Introduction to optical waveguides

Optical waveguides are made from material structures that have a core region which has a higher index of refraction than the surrounding regions. Guided electromagnetic waves propagate in and around the core. The transverse dimensions of the core are comparable to or smaller than the optical wavelength. Figure 1.1(a) illustrates a typical planar waveguide. Figure 1.1(b) illustrates a typical channel waveguide. For rigorous electromagnetic analysis of such guided-wave structures, Maxwell's vector equations should be used. Many of the theoretical methods used in the analysis of optical guided waves are very similar to those used in microwave analysis. For example, modal analysis is again a powerful mathematical tool for analyzing many devices, applications and systems.

However, there are also important differences between optical and microwave waveguides. In microwaves, we usually have closed waveguides inside metallic boundaries. Metals are considered as perfect conductors at most microwave frequencies. Microwaves propagate within the metallic enclosure. Figure 1.2 illustrates a typical microwave rectangular waveguide. In these closed structures, we have only a discrete set of waveguide modes whose electric fields terminate at the metallic boundary. Microwave radiation in the waveguide may be excited either by an electric field or by a current loop. At optical wavelengths, we avoid the use of metallic boundaries because of their strong absorption of radiation. Ideal optical waveguides, such as those illustrated in Fig. 1.1(a) and (b), are considered to have dielectric boundaries extending to infinity. They are called open waveguides.