Optical fiber is a thin, flexible strand of transparent material that guides light along its length. Most communications fibers are made from highly purified glass, while some short-distance applications use plastic. A fiber functions as an optical waveguide, confining optical signals within a small cross-section. Its low transmission loss and capacity to carry high-bandwidth signals make it a principal medium of telecommunications, including the physical networks supporting the Internet. Optical fibers also serve as sensing elements and deliver light in instruments and industrial equipment. (corning.com)
Structure and guiding principles
A conventional glass fiber consists of a central core, surrounding cladding, and protective coating. The core and cladding commonly contain fused silica, with their compositions adjusted to produce different optical properties. The outer coating, usually a polymer, protects the glass surface from damage; it is distinct from the cladding, which participates directly in guiding light. (corning.com)
The core has a slightly higher refractive index than the cladding. In a ray description, light striking their boundary at a sufficiently large angle to the normal undergoes total internal reflection, remaining guided rather than escaping. This explanation is useful for large-core fibers, but a wave description is necessary to characterize single-mode propagation accurately. The guided field is not confined entirely to the core: part extends into the cladding. (thorlabs.com)
A fiber’s numerical aperture describes its acceptance characteristics in the multimode ray model. Core size, index contrast, and wavelength together determine which guided modes the fiber supports. A mode is an allowed spatial distribution of the optical field, not a separate physical channel inside the glass. (thorlabs.com)
Major fiber types
Single-mode fiber supports one spatial mode over its intended operating wavelength range. Communications versions commonly have cores approximately 8–10 micrometers across and glass cladding 125 micrometers across. Eliminating propagation through multiple spatial modes avoids modal pulse spreading, making these fibers suitable for long-distance and high-capacity links. Single-mode operation depends on wavelength as well as fiber geometry. (corning.com)
Multimode fiber has a larger core, commonly 50 or 62.5 micrometers, and supports many modes. It is widely used for shorter connections within buildings and data centers. In step-index fiber, the refractive index changes abruptly at the core boundary. In graded-index fiber, it decreases progressively toward the core edge, reducing differences in modal travel times and increasing usable bandwidth. (thefoa.org)
Plastic optical fiber generally has higher transmission loss than communications-grade silica fiber, but its large core can simplify coupling and handling. It is used in short links, including some consumer audio connections. Specialized glass fibers have compositions, coatings, or structures adapted to particular wavelengths and operating environments. (thefoa.org)
Attenuation and dispersion
Optical attenuation is the reduction of optical power during transmission. It is commonly expressed in decibels per kilometer and arises from absorption, scattering, and light escaping through bends or other disturbances. Impurities can introduce absorption bands, while microscopic variations in glass contribute to scattering. Splices and connectors add further losses to a complete link. (thefoa.org)
Communications systems use wavelength regions where fiber loss and component performance are favorable. Common operating wavelengths include approximately 850 nanometers for multimode links and 1310 or 1550 nanometers for single-mode links. Longer wavelengths do not universally mean lower loss: material absorption and bending sensitivity also influence performance. (thefoa.org)
Dispersion spreads an optical pulse in time. Modal dispersion results from different travel times among modes. Chromatic dispersion occurs because different wavelength components propagate with different group velocities; single-mode transmission therefore does not eliminate all pulse spreading. Polarization-mode dispersion introduces additional timing differences between polarization components. These effects constrain transmission distance and data rate unless accommodated by system design. (thefoa.org)
Manufacture and connection
Glass fiber manufacture begins with a preform, a much larger glass body containing the required core-and-cladding structure. Vapor-deposition processes create carefully controlled material, which is consolidated into transparent glass. During drawing, the preform enters a furnace and a thin strand is pulled from its softened end. Protective coatings are applied, and the resulting fiber undergoes mechanical and optical testing. (corning.com)
Fusion splicing permanently joins prepared fiber ends by aligning and fusing the glass. Connectors provide detachable interfaces. An optical time-domain reflectometer examines returned light to characterize loss along a fiber and locate discontinuities. Because backscattering differences can distort apparent splice loss, measurements from both directions can improve its assessment. (corning.com)
Communication and sensing applications
A communications transmitter uses a laser or light-emitting diode to encode information into light; a receiver detects the arriving signal and converts it into electrical form. Wavelength-division multiplexing sends several optical wavelength channels through one fiber. Fiber links range from local connections to long-haul and submarine networks. Their capacity depends on the complete transmission system, rather than on the glass strand alone. (corning.com)
In sensing, the fiber can itself respond to environmental changes. Fiber Bragg gratings contain periodic refractive-index variations whose reflected wavelength changes with conditions such as strain or temperature. Distributed sensing instead analyzes light returned from positions along a fiber, allowing measurements over its length rather than only at discrete points. (nist.gov)
Historical development
In 1966, Charles K. Kao and George A. Hockham established the feasibility of low-loss glass fibers for communication, identifying material purity as a central challenge. In 1970, researchers at Corning demonstrated a low-loss fiber that helped turn this proposal into practical technology. Subsequent improvements in fibers and optical components enabled large-scale communications deployment. Kao received half of the 2009 Nobel Prize in Physics for achievements concerning light transmission in fibers for optical communication. (nobelprize.org)