Bragg Diffraction Sampling of a Sound Field

Abstract

In this paper we describe a novel method of probing a sound field in three dimensions with a focused light beam. In our method, the spatial-temporal modulation impressed upon the light by the sound is extracted by the use of either polarization sensitive elements or spatial filters placed in front of a photodiode. The technique is inherently capable of high-resolution sampling because, as is shown, phase and amplitude of the resulting rf current are indicative of the complex sound pressure in a narrow region around the focus of the light beam. This is in contrast to earlier methods using a narrow collimated light beam which only give an indication of sound-pressure amplitude and phase integrated over the entire width of the sound field traversed by the light beam. The first part of the paper analyzes the mode of operation and discusses sensitivity and resolution in a heuristic manner. The second part provides experimental results for various frequencies and sound media. The third part treats the rigorous theory.

Full-field mapping of ultrasonic field by light-source-synchronized projection

“…Because of the advantage of noninvasive measurements, various optical techniques, such as the Bragg diffraction, 1 Schlieren imaging, 2 Raman-Nath diffraction, 3 and TV holography, 4 have been used to map ultrasonic fields. Most of these methods except TV holography must use some scanning mechanisms to acquire an image.…”