Optical fiber

An optical fiber is a narrow strand of glass or, in some applications, plastic that guides light along its length. A typical glass fiber has a central core surrounded by cladding, whose refractive index is slightly lower than the core's. At the boundary, light entering within the fiber's acceptance range is guided by total internal reflection, allowing it to follow the fiber rather than escape through the sides. A protective coating helps shield the fine glass from moisture and damage; cables add further protection and structural strength. In communications systems, a transmitter encodes information into a changing light signal, and equipment at the far end detects and decodes it. The fiber carries the light; the transmitters, receivers, connectors and other network components make it part of a complete link.

Fiber is commonly classified by the number of light modes—the permitted patterns of propagation—that its core can support. Single-mode fiber has a very small core and guides one mode. With little modal pulse spreading, it is widely used for telecommunications links that need to carry signals over long distances and at high data rates. Multimode fiber has a larger core and allows many modes; the different paths can arrive at different times and spread a pulse, so this type is generally suited to shorter links, including many local networks. These are design categories rather than separate communication principles: in both, light is guided through a core and cladding, while the choice of fiber, light source and system design depends on distance, capacity and cost.

Glass fibers were used for medical viewing before they could carry information efficiently over long distances: impurities in early glass absorbed too much light. In a 1966 paper, Charles K. Kao and George Hockham set out the performance a glass-fiber waveguide would need for telecommunications and argued that sufficiently pure glass could make low-loss transmission possible. Kao received half of the 2009 Nobel Prize in Physics for groundbreaking achievements concerning the transmission of light in fibers for optical communication. A research team at Corning produced a low-loss fused-silica fiber in 1970, an important step toward practical long-distance fiber systems. Today, optical fiber is used in broadband access, local and long-haul networks, and submarine cables; specialized fibers also serve purposes such as illumination and sensing. Its ability to carry light with low attenuation has made it a central technology for moving voice, video and data across communication networks.

Two important limits in an optical link are attenuation, the reduction of signal strength as light travels, and dispersion, the spreading of a light pulse over time. Loss and pulse spreading constrain how far and how rapidly information can be sent before a system needs amplification, regeneration or other design measures. The history of fiber optics illustrates why material quality matters: Kao and Hockham's proposal required far lower loss than then-available glass could provide, and later advances in glass purity and manufacturing made practical telecom fiber possible. Fiber cables must also be installed, joined and protected carefully; the glass strand itself is only one component of a communications system. Engineers select fiber types and equipment together to meet the needs of a particular route.