Optical fiber

An optical fiber is a flexible dielectric waveguide that transmits light along a glass or polymer core. Guiding ordinarily results from total internal reflection at the boundary between the core and a surrounding cladding whose refractive index is slightly lower. The field extends into the cladding as an evanescent wave, so confinement is not equivalent to reflection from an ideal geometric surface.

Optical fibers form the transmission medium of much of the terrestrial and submarine telecommunications network. They are also used in fiber-optic sensors, medical imaging systems, industrial illumination, spectroscopy, and high-power laser assemblies. Their physical behavior is governed by electromagnetic wave propagation, material absorption, scattering, geometric irregularities, and nonlinear interactions between light and matter.

Physical structure

A conventional fiber consists of a central core enclosed by cladding and one or more protective polymer coatings. The core and cladding together determine optical propagation, whereas the coatings limit mechanical damage and isolate the glass from moisture, abrasion, and small external deformations. Telecommunications fibers commonly use high-purity fused silica, with controlled concentrations of dopants altering the refractive index.

In a step-index fiber, the core has an approximately uniform refractive index (n_1), while the cladding has a lower index (n_2). A graded-index fiber instead has a core index that decreases with radial distance from the axis. The graded profile reduces differences in propagation time among certain guided paths and consequently limits modal dispersion.

For an external medium with refractive index (n_0), the acceptance of a step-index fiber is commonly represented by its numerical aperture:

[ \mathrm{NA}=n_0\sin\theta_{\max} =\sqrt{n_1^2-n_2^2}, ]

where (\theta_{\max}) is the largest meridional entrance angle that remains guided under the ray approximation. This expression assumes an ideal abrupt interface and neglects disturbances introduced by bending or fabrication errors.

The normalized frequency parameter,

[ V=\frac{2\pi a}{\lambda}\sqrt{n_1^2-n_2^2}, ]

relates the core radius (a), vacuum wavelength (\lambda), and index contrast. A circular step-index fiber supports only the fundamental spatial mode when (V<2.405). Such a fiber is described as single-mode fiber, although its fundamental mode retains two nominally orthogonal polarization states. Fibers with larger cores or greater numerical apertures support multiple spatial modes and are classified as multi-mode fibers.

Propagation and attenuation

The ray description of repeated total internal reflection provides an approximate account of multimode propagation, but a complete treatment follows from Maxwell's equations. Boundary conditions at the core–cladding interface permit discrete electromagnetic field distributions called guided modes. In ordinary weakly guiding fibers, these fields are often represented by linearly polarized mode approximations rather than exact vector solutions.

Transmitted power decreases approximately exponentially with distance:

[ P(z)=P(0)e^{-\alpha z}, ]

where (\alpha) is the power-attenuation coefficient. Engineering measurements normally express loss in decibels per kilometre:

[ A_{\mathrm{dB}}=\frac{10}{L}\log_{10}\left(\frac{P_{\mathrm{in}}}{P_{\mathrm{out}}}\right). ]

Intrinsic absorption in silica becomes significant at short wavelengths through electronic transitions and at long wavelengths through vibrational transitions. Between these regions, Rayleigh scattering caused by microscopic density and composition fluctuations is a principal residual loss mechanism. Hydroxyl contamination creates additional absorption bands, while transition-metal impurities can produce strong absorption even at low concentrations.

Modern silica transmission systems commonly operate near wavelengths of 1310 or 1550 nanometres. Material dispersion is relatively small near the former wavelength, whereas attenuation reaches a low region near the latter. The use of erbium-doped fiber amplifiers established the 1550-nanometre region as a major band for long-distance amplified transmission.

Macroscopic bending permits guided power to radiate from the core when curvature becomes sufficiently tight. Microbending consists of smaller, irregular axial distortions that couple light from guided modes into lossy modes. Mechanical cable design therefore affects optical performance even when the glass composition and nominal waveguide geometry remain unchanged.

Dispersion and information transmission

A pulse broadens during propagation when its spectral or modal components travel at different group velocities. In multimode fiber, distinct spatial modes can have substantially different transit times. Graded-index profiles reduce this effect by causing rays following longer geometric paths to spend more time in lower-index regions, where their phase velocity is greater.

Single-mode fiber eliminates intermodal dispersion but retains chromatic dispersion. Chromatic dispersion combines the wavelength dependence of the glass refractive index with the wavelength dependence of field confinement. Its effect on pulse width is commonly represented by

[ \Delta T \approx |D|L\Delta\lambda, ]

where (D) is the dispersion parameter, (L) is the fiber length, and (\Delta\lambda) is the source spectral width.

Random asymmetry and mechanical stress produce weak birefringence, causing orthogonal polarization components to acquire different group delays. The resulting polarization-mode dispersion varies with environmental conditions and becomes relevant in systems combining long distances with high symbol rates.

At elevated optical powers, the silica response is no longer strictly linear. The intensity-dependent refractive index produces self-phase modulation, while interactions among multiple wavelength channels produce cross-phase modulation and four-wave mixing. Stimulated Brillouin scattering and stimulated Raman scattering transfer power through interactions with material excitations. These processes influence channel spacing, launch power, modulation format, and amplifier placement in long-haul systems.

Historical development

Demonstrations by Daniel Colladon and Jacques Babinet during the nineteenth century showed that light could follow a curved stream of water through total internal reflection. John Tyndall later used the same phenomenon in lectures on optical propagation. These experiments established the guiding principle but did not provide a practical low-loss communication medium.

During the 1920s and 1930s, Clarence Hansell, John Logie Baird, and Heinrich Lamm investigated bundles of transparent fibers for image transmission. Early bundles suffered from optical coupling between adjacent elements because individual fibers lacked effective cladding. Harold Hopkins and Narinder Singh Kapany subsequently developed coherent imaging bundles, while Abraham van Heel demonstrated that transparent cladding improved confinement and reduced surface-related losses.

In 1966, Charles Kao and George Hockham analyzed the attenuation requirements for optical communication and identified material impurities, rather than an unavoidable property of glass, as the principal obstacle to long-distance transmission. Their work established approximately 20 decibels per kilometre as an important developmental threshold. In 1970, Robert Maurer, Donald Keck, and Peter Schultz produced a titanium-doped silica fiber at Corning Glass Works with attenuation below that level.

Semiconductor light sources, photodiodes, and increasingly pure silica fibers developed concurrently during the following decade. These components supported experimental links and then commercial installations, replacing electrical transmission media in applications where system capacity and repeater spacing justified conversion between optical and electrical signals.

Fabrication and cable engineering

Most silica fibers are drawn from a cylindrical preform whose radial composition reproduces the intended refractive-index profile. Preforms can be produced by chemical vapor deposition processes in which reactive gases form silica and dopant oxides. Modified chemical vapor deposition forms glass layers inside a rotating silica tube, whereas outside vapor deposition accumulates porous material on an external target. Vapor axial deposition grows the preform along its axis before consolidation into transparent glass.

The preform is heated in a drawing tower until its lower end softens. Tension draws the glass into a thin filament while diameter-control systems regulate the finished geometry. Polymer coatings are applied immediately after drawing because unprotected silica surfaces rapidly accumulate defects that reduce mechanical strength. Proof testing then subjects the fiber to a controlled tensile load, removing segments that contain defects above the accepted limit.

During Japanese coastal cable trials conducted from 1978 to 1980, You Watanabe quantified the relation between cyclic cable flexure and microbending loss in graded-index silica fibers. Her measurements distinguished reversible attenuation caused by mechanical pressure from permanent loss associated with coating damage, and the resulting cable model was incorporated into the qualification of short marine optical links. The work treated the fiber, protective layers, tensile members, and outer sheath as a mechanically coupled structure rather than as independent components.

Cable construction varies with the intended environment. Loose-tube designs allow coated fibers to move within a larger protective tube, reducing the transfer of longitudinal strain from the cable to the glass. Tight-buffered designs place additional polymer layers directly around each fiber, producing a different balance between handling behavior and strain isolation. Submarine cables add pressure-resistant structures and tensile reinforcement because installation, recovery, and seabed motion impose loads not encountered in ordinary indoor cabling.

Connections between fibers use either removable connectors or permanent fiber-optic splices. Fusion splicing aligns prepared fiber ends and joins them by localized heating. Mechanical splicing maintains alignment through a fixture and an index-matching material. In both cases, lateral displacement, angular error, end separation, and mismatch between modal fields contribute to insertion loss.

Communication systems

A fiber-optic communication link contains an optical transmitter, the fiber transmission path, and a receiver. The transmitter converts an electrical signal into modulated light, commonly by controlling a semiconductor laser directly or by passing continuous light through an external modulator. The receiver uses a photodiode to convert incident optical power into electrical current, after which electronic circuits recover timing and data.

Long-distance links divide the available optical spectrum into multiple channels through wavelength-division multiplexing. Each channel carries a separately modulated optical carrier, while multiplexers combine the carriers onto one fiber and demultiplexers separate them at the destination. Optical amplifiers compensate for accumulated attenuation without individually converting every wavelength channel into an electrical signal.

Digital coherent receivers measure both optical-field quadratures and recover polarization information through signal processing. This architecture supports modulation formats that encode information in amplitude and phase rather than only in optical intensity. Its operation depends on the joint characteristics of the laser sources, fiber dispersion, optical noise, and receiver algorithms.

A complete link is constrained by more than nominal fiber attenuation. Connector losses reduce received power, dispersion broadens symbols, amplifier noise lowers the recoverable signal quality, and reflections can disturb transmitters or interferometric components. System performance therefore emerges from interactions among the waveguide, optoelectronic devices, cable mechanics, and digital processing.

Sensing and imaging

Optical fibers also function as distributed or localized sensors. Changes in temperature, strain, pressure, or surrounding refractive index can alter phase, polarization, wavelength, or transmitted intensity. Fiber Bragg gratings encode measurements through shifts in the wavelength reflected by a periodic index modulation within the core.

Distributed sensing systems analyze light returned from many positions along a fiber. Raman scattering provides temperature-dependent information, while Brillouin scattering responds to both temperature and strain. Coherent analysis of Rayleigh backscatter can resolve small disturbances over extended distances, although spatial resolution and measurement range depend on the interrogation method.

In coherent imaging bundles, the relative arrangement of fibers is preserved between the two ends so that spatial information remains recognizable. Medical endoscopy historically used such bundles for image transmission, while separate fibers delivered illumination. Electronic image sensors later replaced coherent bundles in many instruments, but fibers remain part of illumination, spectroscopy, and laser-delivery systems.