Modulation Effects in Light Diffraction by Two Ultrasonic Beams and Application in Signal Processing

Abstract

Among the light-ultrasound interaction phenomena, the modulation of light amplitude, phase, and intensity is a known problem since the beginning of acousto-optics (Debye, and Sears,1 Lucas, and Biquard,2 Raman and Nath3 and others.4–8 In the case of light diffraction by a single ultrasonic beam in the Raman-Nath region4–8 the light amplitude is spatially distributed in diffraction orders in the far field — Fraunhofer-region. In contrast, in the near field — Fresnel — region they form an interference pattern9–11 and a given component of every order (except the O-order) is shifted in frequency due to the phase modulation in-order. In the Fraunhofer region every order is shifted in frequency due to the phase modulations introduced by the Doppler effect according to the known formula $${{\text{v}}_{\text{p}}} = {{\text{v}}_{\text{o}}} \pm {\text{pf}}$$ (1) where vo is the frequency of the incident light, f the frequency of the ultrasonic beam and p is an integer.