Use of phase fluctuations in the scattering of electromagnetic radiation by sound as a highly directional acoustic receiver

Abstract

The phase and amplitude fluctuations of an electromagnetic wave scattered by a sound beam may be used as a highly directional sensitive (in air) acoustic receiver for low-frequency sound. The phase and amplitude changes of the electromagnetic wave as a function of the distance L traveled through the sound beam are calculated using geometrical optics. In the limit λs ≪ L where λs is the wavelength of the sound beam, the phase perturbation caused by variations in the dielectric constant of the medium is nonzero only for normal incidence of the two beams. The maximum phase perturbation at normal incidence is kΔθ = − 12η n0kL cosωst, where kΔθ is the phase change, k is the wave vector for the electromagnetic wave, n0 is the index of refraction, ωs is the angular frequency of the sound beam, and η is the parameter coupling changes in pressure to changes in the dielectric constant. The analysis is extended using Rytov's method of smooth perturbations, and the result is compared with the answer obtained for the analogous problem in the scattering of sound by sound in dispersive media. [This work was supported by the U. S. Office of Naval Research.]