Recently, in order to resolve the conflict between bandwidth and sensitivity in normal ultrasonic FBG sensors, phase-shifted fiber Bragg gratings (PS-FBG) were introduced by the authors and other researchers as a potential alternative to ultrasonic sensors [5,6]. The effective grating length of the PS-FBG sensor is much shorter than its grating length, leading to better responses to high-frequency ultrasonic waves. Additionally, the steep slope in the peak area of PS-FBG effectively improves the sensitivity of the sensing system, because the sensitivity of the demodulation system based on laser-source positively relates to the slope of the spectrum according to [7]. For example, the authors have successfully used PS-FBG in the acousto-ultrasonic [5] and the acoustic emission (AE) methods [8].

In these experiments, the PS-FBG sensor was glued in line to the ultrasonic source, which is a setup similar to other experiments where normal FBGs were used. However, a network with many FBG or PS-FBG sensors is necessary in both the acousto-ultrasonic and the AE method. Thus, to understand the sensitivity distribution properties of FBG or PS-FBG, it is important to optimize the sensor network for NDT and SHM. Furthermore, signal analysis to determine the damage types or identify the impact position or AE positions also demands systematic study in this field.Although, compared to a normal FBG, a PS-FBG has broader bandwidth and higher sensitivity to ultrasonic waves, the theory of ultrasonic detection for PS-FBG is identical because, under the influence of ultrasonic waves, PS-FBGs undergo the same physical changes as optical fibers.

Once we have obtained the sensitivity distribution properties of the PS-FBG, we could also deduce these results for normal fiber sensors in more common cases. The sensitivity distribution properties of an optical fiber sensor are mainly determined by the distance from and the angle to the ultrasonic source, but the distribution is also influenced by many other factors, such as effective sensing length and cladding property. Furthermore, when the fiber is attached to the surface of a material, additional factors should be considered, including coupling performance, the ultrasonic waves’ properties, and the material’s geometric structure. In common cases of NDT or SHM, the target material has a plate shape, while the propagating ultrasonic wave is a Lamb wave.

Thus, in this study, we comprehensively researched the sensitivity distribution properties of PS-FBG when the sensor was attached to the surface of an aluminum plate. The structure of this paper Dacomitinib is as follows: firstly, the experimental setup is introduced; then, theory analysis is explained; and lastly, the results are shown and discussed.2.?Experimental SetupFigure 1 is a schematic diagram of the experimental setup based on the acousto-ultrasonic method.