Acoustic wave-front visualization by use of a Kösters prism in the optical Fresnel diffraction region

Abstract

A simple method for visualizing the wave fronts of a progressive acoustic wave is explained. The Kosters prism works by folding the acoustically phase-modulated optical wave front at its bisecting semitransparent plane. The same temporal frequency components in the folded optical field can interfere with each other in space. Interference fringes spaced by half the acoustic wavelength are obtained over the observation area; the edges of the Kosters prism do not give appreciable diffraction effects. The fringe visibility attains a maximum of 0.54 at the Raman–Nath parameter v = 1.92. Theoretical predictions are fully verified experimentally. The frequency range of acoustic waves to be visualized is discussed; a high-frequency limit is almost nonexistent, but the lowest frequency is limited by the aperture size of the Kosters prism used. This aperture size must be at least four times as large as one acoustic wavelength in the propagation direction of the acoustic wave. Visualization can be made at any planes in the optical Fresnel diffraction region.