Photorefractive Ultrasonic Sensor for Weld Quality Monitoring

Abstract

We report on a laser ultrasonic receiver based on two wave mixings in two nonlinear Bi12SiO20 (BSO) photorefractive crystals and its application for welded joints inspection. The nonlinear photo refractive crystals are operated with inverted voltages in respect to each other. Reversal of the voltage inverts the polarity of detected ultrasonic pulses which are generated by a pulsed laser on a sample surface. Present noise is, however, not inverted. Subtracting both signals after the two photorefractive crystals using a balanced photo-detector leads to suppression of noise and to an increase in the signal/noise ratio. We demonstrate the working principle by using a phase modulator and present laser ultrasonic measurement and compare the results from the proposed technique with a conventional two wave mixing interferometer based on single BSO crystal showing that the signal to noise/rate is significantly enhanced. Finally, we show the experimental results of application of these detectors for defects detection in two industrial samples with welded area.