Optical observation of finite amplitude ultrasonic fields

When a coherent light is passed through an ultrasonic field of finite amplitude, the diffraction spectrum has an asymmetrical form around the Oth order component due to the harmonic generation corresponding to the waveform distortion in the propagation of ultrasound. This paper demonstrates that the asymmetry depends on the phase relationships among the harmonic components of the ultrasound. A method is proposed which can reconstruct the original ultrasonic waveform by a simple measurement and calculation. The method is to separate the symmetric and antimetric components of the diffracted light by calculating the sum and difference of the positive‐ and negative‐order components, and to determine the amplitude and phase from the sum and the difference, respectively. Using the proposed method, the CT techniques are applied to the optically measured data which are a line‐integral of the ultrasonic pressure. The ultrasonic field distribution is visualized for arbitrary cross section for each harmonic component, making it possible to observe the two‐dimensional distribution of the harmonic generation accompanying the propagation of finite amplitude ultrasound.