The optical components and systems we have considered to this point belong to the realm of free-space optics, meaning the optical field is transmitted through a homogeneous medium (such as vacuum or air) without being guided in any manner. The lenses, mirrors, and diffractive structures we have studied are intended to modulate the unguided beams of free-space optics.

A significant branch of optics, however, uses structures to guide an optical field using reflections at an optical interface. The optical structures that accomplish this, waveguides and optical fibers, may be considered to be a form of “optical wire,” in which the field is confined to a small area in two dimensions and transmitted along the third. Waveguides are micro-optical components for light transmission that are typically fabricated on glass, plastic or semiconductor substrates, giving rise to the concept of integrated optics, analogously to integrated electronics (S. Miller, 1969). When combined with integrated active optical devices, such as lasers, modulators or detectors, also waveguide-based, such guided wave chips form the basis for the field of photonics. Finally, optical fibers span several orders of magnitude in their dimensions, with micrometer-sized guiding regions and hundred kilometer lengths; they are also useful as light guides in hybrid micro-optical systems.

We consider guided-wave optics in this chapter, beginning with an overview of how light is transmitted in a waveguide, and introduce the discrete propagation modes that result.