Visualization of a phase structure of sound field in a Bragg cell

Abstract

A phase structure of a sound field in a Bragg cell was investigated experimentally using a dual-beam, dual-frequency laser system. The technique used allows the recording of a spatial phase structure of diffracted light field to visualize phase nonuniformities of a sound field. The underlying physical principle of this technique is to probe an analyzed phase object using two slightly separated in space light beams with different optical frequencies. A difference frequency signal, which is obtained as a photodetector output in an optical heterodyne system, contains information about a phase difference of the adjacent areas of the sound field. When the probing beams are diffracted by a Bragg cell, their optical frequencies are shifted by the same acoustic frequency, so that the difference is the same at any Bragg cell operating frequency. A Bragg cell on paratellurite operating around 100 MHz was investigated. Focused laser beams with a difference of optical frequencies equal to 1 MHz were used as probing beams. Pictures were obtained which demonstrated dependencies of a phase structure of diffracted light on frequency and sound power. Phase nonuniformities across the sound field up to three wavelengths have been estimated. [Work supported by EOARD.]