Laser-based ultrasonic detection by using all-fiber adaptive fiber Bragg grating sensors

This paper introduces a novel nondestructive testing (NDT) method based on laser ultrasonic technology and fiber Bragg grating (FBG) sensors. A FBG sensor is employed as discrete sensing elements to detect the ultrasonic generated by pulsed laser. An all-fiber and adaptive acousto-ultrasonic (AU) monitoring system for the dynamic detection of impact damage is developed for the present work. A tunable distributed feedback semiconductor laser is used to track AU frequencies as a demodulation scheme. To continuously lock the laser wavelength to the middle-reflection wavelength of FBG, a proportional-integral-derivative (PID) device is integrated with the signals from the photodetector in a feedback loop, thus stabilizing the operating points against quasi-state perturbations such as temperature and vibration, while the high frequency dynamic strain shifts the FBG reflection spectral profiles. We demonstrate the all-fiber adaptive AU demodulating system that has the sensing capability to monitor the high frequency AU signals using FBG that is glued on a loaded aluminum plate.