Nonlinearity Considerations in Bragg Imaging of Ultrasonic Waves

Abstract

Two fundamentally different techniques exist for the study of the diffraction of light by finite amplitude ultrasonic waves: (1) Diffraction of light at normal incidence can be used to determine the ultrasonic waveform. (2) Diffraction of light incident at the Bragg angle gives detailed information of a completely different nature. Each diffraction order contains an image of the ultrasonic wavefront. Therefore, objects placed within the ultrasonic field are imaged in the diffraction orders. When a finite amplitude ultrasonic wave is used, an ultrasonic wave transmission filter allows one to distinguish the images produced by the second harmonic from those produced by the fundamental. The image produced by the second harmonic is twice as large as that produced by the fundamental and appears at the position of the fundamental's second order. In addition, at large amplitudes, multiple scattering of light by the fundamental ultrasonic wave can produce higher Bragg diffraction orders. These higher orders contain multiple images. Two images appear in the second order, three in the third, etc. [Research supported by the Office of Naval Research.]