Application of an adaptive two-wave mixing interferometer for detection of surface defects

The technology of non-contact and non-destructive detection of acoustic and ultrasound waveforms play an importance role in industrial application which make it possible to measure acoustic and ultrasonic in hostile environments, such as high temperature circumstance or long distance from test structure. The conventional Optical interferometer cannot detect rough surfaces accurately due to the noise caused by serious scattering. And the environmental noise (low-frequency) arise from machine vibration can also affect the measuring result. Thus, a kind of nonlinear interferometer based on photorefractive effective of photorefractive crystal (BSO) has been proposed to solve these problems. When signal beam (carrying vibration information) and reference beam incident on the photorefractive crystal, A holographic grating is formed due to the photorefractive effect, and the grating diffracts the signal beam and reference beam then the information of the ultrasonic vibration can be demodulated. Meanwhile, the low frequency noise caused by the environmental perturbation can be filtered by the dynamic holographic grating. It is demonstrated that this kind of nonlinear interferometer has the advantages of highly sensitive, low cut-off frequency, simple structure and high resolution. In this paper, we use this nonlinear interferometer to detect the surface defects of the aluminum sheet and get the information of defect location successfully. Samples of aluminum sheet with different size and location of defects are divided into groups to study. Finally we compare with the result concluded from ultrasonic flaw detector. The research findings conclude that this technology provide a potential application for testing surface-breaking cracks and ultrasonic vibration detection in industrial production.