Self-referencing ultrasound detection of fiber Bragg grating sensor remotely bonded at two locations

Ultrasound measurement for damage detection in practical structural health monitoring (SHM) applications is often affected by varying environmental condition. In this case, a baseline reference signal measured under initial conditions may not be valid for comparison with the distorted signal (due to structural damage) that is measured under different conditions. In this study, we investigate a self-referencing ultrasound detection of fiber Bragg grating (FBG) by bonding an optical fiber at two different locations away from the FBG. We first investigate the extraction of ultrasonic waves from two different adhesive bond locations, which are measured with a single FBG sensor located between the two bonds. Based on understanding the ultrasound coupling mechanism through two adhesive bonds, we test the self-referencing ultrasound with the presence of a damage in a structure, examining the output FBG response that contains a combined signal of distorted and reference signals extracted through each bond.