Optical Imaging of the Cross Section of a Microwave Acoustic Beam in Rutile by Bragg Diffraction of a Laser Beam

Abstract

This letter reports some results of an analysis, based on a wave-optical formulation, of the farfield Bragg diffraction from a cylindrical line source light beam by a line-source acoustic wave. The properties of the diffracted light that are relevant to the problem of optically viewing or imaging the acoustic fields of plane or cylindrical transducers are discussed. The most important of these properties are as follows: (1) A line light source exists for the diffracted light and is either real or virtual depending upon the location of the acoustic source relative to the light source; (2) the location of this line source for the diffracted light has a geometrical correspondence to the acoustic source location; and (3) the phase and amplitude of the acoustic source is preserved in the diffracted line source. As the field of a plane or cylindrical transducer can be represented as a superposition of a set of line-source waves, it follows that the total diffracted field is either a real or virtual image of the acoustic field. In either case, a real, enlarged image can be formed by the used of proper lenses. A description and examples of results are given for an experiment in imaging the transverse section of two parallel but physically separated 923 MHz microwave acoustic beams in a rutile crystal. The image resolution obtainable is discussed and compared for the experimental conditions with that obtained using a finely focused, mechanically scanned light probe.