Planar waveguides work on the same principle of total internal reflection as optical fibers, but they come in a different form. A planar waveguide is a thin layer on the surface of a flat material, which has higher refractive index than the bulk material. Typically the high refractive index is produced by doping the substrate material with something that increases its refractive index. Figure below shows the basic idea. The boundaries of the doped area form an interface that guides light, like the core-cladding interface in optical fibers the substrate provides the low-index materials on the sides and bottom, while air is the low-index medium on the top.
An alternative approach is to deposit a layer of high-index material on a lower index substrate. In this case, the waveguide is a raised stripe on the substrate, surrounded by air on top and on the sides, and contacting the substrate only on the bottom. As with the doped waveguide, total internal reflection confines light in the waveguide layer.
From a theoretical standpoint, both types of planar waveguides are dielectric slab waveguides. That means they are made of nonconducting (dielectric) materials, and are rectangular in cross section, rather than round like a fiber. The theory of such waveguides is quite well developed.
From a practical standpoint, planar waveguides also have some attractions. The technology for making thin stripes of material on flat substrates has been well developed by the semiconductor electronics industry. The technology can be used with a wide variety of materials, including silica glass and other compounds as well as semiconductors. Active optical components such as lasers and photodetectors can be made on the semiconductor materials. So can a wide variety of passive optical components, such as demultiplexers and couplers that divide and combine optical signals. That opens the possibility of integrating optical components on a chip.
On the other hand, planar waveguides also suffer serious practical drawbacks. Their attenuation is much higher than optical fibers, so they can’t send signals very far. Their flat, wide geometry differs greatly from the round cores of optical fibers, so light is inevitably lost in transferring from a fiber to a waveguide. Such problems limit the uses of planar waveguides.
They can guide light like optical fibers, and that they can serve as the basis for a variety of important components.