Waveguide (optics)

An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber waveguides, transparent dielectric waveguides made of plastic and glass, liquid light guides, and liquid waveguides. Optical waveguides are used as components in integrated optical circuits or as the transmission medium in local and long-haul optical communication systems. Optical waveguides can be classified according to their geometry (planar, strip, or fiber waveguides), mode structure (single-mode, multi-mode), refractive index distribution (step or gradient index), and material (glass, polymer, semiconductor). The basic principles behind optical waveguides can be described using the concepts of geometrical or ray optics, as illustrated in the diagram. Light passing into a medium with higher refractive index bends toward the normal by the process of refraction (Figure a.). Take, for example, light passing from air into glass. Similarly, light traveling in the opposite direction (from glass into air) takes the same path, bending away from the normal. This is a consequence of time-reversal symmetry. Each ray in air (black) can be mapped to a ray in the glass (blue), as shown in Figure b. There's a one-to-one correspondence. But because of refraction, some of the rays in the glass are left out (red). The remaining rays are trapped in the glass by a process called total internal reflection. They are incident on the glass-air interface at an angle above the critical angle. These extra rays correspond to a higher density of states in more-advanced formulations based on the Green's function. Using total internal reflection, we can trap and guide the light in a dielectric waveguide (Figure c). The red rays bounce off both the top and bottom surface of the high index medium. They're guided even if the slab curves or bends, so long as it bends slowly. This is the basic principle behind fiber optics in which light is guided along a high index glass core in a lower index glass cladding (Figure d).