Silicon Photonics: Fundamentals and Devices

10.1 Introduction

Earlier chapters in this volume discussed the principle of operation of various active devices and problems and the prospects of using Si and its alloys to fabricate the devices. While the field for active devices is not yet well developed, there exists an important area covering passive devices in which Si-based devices are finding widespread use. Mature Si microfabrication facilities are used to fabricate these devices as well as to make optical integrated circuits (OICs).

The basic element in all the passive devices or passive lightwave circuits is an optical waveguide. To understand the light-guiding properties in optical waveguides, one needs to solve the wave equation subject to appropriate boundary conditions. There exists another approach, known as the ray optic theory, which proves to be very useful in understanding the basic principle of how light is confined in the structure. The theory has its limitations, of course, but it forms a good starting point.

In this chapter, the ray optic theory of guidance of light as well as the wave theory will be presented for a simple three-layer planar light guide structure. The different practical planar structures, however, deviate considerably from the ideal structure. The guiding mechanism in these structures will be discussed qualitatively, mentioning how the ideal theory may be made use of in explaining their mode behavior.

The present chapter will also discuss the loss mechanisms in the guides and methods of ...