Improvement of Optical Probe Response to Large-Amplitude Ultrasound

An ultrasound detection system with optical heterodynes shows nonlinear behavior in the large-amplitude region (>λ/8, λ: wavelength of detecting light), and entails some problems in the measurement of laser ultrasound (ultrasound generated by laser deposition), especially for thin specimens. We studied the mechanism of the nonlinear response and found a method for improving it. Through experiments, we devised 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 experiments to investigate the characteristics of our improved demodulation method and the standard demodulation method. The results show that compared with the standard demodulation method, our method is more robust against noise of beat signals. and more useful for measuring laser ultrasound even in the small-amplitude region.