High Frequency Ultrasonic Wave Detection Using Fiber Bragg Gratings

摘要

This paper describes the results of the characterization of Fiber Optic (FO) Bragg Gratings (BG) for detection of high frequency ultrasonic waves. The ultimate goal of this effort is to determine the sensitivity of these FO sensors for Acoustic Emission (AE) detection. Ultrasonic AE signals are typically generated in materials when a pre-existing crack in the material grows in length. In order to characterize these sensors, FOBG's of different lengths were bonded to piezoelectric crystals of comparable thickness. The lengths used in this study where 1 mm, 2 mm, 4 mm and 6 mm. This allowed us to study the effects of the grating-length to ultrasound-wavelength ratio as they relate to detection sensitivity. A large number of frequencies (ranging from 10 kHz to 2 MHz) were used in the study to determine the effect of frequency on System sensitivity. To enhance detection sensitivity, a matched Bragg grating was used in the demodulation side of the detection system. We have also performed theoretical modeling of the acoustic emission detection by fiber Bragg gratings using coupled wave theory. Our modeling results will be compared with the experimental findings. Work is currently underway to apply this detection technique to monitor acoustic wave emission from cracking and sudden damage in composite polymer plates.