Smart Feedback Control for Fiber-optics Acoustic Emission Sensor System

Optical fiber sensors for ultrasonic detection have become a subject of much research in recent years. In this thesis, a fiber-optic acoustic emission (AE) sensor system that is capable of performing AE detection, even when the sensor is experiencing large quasi-static strains, is first described. The system consists of a smart selection of a wavelength notch to which a distributed feedback (DFB) laser is locked for high sensitivity AE signal demodulation. A smart feedback control unit for the DFB laser, which is the focus of this thesis, is designed and investigated. The smart control ensures that the AE signal is monitored without significant disruption even when large external strains are superimposed on the sensor. Using a chirped fiber-Bragg-grating Fabry–Perot interferometer (CFBG-FPI) sensor head, the performance of the designed feedback controller has been examined. Experiments were conducted on an aluminum plate which in the meantime was subject to a large background strain variation. The large external strain shifted the Bragg wavelength by six spectral notches of the CFBG-FPI and thus led to a total of six locking point “jumps” to accommodate the large strain change while the AE signals were continuously monitored with minute disruption. The designed smart control proved to work properly for the fiber-optic acoustic emission system. In the end of this thesis, characteristics of wavelength tuning, in terms of wavelength scanning range and delay as a function of tuning frequency, of a narrow linewidth semiconductor has been investigated. This narrow linewidth laser based on quantum well theory has much lower intensity and frequency noise in comparison with the DFB laser used in our previous experiments, and thus will serve as a promising candidate for next generation AE sensor with enhanced performance. Advisor: Ming Ha