Acoustic Beam Probing Using Optical Techniques

It is well known that acoustic waves in transparent materials can be used to deflect or scatter light beams. 1 As a result, a great deal can be learned about the energy distribution in the acoustic beam by studying the angular and positional dependence of the optical-acoustic interaction. The paper is divided into two parts. Section I is devoted to the theoretical and experimental demonstration of the fact that the amplitude of the light deflected by an advancing sinusoidal acoustic wave, as a function of the angle between the direction of light propagation and the acoustic wavefront, is proportional to the Fourier transform of the amplitude distribution of the acoustic wave in the plane of the wavefront. Thus the angular dependence of the optical-acoustic interaction accurately and directly measures the angular distribution of the acoustic energy. Stated another way, a study of the total power in the scattered light beam as a function of the angle of the light beam relative to the acoustic beam yields directly the far-field or Fraunhofer diffraction pattern of the acoustic beam. The power in the deflected light beam measures the acoustic intensity at the position of the light beam. Absolute determination of the acoustic intensity can be made if the photoelastic constants for the medium are known. This technique is thereby capable of providing more information about the acoustic beam than can be obtained with conventional pulse-echo techniques or acoustic probes, such as described by Fitch and Dean, 3 which must be used at a boundaiy of the acoustic transmission medium.