Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
through an
optical fiberAn optical fiber 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...
. The light forms an
electromagneticElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
carrier waveIn telecommunications, a carrier wave, or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually of much higher frequency than the input signal....
that is
modulatedModulation is the process of varying one waveform in relation to another waveform. In telecommunications, modulation is used to convey a message, or a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Often a high-frequency sinusoid waveform is used...
to carry information. First developed in the 1970s, fiber-optic communication systems have revolutionized the telecommunications industry and have played a major role in the advent of the
Information AgeThe Information Age, also commonly known as the Computer Age or Information Era, is an idea that the current age will be characterized by the ability of individuals to transfer information freely, and to have instant access to knowledge that would have been difficult or impossible to find previously...
. Because of its advantages over electrical transmission, optical fibers have largely replaced copper wire communications in
core networkA core network is the central part of a telecom network that provides various services to customers who are connected by the access network.- Primary Functions :Core networks typically providing the following functionality:...
s in the developed world.
The process of communicating using fiber-optics involves the following basic steps: Creating the optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, receiving the optical signal, and converting it into an electrical signal.
Applications
Optical fiberAn optical fiber 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...
is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Due to much lower
attenuationIn physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, and X-rays are attenuated by lead....
and
interferenceIn physics, interference is the addition of two or more waves that results in a new wave pattern.Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same...
, optical fiber has large advantages over existing copper wire in long-distance and high-demand applications. However, infrastructure development within cities was relatively difficult and time-consuming, and fiber-optic systems were complex and expensive to install and operate. Due to these difficulties, fiber-optic communication systems have primarily been installed in long-distance applications, where they can be used to their full transmission capacity, offsetting the increased cost. Since 2000, the prices for fiber-optic communications have dropped considerably. The price for rolling out fiber to the home has currently become more cost-effective than that of rolling out a copper based network. Prices have dropped to
$850 per subscriber in the US and lower in countries like The Netherlands, where digging costs are low.
Since 1990, when
optical-amplificationAn 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 the cavity is suppressed. Stimulated emission in the...
systems became commercially available, the telecommunications industry has laid a vast network of intercity and transoceanic fiber communication lines. By 2002, an intercontinental network of 250,000 km of
submarine communications cableA submarine communications cable is a cable laid beneath the sea to carry telecommunications between countries.The first submarine communications cables carried telegraphy traffic. Subsequent generations of cables carried first telephony traffic, then data communications traffic...
with a capacity of 2.56
TbA terabit is a unit of information or computer storage, abbreviated Tbit .1 terabit = 1012 bits = 1,000,000,000,000 bits . 1 terabit is equal to 125 gigabytes or 122 gibibytes.The terabit is closely related to the tebibit, which is equal to 240 or 1,099,511,627,776 bits...
/s was completed, and although specific network capacities are privileged information, telecommunications investment reports indicate that network capacity has increased dramatically since 2002.
History
In 1966
Charles K. KaoProfessor Charles Kuen Kao CBE FRS FREng is a pioneer in the development and use of fiber optics in telecommunications...
and George Hockham proposed optical fibers at STC Laboratories (STL),
HarlowHarlow is a former new town and now a borough town and local government district in Essex, England. It is located in the west of the county and on the border with Hertfordshire, on the Stort Valley. The town is near the M11 motorway and forms part of the London commuter belt...
, when they showed that the losses of 1000 db/km in existing glass (compared to 5-10 db/km in coaxial cable) was due to contaminants, which could potentially be removed.
Optical fiber was successfully developed in 1970 by Corning Glass Works, with attenuation low enough for communication purposes (about 20
dBThe decibel is a logarithmic unit of measurement that expresses the magnitude of a physical quantity relative to a specified or implied reference level. Since it expresses a ratio of two quantities with the same unit, it is a dimensionless unit...
/
kmThe kilometre , symbol km is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second....
), and at the same time GaAs
semiconductor lasersA laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common and practical type of laser diode is formed from a p-n junction and powered by injected electric current...
were developed that were compact and therefore suitable for transmitting light through fiber optic cables for long distances.
After a period of research starting from 1975, the first commercial fiber-optic communications system was developed, which operated at a wavelength around 0.8 µm and used GaAs semiconductor lasers. This first-generation system operated at a bit rate of 45 Mbps with repeater spacing of up to 10
kmKM, Km, or km may stand for:*Kilometre *KM - the Michaelis constant in Michaelis-Menten kinetics*Kernel methods*Kettle Moraine High School*Khmer language...
. Soon on 22 April, 1977, General Telephone and Electronics sent the first live telephone traffic through fiber optics at a 6 Mbps throughput in Long Beach, California.
The second generation of fiber-optic communication was developed for commercial use in the early 1980s, operated at 1.3 µm, and used InGaAsP semiconductor lasers. Although these systems were initially limited by dispersion, in 1981 the
single-mode fiberIn fiber-optic communication, a single-mode optical fiber is an optical fiber designed to carry only a single ray of light . This ray of light often contains a variety of different wavelengths...
was revealed to greatly improve system performance. By 1987, these systems were operating at bit rates of up to 1.7
GbGigabit is a unit of digital information storage, with the symbol Gbit .1 gigabit = 109 = 1,000,000,000 bits...
/s with repeater spacing up to 50 km.
The first
transatlantic telephone cableA transatlantic telephone cable is a submarine communications cable that carries telephone traffic under the Atlantic Ocean.When the first transatlantic telegraph cable was laid in 1858 by businessman Cyrus West Field, it operated for only a month; subsequent attempts in 1865 and 1866 were more...
to use optical fiber was
TAT-8TAT-8 was the 8th transatlantic telephone cable,initially carrying 40,000 telephone circuits between USA, England and France. It was constructed in 1988 by a consortium of companies led by AT&T, France Telecom, and British Telecom...
, based on Desurvire optimized laser amplification technology. It went into operation in 1988.
Third-generation fiber-optic systems operated at 1.55 µm and had losses of about 0.2 dB/km. They achieved this despite earlier difficulties with
pulse-spreadingIn optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
at that wavelength using conventional InGaAsP semiconductor lasers. Scientists overcame this difficulty by using dispersion-shifted fibers designed to have minimal dispersion at 1.55 µm or by limiting the laser spectrum to a single
longitudinal modeA longitudinal mode of a resonant cavity is a particular standing wave pattern formed by waves confined in the cavity. The longitudinal modes correspond to the wavelengths of the wave which are reinforced by constructive interference after many reflections from the cavity's reflecting surfaces...
. These developments eventually allowed third-generation systems to operate commercially at 2.5 Gbit/s with repeater spacing in excess of 100 km.
The fourth generation of fiber-optic communication systems used
optical amplificationAn 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 the cavity is suppressed. Stimulated emission in the...
to reduce the need for repeaters and
wavelength-division multiplexingIn fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths of laser light to carry different signals. This allows for a multiplication in capacity, in addition to...
to increase
data capacityIn electrical engineering, computer science and information theory, channel capacity is the tightest upper bound on the amount of information that can be reliably transmitted over a communications channel...
. These two improvements caused a revolution that resulted in the doubling of system capacity every 6 months starting in 1992 until a bit rate of 10
TbA terabit is a unit of information or computer storage, abbreviated Tbit .1 terabit = 1012 bits = 1,000,000,000,000 bits . 1 terabit is equal to 125 gigabytes or 122 gibibytes.The terabit is closely related to the tebibit, which is equal to 240 or 1,099,511,627,776 bits...
/s was reached by 2001. Recently, bit-rates of up to 14 Tbit/s have been reached over a single 160 km line using optical amplifiers.
The focus of development for the fifth generation of fiber-optic communications is on extending the wavelength range over which a WDM system can operate. The conventional wavelength window, known as the C band, covers the wavelength range 1.53-1.57 µm, and the new
dry fiber has a low-loss window promising an extension of that range to 1.30-1.65 µm. Other developments include the concept of "
optical solitonsIn optics, the term soliton is used to refer to any optical field that does not change during propagation because of a delicate balance between nonlinear and linear effects in the medium. There are two main kinds of solitons:...
, " pulses that preserve their shape by counteracting the effects of dispersion with the
nonlinear effectsNonlinear optics is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light. This nonlinearity is typically only observed at very high light intensities such as...
of the fiber by using pulses of a specific shape.
In the late 1990s through 2000, industry promoters, and research companies such as KMI and RHK predicted vast increases in demand for communications bandwidth due to increased use of the
InternetThe Internet is a global system of interconnected computer networks that use the standardized Internet Protocol Suite to serve billions of users worldwide...
, and commercialization of various bandwidth-intensive consumer services, such as
video on demandVideo on Demand or Audio Video on Demand are systems which allow users to select and watch/listen to video or audio content on demand....
.
Internet protocolThe Internet Protocol is a protocol used for communicating data across a packet-switched internetwork using the Internet Protocol Suite, also referred to as TCP/IP....
data traffic was increasing exponentially, at a faster rate than integrated circuit complexity had increased under
Moore's LawMoore's Law describes a long-term trend in the history of computing hardware, in which the number of transistors that can be placed inexpensively on an integrated circuit has doubled approximately every two years...
. From the bust of the dot-com bubble through 2006, however, the main trend in the industry has been
consolidationConsolidation or amalgamation is the act of merging many things into one. In business, it often refers to the mergers or acquisitions of many smaller companies into much larger ones. The financial accounting term of consolidation refers to the aggregated financial statements of a group company as...
of firms and
offshoringOffshoring describes the relocation by a company of a business process from one country to another -- typically an operational process, such as manufacturing, or supporting processes, such as accounting. Even state governments employ offshoring....
of manufacturing to reduce costs. Recently, companies such as
VerizonVerizon FiOS is a bundled communications service, operating over a fiber-optic communications network, that is presently offered in some areas of the United States by Verizon. Verizon has attracted consumer and media attention in the area of broadband Internet access as the first major U.S....
and AT&T have taken advantage of fiber-optic communications to deliver a variety of high-throughput data and broadband services to consumers' homes.
Technology
Modern fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fiber, a
cableAn optical fiber cable is a cable containing one or more optical fibers. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed....
containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal. The information transmitted is typically digital information generated by computers,
telephone systemsDigital telephony is the use of digital electronics in the provision of digital telephone services and systems. Since the 1960s a digital core network has almost entirely replaced the old analog system, and much of the access network has also been digitized...
, and
cable televisionCable television is a system of providing television to consumers via radio frequency signals transmitted to televisions through fixed optical fibers or coaxial cables as opposed to the over-the-air method used in traditional television broadcasting in which a television antenna is required...
companies.
Transmitters
The most commonly-used optical transmitters are semiconductor devices such as
light-emitting diodeA light-emitting diode , is an electronic light source. The first LED was built in the 1920s by Oleg Vladimirovich Losev, a radio technician who noticed that diodes used in radio receivers emitted light when current was passed through them...
s (LEDs) and
laser diodeA laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common and practical type of laser diode is formed from a p-n junction and powered by injected electric current...
s. The difference between LEDs and laser diodes is that LEDs produce incoherent light, while laser diodes produce coherent light. For use in optical communications, semiconductor optical transmitters must be designed to be compact, efficient, and reliable, while operating in an optimal wavelength range, and directly modulated at high frequencies.
In its simplest form, an LED is a forward-biased
p-n junctionA p–n junction is a junction formed by joining P-type and N-type semiconductors together in very close contact.The term junction refers to the region where the two regions of the semiconductor meet...
, emitting light through
spontaneous emissionSpontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to the ground state and emits a photon...
, a phenomenon referred to as
electroluminescenceElectroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to an electric current passed through it, or to a strong electric field...
. The emitted light is incoherent with a relatively wide spectral width of 30-60 nm. LED light transmission is also inefficient, with only about 1 % of input power, or about 100 microwatts, eventually converted into launched power which has been coupled into the optical fiber. However, due to their relatively simple design, LEDs are very useful for low-cost applications.
Communications LEDs are most commonly made from
gallium arsenide phosphideGallium arsenide phosphide is a semiconductor material, an alloy of gallium arsenide and gallium phosphide.Gallium arsenide phosphide is used for manufacturing red, orange and yellow light-emitting diodes. It is often grown on gallium phosphide substrate to form a GaP/GaAsP heterostructure...
(GaAsP) or gallium arsenide (GaAs). Because GaAsP LEDs operate at a longer wavelength than GaAs LEDs (1.3 micrometers vs. 0.81-0.87 micrometers), their output spectrum is wider by a factor of about 1.7. The large spectrum width of LEDs causes higher fiber dispersion, considerably limiting their bit rate-distance product (a common measure of usefulness). LEDs are suitable primarily for
local-area-networkA local area network is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport...
applications with bit rates of 10-100 Mbit/s and transmission distances of a few kilometers. LEDs have also been developed that use several
quantum wellA quantum well is a potential well that confines particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
s to emit light at different wavelengths over a broad spectrum, and are currently in use for local-area
WDMIn fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths of laser light to carry different signals. This allows for a multiplication in capacity, in addition to...
networks.
A semiconductor laser emits light through
stimulated emissionIn optics, stimulated emission is the process by which an electron, perturbed by a photon having the correct energy, may drop to a lower energy level resulting in the creation of another photon. The perturbing photon is seemingly unchanged in the process , and the second photon is created with the...
rather than spontaneous emission, which results in high output power (~100 mW) as well as other benefits related to the nature of coherent light. The output of a laser is relatively directional, allowing high coupling efficiency (~50 %) into single-mode fiber. The narrow spectral width also allows for high bit rates since it reduces the effect of
chromatic dispersionIn optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
. Furthermore, semiconductor lasers can be modulated directly at high frequencies because of short
recombination timeIn the solid state physics of semiconductors, carrier generation and recombination are processes by which mobile charge carriers are created and eliminated. Carrier generation and recombination processes are fundamental to the operation of many optoelectronic semiconductor devices, such as...
.
Laser diodes are often directly
modulatedModulation is the process of varying one waveform in relation to another waveform. In telecommunications, modulation is used to convey a message, or a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Often a high-frequency sinusoid waveform is used...
, that is the light output is controlled by a current applied directly to the device. For very high data rates or very long distance
links, a laser source may be operated continuous wave, and the light modulated by an external device such as an electroabsorption modulator or
Mach-Zehnder interferometerThe Mach-Zehnder interferometer is a device used to determine the phase shift caused by a small sample which is placed in the path of one of two collimated beams from a coherent light source.In contrast to the Michelson interferometer, there are two output ports.-Set-up:A...
. External modulation increases the achievable link distance by eliminating laser
chirpA chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...
, which broadens the linewidth of directly-modulated lasers, increasing the chromatic dispersion in the fiber.
Receivers
The main component of an optical receiver is a
photodetectorPhotosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties: *Optical detectors, which are mostly quantum devices in which an individual photon produces a discrete effect....
, which converts light into electricity using the
photoelectric effectThe photoelectric effect is a phenomenon in which electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as "photoelectrons"...
. The photodetector is typically a semiconductor-based
photodiodeA photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.Photodiodes are similar to regular semiconductor diodes except that they may be either exposed or packaged with a window or optical fiber connection to allow...
. Several types of photodiodes include p-n photodiodes, a p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photodetectors are also used due to their suitability for
circuit integrationIn electronics, an integrated circuit is a miniaturized electronic circuit that has been manufactured in the surface of a thin substrate of semiconductor material...
in
regeneratorsIn telecommunications, signal regeneration is signal processing that restores a signal to its original specifications.The signal may be electrical, as in a repeater on a T-carrier line, or optical, as in an OEO optical cross-connect....
and wavelength-division multiplexers.
The optical-electrical converters are typically coupled with a transimpedance amplifier and a limiting amplifier to produce a digital signal in the electrical domain from the incoming optical signal, which may be attenuated and distorted while passing through the channel. Further signal processing such as clock recovery from data (CDR) performed by a
phase-locked loopA phase-locked loop or phase lock loop is a control system that generates a signal that has a fixed relation to the phase of a "reference" signal...
may also be applied before the data is passed on.
Fiber
An optical fiber consists of a core, cladding, and a buffer (a protective outer coating), in which the cladding guides the light along the core by using the method of
total internal reflectionTotal internal reflection is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through...
. The core and the cladding (which has a lower-
refractive-indexThe refractive index of a medium is a measure of how much the speed of light is reduced inside the medium. For example, typical soda-lime glass has a refractive index close to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed of light in a vacuum...
) are usually made of high-quality
silicaThe chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of ' and has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms...
glass, although they can both be made of plastic as well. Connecting two optical fibers is done by fusion splicing or mechanical splicing and requires special skills and interconnection technology due to the microscopic precision required to align the fiber cores.
Two main types of optical fiber used in fiber optic communications include
multi-mode optical fiberMulti-mode optical fiber is a type of optical fiber mostly used for communication over shorter distances, such as within a building or on a campus...
s and
single-mode optical fiberIn fiber-optic communication, a single-mode optical fiber is an optical fiber designed to carry only a single ray of light . This ray of light often contains a variety of different wavelengths...
s. A
multi-mode optical fiberMulti-mode optical fiber is a type of optical fiber mostly used for communication over shorter distances, such as within a building or on a campus...
has a larger core (≥ 50
micrometreA micrometre or micron is one millionth of a metre,or equivalently one thousandth of a millimetre....
s), allowing less precise, cheaper transmitters and receivers to connect to it as well as cheaper connectors. However, a multi-mode fiber introduces multimode distortion, which often limits the bandwidth and length of the link. Furthermore, because of its higher
dopantA dopant, also called doping agent and dope, is an impurity element added to a crystal lattice in low concentrations in order to alter the optical/electrical properties of the crystal....
content, multimode fibers are usually expensive and exhibit higher attenuation. The core of a single-mode fiber is smaller (<10 micrometres) and requires more expensive components and interconnection methods, but allows much longer, higher-performance links.
In order to package fiber into a commercially-viable product, it is typically protectively-coated by using ultraviolet (UV), light-cured
acrylate polymersAn acrylate polymer belongs to a group of polymers which could be referred to generally as plastics. They are noted for their transparency and resistance to breakage and elasticity...
, then terminated with
optical fiber connectorAn optical fiber connector terminates the end of an optical fiber, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so that light can pass...
s, and finally assembled into a cable. After that, it can be laid in the ground and then run through the walls of a building and deployed aerially in a manner similar to copper cables. These fibers require less maintenance than common copper cables, once they are deployed.
http://observernews.com/stories/current/news/021105/fiber.shtml
Amplifiers
The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than it was before. Because of the high complexity with modern wavelength-division multiplexed signals (including the fact that they had to be installed about once every 20 km), the cost of these repeaters is very high.
An alternative approach is to use an
optical amplifierAn 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 the cavity is suppressed. Stimulated emission in the...
, which amplifies the optical signal directly without having to convert the signal into the electrical domain. It is made by doping a length of fiber with the rare-earth mineral
erbiumErbium is a chemical element with the symbol Er and atomic number 68. A rare, silvery, white metallic lanthanide, erbium is solid in its normal state...
, and
pumpingLaser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...
it with light from a
laserA laser is a device that emits light through a process called stimulated emission. Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses...
with a shorter wavelength than the communications signal (typically 980 nm). Amplifiers have largely replaced repeaters in new installations.
Wavelength-division multiplexing
Wavelength-division multiplexing (WDM) is the practice of multiplying the available capacity of an optical fiber by adding new channels, each channel on a new wavelength of light. This requires a wavelength division multiplexer in the transmitting equipment and a demultiplexer (essentially a
spectrometerA spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
) in the receiving equipment.
Arrayed waveguide gratingArrayed waveguide gratings are commonly used as optical multiplexers in wavelength division multiplexed systems. These devices are capable of multiplexing a large number of wavelengths into a single optical fiber, thereby increasing the transmission capacity of optical networks considerably.The...
s are commonly used for multiplexing and demultiplexing in WDM. Using WDM technology now commercially available, the bandwidth of a fiber can be divided into as many as 160 channels to support a combined bit rate into the range of
terabitA terabit is a unit of information or computer storage, abbreviated Tbit .1 terabit = 1012 bits = 1,000,000,000,000 bits . 1 terabit is equal to 125 gigabytes or 122 gibibytes.The terabit is closely related to the tebibit, which is equal to 240 or 1,099,511,627,776 bits...
s per second.
Bandwidth-distance product
Because the effect of dispersion increases with the length of the fiber, a fiber transmission system is often characterized by its
bandwidth-distance product, often expressed in units of
MHzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
×km. This value is a product of bandwidth and distance because there is a trade off between the bandwidth of the signal and the distance it can be carried. For example, a common multimode fiber with bandwidth-distance product of 500 MHz×km could carry a 500 MHz signal for 1 km or a 1000 MHz signal for 0.5 km.
Through a combination of advances in dispersion management,
wavelength-division multiplexingIn fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths of laser light to carry different signals. This allows for a multiplication in capacity, in addition to...
, and optical amplifiers, modern-day optical fibers can carry information at around 14 Terabits per second over 160 kilometers of fiber
.
Engineers are always looking at current limitations in order to improve fiber-optic communication, and several of these restrictions are currently being researched:
Dispersion
For modern glass optical fiber, the maximum transmission distance is limited not by direct material absorption but by several types of
dispersionIn optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
, or spreading of optical pulses as they travel along the fiber. Dispersion in optical fibers is caused by a variety of factors. Intermodal dispersion, caused by the different axial speeds of different transverse modes, limits the performance of
multi-mode fiberMulti-mode optical fiber is a type of optical fiber mostly used for communication over shorter distances, such as within a building or on a campus...
. Because single-mode fiber supports only one transverse mode, intermodal dispersion is eliminated.
In single-mode fiber performance is primarily limited by chromatic dispersion (also called group velocity dispersion), which occurs because the index of the glass varies slightly depending on the wavelength of the light, and light from real optical transmitters necessarily has nonzero spectral width (due to modulation).
Polarization mode dispersionPolarization mode dispersion is a form of modal dispersion where two different polarizations of light in a waveguide, which normally travel at the same speed, travel at different speeds due to random imperfections and asymmetries, causing random spreading of optical pulses...
, another source of limitation, occurs because although the single-mode fiber can sustain only one transverse mode, it can carry this mode with two different polarizations, and slight imperfections or distortions in a fiber can alter the propagation velocities for the two polarizations. This phenomenon is called
fiber birefringenceBirefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain types of material, such as calcite crystals or boron nitride, depending on the polarization of the light...
and can be counteracted by
polarization-maintaining optical fiberIn fiber optics, polarization-maintaining optical fiber is optical fiber in which the polarization of linearly-polarized light waves launched into the fiber is maintained during propagation, with little or no cross-coupling of optical power between the polarization modes...
. Dispersion limits the bandwidth of the fiber because the spreading optical pulse limits the rate that pulses can follow one another on the fiber and still be distinguishable at the receiver.
Some dispersion, notably chromatic dispersion, can be removed by a 'dispersion compensator'. This works by using a specially prepared length of fiber that has the opposite dispersion to that induced by the transmission fiber, and this sharpens the pulse so that it can be correctly decoded by the electronics.
Attenuation
Fiber attenuation, which necessitates the use of amplification systems, is caused by a combination of
material absorptionIn physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy, for example, to heat. The absorption of light during wave propagation...
,
Rayleigh scatteringRayleigh scattering is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light, which may be individual atoms or molecules. It can occur when light travels in transparent solids and liquids, but is most prominently seen in gases...
,
Mie scatteringMie theory, also called Lorenz-Mie theory or Lorenz-Mie-Debye theory, is an analytical solution of Maxwell's equations for the scattering of electromagnetic radiation by spherical particles . The Mie solution is named after its developer, German physicist Gustav Mie...
, and connection losses. Although material absorption for pure silica is only around 0.03 dB/km (modern fiber has attenuation around 0.3 dB/km), impurities in the original optical fibers caused attenuation of about 1000 dB/km. Other forms of attenuation are caused by physical stresses to the fiber, microscopic fluctuations in density, and imperfect splicing techniques.
Transmission windows
Each of the effects that contributes to attenuation and dispersion depends on the optical wavelength, however wavelength bands exist where these effects are weakest, making these bands, or windows, most favorable for transmission. These windows have been standardized, and the current bands defined are the following:
| Band |
Description |
Wavelength Range |
| O band |
original |
1260 to 1360 nm |
| E band |
extended |
1360 to 1460 nm |
| S band |
short wavelengths |
1460 to 1530 nm |
| C band |
conventional ("erbium window") |
1530 to 1565 nm |
| L band |
long wavelengths |
1565 to 1625 nm |
| U band |
ultralong wavelengths |
1625 to 1675 nm |
Note that this table shows that current technology has managed to bridge the second and third windows- originally the windows were disjoint.
Historically, the first window used was from 800-900 nm; however losses are high in this region and because of that, this is mostly used for short-distance communications.
The second window is around 1300 nm, and has much lower losses. The region has zero dispersion.
The third window is around 1500nm, and is the most widely used. This region has the lowest attenuation losses and hence it achieves the longest range. However it has some dispersion, and dispersion compensators are used to remove this.
Regeneration
When a communications link must span a larger distance than existing fiber-optic technology is capable of, the signal must be
regenerated at intermediate points in the link by
repeatersAn optical communications repeater is used in a fiber-optic communications system to regenerate an optical signal by converting it to an electrical signal, processing that electrical signal and then retransmitting an optical signal...
. Repeaters add substantial cost to a communication system, and so system designers attempt to minimize their use.
Recent advances in fiber and optical communications technology have reduced signal degradation so far that
regeneration of the optical signal is only needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical networking, particularly over undersea spans where the cost and reliability of repeaters is one of the key factors determining the performance of the whole cable system. The main advances contributing to these performance improvements are dispersion management, which seeks to balance the effects of dispersion against non-linearity; and
solitonIn optics, the term soliton is used to refer to any optical field that does not change during propagation because of a delicate balance between nonlinear and linear effects in the medium. There are two main kinds of solitons:...
s, which use nonlinear effects in the fiber to enable dispersion-free propagation over long distances.
Last mile
Although fiber-optic systems excel in high-bandwidth applications, optical fiber has been slow to achieve its goal of fiber to the premises or to solve the
last mileThe "last mile" is the final leg of delivering connectivity from a communications provider to a customer. The phrase is therefore often used by the telecommunications and cable television industries. The actual distance of this leg may be considerably more than a mile, especially in rural areas...
problem. However, as bandwidth demand increases, more and more progress towards this goal can be observed. In Japan, for instance, fiber-optic systems are beginning to replace wire-based DSL as a broadband Internet source. The same applies to the Scandinavian countries. South Korea’s KT also provides a service called FTTH (Fiber To The Home), which provides 100 percent fiber-optic connections to the subscriber’s home. Verizon, a US based telecom company, provides a service called
FiOSFíos is one of 17 parishes in Parres, a municipality within the province and autonomous community of Asturias, in northern Spain....
which offers TV, high-speed internet, and telephone communications on a 100 percent fiber-optic network to a junction box mounted in a subscriber’s home.
Comparison with electrical transmission
The choice between optical fiber and electrical (or
copperCopper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
) transmission for a particular system is made based on a number of trade-offs. Optical fiber is generally chosen for systems requiring higher
bandwidthIn computer networking and computer science, digital bandwidth, network bandwidth or just bandwidth is a measure of available or consumed data communication resources expressed in bit/s or multiples of it ....
or spanning longer distances than electrical cabling can accommodate. The main benefits of fiber are its exceptionally low loss, allowing long distances between amplifiers or repeaters; and its inherently high data-carrying capacity, such that thousands of electrical links would be required to replace a single high bandwidth fiber cable. Another benefit of fibers is that even when run alongside each other for long distances, fiber cables experience effectively no crosstalk, in contrast to some types of electrical
transmission lineA transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission....
s. Fiber can be installed in areas with high
electromagnetic interferenceElectromagnetic interference is a disturbance that affects an electrical circuit due to either electromagnetic conduction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
(EMI),(along the sides of utility lines, power-carrying lines, and railroad tracks). All-dielectric cables are also ideal for areas of high lightning-strike incidence.
For comparison, while single-line, voice-grade copper systems longer than a couple of kilometers require in-line signal repeaters for satisfactory performance; it is not unusual for optical systems to go over 100 kilometers (60 miles), with no active or passive processing. Single-mode fiber cables are commonly available in 12 km lengths, minimizing the number of splices required over a long cable run. Multi-mode fiber is available in lengths up to 4 km, although industrial standards only mandate 2 km unbroken runs.
In short distance and relatively low bandwidth applications, electrical transmission is often preferred because of its
- Lower material cost, where large quantities are not required
- Lower cost of transmitters and receivers
- Capability to carry electrical power
Electric power is defined as the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt.When electric current flows in a circuit, it can transfer energy to do mechanical or thermodynamic work...
as well as signals (in specially-designed cables)
- Ease of operating transducers in linear mode.
- Optical Fibers are more difficult and expensive to splice
Splice may refer to:* Connection of two or more pieces of linear material:** Rope splicing, joining two pieces of rope or cable by weaving the strands of each into the other...
.
- At higher optical powers, Optical Fibers are susceptible to fiber fuse wherein a bit too much light meeting with an imperfection can destroy several meters per second. The installation of fiber fuse detection circuity at the transmitter can break the circuit and halt the failure to minimize damage.
Because of these benefits of electrical transmission, optical communication is not common in short box-to-box,
backplaneA backplane is a circuit board that connects several connectors in parallel to each other, so that each pin of each connector is linked to the same relative pin of all the other connectors, forming a computer bus. It is used as a backbone to connect several printed circuit boards together to...
, or chip-to-chip applications; however, optical systems on those scales have been demonstrated in the laboratory.
In certain situations fiber may be used even for short distance or low bandwidth applications, due to other important features:
- Immunity to electromagnetic interference, including nuclear electromagnetic pulse
The term electromagnetic pulse has the following meanings:# A burst of electromagnetic radiation from an explosion or a suddenly fluctuating magnetic field. The resulting electric and magnetic fields may couple with electrical/electronic systems to produce damaging current and voltage...
s (although fiber can be damaged by alphaAlpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus; hence, it can be written as or . They have a net spin of zero, and normally a total energy of about 5 MeV...
and betaBeta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
radiation).
- High electrical resistance
The electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the...
, making it safe to use near high-voltage equipment or between areas with different earth potentials.
- Lighter weight—important, for example, in aircraft.
- No sparks—important in flammable or explosive gas environments.
- Not electromagnetically radiating, and difficult to tap without disrupting the signal—important in high-security environments.
- Much smaller cable size—important where pathway is limited, such as networking an existing building, where smaller channels can be drilled and space can be saved in existing cable ducts and trays.
Optical fiber cables can be installed in buildings with the same equipment that is used to install copper and coaxial cables, with some modifications due to the small size and limited pull tension and bend radius of optical cables. Optical cables can typically be installed in duct systems in spans of 6000 meters or more depending on the duct's condition, layout of the duct system, and installation technique. Longer cables can be coiled at an intermediate point and pulled farther into the duct system as necessary.
Governing standards
In order for various manufacturers to be able to develop components that function compatibly in fiber optic communication systems, a number of standards have been developed. The International Telecommunications Union publishes several standards related to the characteristics and performance of fibers themselves, including
- ITU-T G.651, "Characteristics of a 50/125 µm multimode graded index optical fibre cable"
- ITU-T G.652, "Characteristics of a single-mode optical fibre cable"
Other standards, produced by a variety of standards organizations, specify performance criteria for fiber, transmitters, and receivers to be used together in conforming systems. Some of these standards are the following:
- 10 Gigabit Ethernet
The 10 Gigabit Ethernet or 10GE or 10GbE or 10 GigE standard was first published in 2002 as IEEE Std 802.3ae-2002 and is the fastest of the Ethernet standards...
- FDDI
- Fibre Channel
Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
- Gigabit Ethernet
Gigabit Ethernet is a term describing various technologies for transmitting Ethernet frames at a rate of a gigabit per second, as defined by the IEEE 802.3-2008 standard...
- HIPPI
HIPPI is a computer bus for the attachment of high speed storage devices to supercomputers. SCSI and Fibre Channel....
- Synchronous Digital Hierarchy
- Synchronous Optical Networking
Sonet may refer to:* Sonet Records, European record label* Synchronous optical networking...
TOSLINKTOSLINK or Optical Cable is a standardized optical fiber connection system. Its most common use is in consumer audio equipment , where it carries a digital audio stream between components such as MiniDisc and CD players and DAT recorders...
is the most common format for
digital audioDigital audio uses digital signals for sound reproduction. This includes analog-to-digital conversion, digital-to-analog conversion, storage, and transmission. In effect, the system commonly referred to as digital is in fact a discrete-time, discrete-level analog of a previous electrical analog...
cable using
plastic optical fiberPlastic optical fiber is an optical fiber which is made out of plastic. Traditionally PMMA is the core material, and fluorinated polymers are the cladding material...
to connect digital sources to digital
receiverThis article is about a radio receiver, for other uses see Radio .A radio receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for...
s.
See also
- Free-space optical communication
In telecommunications, Free Space Optics is an optical communication technology that uses light propagating in free space to transmit data between two points...
- Information theory
Information theory is a branch of applied mathematics and electrical engineering involving the quantification of information. Historically, information theory was developed by Claude E. Shannon to find fundamental limits on compressing and reliably storing and communicating data...
- Verizon FiOS
Verizon FiOS is a bundled communications service, operating over a fiber-optic communications network, that is presently offered in some areas of the United States by Verizon. Verizon has attracted consumer and media attention in the area of broadband Internet access as the first major U.S....
- Dark fiber
In fiber optic communications, dark fiber or unlit fiber refers to unused optical fibers, available for use.It may also be used to identify fiber transporting a radiation with a wave length that cannot be seen by the human eye, or a fiber that is connected to equipment that a service provider...
External links