The electro-optic and acousto-optic effects and modulation of light beams

Abstract

In this chapter we shall explain how the distortion produced in a crystal lattice by the application of an electric field or by the passage of a sound wave affects the propagation of light through the crystal. These effects – the electro-optic and acousto-optic effects, and related effects such as field-induced changes in the absorption of a material – are of considerable practical importance since they can be used to amplitude- and phase-modulate light beams, shift their frequencies, and alter the direction in which they travel. Introduction to the electro-optic effect When an electric field is applied to a crystal, the ionic constituents move to new locations determined by the field strength, the charge on the ions, and the restoring force. As we saw in Chapter 17, unequal restoring forces along three mutually perpendicular axes in the crystal lead to anisotropy in the optical properties of the medium. When an electric field is applied to such a crystal, in general, it causes a change in the anisotropy. These changes can be described in terms of the modification of the indicatrix by the field – both in terms of the principal refractive indices of the medium and in terms of the orientation of the indicatrix. If these effects can be described, to first order, as being linearly proportional to the applied field then the crystal exhibits the linear electro-optic effect. We shall see that this results only if the crystal lattice lacks a center of symmetry. So, some cubic crystals can exhibit the linear electro-optic effect.