not located at the bond, but instead at a remote location along the optical fiber. The Lamb waves are converted into propagating acoustic waves along the fiber, which are measured by the FBG. This article presents a discussion of the primary benefits of remote bonding, including the higher sensitivity to low-amplitude Lamb waves, the fact that the FBG can situated in a less harsh environment than the sensing region, and the insensitivity to quasi-static strain. The properties of ultrasonic modes in optical fibers and their conversion from Lamb waves is first reviewed. Strategies to detect these waves with FBGs and the associated instrumentation is also presented. Recent examples from the literature utilizing remote bonded FBGs are then presented. Finally, acoustic couplers to transfer the ultrasonic modes from one or more optical fibers to another are also reviewed.