Slab-Coupled Waveguides

Descriptions of three novel dielectric waveguides of wide potential use in long-distance optical transmission and in integrated optics have appeared in the literature recently. These guides are the single-material fiber1'2 (Fig. 1) made of low-loss undoped fused silica; the rib waveguide3 (Fig. 2) made of two materials, and the strip-loaded film guide4,5 (Fig. 3) made of three materials. In all these fibers n3 is air or an inert atmosphere, while in a more general guide it could be another dielectric. Although these guides have different shapes and different distributions of refractive indices, they have essential elements in common that make them close relatives of the same family. A more generic member of this family of waveguides (Fig. 4), from which all the others can be deduced, is a fiber of arbitrary cross section at a distance I from a slab mounted on a substrate. The way in which this guide operates is simpler to understand than the others, and is described below. The modal spectrum of the fiber (I = is shown in Fig. 5(a). In this example, five modes are guided and their axial propagation constants kz lie between kn3 and kn2 where k is the free-space propagation constant 2ir/. Smaller propagation constants than kn3 belong to a 645