Fiber-optic communication
Encyclopedia
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light
through an optical fiber
. The light forms an electromagnetic
carrier wave
that is modulated
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 Age
. Because of its advantages over electrical transmission, optical fibers have largely replaced copper wire communications in core network
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.
is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Due to much lower attenuation
and interference
, 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-amplification
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 cable
with a capacity of 2.56 Tb
/s was completed, and although specific network capacities are privileged information, telecommunications investment reports indicate that network capacity has increased dramatically since 2004.
and his assistant Charles Sumner Tainter
created a very early precursor to fiber-optic communications, the Photophone
, at Bell's newly established Volta Laboratory in Washington, D.C.
Bell considered it his most important invention. The device allowed for the transmission
of sound on a beam of light. On June 3, 1880, Bell conducted the world's first wireless telephone
transmission between two buildings, some 213 meters apart. Due to its use of an atmospheric transmission medium, the Photophone would not prove practical until advances in laser and optical fiber technologies permitted the secure transport of light. The Photophone's first practical use came in military communication systems many decades later.
In 1966 Charles K. Kao
and George Hockham
proposed optical fibers at STC Laboratories (STL) at Harlow
, England, 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 20dB
/km), and at the same time GaAs semiconductor lasers
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 km. Soon on 22 April 1977, General Telephone and Electronics sent the first live telephone traffic through fiber optics at a 6 Mbit/s 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. These early systems were initially limited by multi mode fiber dispersion, and in 1981 the single-mode fiber
was revealed to greatly improve system performance, however practical connectors capable of working with single mode fiber proved difficult to develop. By 1987, these systems were operating at bit rates of up to 1.7 Gb
/s with repeater spacing up to 50 km.
The first transatlantic telephone cable
to use optical fiber was TAT-8
, 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-spreading
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 mode
. 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 amplification
to reduce the need for repeaters and wavelength-division multiplexing
to increase data capacity
. 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 Tb
/s was reached by 2001. In 2006 a bit-rate of 14 Tbit/s was 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 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 solitons
, " pulses that preserve their shape by counteracting the effects of dispersion with the nonlinear effects
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 massive increases in demand for communications bandwidth due to increased use of the Internet
, and commercialization of various bandwidth-intensive consumer services, such as video on demand
. Internet protocol
data traffic was increasing exponentially, at a faster rate than integrated circuit complexity had increased under Moore's Law
. From the bust of the dot-com bubble through 2006, however, the main trend in the industry has been consolidation
of firms and offshoring
of manufacturing to reduce costs. Companies such as Verizon
and AT&T have taken advantage of fiber-optic communications to deliver a variety of high-throughput data and broadband services to consumers' homes.
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 systems
, and cable television
companies.
The most commonly-used optical transmitters are semiconductor devices such as light-emitting diode
s (LEDs) and laser diode
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 junction
, emitting light through spontaneous emission
, a phenomenon referred to as electroluminescence
. 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 phosphide
(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-network
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 well
s to emit light at different wavelengths over a broad spectrum, and are currently in use for local-area WDM
networks.
Today, LEDs have been largely superseded by VCSEL (Vertical Cavity Surface Emitting Laser) devices, which offer improved speed, power and spectral properties, at a similar cost. Common VCSEL devices couple well to multi mode fiber.
A semiconductor laser emits light through stimulated emission
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 dispersion
. Furthermore, semiconductor lasers can be modulated directly at high frequencies because of short recombination time
.
Commonly used classes of semiconductor laser transmitters used in fiber optics include VCSEL (Vertical Cavity Surface Emitting Laser), Fabry–Pérot and DFB
(Distributed Feed Back).
Laser diodes are often directly modulated
, 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 electro-absorption modulator
or Mach–Zehnder interferometer. External modulation increases the achievable link distance by eliminating laser chirp
, which broadens the linewidth of directly-modulated lasers, increasing the chromatic dispersion in the fiber.
, which converts light into electricity using the photoelectric effect
. The photodetector is typically a semiconductor-based photodiode
. Several types of photodiodes include p-n photodiodes, p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photodetectors are also used due to their suitability for circuit integration
in regenerators
and wavelength-division multiplexers.
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 loop
may also be applied before the data is passed on.
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 reflection
. The core and the cladding (which has a lower-refractive-index
) are usually made of high-quality silica
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 optic communications include multi-mode optical fiber
s and single-mode optical fiber
s. A multi-mode optical fiber has a larger core (≥ 50 micrometre
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 dopant
content, multi-mode 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 polymers
, then terminated with optical fiber connector
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 twisted pair wires, once they are deployed.
Specialized cables are used for long distance subsea data transmission, e.g. transatlantic communications cable. New (2011-2013) cables operated by commercial enterprises (Emerald Atlantis, Hibernia Atlantic
) typically have four strands of fiber and cross the Atlantic (NYC-London) in 60-70ms. Cost of each such cable was about $300M in 2011. source: Halifax Chronicle-Herald.
Another common practice is to bundle many fiber optic strands within long-distance power transmission
cable. This exploits power transmission rights of way effectively, ensures a power company can own and control the fiber required to monitor its own devices and lines, is effectively immune to tampering, and simplifies the deployment of smart grid technology.
An alternative approach is to use an optical amplifier
, 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 erbium
, and pumping
it with light from a laser
with a shorter wavelength than the communications signal (typically 980 nm). Amplifiers have largely replaced repeaters in new installations.
) in the receiving equipment. Arrayed waveguide grating
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 terabit
s per second.
×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 multi-mode 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.
Engineers are always looking at current limitations in order to improve fiber-optic communication, and several of these restrictions are currently being researched. Each fiber can carry many independent channels, each using a different wavelength of light (wavelength-division multiplexing
(WDM)). 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 eighty in commercial dense WDM systems ). For instance, NTT was able to achieve 69.1 Tbit/s transmission by applying wavelength division multiplex (WDM) of 432 wavelengths with a capacity of 171 Gbit/s over a single 240 km-long optical fiber on March 25, 2010. This was the highest optical transmission speed recorded at that time.
, 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 fiber
. 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 dispersion
, 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 birefringence
and can be counteracted by polarization-maintaining optical fiber
. 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.
, Rayleigh scattering
, Mie scattering
, 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.
Note that this table shows that current technology has managed to bridge the second and third windows that were originally disjoint.
Historically, there was a window used below the O band, called the first window, at 800-900 nm; however, losses are high in this region so this window is used primarily for short-distance communications. The current lower windows (O and E) around 1300 nm have much lower losses. This region has zero dispersion. The middle windows (S and C) around 1500 nm are the most widely used. This region has the lowest attenuation losses and achieves the longest range. It does have some dispersion, so dispersion compensator devices are used to remove this.
. 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 soliton
s, which use nonlinear effects in the fiber to enable dispersion-free propagation over long distances.
problem. However, as bandwidth demand increases, more and more progress towards this goal can be observed. In Japan, for instance EPON
has largely replaced DSL as a broadband Internet source. South Korea’s KT also provides a service called FTTH (Fiber To The Home), which provides fiber-optic connections to the subscriber’s home. The largest FTTH deployments are in Japan, Korea, and China. Singapore started implementation of their all-fibre Next Generation Nationwide Broadband Network (Next Gen NBN), which is slated for completion in 2012 and is being installed by OpenNet. Since they began rolling out services in September 2010, Network coverage in Singapore has reached 60% nationwide.
In the US, Verizon Communications provides a FTTH service called FiOS
to select high-ARPU (Average Revenue Per User) markets within its existing territory. The other major surviving ILEC (or Incumbent Local Exchange Carrier), AT&T, uses a FTTN (Fiber To The Node) service called U-verse
with twisted-pair to the home. Their MSO competitors employ FTTN with coax using HFC
. All of the major access networks use fiber for the bulk of the distance from the service provider's network to the customer.
The globally dominant access network technology is EPON (Ethernet Passive Optical Network). In Europe, and among telcos in the United States, BPON (ATM-based Broadband PON) and GPON (Gigabit PON) had roots in the FSAN (Full Service Access Network) and ITU-T standards organizations under their control.
The choice between optical fiber and electrical (or copper
) transmission for a particular system is made based on a number of trade-offs. Optical fiber is generally chosen for systems requiring higher bandwidth
or spanning longer distances than electrical cabling can accommodate.
The main benefits of fiber are its exceptionally low loss (allowing long distances between amplifiers/repeaters), its absence of ground currents and other parasite signal and power issues common to long parallel electric conductor runs (due to its reliance on light rather than electricity for transmission, and the dielectric nature of fiber optic), and its inherently high data-carrying capacity. 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 line
s. Fiber can be installed in areas with high electromagnetic interference
(EMI), such as alongside utility lines, power lines, and railroad tracks. Nonmetallic 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 kilometres (62.1 mi), 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
Optical fibers are more difficult and expensive to splice
than electrical conductors. And at higher powers, optical fibers are susceptible to fiber fuse, resulting in catastrophic destruction of the fiber core and damage to transmission components.
Because of these benefits of electrical transmission, optical communication is not common in short box-to-box, backplane
, 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:
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.
Other standards specify performance criteria for fiber, transmitters, and receivers to be used together in conforming systems. Some of these standards are:
TOSLINK
is the most common format for digital audio
cable using plastic optical fiber
to connect digital sources to digital receiver
s.
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
through an optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
. The light forms an electromagnetic
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
carrier wave
Carrier wave
In 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 a much higher frequency than the input signal...
that is modulated
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
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 Age
Information Age
The Information Age, also commonly known as the Computer Age or Digital Age, 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 network
Core network
A core network, or network core, is the central part of a telecommunication network that provides various services to customers who are connected by the access network. One of the main functions is to route telephone calls across the PSTN....
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 fiberOptical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Due to much lower attenuation
Attenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
and interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
, 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-amplification
Optical 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 the cavity is suppressed...
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 cable
Submarine communications cable
A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean....
with a capacity of 2.56 Tb
Terabit
The terabit is a multiple of the unit bit for digital information or computer storage. The prefix tera is defined in the International System of Units as a multiplier of 1012 , and therefore...
/s was completed, and although specific network capacities are privileged information, telecommunications investment reports indicate that network capacity has increased dramatically since 2004.
History
In 1880 Alexander Graham BellAlexander Graham Bell
Alexander Graham Bell was an eminent scientist, inventor, engineer and innovator who is credited with inventing the first practical telephone....
and his assistant Charles Sumner Tainter
Charles Sumner Tainter
Charles Sumner Tainter was an American scientific instrument maker, engineer and inventor, best known for his collaborations with Alexander Graham Bell, Chichester Bell, Alexander's father-in-law Gardiner Hubbard, and for his significant improvements to Thomas Edison's phonograph, resulting in the...
created a very early precursor to fiber-optic communications, the Photophone
Photophone
The photophone, also known as a radiophone, was invented jointly by Alexander Graham Bell and his then-assistant Charles Sumner Tainter on February 19, 1880, at Bell's 1325 'L' Street laboratory in Washington, D.C...
, at Bell's newly established Volta Laboratory in Washington, D.C.
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....
Bell considered it his most important invention. The device allowed for the transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
of sound on a beam of light. On June 3, 1880, Bell conducted the world's first wireless telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
transmission between two buildings, some 213 meters apart. Due to its use of an atmospheric transmission medium, the Photophone would not prove practical until advances in laser and optical fiber technologies permitted the secure transport of light. The Photophone's first practical use came in military communication systems many decades later.
In 1966 Charles K. Kao
Charles K. Kao
The Honorable Sir Charles Kuen Kao, GBM, KBE, FRS, FREng is a pioneer in the development and use of fiber optics in telecommunications...
and George Hockham
George Hockham
George Alfred Hockham is an engineer who has worked for over 40 years in theoretical analysis and design techniques applied to the solution of electromagnetic problems covering many different antenna types for radar, electronic warfare and communication systems...
proposed optical fibers at STC Laboratories (STL) at Harlow
Harlow
Harlow is a new 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.The district has a current population of 78,889...
, England, 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 20dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
/km), and at the same time GaAs semiconductor lasers
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common 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 km. Soon on 22 April 1977, General Telephone and Electronics sent the first live telephone traffic through fiber optics at a 6 Mbit/s 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. These early systems were initially limited by multi mode fiber dispersion, and in 1981 the single-mode fiber
Single-mode optical fiber
In fiber-optic communication, a single-mode optical fiber is an optical fiber designed to carry only a single ray of light . Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining Maxwell's equations and the boundary conditions...
was revealed to greatly improve system performance, however practical connectors capable of working with single mode fiber proved difficult to develop. By 1987, these systems were operating at bit rates of up to 1.7 Gb
Gigabit
The gigabit is a multiple of the unit bit for digital information or computer storage. The prefix giga is defined in the International System of Units as a multiplier of 109 , and therefore...
/s with repeater spacing up to 50 km.
The first transatlantic telephone cable
Transatlantic telephone cable
A transatlantic telecommunications cable is a submarine communications cable running under the Atlantic Ocean. All modern cables use fibre optic technology....
to use optical fiber was TAT-8
TAT-8
TAT-8 was the 8th transatlantic telecommunications 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-spreading
Dispersion (optics)
In 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 mode
Longitudinal mode
For the longitudinal mode of conduction of electric currents, see Common 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...
. 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 amplification
Optical 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 the cavity is suppressed...
to reduce the need for repeaters and wavelength-division multiplexing
Wavelength-division multiplexing
In fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light...
to increase data capacity
Channel capacity
In 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 Tb
Terabit
The terabit is a multiple of the unit bit for digital information or computer storage. The prefix tera is defined in the International System of Units as a multiplier of 1012 , and therefore...
/s was reached by 2001. In 2006 a bit-rate of 14 Tbit/s was 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
Wavelength-division multiplexing
In fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light...
system can operate. The conventional wavelength window, known as the C band, covers the wavelength range 1.53-1.57 µm, and 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 solitons
Soliton (optics)
In 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 effects
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
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 massive increases in demand for communications bandwidth due to increased use of the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, and commercialization of various bandwidth-intensive consumer services, such as video on demand
Video on demand
Video on Demand or Audio and Video On Demand are systems which allow users to select and watch/listen to video or audio content on demand...
. Internet protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
data traffic was increasing exponentially, at a faster rate than integrated circuit complexity had increased under Moore's Law
Moore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....
. From the bust of the dot-com bubble through 2006, however, the main trend in the industry has been consolidation
Consolidation (business)
Consolidation or amalgamation is the act of merging many things into one. In business, it often refers to the mergers and acquisitions of many smaller companies into much larger ones. In the context of financial accounting, consolidation refers to the aggregation of financial statements of a group...
of firms and offshoring
Offshoring
Offshoring 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. Companies such as Verizon
Verizon FiOS
Verizon FiOS is a bundled Internet access, telephone, and television service which operates over a fiber-optic communications network. It is offered in some areas of the United States by Verizon Communications. Verizon was one of 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 cableOptical fiber cable
An 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 systems
Digital telephony
Digital 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 television
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
companies.
Transmitters
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
s (LEDs) and laser diode
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common 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 junction
P-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
, emitting light through spontaneous emission
Spontaneous emission
Spontaneous 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 a state with a lower energy, e.g., the ground state and emits a photon...
, a phenomenon referred to as electroluminescence
Electroluminescence
Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current 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 phosphide
Gallium arsenide phosphide
Gallium arsenide phosphide is a semiconductor material, an alloy of gallium arsenide and gallium phosphide. It exists in various composition ratios indicated in its formula by the fraction x....
(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-network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
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 well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine 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 WDM
Wavelength-division multiplexing
In fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light...
networks.
Today, LEDs have been largely superseded by VCSEL (Vertical Cavity Surface Emitting Laser) devices, which offer improved speed, power and spectral properties, at a similar cost. Common VCSEL devices couple well to multi mode fiber.
A semiconductor laser emits light through stimulated emission
Stimulated emission
In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...
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 dispersion
Dispersion (optics)
In 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 time
Carrier generation and recombination
In 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...
.
Commonly used classes of semiconductor laser transmitters used in fiber optics include VCSEL (Vertical Cavity Surface Emitting Laser), Fabry–Pérot and DFB
Distributed feedback laser
A distributed feedback laser is a type of laser diode, quantum cascade laser or optical fibre laser where the active region of the device is periodically structured as a diffraction grating...
(Distributed Feed Back).
Laser diodes are often directly modulated
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
, 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 electro-absorption modulator
Electro-absorption modulator
Electro-absorption modulator is a semiconductor device which can be used for modulating the intensity of a laser beam via an electric voltage...
or Mach–Zehnder interferometer. External modulation increases the achievable link distance by eliminating laser chirp
Chirp
A 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 photodetectorPhotodetector
Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:*Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier...
, which converts light into electricity using the photoelectric effect
Photoelectric effect
In the photoelectric effect, 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 photodiode
Photodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
. Several types of photodiodes include p-n photodiodes, p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photodetectors are also used due to their suitability for circuit integration
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
in regenerators
Signal regeneration
In telecommunications, signal regeneration is signal processing that restores a signal, recovering its original characteristics.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.
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 loop
Phase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
may also be applied before the data is passed on.
Fiber cable types
An optical fiber consists of a core, cladding
Cladding (fiber optics)
Cladding is one or more layers of material of lower refractive index, in intimate contact with a core material of higher refractive index. The cladding causes light to be confined to the core of the fiber by total internal reflection at the boundary between the two. Light propagation in the...
, and a buffer (a protective outer coating), in which the cladding guides the light along the core by using the method of total internal reflection
Total internal reflection
Total internal reflection is an optical phenomenon that happens 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 and the incident angle is...
. The core and the cladding (which has a lower-refractive-index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
) are usually made of high-quality silica
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
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 optic communications include multi-mode optical fiber
Multi-mode optical fiber
Multi-mode optical fiber is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus...
s and single-mode optical fiber
Single-mode optical fiber
In fiber-optic communication, a single-mode optical fiber is an optical fiber designed to carry only a single ray of light . Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining Maxwell's equations and the boundary conditions...
s. A multi-mode optical fiber has a larger core (≥ 50 micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
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 dopant
Dopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
content, multi-mode 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 polymers
Acrylate polymers
An 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 connector
Optical fiber connector
An 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 twisted pair wires, once they are deployed.
Specialized cables are used for long distance subsea data transmission, e.g. transatlantic communications cable. New (2011-2013) cables operated by commercial enterprises (Emerald Atlantis, Hibernia Atlantic
Hibernia Atlantic
Hibernia Atlantic is a privately held, US-owned, transatlantic submarine communications cable system in the North Atlantic Ocean which connects Canada, the United States, Ireland, the United Kingdom and Europe....
) typically have four strands of fiber and cross the Atlantic (NYC-London) in 60-70ms. Cost of each such cable was about $300M in 2011. source: Halifax Chronicle-Herald.
Another common practice is to bundle many fiber optic strands within long-distance power transmission
Power transmission
Power transmission is the movement of energy from its place of generation to a location where it is applied to performing useful work.Power is defined formally as units of energy per unit time...
cable. This exploits power transmission rights of way effectively, ensures a power company can own and control the fiber required to monitor its own devices and lines, is effectively immune to tampering, and simplifies the deployment of smart grid technology.
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 amplifier
Optical 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 the cavity is suppressed...
, 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 erbium
Erbium
Erbium is a chemical element in the lanthanide series, 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 on Earth...
, and pumping
Laser pumping
Laser 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 laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
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 spectrometerSpectrometer
A 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 grating
Arrayed waveguide grating
Arrayed 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 terabit
Terabit
The terabit is a multiple of the unit bit for digital information or computer storage. The prefix tera is defined in the International System of Units as a multiplier of 1012 , and therefore...
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, usually expressed in units of MHzHertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
×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 multi-mode 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.
Engineers are always looking at current limitations in order to improve fiber-optic communication, and several of these restrictions are currently being researched. Each fiber can carry many independent channels, each using a different wavelength of light (wavelength-division multiplexing
Wavelength-division multiplexing
In fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light...
(WDM)). 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 eighty in commercial dense WDM systems ). For instance, NTT was able to achieve 69.1 Tbit/s transmission by applying wavelength division multiplex (WDM) of 432 wavelengths with a capacity of 171 Gbit/s over a single 240 km-long optical fiber on March 25, 2010. This was the highest optical transmission speed recorded at that time.
Dispersion
For modern glass optical fiber, the maximum transmission distance is limited not by direct material absorption but by several types of dispersionDispersion (optics)
In 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 fiber
Multi-mode optical fiber
Multi-mode optical fiber is a type of optical fiber mostly used for communication over short 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 dispersion
Polarization mode dispersion
Polarization 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 birefringence
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
and can be counteracted by polarization-maintaining optical fiber
Polarization-maintaining optical fiber
In 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 absorptionAbsorption (electromagnetic radiation)
In 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 is...
, Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
, Mie scattering
Mie theory
The Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere...
, 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 effect that contributes to attenuation and dispersion depends on the optical wavelength. The wavelength bands (or windows) that exist where these effects are weakest are the most favorable for transmission. These windows have been standardized, and the currently defined bands 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 that were originally disjoint.
Historically, there was a window used below the O band, called the first window, at 800-900 nm; however, losses are high in this region so this window is used primarily for short-distance communications. The current lower windows (O and E) around 1300 nm have much lower losses. This region has zero dispersion. The middle windows (S and C) around 1500 nm are the most widely used. This region has the lowest attenuation losses and achieves the longest range. It does have some dispersion, so dispersion compensator devices 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 repeatersOptical communications repeater
An 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 soliton
Soliton (optics)
In 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 mileLast mile
The "last mile" or "last kilometer" 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,...
problem. However, as bandwidth demand increases, more and more progress towards this goal can be observed. In Japan, for instance EPON
Passive optical network
A passive optical network is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 16-128. A PON consists of an optical line terminal at the service provider's central...
has largely replaced DSL as a broadband Internet source. South Korea’s KT also provides a service called FTTH (Fiber To The Home), which provides fiber-optic connections to the subscriber’s home. The largest FTTH deployments are in Japan, Korea, and China. Singapore started implementation of their all-fibre Next Generation Nationwide Broadband Network (Next Gen NBN), which is slated for completion in 2012 and is being installed by OpenNet. Since they began rolling out services in September 2010, Network coverage in Singapore has reached 60% nationwide.
In the US, Verizon Communications provides a FTTH service called FiOS
Fíos
Fíos is one of 17 parishes in Parres, a municipality within the province and autonomous community of Asturias, in northern Spain....
to select high-ARPU (Average Revenue Per User) markets within its existing territory. The other major surviving ILEC (or Incumbent Local Exchange Carrier), AT&T, uses a FTTN (Fiber To The Node) service called U-verse
U-Verse
AT&T U-verse is a registered service mark under which AT&T offers Internet access, television, and telephone services in various parts of the United States. It began in 2008 to serve mostly residences and small businesses in urban and suburban areas.-Services:...
with twisted-pair to the home. Their MSO competitors employ FTTN with coax using HFC
Hybrid fibre-coaxial
Hybrid fiber-coaxial is a telecommunications industry term for a broadband network which combines optical fibre and coaxial cable. It was commonly employed globally by cable television operators since the early 1990s.-Description:...
. All of the major access networks use fiber for the bulk of the distance from the service provider's network to the customer.
The globally dominant access network technology is EPON (Ethernet Passive Optical Network). In Europe, and among telcos in the United States, BPON (ATM-based Broadband PON) and GPON (Gigabit PON) had roots in the FSAN (Full Service Access Network) and ITU-T standards organizations under their control.
Comparison with electrical transmission
Copper
Copper 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 soft and malleable; an exposed surface has a reddish-orange tarnish...
) transmission for a particular system is made based on a number of trade-offs. Optical fiber is generally chosen for systems requiring higher bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
or spanning longer distances than electrical cabling can accommodate.
The main benefits of fiber are its exceptionally low loss (allowing long distances between amplifiers/repeaters), its absence of ground currents and other parasite signal and power issues common to long parallel electric conductor runs (due to its reliance on light rather than electricity for transmission, and the dielectric nature of fiber optic), and its inherently high data-carrying capacity. 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 line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
s. Fiber can be installed in areas with high electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction 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), such as alongside utility lines, power lines, and railroad tracks. Nonmetallic 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 kilometres (62.1 mi), 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 powerElectric powerElectric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
as well as signals (in specially-designed cables) - Ease of operating transducers in linear mode.
Optical fibers are more difficult and expensive to splice
Fusion splicing
Fusion splicing is the act of joining two optical fibers end-to-end using heat. The goal is to fuse the two fibers together in such a way that light passing through the fibers is not scattered or reflected back by the splice, and so that the splice and the region surrounding it are almost as...
than electrical conductors. And at higher powers, optical fibers are susceptible to fiber fuse, resulting in catastrophic destruction of the fiber core and damage to transmission components.
Because of these benefits of electrical transmission, optical communication is not common in short box-to-box, backplane
Backplane
A backplane is a group of connectors connected in parallel with 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 make up a complete...
, 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 pulseElectromagnetic pulseAn electromagnetic pulse is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field...
s (although fiber can be damaged by alphaAlpha particleAlpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
and betaBeta particleBeta 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 resistanceElectrical resistanceThe electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
, 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 specify performance criteria for fiber, transmitters, and receivers to be used together in conforming systems. Some of these standards are:
- 100 Gigabit Ethernet100 Gigabit Ethernet40 Gigabit Ethernet, or 40GbE, and 100 Gigabit Ethernet, or 100GbE, are high-speed computer network standards developed by the Institute of Electrical and Electronics Engineers . They support sending Ethernet frames at 40 and 100 gigabits per second over multiple 10 Gbit/s or 25 Gbit/s lanes...
- 10 Gigabit Ethernet10 Gigabit EthernetThe 10 gigabit Ethernet computer networking standard was first published in 2002. It defines a version of Ethernet with a nominal data rate of 10 Gbit/s , ten times faster than gigabit Ethernet.10 gigabit Ethernet defines only full duplex point to point links which are generally connected by...
- Fibre ChannelFibre ChannelFibre 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 EthernetGigabit EthernetGigabit 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. It came into use beginning in 1999, gradually supplanting Fast Ethernet in wired local networks where it performed...
- HIPPIHIPPIHIPPI is a computer bus for the attachment of high speed storage devices to supercomputers. It was popular in the late 1980s and into the mid-to-late 1990s, but has since been replaced by ever-faster standard interfaces like SCSI and Fibre Channel.The first HIPPI standard defined a 50-wire...
- Synchronous Digital Hierarchy
- Synchronous Optical NetworkingSonetSonet may refer to:* Sonet Records, European record label* Synchronous optical networking * Saab Sonett...
- Optical Transport NetworkOptical Transport NetworkITU-T defines an Optical Transport Network as a set of Optical Network Elements connected by optical fibre links, able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals...
(OTN)
TOSLINK
TOSLINK
TOSLINK is a standardized optical fiber connection system. Also known generically as an "optical audio cable," its most common use is in consumer audio equipment , where it carries a digital audio stream from components such as MiniDisc, CD and DVD players, DAT recorders, computers, and modern...
is the most common format for digital audio
Digital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
cable using plastic optical fiber
Plastic optical fiber
Plastic 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 receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
s.
See also
- Fibre to the x
- Free-space optical communicationFree-space optical communicationFree-space optical communication is an optical communication technology that uses light propagating in free space to transmit data for telecommunications or computer networking."Free space" means air, outer space, vacuum, or something similar...
- Information theoryInformation theoryInformation theory is a branch of applied mathematics and electrical engineering involving the quantification of information. Information theory was developed by Claude E. Shannon to find fundamental limits on signal processing operations such as compressing data and on reliably storing and...
- Passive optical networkPassive optical networkA passive optical network is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 16-128. A PON consists of an optical line terminal at the service provider's central...
- Dark fiberDark fiberA dark fiber or unlit fiber is an unused Optical fiber, available for use in fiber-optic communication.The term dark fiber was originally used when referring to the potential network capacity of telecommunication infrastructure, but now also refers to the increasingly common practice of leasing...