A multimode optical fiber in which the refractive index of the core declines from its highest value at the center of the core to a value at the edge of the core that equals the refractive index of the cladding. This design compensates for modal dispersion by allowing light rays in the outer zones of the core to travel faster than those in the center of core. It is typically used for transmitting distances of a couple of kilometers. Standard graded-index fibers typically have a core diameter of 50 or 62.5 µm and a cladding diameter of 125 µm. See step-index fiber and dispersion-shifted fiber.
A type of glass optical fiber (GOF) characterized by many layers of cladding surrounding the inner core, as illustrated in Figure G-1. From the core outward, the layers of cladding are of doped silica that are gradually and successively lower in index of refraction (IOR). Multi-mode fiber (MMF) commonly is of graded-index fiber construction and couples to a light-emitting diode (LED) light source. Unlike more sophisticated laser diodes, LEDs do not tightly focus a collimated beam of light. Rather, they emit a poorly focused and physically broad beam, which they inject into the relatively thick inner core of a MMF. Some light rays strike the core/cladding interface at sharp angles that exceed the critical angle. Rather than reflecting off the interface, they penetrate it and enter the cladding. As the fiber is not perfectly constructed, there may be numerous physical anomalies in the core/cladding interface over the length of a cable run, numerous points at which the light rays encounter extreme angles in the interface, and, therefore, numerous opportunities for the light rays to enter the cladding. Also, a typical cable installation is not perfectly straight, but takes numerous twists and turns that create odd angles, once again causing light rays to enter the cladding. Some light rays enter the cladding at extreme angles and are simply lost in it.Those errant light rays that enter at more modest angles gain in velocity and refract, or bend, as they travel through it as the cladding has a slightly lower IOR than the core. As they do so through many very thin layers of glass with slightly and successively lower refractive indexes, they gradually gain in speed and gradually bend back into the core, which is the primary light conducting medium. So, the graded-index fiber variously reflects or refracts light rays, guiding them to propagate through the core through a process known as total internal reflection. Graded-index multimode fiber (MMF) generally is used in relatively short haul, low-bandwidth applications such as local area networks (LANs) running at 1 Gbps or less. Step-index single-mode fiber (SMF) is used in long haul, high-bandwidth applications such as carrier backbones. See also collimation, critical angle, GOF, laser diode, LED, MMF, reflection, refraction, SMF, step-index fiber, total internal reflection, and VCSEL.