IMPROVEMENT OF OPTICAL PROBE RESPONSE TO LARGE-AMPLITUDE ULTRASOUND

An ultrasound detection system with optical heterodynes shows nonlinea r behavior ill the large-amplitude region (>lambda/8, lambda: waveleng th of detecting light), and entails some problems in the measurement o f laser ultrasound (ultrasound generated by laser deposition), especia lly for thin specimens. We studied the mechanism of the nonlinear resp onse and found a method for improving it. Through experiments, we devi sed an improved demodulation method using signal processing techniques to extract actual ultrasonic waveforms in the large-amplitude region, and confirmed its effectiveness. We also carried out numerical experi ments to investigate the characteristics of our improved demodulation method and the standard demodulation method. The results show that com pared with the standard demodulation method, our method is more robust against noise of beat signals, and more useful for measuring laser ul trasound even in the small-amplitude region.