Introduction

It has been suggested that a focused beam in the neighborhood of the focal point behaves just like a plane wave. This misconception is based on the fact that the focused beam at its waist has a flat wave-front. The finite size of the beam, however, causes the plane-wave analogy to fail: Fourier analysis shows that a focused light spot is the superposition of a multitude of plane waves with varying directions of propagation. At the high values of numerical aperture typically used in optical recording, the fraction of light having a sizeable obliquity factor is simply too large to be ignored. The reflection coefficients and magneto-optical conversion factors of optical disks are rather strong functions of the angle of incidence; thus the presence of oblique rays within the angular spectrum of the focused light must influence the outcome of the readout process. The goal of the present chapter is to investigate the consequences of sharp focus in optical disk systems, and to clarify the extent of departure of the readout signals from those predicted in the preceding chapter.

The vector diffraction theory of Chapter 3 and the methods of Chapter 5 for computing multilayer reflection coefficients are used here to analyze the effects of high-NA focusing. The focused incident beam is decomposed into its spectrum of plane waves, and the reflected beam is obtained by the superposition of these plane waves after they are independently reflected from the multilayer surface.