Focused sound field measurements typically involve needle- or membrane-type transducers stepped across the sound field. This process produces an apparent image of the sound field limited by probe linearity, effective aperture size, and experimental alignment. Historically, acousto-optic schlieren imaging has provided an effective, qualitative technique for examining a sound-pressure field. In this paper the schlieren technique is extended to provide quantitative measurements of the peak focused sound-pressure field without in situ disruptions. A numerical solution of the Khokhlov–Zabolotskaya–Kuznetsov parabolic equation is used to predict the sound-pressure field for a line focused transducer. Enhancements in the standard numerical solution include a floating boundary condition and an adaptive technique for adjusting the computation effectiveness consistent with nonlinearity induced harmonic growth. Key features of the experimental setup are outlined and theoretical predictions compared with experimental measurements made with the extended schlieren technique.
Finite-amplitude pulses are examined acousto-optically using a newly developed lightdiffraction apparatus. Based on an optical analysis of ultrasonic transducer response to continuous-wave excitation at and near the fundamental frequency, pulse Fourier spectra are derived for input to a light-diffraction model, providing quantitative agreement between experiment and theory. The diffraction theory predicts that a light-diffraction pattern produced by a harmonically distorted acoustic pulse train will exhibit asymmetry in the intensity distribution with respect to the zero order. To simulate harmonic distortion, pulse frequency spectra are used for input to a computational model that is based on the Burgers' equation for propagation of finite-amplitude acoustic waves in a nonlinear medium. The spectrum, propagation, and light-diffraction models give a complete description of light diffraction by finite-amplitude pulses and provide good agreement with experimentally obtained diffraction patterns.
The diffraction pattern produced by reflection of monochromatic light from an ultrasonic surface wave composed of a fundamental and higher harmonic frequencies is calculated for a Gaussian and a uniform light surface illumination. The diffraction pattern is found to be asymmetric with respect to the central order. This asymmetry is calculated from available diffraction data on LiNb0 3 .
Low-MHz, continuous ultrasonic waves traveling in a transparent medium cause light to be diffracted into discrete diffraction orders when light and sound propagation directions are normal to each other. When pulsed ultrasonic waves are used the diffraction orders split into secondary orders which are asymmetric with respect to the central diffraction order. This splitting is derived and a general expression provided for the intensity as a function of the ultrasonic pulse Fourier spectra. Examples are provided which demonstrate the degree of asymmetry for an exponential driving pulse and the convergence to the classic Raman–Nath results when the pulse approaches a continuous wave.
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