Self-referencing ultrasound detection of fiber Bragg grating sensor with two adhesive bonds

In practical structural health monitoring applications, the environmental conditions typically vary over time. This variation can potentially affect the ultrasound detection of a sensor for damage detection, because ultrasound propagation as well as its coupling through adhesive bond layer to the sensor can be altered. In this case, a reference signal measured without structural defect under some initial condition may not be valid for comparison with the distorted signal (due to defect) that is measured under a different condition. In order to extract reference and distorted signals under the same condition at the same time, in this work 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 demonstrate that ultrasounds in a thin structure can couple to optical fiber guided ultrasounds through two adhesive bonds. We examine the ultrasounds trapped within the optical fiber, which are producing signal interference and beating behavior of the output FBG response. Based on understanding the ultrasound coupling mechanism through two adhesive bonds, finally we test the self-referencing ultrasound detection case, investigating the output FBG response that contains a combined signal of distorted and reference signals extracted through each bond.