Study On The Stability Of The Fiber-optic Acoustic Emission Testing Systems

Acoustic emission testing for nondestructive detection and evaluation is a very important subject for domestic and international researchers. As the combination of the fiber-optic acoustic emission testing and the fiber optic sensing, the acoustic emission testing would have more widely application in future. Because the fiber-optic sensor is inclined to be influenced by the conditions, temperature and some other factors, solving the problems about the stability of the fiber-optic sensor becomes a key issue to improve the stability of the acoustic emission testing systems.To improve the stability of the acoustic emission testing system, this dissertation proceeding from the principles of the Mach-Zehnder fiber-optic acoustic emission sensor which is applied in the acoustic emission testing system, studied the phase characteristics of that sensor. The formula of the output light intensity is derived by Jones matrix. The initial phase difference, the characteristics of the output signal waveforms and of the frequency gotten at the modulation of large signal and small signal respectively, of the fiber-optic sensor are analyzed. Otherwise, reasons for the phase drift and the frequency multiplication are discussed. Finally, the optimal working state of the interferometer is obtained, and the tracing modulation principle of the phase using PZT is analyzed.In addition, this dissertation proceeding from the polarization properties of the fiber-optic sensor, analyzed the reason of instability of polarization state for the single mode fiber, and discussed the influence of the bending stress and the torsion stress on the polarization respectively, furthermore, the decline factors of polarization for output light intensity signal and the appended phase shift noise are discussed, based on the Mach-Zehnder fiber-optic acoustic emission sensor.On the basis of theoretical analysis, the Mach-Zehnder fiber-optic acoustic emission testing system is built and studied from the aspect of phase features, polarization properties, temperature, circuits, and other environmental noise. Influences of the large signal and the small signal on the interference signal are given by applying different modulating signal to the fiber-optic sensor using PZT. And influence of the polarization on the visibility of the interference signals is also obtained by the torsion of the fiber.