3 - Theory of Diffraction

Summary

Introduction

The diffraction of light plays an important role in optical data storage systems. The rapid divergence of the beam as it emerges from the front facet of the diode laser is due to diffraction, as is the finite size of the focused spot at the focal plane of the objective lens. Diffraction of light from the edges of the grooves on an optical disk surface is used to generate the tracking-error signal. Some of the schemes for readout of the recorded data also utilize diffraction from the edges of pits and magnetic domains. A deep understanding of the theory of diffraction is therefore essential for the design and optimization of optical data storage systems.

Diffraction and related phenomena have been the subject of investigation for many years, and one can find many excellent books and papers on the subject. The purpose of the present chapter is to give an overview of the basic principles of diffraction, and to derive those equations that are of particular interest in optical data storage. We begin by introducing in section 3.1 the concept of the stationary-phase approximation as applied to two-dimensional integrals. Then in section 3.2 we approximate two integrals that appear frequently in diffraction problems using the stationary-phase method.

An arbitrary light amplitude distribution in a plane can be decomposed into a spectrum of plane waves. These plane waves propagate independently in space and, at the final destination, are superimposed to form a diffraction pattern.