光电子学-声光相互作用(English).ppt

Major：电子与通信工程专业 Interaction of Light and Sound 声-光相互作用 Chapter：12.0 —— 12.4 Outline 12.0 Introduction 12.1 Scattering of Light By Sound 12.2 Particle Picture of Bragg Diffraction Of Light By Sound 12.3 Bragg Diffraction Of Light By Acoustic Waves—Analysis 12.4 Deflection Of Light By Sound A sound wave consists of a sinusoidal perturbation of the density of the material , or strain, that travels at the sound velocity , as shown in Figure 12-1. 12.1 Scattering of Light By Sound A change in the density of the medium causes a change in its index of refraction, which , to first order, is proportional to it. We can represent the sound wave shown if Figure 12-1 by (12.1-1) where . Next consider an optical beam incident on a sound wave at an angle as in Figure12-2. We can characterize the sound wave as a series of partially reflecting mirrors, separated by the sound wave-length ，that are moving at a velocity . A necessary condition for diffraction in a given direction is that all the points on a given mirror contribute in phase to the diffraction along the direction. Considering the diffraction from two points, such as C and B in Figure 12-2, it’s necessary that the optical path AC-BD be some multiple of the wavelength for diffraction along to occur. (12.1-2) The only way in which (12.1-2) can be satisfied simultaneously for all points x along a given reflector is if m=0, from which it follows that (12.1-3) In addition to the requirement that the different parts of a given acoustic phase front interfere constructively, which leads to (12.1-3), we require that the diffraction from any two acoustic phase fronts add up in phase along the direction of the reflected beam. The path difference, AO+BO, of a given optical wave front resulting from reflection from two equivalent acoustic wave fronts must thus be equal to the optical wavelength . We find this conditio