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Exploration of Ultrasonic Guided Wave Detection with Optical Fiber Sensors and Piezoelectric Transducers



This paper presents a comparative study of ultrasonic guided wave detection with optical fiber sensors and piezoelectric transducers. In recent years, fiber Bragg gratings (FBG) have been investigated by several researchers as an alternative to piezoelectric sensors for the detection of ultrasonic waves. FBG have the advantage of being durable, lightweight, and easily embeddable into composite structures as well as being immune to electromagnetic interference and optically multiplexed. However, there is no turn-key commercially available product that uses this promising technology for the detection of ultrasonic guided waves because: (a) the frequency is high (hundreds of kHz and above); (b) the strains are very small (less than one microstrain); (c) the operational loads may also induce very large quasistatic strains (the detection of very small ultrasonic strains superposed on large quasi-static strain presents a quite challenge). Although no turn-key optical system exists for ultrasonic guided wave detection, two commercially available FBG interrogation systems with various frequency ranges and strain resolutions (Micron Optics SM690 and Redondo Optics M200) are available and have been evaluated by us. This paper starts with a literature review of the state of the art in optical technologies for ultrasonic wave detection. Subsequently, this paper describes our comparative tests with several fiber optics systems. The measurement resolution and sampling speed were considered as the most important criteria in our test. In the final part, a commercially available tunable laser source from LUNA was customized for ultrasonic sensing. Parallel strain measurements were done with different systems including FBG and piezoelectric wafer active sensors (PWAS). The measurements were compared. Possible calibration and performance improvements for the optical interrogation system are also developed and discussed. The paper ends with conclusions and suggestions for further work.

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