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In structural health monitoring (SHM) applications, fiber Bragg grating (FBG) sensors are typically bonded directly to the surface of a structure to detect ultrasonic waves for damage identification. The sensitivity of the bonded FBGs to guided waves varies significantly with input ultrasound wavelength (λ)-to-FBG grating length (L) ratio, i.e., λ/L. Recently, the authors have demonstrated that the detection sensitivity of an FBG can be potentially increased by remotely bonding the optical fiber at a distance away from the FBG, however its response as a function of λ/L has not been studied in detail. In this work, we investigate the ultrasound detection of directly bonded FBG and remotely bonded FBG with varying λ/L. Specifically, we maintain L constant and change λ by varying the ultrasound excitation frequency. Using a 3D laser Doppler vibrometer (LDV) we first characterize input Lamb waves as a function of frequency, which are excited in a thin plate using a broadband transducer. Next, we measure the output response of directly bonded FBG and remotely bonded FBG to the same input Lamb waves. Finally, we examine the output FBG responses normalized with the LDV measurements of input waves, investigating the FBG sensitivity as a function of λ/L ratio. Understanding this sensitivity is important because many guided wave signals, for example generated by acoustic emission, are broadband. Additionally, multiple frequencies are often used for guided wave imaging of structures.
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