BRAGG-PHENOMENA IN THE FRESNEL REGION OF LIGHT DIFFRACTED BY ULTRASOUND

The Fresnel region of light diffracted by a plane harmonic ultrasonic wave is examined under Bragg and near-Bragg conditions. For diffraction in the Bragg regime, the intensity of the diffracted light is sinusoidally modulated as a consequence of a two beam interference. In that case, the amplitude of the modulation does not depend on the plane of observation. For diffraction parameters in the near-Bragg regime, the light intensity modulation is produced by a multiple beam interference and its harmonic character disappears. In that case, the shape of the modulation also depends on the specific plane of observation.