Phase-Shifted Fiber Bragg Grating Sensing Network and its Ultrasonic Sensing Application

Optical fiber sensors have been utilized as an alternative to piezoelectric sensors in ultrasonic nondestructive testing due to their advantages such as a light weight, small size, and anti-electromagnetic interference. Although phase-shifted fiber Bragg gratings (PSFBGs) have better performance in sensitivity and bandwidth compared to conventional uniform fiber Bragg grating, the problem of PSFBG multiplexing originating from the demodulation technique should be addressed, and their practical applications should be further explored. We designed a novel PSFBG ultrasonic sensing network by integrating a proportional-integral-derivative controller to maintain system stability in this study. The proposed sensing network can have two different layouts, namely, rosette and array. Its sensing performance was demonstrated with ultrasonic detection of Lamb waves propagating on an aluminum plate and a composite laminate. The PSFBG rosette manifested omni-directional sensitivity because the orientation-dependent sensitivity of each PSFBG sensor in the rosette can be mutually compensated. The PSFBG array monitored the mode dispersion and amplitude change along the wave propagation. Since the sensitivity of the PSFBG sensor is comparable to that of a conventional lead-zirconate-titanate sensor, the PSFBG sensor succeeded in detecting Lamb waves in the composite laminate and evaluating the number of matrix cracks generated in a three-point bending test by monitoring the amplitude decrease of the Lamb waves. These experiments demonstrated that the proposed PSFBG network has a good application prospect in ultrasonic nondestructive testing.