An optical fiber (or fibre) is a
glass or
plastic fiber that carries
light along its length. Fib
er optics is the overlap of
applied science and
engineering concerned with the design and plication of optical fibers. Optical fibers are widely used in
fiber-optic communications, which permits transmission over longer distances and at higher data rates (a.k.a "
bandwidth") than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less
loss, and they are also immune to
electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including
sensors and
fiber lasers.
Light is kept in the "core" of the optical fiber by
total internal reflection. This causes the fiber to act as a
waveguide. Fibers which support many propagation paths or
transverse modes are called
multi-mode fibers (MMF). Fibers which can only support a single mode are called
single-mode fibers (SMF). Multi-mode fibers generally have a larger 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 200 meters.
Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully
cleaved, and then spliced together either
mechanically or by
fusing them together with an
electric arc. Special
connectors are used to make removable connections.
History
Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by
Daniel Colladon and
Jacques Babinet in Paris in the 1840s, with Irish inventor
John Tyndall offering public displays using water-fountains ten years later.
[1] Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter
Clarence Hansell and the television pioneer
John Logie Baird in the 1920s. The principle was first used for internal medical examinations by
Heinrich Lamm in the following decade. In 1952, physicist
Narinder Singh Kapany conducted experiments that led to the invention of optical fiber, based on Tyndall's earlier studies; modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable
refractive index, appeared later in the decade.
[1] Development then focused on fiber bundles for image transmission. The first fiber optic semi-flexible gastroscope was patented by
Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the
University of Michigan, 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. A variety of other image transmission applications soon followed.
In 1965,
Charles K. Kao and George A. Hockham of the British company
Standard Telephones and Cables (STC) were the first to promote the idea that the
attenuation in optical fibers could be reduced below 20
dB per kilometer, allowing fibers to be a practical medium for communication.
[2] They proposed that the attenuation in fibers available at the time was caused by impurities, which could be removed, rather than fundamental physical effects such as scattering. The crucial attenuation level of 20 dB was first achieved in 1970, by researchers
Robert D. Maurer,
Donald Keck,
Peter C. Schultz, and Frank Zimar working for American glass maker Corning Glass Works, now
Corning Incorporated They demonstrated a fiber with 17 dB optic attenuation per kilometer by
doping silica glass with
titanium. A few years later they produced a fiber with only 4 dB/km using
germanium dioxide as the core dopant. Such low attenuations ushered in optical fiber telecommunications and enabled the Internet. In 1981,
General Electric produced fused quartz ingots that could be drawn into fiber optic strands 25 miles long.
[3]Attenuations in modern optical cables are far less than those in electrical copper cables, leading to long-haul fiber connections with repeater distances of 50–80 km. The
erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by reducing or even in many cases eliminating the need for optical-electrical-optical repeaters, was co-developed by teams led by
David N. Payne of the
University of Southampton, and
Emmanuel Desurvire at
Bell Laboratories in 1986. The more robust optical fiber commonly used today utilizes glass for both core and sheath and is therefore less prone to aging processes. It was invented by Gerhard Bernsee in 1973 of
Schott Glass in Germany.
[4]In 1991, the emerging field of
photonic crystals led to the development of
photonic crystal fiber [5] which guides light by means of diffraction from a periodic structure, rather than total internal reflection. The first photonic crystal fibers became commercially available in 2000.
[6] Photonic crystal fibers can be designed to carry higher power than conventional fiber, and their wavelength dependent properties can be manipulated to improve their performance in certain applications.
Optical fiber communication
Optical fiber can be used as a medium for telecommunication and
networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little
attenuation compared to electrical cables. This allows long distances to be spanned with few
repeaters. Additionally, the per channel light signals propagating in the fiber can be modulated at rates as high as 111
Gb/s
[7]. (In 2001 the limit was at 40
Gb/s
[8]. In today's
DWDM systems the net data rate (data rate without overhead bytes) per fiber is the per channel data rate reduced by the FEC overhead multiplied by the number of channels (usually up to 80 channels in commercially available systems as of 2008). (Some communication companies are revealing that net data rates as fast as 1
Tb/s are currently being developed.[
citation needed]), and each fiber can carry many independent channels, each by a different wavelength of light (
wavelength-division multiplexing). Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable. Fiber is also immune to electrical interference, which makes using a non-armor jacketed fiber optic cable a good solution for protecting communications equipment located in high voltage environments such as power generation facilities normally served by a copper telephone cable or metal communication structures prone to lightning strikes . Fiber prevents high voltage from traveling from one end of the fiber system to the other. Fiber also eliminates cross-talk between signals in different cables and pickup of environmental noise. Also,
wiretapping is more difficult compared to electrical connections, and there are concentric dual core fibers that are said to be tap-proof. Because they are non-electrical, fiber cables can bridge very high electrical potential differences and can be used in environments where explosive fumes are present, without danger of ignition.
Although fibers can be made out of transparent
plastic,
glass, or a
combination of the two, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers (core diameter about 10 micrometers), single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.
Application examples:
TOSLINK,
Fiber distributed data interface,
Synchronous optical networkingFiber optic sensors
Main article:
Fiber optic sensorFibers have many uses in remote sensing. In some applications, the sensor is itself an optical fiber. In other cases, fiber is used to connect a non-fiberoptic sensor to a measurement system. Depending on the application, fiber may be used because of its small size, or the fact that no
electrical power is needed at the remote location, or because many sensors can be
multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an
optical time-domain reflectometer.
Optical fibers can be used as sensors to measure
strain,
temperature,
pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the
intensity,
phase,
polarization,
wavelength 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.
Extrinsic fiber optic sensors use an
optical fiber cable, normally a multimode one, to transmit
modulated 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 places which are otherwise inaccessible. An example is the measurement of temperature inside
aircraft jet engines by using a fiber to transmit
radiation into a radiation
pyrometer located outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature of
electrical transformers, where the extreme
electromagnetic fields present make other measurement techniques impossible. Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting.
Optical fibers
A
TOSLINK fiber optic audio cable being illuminated on one end
An optical fiber (or fibre) is a
glass or
plastic fiber that carries
light along its length. Fiber optics is the overlap of
applied science and
engineering concerned with the design and application of optical fibers. Optical fibers are widely used in
fiber-optic communications, which permits transmission over longer distances and at higher data rates (a.k.a "
bandwidth") than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less
loss, and they are also immune to
electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including
sensors and
fiber lasers.
Light is kept in the "core" of the optical fiber by
total internal reflection. This causes the fiber to act as a
waveguide. Fibers which support many propagation paths or
transverse modes are called
multi-mode fibers (MMF). Fibers which can only support a single mode are called
single-mode fibers (SMF). Multi-mode fibers generally have a larger 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 200 meters.
Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully
cleaved, and then spliced together either
mechanically or by
fusing them together with an
electric arc. Special
connectors are used to make removable connections.
History Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by
Daniel Colladon and
Jacques Babinet in Paris in the 1840s, with Irish inventor
John Tyndall offering public displays using water-fountains ten years later.
[1] Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter
Clarence Hansell and the television pioneer
John Logie Baird in the 1920s. The principle was first used for internal medical examinations by
Heinrich Lamm in the following decade. In 1952, physicist
Narinder Singh Kapany conducted experiments that led to the invention of optical fiber, based on Tyndall's earlier studies; modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable
refractive index, appeared later in the decade.
[1] Development then focused on fiber bundles for image transmission. The first fiber optic semi-flexible gastroscope was patented by
Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the
University of Michigan, 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. A variety of other image transmission applications soon followed.
In 1965,
Charles K. Kao and George A. Hockham of the British company
Standard Telephones and Cables (STC) were the first to promote the idea that the
attenuation in optical fibers could be reduced below 20
dB per kilometer, allowing fibers to be a practical medium for communication.
[2] They proposed that the attenuation in fibers available at the time was caused by impurities, which could be removed, rather than fundamental physical effects such as scattering. The crucial attenuation level of 20 dB was first achieved in 1970, by researchers
Robert D. Maurer,
Donald Keck,
Peter C. Schultz, and Frank Zimar working for American glass maker Corning Glass Works, now
Corning Incorporated They demonstrated a fiber with 17 dB optic attenuation per kilometer by
doping silica glass with
titanium. A few years later they produced a fiber with only 4 dB/km using
germanium dioxide as the core dopant. Such low attenuations ushered in optical fiber telecommunications and enabled the Internet. In 1981,
General Electric produced fused quartz ingots that could be drawn into fiber optic strands 25 miles long.
[3]Attenuations in modern optical cables are far less than those in electrical copper cables, leading to long-haul fiber connections with repeater distances of 50–80 km. The
erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by reducing or even in many cases eliminating the need for optical-electrical-optical repeaters, was co-developed by teams led by
David N. Payne of the
University of Southampton, and
Emmanuel Desurvire at
Bell Laboratories in 1986. The more robust optical fiber commonly used today utilizes glass for both core and sheath and is therefore less prone to aging processes. It was invented by Gerhard Bernsee in 1973 of
Schott Glass in Germany.
[4]In 1991, the emerging field of
photonic crystals led to the development of
photonic crystal fiber [5] which guides light by means of diffraction from a periodic structure, rather than total internal reflection. The first photonic crystal fibers became commercially available in 2000.
[6] Photonic crystal fibers can be designed to carry higher power than conventional fiber, and their wavelength dependent properties can be manipulated to improve their performance in certain applications.
Applications
Optical fiber communicationOptical fiber can be used as a medium for telecommunication and
networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little
attenuation compared to electrical cables. This allows long distances to be spanned with few
repeaters. Additionally, the per channel light signals propagating in the fiber can be modulated at rates as high as 111
Gb/s
[7]. (In 2001 the limit was at 40
Gb/s
[8]. In today's
DWDM systems the net data rate (data rate without overhead bytes) per fiber is the per channel data rate reduced by the FEC overhead multiplied by the number of channels (usually up to 80 channels in commercially available systems as of 2008). (Some communication companies are revealing that net data rates as fast as 1
Tb/s are currently being developed.[
citation needed]), and each fiber can carry many independent channels, each by a different wavelength of light (
wavelength-division multiplexing). Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable. Fiber is also immune to electrical interference, which makes using a non-armor jacketed fiber optic cable a good solution for protecting communications equipment located in high voltage environments such as power generation facilities normally served by a copper telephone cable or metal communication structures prone to lightning strikes . Fiber prevents high voltage from traveling from one end of the fiber system to the other. Fiber also eliminates cross-talk between signals in different cables and pickup of environmental noise. Also,
wiretapping is more difficult compared to electrical connections, and there are concentric dual core fibers that are said to be tap-proof. Because they are non-electrical, fiber cables can bridge very high electrical potential differences and can be used in environments where explosive fumes are present, without danger of ignition.
Although fibers can be made out of transparent
plastic,
glass, or a
combination of the two, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers (core diameter about 10 micrometers), single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.
Application examples:
TOSLINK,
Fiber distributed data interface,
Synchronous optical networkingFiber optic sensors
Main article:
Fiber optic sensorFibers have many uses in remote sensing. In some applications, the sensor is itself an optical fiber. In other cases, fiber is used to connect a non-fiberoptic sensor to a measurement system. Depending on the application, fiber may be used because of its small size, or the fact that no
electrical power is needed at the remote location, or because many sensors can be
multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an
optical time-domain reflectometer.
Optical fibers can be used as sensors to measure
strain,
temperature,
pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the
intensity,
phase,
polarization,
wavelength 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.
Extrinsic fiber optic sensors use an
optical fiber cable, normally a multimode one, to transmit
modulated 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 places which are otherwise inaccessible. An example is the measurement of temperature inside
aircraft jet engines by using a fiber to transmit
radiation into a radiation
pyrometer located outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature of
electrical transformers, where the extreme
electromagnetic fields present make other measurement techniques impossible. Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting.
Other uses of optical fibersFibers are widely used in illumination applications. They 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. In some buildings, optical fibers are used to route sunlight from the roof to other parts of the building (see
non-imaging optics). Optical fiber illumination is also used for
decorative applications, including
signs,
art, and artificial
Christmas trees.
Swarovski boutiques use optical fibers to illuminate their crystal showcases from many different angles while only employing one light source. Optical fiber is an intrinsic part of the light-transmitting concrete building product,
LiTraCon.
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, which is used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures (
endoscopy). Industrial endoscopes (see
fiberscope or
borescope) are used for inspecting anything hard to reach, such as jet engine interiors.
An optical fiber
doped with certain
rare-earth elements such as
erbium can be used as the
gain medium of a
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 fibers doped with a
wavelength shifter are used to collect
scintillation light in
physics experiments.
Optical fiber can be used to supply a low level of power (around one watt) to electronics situated in a difficult electrical environment. Examples of this are electronics in high-powered antenna elements and measurement devices used in high voltage transmission equipment.
Principle of operationAn optical fiber is a cylindrical
dielectric waveguide that transmits light along its axis, by the process of
total internal reflection. The fiber consists of a core surrounded by a
cladding layer. To confine the optical signal in the core, the
refractive index 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.
Index of refraction
Main article:
Refractive indexThe index of refraction is a way of measuring the
speed of light in a material. Light travels fastest in a
vacuum, such as outer space. The actual speed of light in a vacuum is 299,792 kilometers per second, or 186,282 miles per second.
[9] Index of refraction is calculated by dividing the speed of light in a vacuum by the speed of light in some other medium. The index of refraction of a vacuum is therefore 1, by definition. The typical value for the cladding of an optical fiber is 1.46. The core value is typically 1.48. The larger the index of refraction, the more slowly light travels in that medium.
Total internal reflection
Main article:
Total internal reflectionWhen light traveling in a dense medium hits a boundary at a steep angle (larger than the "critical angle" for the boundary), the light will be completely reflected. This effect is used in optical fibers to confine light in the core. Light travels along the fiber bouncing back and forth off of the boundary. Because the light must strike the boundary with an angle less 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 of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding.
In simpler terms, 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 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. Single-mode fiber has a small NA.
Multimode fiber 
Fiber with large (greater than 10
μm) core diameter may be analyzed by
geometric optics. Such fiber is called
multimode fiber, from the electromagnetic analysis (see below). In a step-index multimode fiber,
rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line
normal to the boundary), greater than the
critical angle for this boundary, are completely reflected. The critical angle (minimum angle for total in
ternal reflection) is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the
core into the cladding, and do not convey light and hence information along the fiber. 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 as rays at different angles have different
path lengths and therefore take different times to traverse the fiber. A low numerical aperture may therefore be desirable.
Optical fiber types.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.
Singlemode fiber
Fiber with a core diameter less than about ten times the
wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an
electromagnetic structure, by solution of
Maxwell's equations as reduced to the
electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as
speckle that occur when
coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined
transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called
single-mode or mono-mode fiber. The behavior of larger-core multimode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation (hence the name). The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes.
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 to 10 μm and is designed for use in the
near infrared. The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometres and as large as hundreds of micrometres.
Special-purpose fiber
Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include
polarization-maintaining fiber and fiber designed to suppress
whispering gallery mode propagation.
Photonic crystal fiber 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 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.
The TU12 is multiplexed into a TUG 2 along with 2 other TU12s. This is achieved by interleving the bytes of each TU12 in turn. Next, seven TUG 2s are byte interleaved into a TUG 3 and then three TUG 3s can be byte interleaved to form the VC4 (see the SDH diagram above). You can see that 3 x 7 x 3 = 63 2Mb/s circuits can be contained in VC 4.
