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An optical fiber, or optical fibre in
Commonwealth English The use of the English language in current and former Member states of the Commonwealth of Nations, member countries of the Commonwealth of Nations was largely inherited from British Empire, British colonisation, with some exceptions. English s ...
, is a flexible,
transparent Transparency, transparence or transparent most often refer to: * Transparency (optics), the physical property of allowing the transmission of light through a material They may also refer to: Literal uses * Transparency (photography), a still, ...
fiber Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
made by
drawing Drawing is a form of visual art in which an artist uses instruments to mark paper or other two-dimensional surface. Drawing instruments include graphite pencils, pen and ink, various kinds of paints, inked brushes, colored pencils, crayons, ...
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
(
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
) or plastic to a diameter slightly thicker than that of a
human hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine ...
. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in
fiber-optic communication Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pref ...
s, where they permit transmission over longer distances and at higher bandwidths (data transfer rates) than electrical cables. Fibers are used instead of
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
wires because signals travel along them with less
loss Loss may refer to: Arts, entertainment, and media Music * ''Loss'' (Bass Communion album) (2006) * ''Loss'' (Mull Historical Society album) (2001) *"Loss", a song by God Is an Astronaut from their self-titled album (2008) * Losses "(Lil Tjay son ...
; in addition, fibers are immune to electromagnetic interference, a problem from which metal wires suffer. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a
fiberscope A fiberscope is a flexible optical fiber bundle with an eyepiece on one end and a lens on the other that is used to examine and inspect small, difficult-to-reach places such as the insides of machines, locks, and the human body. History Guid ...
. Specially designed fibers are also used for a variety of other applications, some of them being
fiber optic sensor A fiber-optic sensor is a sensor that uses optical fiber either as the sensing element ("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the electronics that process the signals ("extrinsic sensors"). Fibers have ...
s and fiber lasers. Optical fibers typically include a
core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...
surrounded by a transparent
cladding Cladding is an outer layer of material covering another. It may refer to the following: *Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell *Cladding (construction), materials applied to the exterior of buildings ...
material with a lower index of refraction. Light is kept in the core by the phenomenon of
total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflect ...
which causes the fiber to act as a
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
. Fibers that support many propagation paths or
transverse mode A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in the plane perpendicular (i.e., transverse) to the radiation's propagation direction. Transverse modes occur in radio waves and microwav ...
s are called multi-mode fibers, while those that support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a wider core diameter and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than . Being able to join optical fibers with low loss is important in fiber optic communication. This is more complex than joining electrical wire or cable and involves careful cleaving of the fibers, precise alignment of the fiber cores, and the coupling of these aligned cores. For applications that demand a permanent connection a
fusion splice Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole. Fusion may also refer to: Science and technology Physics *Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nuc ...
is common. In this technique, an electric arc is used to melt the ends of the fibers together. Another common technique is a
mechanical splice A mechanical splice is a junction of two or more optical fibers that are aligned and held in place by a self-contained assembly (usually the size of a large carpenter's nail). The fibers are not permanently joined, just precisely held together so th ...
, where the ends of the fibers are held in contact by mechanical force. Temporary or semi-permanent connections are made by means of specialized
optical fiber connector An optical fiber connector joins optical fibers, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so light can pass. Better connectors lose very little light due ...
s. The field of applied science and engineering concerned with the design and application of optical fibers is known as ''fiber optics''. The term was coined by Indian-American physicist Narinder Singh Kapany.


History

Daniel Colladon and
Jacques Babinet Jacques Babinet (; 5 March 1794 – 21 October 1872) was a French physicist, mathematician, and astronomer who is best known for his contributions to optics. Biography His father was Jean Babinet and mother, Marie‐Anne Félicité Bonneau d ...
first demonstrated the guiding of light by refraction, the principle that makes fiber optics possible, in
Paris Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...
in the early 1840s. John Tyndall included a demonstration of it in his public lectures in
London London is the capital and largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary down to the North Sea, and has been a majo ...
, 12 years later. Tyndall also wrote about the property of
total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflect ...
in an introductory book about the nature of light in 1870: In the late 19th century, a team of Viennese doctors guided light through bent glass rods to illuminate body cavities. Practical applications such as close internal illumination during dentistry followed, early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter
Clarence Hansell Clarence Weston Hansell (January 20, 1898 – ) was an American research engineer who pioneered investigation into the biological effects of ion air. He was granted over 300 US patents, including, in the 1930s, a precursor to the modern ink jet pri ...
and the television pioneer
John Logie Baird John Logie Baird FRSE (; 13 August 188814 June 1946) was a Scottish inventor, electrical engineer, and innovator who demonstrated the world's first live working television system on 26 January 1926. He went on to invent the first publicly demo ...
in the 1920s. In the 1930s,
Heinrich Lamm Heinrich Lamm (January 19, 1908 – July 12, 1974), a Jewish German-American physician, was a pioneer in using optical fibers for image transmission, and was the first to make a fiber-optic endoscope. When Lamm was a medical student in 1930, he ...
showed that one could transmit images through a bundle of unclad optical fibers and used it for internal medical examinations, but his work was largely forgotten. In 1953, Dutch scientist first demonstrated image transmission through bundles of optical fibers with a transparent cladding. That same year, Harold Hopkins and Narinder Singh Kapany at Imperial College in London succeeded in making image-transmitting bundles with over 10,000 fibers, and subsequently achieved image transmission through a 75 cm long bundle which combined several thousand fibers. The first practical fiber optic semi-flexible
gastroscope Esophagogastroduodenoscopy (EGD) or oesophagogastroduodenoscopy (OGD), also called by various other names, is a diagnostic endoscopic procedure that visualizes the upper part of the gastrointestinal tract down to the duodenum. It is considere ...
was patented by
Basil Hirschowitz Basil Isaac Hirschowitz (29 May 1925 – 19 January 2013) was an academic gastroenterologist from the University of Alabama at Birmingham (UAB) best known in the field for having invented an improved optical fiber which allowed the creation of ...
, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the
University of Michigan , mottoeng = "Arts, Knowledge, Truth" , former_names = Catholepistemiad, or University of Michigania (1817–1821) , budget = $10.3 billion (2021) , endowment = $17 billion (2021)As o ...
, in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material. Kapany coined the term ''fiber optics'' after writing a 1960 article in ''Scientific American'' that introduced the topic to a wide audience. He subsequently wrote the first book about the new field. The first working fiber-optic data transmission system was demonstrated by German physicist
Manfred Börner Manfred Börner (16 March 1929 in Rochlitz – 15 January 1996) was a German physicist he holds nearly 60 patents and is best known for his contributions to the development of fibre optic technology. He developed the first working optical fi ...
at
Telefunken Telefunken was a German radio and television apparatus company, founded in Berlin in 1903, as a joint venture of Siemens & Halske and the ''Allgemeine Elektrizitäts-Gesellschaft'' (AEG) ('General electricity company'). The name "Telefunken" ap ...
Research Labs in Ulm in 1965, followed by the first patent application for this technology in 1966. In 1968, NASA used fiber optics in the television cameras that were sent to the moon. At the time, the use in the cameras was classified ''confidential'', and employees handling the cameras had to be supervised by someone with an appropriate security clearance.
Charles K. Kao Sir Charles Kao Kuen Charles K. Kao was elected in 1990
as a memb ...
and
George A. Hockham George Alfred Hockham FREng FIET (7 December 1938 – 16 September 2013) was a British engineer. He worked for over 40 years in theoretical analysis and design techniques applied to the solution of electromagnetic problems covering many diff ...
of the British company
Standard Telephones and Cables Standard Telephones and Cables Ltd (later STC plc) was a British manufacturer of telephone, telegraph, radio, telecommunications, and related equipment. During its history, STC invented and developed several groundbreaking new technologies incl ...
(STC) were the first to promote the idea that the
attenuation In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variabl ...
in optical fibers could be reduced below 20
decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...
s per kilometer (dB/km), making fibers a practical communication medium, in 1965. They proposed that the attenuation in fibers available at the time was caused by impurities that could be removed, rather than by fundamental physical effects such as scattering. They correctly and systematically theorized the light-loss properties for optical fiber and pointed out the right material to use for such fibers— silica glass with high purity. This discovery earned Kao the
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
in 2009. The crucial attenuation limit of 20 dB/km was first achieved in 1970 by researchers
Robert D. Maurer Robert D. Maurer (born July 20, 1924) is an American industrial physicist noted for his leadership in the invention of optical fiber. In 1979, Maurer was elected a member of the National Academy of Engineering for contributions to the technolog ...
,
Donald Keck Donald B. Keck (born January 2, 1941) is an American research physicist and engineer most noted for his involvement in developing low-loss optical fiber. Keck grew up in Lansing, Michigan and attended Michigan State University, after which he j ...
,
Peter C. Schultz Peter C. Schultz, Ph.D. (born 1942), is co-inventor of the fiber optics now used worldwide for telecommunications. He is retired President (1988 to 2001) of Heraeus Tenevo Inc., a $200 million technical glass manufacturer specializing in fiber opt ...
, and Frank Zimar working for American glass maker
Corning Glass Works Corning Incorporated is an American multinational technology company that specializes in specialty glass, ceramics, and related materials and technologies including advanced optics, primarily for industrial and scientific applications. The co ...
. They demonstrated a fiber with 17 dB/km attenuation by doping silica glass with
titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
. A few years later they produced a fiber with only 4 dB/km attenuation using
germanium dioxide Germanium dioxide, also called germanium(IV) oxide, germania, and salt of germanium, is an inorganic compound with the chemical formula Ge O2. It is the main commercial source of germanium. It also forms as a passivation layer on pure germaniu ...
as the core dopant. In 1981,
General Electric General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
produced fused
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
ingots that could be drawn into strands long. Initially, high-quality optical fibers could only be manufactured at 2 meters per second. Chemical engineer Thomas Mensah joined Corning in 1983 and increased the speed of manufacture to over 50 meters per second, making optical fiber cables cheaper than traditional copper ones. These innovations ushered in the era of optical fiber telecommunication. The Italian research center
CSELT Centro Studi e Laboratori Telecomunicazioni (CSELT) was an Italian research center for telecommunication based in Torino, the biggest in Italy and one of the most important in Europe. It played a major role internationally especially in the stand ...
worked with Corning to develop practical optical fiber cables, resulting in the first metropolitan fiber optic cable being deployed in Turin in 1977. CSELT also developed an early technique for splicing optical fibers, called Springroove. Attenuation in modern optical cables is far less than in electrical copper cables, leading to long-haul fiber connections with repeater distances of . Two teams, led by
David N. Payne Sir David Neil Payne CBE FRS FREng (born 13 August 1944) is a British professor of photonics who is director of the Optoelectronics Research Centre at the University of Southampton. He has made several contributions in areas of optical fibre ...
of the
University of Southampton , mottoeng = The Heights Yield to Endeavour , type = Public research university , established = 1862 – Hartley Institution1902 – Hartley University College1913 – Southampton University Coll ...
and
Emmanuel Desurvire Emmanuel Desurvire (born 1955) is a French researcher and writer. He is the recipient of 2007 John Tyndall Award. Early life Desurvire was born in 1955, in Boulogne, France to Raymond Desurvire, an aircraft engineer and Marcelle Desurvire, a p ...
at
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mult ...
, developed the
erbium-doped fiber amplifier An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from ...
, which reduced the cost of long-distance fiber systems by reducing or eliminating optical-electrical-optical repeaters, in 1986 and 1987 respectively. The emerging field of
photonic crystal A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of Crystal structure, natural crystals gives rise to X-ray crystallograp ...
s led to the development in 1991 of
photonic-crystal fiber Photonic-crystal fiber (PCF) is a class of optical fiber based on the properties of photonic crystals. It was first explored in 1996 at University of Bath, UK. Because of its ability to confine light in hollow cores or with confinement charact ...
, which guides light by
diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
from a periodic structure, rather than by total internal reflection. The first photonic crystal fibers became commercially available in 2000. Photonic crystal fibers can carry higher power than conventional fibers and their wavelength-dependent properties can be manipulated to improve performance.


Uses


Communication

Optical fiber is used as a medium for
telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
and
computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...
ing because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because
infrared light Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...
propagates through the fiber with much lower
attenuation In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variabl ...
compared to electricity in electrical cables. This allows long distances to be spanned with few
repeater In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...
s. 10 or 40 Gbit/s is typical in deployed systems. Through the use of wavelength-division multiplexing (WDM), each fiber can carry many independent channels, each using a different wavelength of light. The net data rate (data rate without overhead bytes) per fiber is the per-channel data rate reduced by the
forward error correction In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is ...
(FEC) overhead, multiplied by the number of channels (usually up to 80 in commercial
dense WDM In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This techniq ...
systems ). For short-distance applications, such as a network in an office building (see
fiber to the office Fiber to the office (FTTO) is an alternative cabling concept for local area network (LAN) network office environments. It combines passive elements ( fibre optic cabling, patch panels, splice boxes, connectors and standard copper 8P8C patch cords) ...
), fiber-optic cabling can save space in cable ducts. This is because a single fiber can carry much more data than electrical cables such as standard
category 5 cable Category 5 cable (Cat 5) is a twisted pair cable for computer networks. Since 2001, the variant commonly in use is the Category 5e specification (Cat 5e). The cable standard provides performance of up to 100 MHz and is ...
, which typically runs at 100 Mbit/s or 1 Gbit/s speeds. Fibers are often also used for short-distance connections between devices. For example, most
high-definition television High-definition television (HD or HDTV) describes a television system which provides a substantially higher image resolution than the previous generation of technologies. The term has been used since 1936; in more recent times, it refers to the g ...
s offer a digital audio optical connection. This allows the streaming of audio over light, using the
S/PDIF S/PDIF (Sony/Philips Digital Interface) is a type of digital audio interface used in consumer audio equipment to output audio over relatively short distances. The signal is transmitted over either a coaxial cable (using RCA or BNC connectors) ...
protocol over an optical
TOSLINK TOSLINK (from ''Toshiba Link'') is a standardized optical fiber connector system. Also known generically as optical audio, its most common use is in consumer audio equipment (via a "digital optical" socket), where it carries a digital audio st ...
connection.


Sensors

Fibers have many uses in remote sensing. In some applications, the sensor is itself an optical fiber. Fibers are used to channel radiation to a sensor where it is measured. In other cases, fiber is used to connect a sensor to a measurement system. Optical fibers can be used as sensors to measure
strain Strain may refer to: Science and technology * Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes * Strain (chemistry), a chemical stress of a molecule * Strain (injury), an injury to a mu ...
,
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
,
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
, and other quantities by modifying a fiber so that the property being measured modulates the intensity,
phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...
, polarization,
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
, or transit time of light in the fiber. Sensors that vary the intensity of light are the simplest since only a simple source and detector are required. A particularly useful feature of such fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to one meter.
Distributed acoustic sensing Rayleigh scattering based distributed acoustic sensing (DAS) systems use fiber optic cables to provide distributed strain sensing. In DAS, the optical fiber cable becomes the sensing element and measurements are made, and in part processed, using an ...
is one example of this. In contrast, highly localized measurements can be provided by integrating miniaturized sensing elements with the tip of the fiber. These can be implemented by various micro- and
nanofabrication Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering (patterning e.g. etching, depositing, writing, printing etc) of nanometer-scale structures on various materials. The modern term reflects on a ...
technologies, such that they do not exceed the microscopic boundary of the fiber tip, allowing for such applications as insertion into blood vessels via hypodermic needle. Extrinsic fiber optic sensors use an
optical fiber cable A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable, but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with ...
, normally a multi-mode one, to transmit
modulated In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
light from either a non-fiber optical sensor—or an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach otherwise inaccessible places. An example is the measurement of temperature inside
jet engine A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, Pump-jet, water jet, and hybrid propulsion, the term ...
s by using a fiber to transmit
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
into a
pyrometer A pyrometer is a type of remote-sensing thermometer used to measure the temperature of distant objects. Various forms of pyrometers have historically existed. In the modern usage, it is a device that from a distance determines the temperature of ...
outside the engine. Extrinsic sensors can be used in the same way to measure the internal temperature of
electrical transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's c ...
s, where the extreme
electromagnetic field An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field produced by (stationary or moving) electric charges. It is the field described by classical electrodynamics (a classical field theory) and is the classical c ...
s present make other measurement techniques impossible. Extrinsic sensors measure vibration, rotation, displacement, velocity, acceleration, torque, and torsion. A solid-state version of the gyroscope, using the interference of light, has been developed. The
fiber optic gyroscope A fibre-optic gyroscope (FOG) senses changes in orientation using the Sagnac effect, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed throu ...
(FOG) has no moving parts and exploits the
Sagnac effect The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferomet ...
to detect mechanical rotation. Common uses for fiber optic sensors include advanced intrusion detection security systems. The light is transmitted along a fiber optic sensor cable placed on a fence, pipeline, or communication cabling, and the returned signal is monitored and analyzed for disturbances. This return signal is digitally processed to detect disturbances and trip an alarm if an intrusion has occurred. Optical fibers are widely used as components of optical chemical sensors and optical
biosensors A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector. The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell rece ...
.


Power transmission

Optical fiber can be used to transmit power using a
photovoltaic cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physics, physical and Chemical substance, chemical phenomenon. Optical fibers are used as light guides in medical and other applications where bright light needs to be shone on a target without a clear line-of-sight path. Many
microscopes A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisib ...
use fiber-optic light sources to provide intense illumination of samples being studied. Optical fiber is also used in imaging optics. A coherent bundle of fibers is used, sometimes along with lenses, for a long, thin imaging device called an
endoscope An endoscope is an inspection instrument composed of image sensor, optical lens, light source and mechanical device, which is used to look deep into the body by way of openings such as the mouth or anus. A typical endoscope applies several modern t ...
, which is used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures. Industrial endoscopes (see
fiberscope A fiberscope is a flexible optical fiber bundle with an eyepiece on one end and a lens on the other that is used to examine and inspect small, difficult-to-reach places such as the insides of machines, locks, and the human body. History Guid ...
or borescope) are used for inspecting anything hard to reach, such as jet engine interiors. In some buildings, optical fibers route sunlight from the roof to other parts of the building (see
nonimaging optics Nonimaging optics (also called anidolic optics)Roland Winston et al., ''Nonimaging Optics'', Academic Press, 2004 R. John Koshel (Editor), ''Illumination Engineering: Design with Nonimaging Optics'', Wiley, 2013 is the branch of optics concerned wi ...
).
Optical-fiber lamp An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means ...
s are used for illumination in decorative applications, including signs,
art Art is a diverse range of human activity, and resulting product, that involves creative or imaginative talent expressive of technical proficiency, beauty, emotional power, or conceptual ideas. There is no generally agreed definition of wha ...
, toys and artificial
Christmas tree A Christmas tree is a decorated tree, usually an evergreen conifer, such as a spruce, pine or fir, or an artificial tree of similar appearance, associated with the celebration of Christmas. The custom was further developed in early modern ...
s. Optical fiber is an intrinsic part of the light-transmitting concrete building product
LiTraCon LiTraCon is a translucent concrete building material. The name is short for "light-transmitting concrete". The material is made of 96% concrete and 4% by weight of optical fibers. It was developed in 2001 by Hungarian architect Áron Losonczi wor ...
. Optical fiber can also be used in
structural health monitoring Structural health monitoring (SHM) involves the observation and analysis of a system over time using periodically sampled response measurements to monitor changes to the material and geometric properties of engineering structures such as bridges an ...
. This type of
sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
can detect stresses that may have a lasting impact on
structures A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
. It is based on the principle of measuring analog attenuation. In
spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
, optical fiber bundles transmit light from a spectrometer to a substance that cannot be placed inside the spectrometer itself, in order to analyze its composition. A spectrometer analyzes substances by bouncing light off and through them. By using fibers, a spectrometer can be used to study objects remotely. An optical fiber doped with certain
rare-earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides ( yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silv ...
s such as
erbium Erbium is a chemical element with the symbol Er and atomic number 68. A silvery-white solid metal when artificially isolated, natural erbium is always found in chemical combination with other elements. It is a lanthanide, a rare-earth element, or ...
can be used as the
gain medium The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a h ...
of a fiber laser or optical amplifier. Rare-earth-doped optical fibers can be used to provide signal amplification by splicing a short section of doped fiber into a regular (undoped) optical fiber line. The doped fiber is optically pumped with a second laser wavelength that is coupled into the line in addition to the signal wave. Both wavelengths of light are transmitted through the doped fiber, which transfers energy from the second pump wavelength to the signal wave. The process that causes the amplification is stimulated emission. Optical fiber is also widely exploited as a nonlinear medium. The glass medium supports a host of nonlinear optical interactions, and the long interaction lengths possible in fiber facilitate a variety of phenomena, which are harnessed for applications and fundamental investigation. Conversely, fiber nonlinearity can have deleterious effects on optical signals, and measures are often required to minimize such unwanted effects. Optical fibers doped with a
wavelength shifter A wavelength shifter is a photofluorescent material that absorbs higher frequency photons and emits lower frequency photons. The material absorbs one photon, and emits one or multiple lower-energy photons. The relaxation time of the excited molecul ...
collect scintillation light in physics experiments. Fiber-optic sights for handguns, rifles, and shotguns use pieces of optical fiber to improve the visibility of markings on the sight.


Principle of operation

An optical fiber is a cylindrical
dielectric waveguide An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber waveguides, transparent dielectric waveguides made of plastic and glass, liquid light ...
( nonconducting waveguide) that transmits light along its axis through the process of total internal reflection. The fiber consists of a ''core'' surrounded by a
cladding Cladding is an outer layer of material covering another. It may refer to the following: *Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell *Cladding (construction), materials applied to the exterior of buildings ...
layer, both of which are made of
dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
materials. To confine the optical signal in the core, the
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...
of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in '' step-index fiber'', or gradual, in ''
graded-index fiber A graded-index or gradient-index fiber is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the optical axis of the fiber. Because parts of the core closer to the fiber axis have a higher re ...
''. Light can be fed into optical fibers using
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
s or
LED A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
s. Fiber is immune to electrical interference; there is no cross-talk between signals in different cables and no pickup of environmental noise. Information traveling inside the optical fiber is even immune to
electromagnetic pulse An electromagnetic pulse (EMP), also a transient electromagnetic disturbance (TED), is a brief burst of electromagnetic energy. Depending upon the source, the origin of an EMP can be natural or artificial, and can occur as an electromagnetic fi ...
s generated by nuclear devices. Fiber cables do not conduct electricity, which makes fiber useful for protecting communications equipment in
high voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant spec ...
environments such as
power generation Electricity generation is the process of generating electric power from sources of primary energy. For utilities in the electric power industry, it is the stage prior to its delivery ( transmission, distribution, etc.) to end users or its stor ...
facilities or applications prone to
lightning Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
strikes. The electrical isolation also prevents problems with ground loops. Because there is no electricity in optical cables that could potentially generate sparks, they can be used in environments where explosive fumes are present.
Wiretapping Telephone tapping (also wire tapping or wiretapping in American English) is the monitoring of telephone and Internet-based conversations by a third party, often by covert means. The wire tap received its name because, historically, the monitorin ...
(in this case,
fiber tapping Fiber tapping uses a network tap method that extracts signal from an optical fiber without breaking the connection. Tapping of optical fiber allows diverting some of the signal being transmitted in the core of the fiber into another fiber or ...
) is more difficult compared to electrical connections. Fiber cables are not targeted for
metal theft Metal theft is "the theft of items for the value of their constituent metals". It usually increases when worldwide prices for scrap metal rise, as has happened dramatically due to rapid industrialization in India and China. Apart from precious m ...
. In contrast, copper cable systems use large amounts of copper and have been targeted since the 2000s commodities boom.


Refractive index

The
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...
is a way of measuring the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
in a material. Light travels fastest in a
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
, such as in outer space. The speed of light in a vacuum is about 300,000
kilometers The kilometre ( SI symbol: km; or ), spelt kilometer in American English, is a unit of length in the International System of Units (SI), equal to one thousand metres (kilo- being the SI prefix for ). It is now the measurement unit used for ex ...
(186,000 miles) per second. The refractive index of a medium is calculated by dividing the speed of light in a vacuum by the speed of light in that medium. The refractive index of a vacuum is therefore 1, by definition. A typical single-mode fiber used for telecommunications has a cladding made of pure silica, with an index of 1.444 at 1500 nm, and a core of doped silica with an index around 1.4475. The larger the index of refraction, the slower light travels in that medium. From this information, a simple rule of thumb is that a signal using optical fiber for communication will travel at around 200,000 kilometers per second. Thus a phone call carried by fiber between Sydney and New York, a 16,000-kilometer distance, means that there is a minimum delay of 80 milliseconds (about \tfrac of a second) between when one caller speaks and the other hears.


Total internal reflection

When light traveling in an optically dense medium hits a boundary at a steep angle (larger than the
critical angle Critical angle may refer to: *Critical angle (optics), the angle of incidence above which total internal reflection occurs *Critical angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference lin ...
for the boundary), the light is completely reflected. This is called
total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflect ...
. This effect is used in optical fibers to confine light in the core. Most modern optical fiber is ''weakly guiding'', meaning that the difference in refractive index between the core and the cladding is very small (typically less than 1%). Light travels through the fiber core, bouncing back and forth off the boundary between the core and cladding. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. This range of angles is called the
acceptance cone A guided ray (also bound ray or trapped ray) is a ray of light in a multi-mode optical fiber, which is confined by the core. For step index fiber, light entering the fiber will be guided if it falls within the acceptance cone of the fiber, that i ...
of the fiber. There is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The
sine In mathematics, sine and cosine are trigonometric functions of an angle. The sine and cosine of an acute angle are defined in the context of a right triangle: for the specified angle, its sine is the ratio of the length of the side that is oppo ...
of this maximum angle is the numerical aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding. Single-mode fiber has a small NA.


Multi-mode fiber

Fiber with large core diameter (greater than 10 micrometers) may be analyzed by
geometrical optics Geometrical optics, or ray optics, is a model of optics that describes light propagation in terms of ''rays''. The ray in geometrical optics is an abstraction useful for approximating the paths along which light propagates under certain circumstan ...
. Such fiber is called ''multi-mode fiber'', from the electromagnetic analysis (see below). In a step-index multi-mode fiber,
rays Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gra ...
of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at an angle (measured relative to a line
normal Normal(s) or The Normal(s) may refer to: Film and television * ''Normal'' (2003 film), starring Jessica Lange and Tom Wilkinson * ''Normal'' (2007 film), starring Carrie-Anne Moss, Kevin Zegers, Callum Keith Rennie, and Andrew Airlie * ''Norma ...
to the boundary) greater than the
critical angle Critical angle may refer to: *Critical angle (optics), the angle of incidence above which total internal reflection occurs *Critical angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference lin ...
for this boundary, are completely reflected. The critical angle is determined by the difference in the index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the
core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...
into the cladding where they terminate. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of
dispersion Dispersion may refer to: Economics and finance * Dispersion (finance), a measure for the statistical distribution of portfolio returns * Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variat ...
as rays at different angles have different path lengths and therefore take different amounts of time to traverse the fiber. In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high-angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.


Single-mode fiber

Fiber with a core diameter less than about ten times the
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic waveguide structure, according to
Maxwell's equations Maxwell's equations, or Maxwell–Heaviside equations, are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits. ...
as reduced to the electromagnetic wave equation. As an optical waveguide, the fiber supports one or more confined
transverse mode A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in the plane perpendicular (i.e., transverse) to the radiation's propagation direction. Transverse modes occur in radio waves and microwav ...
s by which light can propagate along the fiber. Fiber supporting only one mode is called ''single-mode''. The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave. The most common type of single-mode fiber has a core diameter of 8–10 micrometers and is designed for use in the
near infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...
. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometers and as large as hundreds of micrometers.


Special-purpose fiber

Some special-purpose optical fiber is constructed with a non-cylindrical core or cladding layer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber used in fiber optic sensors and fiber designed to suppress
whispering gallery mode Whispering-gallery waves, or whispering-gallery modes, are a type of wave that can travel around a concave surface. Originally discovered for sound waves in the whispering gallery of St Paul's Cathedral, they can exist for light and for other wave ...
propagation.
Photonic-crystal fiber Photonic-crystal fiber (PCF) is a class of optical fiber based on the properties of photonic crystals. It was first explored in 1996 at University of Bath, UK. Because of its ability to confine light in hollow cores or with confinement charact ...
is made with a regular pattern of index variation (often in the form of cylindrical holes that run along the length of the fiber). Such fiber uses
diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
effects instead of or in addition to total internal reflection, to confine light to the fiber's core. The properties of the fiber can be tailored to a wide variety of applications.


Mechanisms of attenuation

Attenuation in fiber optics, also known as transmission loss, is the reduction in the intensity of the light signal as it travels through the transmission medium. Attenuation coefficients in fiber optics are usually expressed in units of dB/km. The medium is usually a fiber of silica glass that confines the incident light beam within. Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. The four orders of magnitude reduction in the attenuation of silica optical fibers over four decades was the result of constant improvement of manufacturing processes, raw material purity, preform, and fiber designs, which allowed for these fibers to approach the theoretical lower limit of attenuation. Single-mode optical fibers can be made with extremely low loss. Corning's SMF-28 fiber, a standard single-mode fiber for telecommunications wavelengths, has a loss of 0.17 dB/km at 1550 nm. For example, an 8 km length of SMF-28 transmits nearly 75% of light at 1,550 nm. It has been noted that if ocean water was as clear as fiber, one could see all the way to the bottom even of the
Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maximum know ...
in the Pacific Ocean, a depth of . Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption.


Light scattering

The propagation of light through the core of an optical fiber is based on the total internal reflection of the lightwave. Rough and irregular surfaces, even at the molecular level, can cause light rays to be reflected in random directions. This is called
diffuse reflection Diffuse reflection is the reflection (physics), reflection of light or other radiation, waves or particles from a surface such that a ray (optics), ray incident on the surface is scattering, scattered at many angles rather than at just one angle ...
or scattering, and it is typically characterized by a wide variety of reflection angles. Scattering depends on the
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of the light being scattered. Thus, limits to spatial scales of visibility arise, depending on the frequency of the incident light wave and the physical dimension (or spatial scale) of the scattering center, which is typically in the form of some specific micro-structural feature. Since
visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
has a wavelength of the order of one
micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
(one-millionth of a meter) scattering centers will have dimensions on a similar spatial scale. Thus, attenuation results from the
incoherent scatter Incoherent scattering is a type of scattering phenomenon in physics. The term is most commonly used when referring to the scattering of an electromagnetic wave (usually light or radio frequency) by random fluctuations in a gas of particles (most o ...
ing of light at internal surfaces and interfaces. In (poly)crystalline materials such as metals and ceramics, in addition to pores, most of the internal surfaces or interfaces are in the form of
grain boundaries In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and thermal ...
that separate tiny regions of crystalline order. It has been shown that when the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent. This phenomenon has given rise to the production of transparent ceramic materials. Similarly, the scattering of light in optical quality glass fiber is caused by molecular level irregularities (compositional fluctuations) in the glass structure. Indeed, one emerging school of thought is that glass is simply the limiting case of a polycrystalline solid. Within this framework, ''domains'' exhibiting various degrees of short-range order become the building blocks of metals as well as glasses and ceramics. Distributed both between and within these domains are micro-structural defects that provide the most ideal locations for light scattering. This same phenomenon is seen as one of the limiting factors in the transparency of IR missile domes. At high optical powers, scattering can also be caused by nonlinear optical processes in the fiber.


UV-Vis-IR absorption

In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths. Primary material considerations include both electrons and molecules as follows: * At the electronic level, it depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific wavelength or frequency in the ultraviolet (UV) or visible ranges. This is what gives rise to color. * At the atomic or molecular level, it depends on the frequencies of atomic or molecular vibrations or chemical bonds, how closely packed its atoms or molecules are, and whether or not the atoms or molecules exhibit long-range order. These factors will determine the capacity of the material to transmit longer wavelengths in the infrared (IR), far IR, radio, and microwave ranges. The design of any optically transparent device requires the selection of materials based upon knowledge of its properties and limitations. The
crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...
absorption characteristics observed at the lower frequency regions (mid- to far-IR wavelength range) define the long-wavelength transparency limit of the material. They are the result of the interactive
coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...
between the motions of thermally induced vibrations of the constituent
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...
s and molecules of the solid lattice and the incident light wave radiation. Hence, all materials are bounded by limiting regions of absorption caused by atomic and molecular vibrations (bond-stretching) in the far-infrared (>10 µm). In other words, the selective absorption of IR light by a particular material occurs because the selected frequency of the light wave matches the frequency (or an integer multiple of the frequency, i.e.
harmonic A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...
) at which the particles of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of IR light. Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects. When IR light of these frequencies strikes an object, the energy is either reflected or transmitted.


Loss budget

Attenuation over a cable run is significantly increased by the inclusion of connectors and splices. When computing the acceptable attenuation (loss budget) between a transmitter and a receiver one includes: * dB loss due to the type and length of fiber optic cable, * dB loss introduced by connectors, and * dB loss introduced by splices. Connectors typically introduce 0.3 dB per connector on well-polished connectors. Splices typically introduce less than 0.2 dB per splice. The total loss can be calculated by: :Loss = dB loss per connector × number of connectors + dB loss per splice × number of splices + dB loss per kilometer × kilometers of fiber, where the dB loss per kilometer is a function of the type of fiber and can be found in the manufacturer's specifications. For example, a typical 1550 nm single-mode fiber has a loss of 0.3 dB per kilometer. The calculated loss budget is used when testing to confirm that the measured loss is within the normal operating parameters.


Manufacturing


Materials

Glass optical fibers are almost always made from
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
, but some other materials, such as fluorozirconate, fluoroaluminate, and
chalcogenide glass Chalcogenide glass (pronounced hard ''ch'' as in ''chemistry'') is a glass containing one or more chalcogens (sulfur, selenium and tellurium, but excluding oxygen). Such glasses are covalently bonded materials and may be classified as covalent netw ...
es as well as crystalline materials like
sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, chromium, vanadium, or magnesium. The name sapphire is derived via the Latin "sapphir ...
, are used for longer-wavelength infrared or other specialized applications. Silica and fluoride glasses usually have refractive indices of about 1.5, but some materials such as the
chalcogenide : 220px, Cadmium sulfide, a prototypical metal chalcogenide, is used as a yellow pigment. A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements ...
s can have indices as high as 3. Typically the index difference between core and cladding is less than one percent. Plastic optical fibers (POF) are commonly step-index multi-mode fibers with a core diameter of 0.5 millimeters or larger. POF typically have higher attenuation coefficients than glass fibers, 1 dB/m or higher, and this high attenuation limits the range of POF-based systems.


Silica

Silica exhibits fairly good optical transmission over a wide range of wavelengths. In the
near-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
(near IR) portion of the spectrum, particularly around 1.5 μm, silica can have extremely low absorption and scattering losses of the order of 0.2 dB/km. Such low losses depend on using ultra-pure silica. A high transparency in the 1.4-μm region is achieved by maintaining a low concentration of hydroxyl groups (OH). Alternatively, a high OH
concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', ''number concentration'', an ...
is better for transmission in the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
(UV) region. Silica can be drawn into fibers at reasonably high temperatures and has a fairly broad
glass transformation range The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rub ...
. One other advantage is that fusion splicing and cleaving of silica fibers is relatively effective. Silica fiber also has high mechanical strength against both pulling and even bending, provided that the fiber is not too thick and that the surfaces have been well prepared during processing. Even simple cleaving of the ends of the fiber can provide nicely flat surfaces with acceptable optical quality. Silica is also relatively
chemically inert In chemistry, the term chemically inert is used to describe a substance that is not chemically reactive. From a thermodynamic perspective, a substance is inert, or nonlabile, if it is thermodynamically unstable (positive standard Gibbs free en ...
. In particular, it is not
hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substan ...
(does not absorb water). Silica glass can be doped with various materials. One purpose of doping is to raise the
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...
(e.g. with
germanium dioxide Germanium dioxide, also called germanium(IV) oxide, germania, and salt of germanium, is an inorganic compound with the chemical formula Ge O2. It is the main commercial source of germanium. It also forms as a passivation layer on pure germaniu ...
(GeO2) or aluminium oxide (Al2O3)) or to lower it (e.g. with
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
or
boron trioxide Boron trioxide or diboron trioxide is the oxide of boron with the formula . It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called boric oxide or boria. It h ...
(B2O3)). Doping is also possible with laser-active ions (for example, rare-earth-doped fibers) in order to obtain active fibers to be used, for example, in fiber amplifiers or
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
applications. Both the fiber core and cladding are typically doped, so that the entire assembly (core and cladding) is effectively the same compound (e.g. an
aluminosilicate Aluminosilicate minerals ( IMA symbol: Als) are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals. Andalusite, kyanite, and sillimanite are naturall ...
, germanosilicate, phosphosilicate or
borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), ma ...
). Particularly for active fibers, pure silica is usually not a very suitable host glass, because it exhibits a low solubility for rare-earth ions. This can lead to quenching effects due to the clustering of dopant ions. Aluminosilicates are much more effective in this respect. Silica fiber also exhibits a high threshold for optical damage. This property ensures a low tendency for laser-induced breakdown. This is important for fiber amplifiers when utilized for the amplification of short pulses. Because of these properties, silica fibers are the material of choice in many optical applications, such as communications (except for very short distances with plastic optical fiber), fiber lasers, fiber amplifiers, and fiber-optic sensors. Large efforts put forth in the development of various types of silica fibers have further increased the performance of such fibers over other materials.


Fluoride glass

Fluoride glass Fluoride glass is a class of non-oxide optical glasses composed of fluorides of various metals. They can contain heavy metals such as zirconium, or be combined with lighter elements like aluminum and beryllium. These heavier elements cause the g ...
is a class of non-oxide optical quality glasses composed of fluorides of various
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
s. Because of the low
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
of these glasses, it is very difficult to completely avoid
crystallization Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...
while processing it through the glass transition (or drawing the fiber from the melt). Thus, although heavy metal fluoride glasses (HMFG) exhibit very low optical attenuation, they are not only difficult to manufacture, but are quite fragile, and have poor resistance to moisture and other environmental attacks. Their best attribute is that they lack the absorption band associated with the
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
(OH) group (3,200–3,600 cm−1; i.e., 2,777–3,125 nm or 2.78–3.13 μm), which is present in nearly all oxide-based glasses. Such low losses were never realized in practice, and the fragility and high cost of fluoride fibers made them less than ideal as primary candidates. Fluoride fibers are used in mid-
IR spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or function ...
,
fiber optic sensor A fiber-optic sensor is a sensor that uses optical fiber either as the sensing element ("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the electronics that process the signals ("extrinsic sensors"). Fibers have ...
s,
thermometry Temperature measurement (also known as thermometry) describes the process of measuring a current local temperature for immediate or later evaluation. Datasets consisting of repeated standardized measurements can be used to assess temperature tren ...
, and
imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image). Imaging technology is the application of materials and methods to create, preserve, or duplicate images. ...
. Fluoride fibers can be used for guided lightwave transmission in media such as YAG (
yttrium aluminium garnet Yttrium aluminium garnet (YAG, Y3 Al5 O12) is a synthetic crystalline material of the garnet group. It is a cubic yttrium aluminium oxide phase, with other examples being YAlO3 (YAP) in a hexagonal or an orthorhombic, perovskite-like form, and ...
)
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
s at 2.9 μm, as required for medical applications (e.g.
ophthalmology Ophthalmology ( ) is a surgical subspecialty within medicine that deals with the diagnosis and treatment of eye disorders. An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a medic ...
and
dentistry Dentistry, also known as dental medicine and oral medicine, is the branch of medicine focused on the teeth, gums, and mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, disorders, and conditions o ...
). An example of a heavy metal fluoride glass is the
ZBLAN ZBLAN is the most stable, and consequently the most used, fluoride glass, a subcategory of the heavy metal fluoride glass (HMFG) group. Typically its composition is 53% ZrF4, 20% BaF2, 4% LaF3, 3% AlF3 and 20% NaF. ZBLAN is not a single mat ...
glass group, composed of
zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
, barium,
lanthanum Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lant ...
,
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, and
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
fluorides. Their main technological application is as optical waveguides in both planar and fiber forms. They are advantageous especially in the mid-infrared (2,000–5,000 nm) range.


Phosphate glass

Phosphate glass Phosphate glass is a class of optical glasses composed of metaphosphates of various metals. Instead of SiO2 in silicate glasses, the glass forming substrate is P2O5. Discovery Dr. Alexis G. Pincus of the American Optical Company supplied alum ...
constitutes a class of optical glasses composed of
metaphosphate A metaphosphate ion is an oxyanion that has the empirical formula . It was first postulated in 1955 but was not observed until 1979, when it was detected by mass spectrometry. Metaphosphate is an intermediate in the hydrolysis of phosphate esters b ...
s of various metals. Instead of the SiO4
tetrahedra In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...
observed in silicate glasses, the building block for this glass former is
phosphorus pentoxide Phosphorus pentoxide is a chemical compound with molecular formula P4 O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydra ...
(P2O5), which crystallizes in at least four different forms. The most familiar polymorph (see figure) comprises molecules of P4O10. Phosphate glasses can be advantageous over silica glasses for optical fibers with a high concentration of doping rare-earth ions. A mix of fluoride glass and phosphate glass is fluorophosphate glass.


Chalcogenide glass

The
chalcogen The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioac ...
s—the elements in
group 16 The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioa ...
of the
periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...
—particularly
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
(S),
selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
(Se) and
tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...
(Te)—react with more
electropositive Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
elements, such as
silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
, to form
chalcogenides : 220px, Cadmium sulfide, a prototypical metal chalcogenide, is used as a yellow pigment. A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements ...
. These are extremely versatile compounds, in that they can be crystalline or amorphous, metallic or semiconducting, and conductors of ions or electrons. Chalcogenide glass, Glass containing chalcogenides can be used to make fibers for far infrared transmission.


Process


Preform

Standard optical fibers are made by first constructing a large-diameter "preform" with a carefully controlled refractive index profile, and then "pulling" the preform to form the long, thin optical fiber. The preform is commonly made by three chemical vapor deposition methods: ''inside vapor deposition'', ''outside vapor deposition'', and ''vapor axial deposition''. With ''inside vapor deposition'', the preform starts as a hollow glass tube approximately long, which is placed horizontally and rotated slowly on a lathe. Gases such as silicon tetrachloride (SiCl4) or germanium tetrachloride (GeCl4) are injected with oxygen in the end of the tube. The gases are then heated by means of an external hydrogen burner, bringing the temperature of the gas up to 1,900 Kelvin, K (1,600 °C, 3,000 °F), where the tetrachlorides react with oxygen to produce
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
or germanium dioxide, germania (germanium dioxide) particles. When the reaction conditions are chosen to allow this reaction to occur in the gas phase throughout the tube volume, in contrast to earlier techniques where the reaction occurred only on the glass surface, this technique is called ''modified chemical vapor deposition (MCVD)''. The oxide particles then agglomerate to form large particle chains, which subsequently deposit on the walls of the tube as soot. The deposition is due to the large difference in temperature between the gas core and the wall causing the gas to push the particles outward (this is known as thermophoresis). The torch is then traversed up and down the length of the tube to deposit the material evenly. After the torch has reached the end of the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to form a solid layer. This process is repeated until a sufficient amount of material has been deposited. For each layer the composition can be modified by varying the gas composition, resulting in precise control of the finished fiber's optical properties. In outside vapor deposition or vapor axial deposition, the glass is formed by ''flame hydrolysis'', a reaction in which silicon tetrachloride and germanium tetrachloride are oxidized by reaction with water (H2O) in an oxyhydrogen flame. In outside vapor deposition, the glass is deposited onto a solid rod, which is removed before further processing. In vapor axial deposition, a short ''seed rod'' is used, and a porous preform, whose length is not limited by the size of the source rod, is built up on its end. The porous preform is consolidated into a transparent, solid preform by heating to about 1,800 K (1,500 °C, 2,800 °F). Typical communications fiber uses a circular preform. For some applications such as double-clad fibers another form is preferred. In fiber lasers based on double-clad fiber, an asymmetric shape improves the filling factor for laser pumping. Because of the surface tension, the shape is smoothed during the drawing process, and the shape of the resulting fiber does not reproduce the sharp edges of the preform. Nevertheless, careful polishing of the preform is important, since any defects of the preform surface affect the optical and mechanical properties of the resulting fiber. In particular, the preform for the test fiber shown in the figure was not polished well, and cracks are seen with the confocal optical microscope.


Drawing

The preform, regardless of construction, is placed in a device known as a drawing tower, where the preform tip is heated and the optical fiber is pulled out as a string. By measuring the resultant fiber width, the tension on the fiber can be controlled to maintain the fiber thickness.


Coatings

The light is guided down the core of the fiber by an optical cladding with a lower
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...
that traps light in the core through total internal reflection. The cladding is coated by a buffer that protects it from moisture and physical damage. The buffer coating is what gets stripped off the fiber for termination or splicing. These coatings are UV-cured urethane acrylate composite or polyimide materials applied to the outside of the fiber during the drawing process. The coatings protect the very delicate strands of glass fiber—about the size of a human hair—and allow it to survive the rigors of manufacturing, proof testing, cabling, and installation. Today’s glass optical fiber draw processes employ a dual-layer coating approach. An inner primary coating is designed to act as a shock absorber to minimize attenuation caused by microbending. An outer secondary coating protects the primary coating against mechanical damage and acts as a barrier to lateral forces, and may be colored to differentiate strands in bundled cable constructions. These fiber optic coating layers are applied during the fiber draw, at speeds approaching . Fiber optic coatings are applied using one of two methods: ''wet-on-dry'' and ''wet-on-wet''. In wet-on-dry, the fiber passes through a primary coating application, which is then UV cured—then through the secondary coating application, which is subsequently cured. In wet-on-wet, the fiber passes through both the primary and secondary coating applications, then goes to UV curing. Fiber optic coatings are applied in concentric layers to prevent damage to the fiber during the drawing application and to maximize fiber strength and microbend resistance. Unevenly coated fiber will experience non-uniform forces when the coating expands or contracts and is susceptible to greater signal attenuation. Under proper drawing and coating processes, the coatings are concentric around the fiber, continuous over the length of the application, and have a constant thickness. The thickness of the coating is taken into account when calculating the stress that the fiber experiences under different bend configurations. When a coated fiber is wrapped around a mandrel, the stress experienced by the fiber is given by :\sigma = E , where is the fiber’s Young’s modulus, is the diameter of the mandrel, is the diameter of the cladding and is the diameter of the coating. In a two-point bend configuration, a coated fiber is bent in a U-shape and placed between the grooves of two faceplates, which are brought together until the fiber breaks. The stress in the fiber in this configuration is given by :\sigma = 1.198E , where is the distance between the faceplates. The coefficient 1.198 is a geometric constant associated with this configuration. Fiber optic coatings protect the glass fibers from scratches that could lead to strength degradation. The combination of moisture and scratches accelerates the aging and deterioration of fiber strength. When fiber is subjected to low stresses over a long period, fiber fatigue can occur. Over time or in extreme conditions, these factors combine to cause microscopic flaws in the glass fiber to propagate, which can ultimately result in fiber failure. Three key characteristics of fiber optic waveguides can be affected by environmental conditions: strength, attenuation, and resistance to losses caused by microbending. External
optical fiber cable A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable, but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with ...
jackets and buffer tubes protect glass optical fiber from environmental conditions that can affect the fiber’s performance and long-term durability. On the inside, coatings ensure the reliability of the signal being carried and help minimize attenuation due to microbending.


Cable construction

In practical fibers, the cladding is usually coated with a tough resin coating and an additional ''buffer (optical fiber), buffer'' layer, which may be further surrounded by a ''jacket'' layer, usually plastic. These layers add strength to the fiber but do not contribute to its optical wave guide properties. Rigid fiber assemblies sometimes put light-absorbing ("dark") glass between the fibers, to prevent light that leaks out of one fiber from entering another. This reduces crosstalk between the fibers, or reduces Lens flare, flare in fiber bundle imaging applications. Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, high voltage isolation, dual use as power lines, installation in conduit, lashing to aerial telephone poles, Submarine communications cable, submarine installation, and insertion in paved streets. Multi-fiber cable usually uses colored coatings and/or buffers to identify each strand. The cost of small fiber-count pole-mounted cables has greatly decreased due to the high demand for fiber to the home (FTTH) installations in Japan and South Korea. Some fiber optic cable versions are reinforced with aramid yarns or glass yarns as an intermediary strength member. In commercial terms, usage of the glass yarns are more cost-effective while no loss in mechanical durability of the cable. Glass yarns also protect the cable core against rodents and termites.


Practical issues


Installation

Fiber cable can be very flexible, but traditional fiber's loss increases greatly if the fiber is bent with a radius smaller than around 30 mm. This creates a problem when the cable is bent around corners or wound around a spool, making FTTX installations more complicated. "Bendable fibers", targeted toward easier installation in home environments, have been standardized as ITU-T G.657. This type of fiber can be bent with a radius as low as 7.5 mm without adverse impact. Even more bendable fibers have been developed. Bendable fiber may also be resistant to fiber hacking, in which the signal in a fiber is surreptitiously monitored by bending the fiber and detecting the leakage. Another important feature of cable is cable's ability to withstand horizontally applied force. It is technically called max tensile strength defining how much force can be applied to the cable during the installation period.


Termination and splicing

Optical fibers are connected to terminal equipment by
optical fiber connector An optical fiber connector joins optical fibers, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so light can pass. Better connectors lose very little light due ...
s. These connectors are usually of a standard type such as ''FC'', ''SC'', ''ST'', ''LC'', ''MTRJ'', ''MPO'' or ''SMA''. Optical fibers may be connected by connectors, or permanently by ''splicing'', that is, joining two fibers together to form a continuous optical waveguide. The generally accepted splicing method is fusion splicing, arc fusion splicing, which melts the fiber ends together with an electric arc. For quicker fastening jobs, a “mechanical splice” is used. Fusion splicing is done with a specialized instrument. The fiber ends are first stripped of their protective polymer coating (as well as the more sturdy outer jacket, if present). The ends are ''cleaved'' (cut) with a precision cleaver to make them perpendicular, and are placed into special holders in the fusion splicer. The splice is usually inspected via a magnified viewing screen to check the cleaves before and after the splice. The splicer uses small motors to align the end faces together, and emits a small spark between electrodes at the gap to burn off dust and moisture. Then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding do not mix, and this minimizes optical loss. A splice loss estimate is measured by the splicer, by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side. A splice loss under 0.1 dB is typical. The complexity of this process makes fiber splicing much more difficult than splicing copper wire. Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning, and precision cleaving. The fiber ends are aligned and held together by a precision-made sleeve, often using a clear index-matching gel that enhances the transmission of light across the joint. Such joints typically have a higher optical loss and are less robust than fusion splices, especially if the gel is used. All splicing techniques involve installing an enclosure that protects the splice. Fibers are terminated in connectors that hold the fiber end precisely and securely. A fiber-optic connector is a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. The mating mechanism can be ''push and click'', ''turn and latch'' (''bayonet mount''), or ''screw-in'' (''threaded''). The barrel is typically free to move within the sleeve and may have a key that prevents the barrel and fiber from rotating as the connectors are mated. A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick-set adhesive is usually used to hold the fiber securely, and a strain relief is secured to the rear. Once the adhesive sets, the fiber's end is polished to a mirror finish. Various polish profiles are used, depending on the type of fiber and the application. For single-mode fiber, fiber ends are typically polished with a slight curvature that makes the mated connectors touch only at their cores. This is called a ''physical contact'' (PC) polish. The curved surface may be polished at an angle, to make an ''angled physical contact (APC)'' connection. Such connections have higher loss than PC connections but greatly reduced back reflection, because light that reflects from the angled surface leaks out of the fiber core. The resulting signal strength loss is called ''gap loss''. APC fiber ends have low back reflection even when disconnected. In the 1990s, terminating fiber optic cables was labor-intensive. The number of parts per connector, polishing of the fibers, and the need to oven-bake the epoxy in each connector made terminating fiber optic cables difficult. Today, many connector types are on the market that offer easier, less labor-intensive ways of terminating cables. Some of the most popular connectors are pre-polished at the factory and include a gel inside the connector. Those two steps help save money on labor, especially on large projects. A cleave (fiber), cleave is made at a required length, to get as close to the polished piece already inside the connector. The gel surrounds the point where the two pieces meet inside the connector for very little light loss. Long-term performance of the gel is a design consideration, so for the most demanding installations, factory pre-polished pigtails of sufficient length to reach the first fusion splice enclosure is normally the safest approach that minimizes on-site labor.


Free-space coupling

It is often necessary to align an optical fiber with another optical fiber or with an optoelectronic device such as a light-emitting diode, a laser diode, or a modulator. This can involve either carefully aligning the fiber and placing it in contact with the device, or can use a lens (optics), lens to allow coupling over an air gap. Typically the size of the fiber mode is much larger than the size of the mode in a laser diode or a Silicon photonics, silicon optical chip. In this case, a Tapered fiber, tapered or lensed fiber is used to match the fiber mode field distribution to that of the other element. The lens on the end of the fiber can be formed using polishing, laser cutting or fusion splicing. In a laboratory environment, a bare fiber end is coupled using a fiber launch system, which uses a microscope objective lens to focus the light down to a fine point. A precision translation stage (micro-positioning table) is used to move the lens, fiber, or device to allow the coupling efficiency to be optimized. Fibers with a connector on the end make this process much simpler: the connector is simply plugged into a pre-aligned fiber-optic collimator, which contains a lens that is either accurately positioned to the fiber or is adjustable. To achieve the best injection efficiency into a single-mode fiber, the direction, position, size, and divergence of the beam must all be optimized. With good beams, 70 to 90% coupling efficiency can be achieved. With properly polished single-mode fibers, the emitted beam has an almost perfect Gaussian shape—even in the far field—if a good lens is used. The lens needs to be large enough to support the full numerical aperture of the fiber, and must not introduce optical aberration, aberrations in the beam. Aspheric lenses are typically used.


Fiber fuse

At high optical intensities, above 2 megawatts per square centimeter, when a fiber is subjected to a shock or is otherwise suddenly damaged, a ''fiber fuse'' can occur. The reflection from the damage vaporizes the fiber immediately before the break, and this new defect remains reflective so that the damage propagates back toward the transmitter at 1–3 meters per second (4–11 km/h, 2–8 mph). The open fiber control system, which ensures laser safety, laser eye safety in the event of a broken fiber, can also effectively halt propagation of the fiber fuse. In situations, such as undersea cables, where high power levels might be used without the need for open fiber control, a "fiber fuse" protection device at the transmitter can break the circuit to keep damage to a minimum.


Chromatic dispersion

The refractive index of fibers varies slightly with the frequency of light, and light sources are not perfectly monochromatic. Modulation of the light source to transmit a signal also slightly widens the frequency band of the transmitted light. This has the effect that, over long distances and at high modulation speeds, the different frequencies of light can take different times to arrive at the receiver, ultimately making the signal impossible to discern, and requiring extra repeaters.G. P. Agrawal, Fiber Optic Communication Systems, Wiley-Interscience, 1997. This problem can be overcome in several ways, including the use of a relatively short length of fiber that has the opposite refractive index gradient.


See also

* Fiber Bragg grating * Fiber management system * The Fiber Optic Association * Gradient-index optics * Interconnect bottleneck * Leaky mode * Li-Fi * Light tube * Modal bandwidth * Optical communication * Optical mesh network * Optical power meter * Radiation effects on optical fibers * Return loss * Subwavelength-diameter optical fibre


Notes


References


Further reading

* * * Mirabito, Michael M. A.; and Morgenstern, Barbara L., ''The New Communications Technologies: Applications, Policy, and Impact'', 5th Edition. Focal Press, 2004. (). * Mitschke F., ''Fiber Optics: Physics and Technology'', Springer, 2009 () * *
''Lennie Lightwave's Guide to Fiber Optics''
The Fiber Optic Association, 2016. * The book discusses how fiber optics has contributed to globalization, and has revolutionized communications, business, and even the distribution of capital among countries.
GR-771, ''Generic Requirements for Fiber Optic Splice Closures''
Telcordia Technologies, Issue 2, July 2008. Discusses fiber optic splice closures and the associated hardware intended to restore the mechanical and environmental integrity of one or more fiber cables entering the enclosure. *


External links


The Fiber Optic Association
*

, article in RP Photonics' ''Encyclopedia of Laser Physics and Technology'' *

, Mercury Communications Ltd, August 1992. *

, Mercury Communications Ltd, March 1993. *

Educational site from Arc Electronics
MIT Video Lecture: Understanding Lasers and Fiberoptics

Fundamentals of Photonics: Module on Optical Waveguides and Fibers

Webdemo for chromatic dispersion
at the Institute of Telecommunicatons, University of Stuttgart {{Authority control Optical fiber, Fiber optics, Articles containing video clips Glass engineering and science Glass production Telecommunications equipment