What Is Optical Fiber?

Optical fiber is a thin, flexible strand of highly transparent material, typically silica glass or polymer, designed to guide light along its length through the principle of total internal reflection. The fiber consists of a core with a refractive index of approximately 1.46 surrounded by a cladding layer with a slightly lower index of about 1.45. Light entering the core at angles within the acceptance cone reflects off the core-cladding interface repeatedly and propagates along the fiber with very low loss, independent of the fiber's route. The small but critical difference in refractive indices confines the light while the outer buffer coating provides mechanical protection against abrasion and moisture.

Optical fibers offer a combination of characteristics unavailable in copper conductors: immunity to electromagnetic interference, very low attenuation over long distances, and the ability to carry enormous information bandwidths through wavelength-division multiplexing. The Fiber Optic Association's reference guide on total internal reflection explains the fundamental physics that makes fiber-guided transmission practical, tracing the acceptance angle and numerical aperture back to Snell's law applied at the core-cladding boundary.

Fiber Types and Propagation Modes

Optical fibers are classified by the number of spatial modes they support. Single-mode fibers have a small core diameter of 8 to 10 micrometers and support only the lowest-order propagation mode, eliminating intermodal dispersion and allowing transmission distances of tens to hundreds of kilometers at attenuation values as low as 0.25 dB/km at 1550 nm. These fibers are the backbone of long-haul telecommunications and submarine cable networks. Multimode fibers have larger core diameters of 50 to 62.5 micrometers and support many simultaneous modes; their attenuation of roughly 2.5 dB/km at 850 nm limits them to shorter links, typically within data centers and enterprise campus networks. The Cisco technical note on OM4 and OM5 multimode fiber describes how ISO/IEC 11801 defines five multimode fiber classes, OM1 through OM5, with OM3 closely tied to the IEEE 802.3 10 Gigabit Ethernet standard published in 2002. Graded-index multimode fibers use a parabolic refractive index profile across the core to equalize the group velocities of different modes, substantially increasing the usable bandwidth compared to step-index designs.

Optical Fiber in Communications

Single-mode fiber forms the physical medium of nearly all long-distance and metropolitan-area optical communications networks. Wavelength-division multiplexing (WDM) systems pack dozens to hundreds of separate channels onto a single fiber by assigning each channel a distinct wavelength in the 1530 to 1625 nm C and L bands, where silica fiber losses are lowest. Dense WDM systems with 50 GHz channel spacing, standardized by ITU-T G.694.1, can carry aggregate capacities exceeding tens of terabits per second per fiber pair. Coherent detection formats, including quadrature phase-shift keying and higher-order quadrature amplitude modulation, extract both amplitude and phase information from the received optical field, pushing spectral efficiency beyond what intensity modulation could achieve. Fiber Bragg gratings, dispersion-compensating fibers, and erbium-doped fiber amplifiers are key components that manage chromatic dispersion and signal attenuation over transoceanic distances. The ITU-T G.694.1 standard for dense WDM frequency grids provides the channel spacing specifications that underpin the capacity scaling of modern fiber networks.

Optical Fiber Sensing

Beyond communications, optical fiber is an effective sensing medium because the guided light responds measurably to temperature, strain, pressure, and chemical exposure. Fiber Bragg gratings shift their reflected wavelength in proportion to local strain or temperature, enabling quasi-distributed sensing along structural members in bridges, pipelines, and aircraft fuselages. Distributed sensing techniques based on Rayleigh, Raman, or Brillouin scattering can map temperature or strain continuously along fibers tens of kilometers long with spatial resolution of centimeters to meters. Fiber-optic gyroscopes exploit the Sagnac effect to measure rotation rates without moving parts and are used in aircraft inertial navigation and autonomous vehicle platforms.

Applications

Optical fiber has applications across a wide range of fields, including:

• Long-haul and submarine telecommunications backbone networks
• Data center interconnects and high-speed local area networks
• Structural health monitoring in civil infrastructure and aerospace
• Medical endoscopy and minimally invasive surgical imaging
• Cable television and broadband internet access distribution
• Power delivery and remote sensing in high-voltage and hazardous environments