Fiber-optic communication is a form ofoptical communication fortransmitting information from one place to another by sending pulses ofinfrared orvisible light through anoptical fiber.[1][2] The light is a form ofcarrier wave that ismodulated to carry information.[3] Fiber ispreferred over electrical cabling when highbandwidth, long distance, or immunity toelectromagnetic interference is required.[4] This type of communication can transmit voice, video, and telemetry through local area networks or across long distances.[5]
Optical fiber is used by many telecommunications companies to transmit telephone signals, internet communication, and cable television signals. Researchers atBell Labs have reached a recordbandwidth–distance product of over100petabit × kilometers per second using fiber-optic communication.[6][better source needed]
First developed in the 1970s, fiber-optics have revolutionized thetelecommunications industry and have played a major role in the advent of theInformation Age.[7] Because of itsadvantages over electrical transmission, optical fibers have largely replaced copper wire communications inbackbone networks in thedeveloped world.[8]
The process of communicating using fiber optics involves the following basic steps:
Optical fiber is used by telecommunications companies to transmit telephone signals, Internet communication and cable television signals. It is also used in other industries, including medical, defense, government, industrial and commercial. In addition to serving the purposes of telecommunications, it is used as light guides, for imaging tools, lasers, hydrophones for seismic waves, SONAR, and as sensors to measure pressure and temperature.
Due to lowerattenuation andinterference, optical fiber has advantages over copper wire in long-distance, high-bandwidth applications. However, infrastructure development within cities is relatively difficult and time-consuming, and fiber-optic systems can be complex and expensive to install and operate. Due to these difficulties, early fiber-optic communication systems were primarily installed in long-distance applications, where they can be used to their full transmission capacity, offsetting the increased cost. The prices of fiber-optic communications have dropped considerably since 2000.[10]
The price for rolling out fiber to homes 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 and housing density is high.[citation needed]
Since 1990, whenoptical-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 ofsubmarine communications cable with a capacity of 2.56Tb/s was completed, and although specific network capacities are privileged information, telecommunications investment reports indicate that network capacity has increased dramatically since 2004.[11] As of 2020, over 5 billion kilometers of fiber-optic cable has been deployed around the globe.[12]
In 1880Alexander Graham Bell and his assistantCharles Sumner Tainter created a very early precursor to fiber-optic communications, thePhotophone, at Bell's newly establishedVolta Laboratory inWashington, D.C. Bell considered it his most important invention. The device allowed for thetransmission of sound on a beam of light. On June 3, 1880, Bell conducted the world's first wirelesstelephone transmission between two buildings, some 213 meters apart.[13][14] 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.[15]
In 1954Harold Hopkins andNarinder Singh Kapany showed that rolled fiber glass allowed light to be transmitted.[16]Jun-ichi Nishizawa, a Japanese scientist atTohoku University, proposed the use of optical fibers for communications in 1963.[17] Nishizawa invented thePIN diode and thestatic induction transistor, both of which contributed to the development of optical fiber communications.[18][19]
In 1966Charles K. Kao andGeorge Hockham atStandard Telecommunication Laboratories showed that the losses of 1,000 dB/km in existing glass (compared to 5–10 dB/km in coaxial cable) were due to contaminants which could potentially be removed.
Optical fiber with attenuation low enough for communication purposes (about 20 dB/km) was developed in 1970 byCorning Glass Works. At the same time,GaAssemiconductor lasers were developed that were compact and therefore suitable for transmitting light through fiber optic cables for long distances.[citation needed]
In 1973,Optelecom, Inc., co-founded by the inventor of the laser,Gordon Gould, received a contract from ARPA for one of the first optical communication systems. Developed forArmy Missile Command in Huntsville, Alabama, the system was intended to allow a short-range missile with video processing to communicate by laser to the ground by means of a five-kilometer long optical fiber that unspooled from the missile as it flew.[20] Optelecom then delivered the first commercial optical communications system to Chevron.[21]
After a period of research starting from 1975, the first commercial fiber-optic telecommunications 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 of45 Mbit/s 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 a6 Mbit/s throughput in Long Beach, California.[22]
In October 1973, Corning Glass signed a development contract withCSELT andPirelli aimed to test fiber optics in an urban environment: in September 1977, the second cable in this test series, named COS-2, was experimentally deployed in two lines (9 km) inTurin, for the first time in a big city, at a speed of140 Mbit/s.[23]
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 bymulti-mode fiber dispersion, and in 1981 thesingle-mode fiber was revealed to greatly improve system performance, however practical connectors capable of working with single mode fiber proved difficult to develop. Canadian service provider SaskTel had completed construction of what was then the world's longest commercial fiber optic network, which covered 3,268 km (2,031 mi) and linked 52 communities.[24] By 1987, these systems were operating at bit rates of up to1.7 Gbit/s with repeater spacing up to 50 km (31 mi).
The firsttransatlantic telephone cable to use optical fiber wasTAT-8, based onDesurvire 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. This development was spurred by the discovery ofindium gallium arsenide and the development of the indium gallium arsenide photodiode by Pearsall. Engineers overcame earlier difficulties withpulse-spreading using conventional InGaAsP semiconductor lasers at that wavelength by usingdispersion-shifted fibers designed to have minimal dispersion at 1.55 μm or by limiting the laser spectrum to a singlelongitudinal mode. These developments eventually allowed third-generation systems to operate commercially at2.5 Gbit/s with repeater spacing in excess of 100 km (62 mi).
The fourth generation of fiber-optic communication systems usedoptical amplification to reduce the need for repeaters andwavelength-division multiplexing (WDM) to increasedata capacity. The introduction of WDM was the start ofoptical networking, as WDM became the technology of choice for fiber-optic bandwidth expansion.[25] The first to market with a dense WDM system was Ciena Corp., in June 1996.[26] The introduction of optical amplifiers and WDM caused system capacity to double every six months from 1992 until a bit rate of10 Tb/s was reached by 2001. In 2006 a bit-rate of14 Tb/s was reached over a single 160 km (99 mi) line using optical amplifiers.[27] As of 2021[update], Japanese scientists transmitted 319 terabits per second over 3,000 kilometers with four-core fiber cables with standard cable diameter.[28]
The focus of development for the fifth generation of fiber-optic communications is on extending the wavelength range over which aWDM system can operate. The conventional wavelength window, known as the C band, covers the wavelength range 1525–1565 nm, anddry fiber has a low-loss window promising an extension of that range to 1300–1650 nm.[citation needed] Other developments include the concept ofoptical solitons, pulses that preserve their shape by counteracting the effects of dispersion with thenonlinear 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 theInternet, and commercialization of various bandwidth-intensive consumer services, such asvideo on demand.Internet Protocol data traffic was increasing exponentially, at a faster rate than integrated circuit complexity had increased underMoore's Law. From the bust of thedot-com bubble through 2006, however, the main trend in the industry has beenconsolidation of firms andoffshoring of manufacturing to reduce costs. Companies such asVerizon andAT&T have taken advantage of fiber-optic communications to deliver a variety of high-throughput data and broadband services to consumers' homes.
The2022 Russian Invasion of Ukraine has seen the usage of fiber optics for communication in drones. Their resilience toelectronic warfare jamming has seen them being used by both sides.[29]
Modern fiber-optic communication systems generally include optical transmitters that convert electrical signals into optical signals,optical fiber cables to carry the signal, optical amplifiers, and optical receivers to convert the signal back into an electrical signal. The information transmitted is typicallydigital information generated by computers ortelephone systems.
The most commonly used optical transmitters are semiconductor devices such aslight-emitting diodes (LEDs) andlaser diodes. The difference between LEDs and laser diodes is that LEDs produceincoherent 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 emits light throughspontaneous emission, a phenomenon referred to aselectroluminescence. The emitted light is incoherent with a relatively wide spectral width of 30–60 nm.[a] The large spectrum width of LEDs is subject to higher fiber dispersion, considerably limiting their bit rate-distance product (a common measure of usefulness). LEDs are suitable primarily forlocal-area-network applications with bit rates of 10–100 Mbit/s and transmission distances of a few kilometers.
LED light transmission is inefficient, with only about 1% of input power, or about 100 microwatts, eventually converted into launched power coupled into the optical fiber.[30]
LEDs have been developed that use severalquantum wells to emit light at different wavelengths over a broad spectrum and are currently in use for local-areawavelength-division multiplexing (WDM) applications.
LEDs have been largely superseded byvertical-cavity surface-emitting laser (VCSEL) devices, which offer improved speed, power and spectral properties, at a similar cost. However, due to their relatively simple design, LEDs are very useful for very low-cost applications. Commonly used classes of semiconductor laser transmitters used in fiber optics include VCSEL,Fabry–Pérot anddistributed-feedback laser.
A semiconductor laser emits light throughstimulated 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. Common VCSEL devices also couple well to multimode fiber. The narrow spectral width also allows for high bit rates since it reduces the effect ofchromatic dispersion. Furthermore, semiconductor lasers can be modulated directly at high frequencies because of shortrecombination time.
Laser diodes are often directlymodulated, 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 operatedcontinuous wave, and the light modulated by an external device, anoptical modulator, such as anelectro-absorption modulator orMach–Zehnder interferometer. External modulation increases the achievable link distance by eliminating laserchirp, which broadens thelinewidth in directly modulated lasers, increasing the chromatic dispersion in the fiber. For very high bandwidth efficiency, coherent modulation can be used to vary the phase of the light in addition to the amplitude, enabling the use ofQPSK,QAM, andOFDM. "Dual-polarization quadrature phase shift keying is a modulation format that effectively sends four times as much information as traditional optical transmissions of the same speed."[31]
The main component of an optical receiver is aphotodetector which converts light into electricity using thephotoelectric effect. The primary photodetectors for telecommunications are made fromIndium gallium arsenide. The photodetector is typically a semiconductor-basedphotodiode. 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 forcircuit integration inregenerators and wavelength-division multiplexers.
Since light may be attenuated and distorted while passing through the fiber, photodetectors are typically coupled with atransimpedance amplifier and a limitingamplifier to produce a digital signal in the electrical domain recovered from the incoming optical signal. Further signal processing such asclock recovery from data performed by aphase-locked loop may also be applied before the data is passed on.
Coherent receivers use a local oscillator laser in combination with a pair of hybrid couplers and four photodetectors per polarization, followed by high-speed ADCs and digital signal processing to recover data modulated with QPSK, QAM, or OFDM.[citation needed]
An optical communication systemtransmitter consists of adigital-to-analog converter (DAC), adriver amplifier and aMach–Zehnder modulator. The deployment of highermodulation formats (>4-QAM) or higherbaud Rates (>32 GBd) diminishes the system performance due to linear and non-linear transmitter effects. These effects can be categorized as linear distortions due to DAC bandwidth limitation and transmitter I/Qskew as well as non-linear effects caused by gain saturation in the driver amplifier and the Mach–Zehnder modulator. Digitalpredistortion counteracts the degrading effects and enables Baud rates up to56 GBd and modulation formats like64-QAM and128-QAM with the commercially available components. The transmitterdigital signal processor performs digital predistortion on the input signals using the inverse transmitter model before sending the samples to the DAC.
Older digital predistortion methods only addressed linear effects. Recent publications also consider non-linear distortions.Berenguer et al models the Mach–Zehnder modulator as an independentWiener system and the DAC and the driver amplifier are modeled by a truncated, time-invariantVolterra series.[32]Khanna et al use a memory polynomial to model the transmitter components jointly.[33] In both approaches the Volterra series or the memory polynomial coefficients are found usingindirect-learning architecture.Duthel et al records, for each branch of the Mach-Zehnder modulator, several signals at different polarity and phases. The signals are used to calculate the optical field.Cross-correlating in-phase and quadrature fields identifies thetiming skew. Thefrequency response and the non-linear effects are determined by the indirect-learning architecture.[34]
Anoptical fiber cable 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 oftotal internal reflection. The core and the cladding (which has a lower-refractive-index) are usually made of high-qualitysilica glass, although they can both be made of plastic as well. Connecting two optical fibers is done byfusion splicing ormechanical splicing and requires special skills and interconnection technology due to the microscopic precision required to align the fiber cores.[35]
Two main types of optical fiber used in optic communications includemulti-mode optical fibers andsingle-mode optical fibers. A multi-mode optical fiber has a larger core (≥ 50micrometers), allowing less precise, cheaper transmitters and receivers to connect to it as well as cheaper connectors. However, a multi-mode fiber introducesmultimode distortion, which often limits the bandwidth and length of the link. Furthermore, because of its higherdopant content, multi-mode fibers are usually expensive and exhibit higher attenuation. The core of a single-mode fiber is smaller (< 10 micrometers) and requires more expensive components and interconnection methods, but allows much longer and higher-performance links. Both single- and multi-mode fiber is offered in different grades.
| Fibre type | Introduced | Performance |
|---|---|---|
| MMF FDDI 62.5/125 µm | 1987 | 160 MHz·km @ 850 nm |
| MMF OM1 62.5/125 µm | 1989 | 200 MHz·km @ 850 nm |
| MMF OM2 50/125 µm | 1998 | 500 MHz·km @ 850 nm |
| MMF OM3 50/125 µm | 2003 | 1500 MHz·km @ 850 nm |
| MMF OM4 50/125 µm | 2008 | 3500 MHz·km @ 850 nm |
| MMF OM5 50/125 µm | 2016 | 3500 MHz·km @ 850 nm + 1850 MHz·km @ 950 nm |
| SMF OS1 9/125 µm | 1998 | 1.0 dB/km @ 1300/1550 nm |
| SMF OS2 9/125 µm | 2000 | 0.4 dB/km @ 1300/1550 nm |
In order to package fiber into a commercially viable product, it typically is protectively coated by using ultraviolet curedacrylate polymers[citation needed] and 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.[37]
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 signals cross the Atlantic (NYC-London) in 60–70 ms. The cost of each such cable was about $300M in 2011.[38]
Another common practice is to bundle many fiber optic strands within long-distancepower transmission cable using, for instance, anoptical ground wire. 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 ofsmart grid technology.
The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. By usingoptoelectronic 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 was received, thus counteracting the loss incurred in the previous segment. 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 (12 mi), the cost of these repeaters is very high.
An alternative approach is to useoptical amplifiers which amplify the optical signal directly without having to convert the signal to the electrical domain. One common type of optical amplifier is anerbium-doped fiber amplifier (EDFA). These are made bydoping a length of fiber with the rare-earth mineralerbium andlaser pumping it with light with a shorter wavelength than the communications signal (typically 980 nm). EDFAs provide gain in the ITU C band at 1550 nm.
Optical amplifiers have several significant advantages over electrical repeaters. First, an optical amplifier can amplify a very wide band at once which can include hundreds ofmultiplexed channels, eliminating the need to demultiplex signals at each amplifier. Second, optical amplifiers operate independently of the data rate and modulation format, enabling multiple data rates and modulation formats to co-exist and enabling upgrading of the data rate of a system without having to replace all of the repeaters. Third, optical amplifiers are much simpler than a repeater with the same capabilities and are therefore significantly more reliable. Optical amplifiers have largely replaced repeaters in new installations, although electronic repeaters are still widely used when signal conditioning beyond amplification is required.
Wavelength-division multiplexing (WDM) is the technique of transmitting multiple channels of information through a single optical fiber by sending multiple light beams of different wavelengths through the fiber, each modulated with a separate information channel. This allows the available capacity of optical fibers to be multiplied. This requires a wavelength division multiplexer in the transmitting equipment and a demultiplexer (essentially aspectrometer) in the receiving equipment.Arrayed waveguide gratings are commonly used for multiplexing and demultiplexing in WDM.[39] Using WDM technology now commercially available, the bandwidth of a fiber can be divided into as many as 160 channels[40] to support a combined bit rate in the range of1.6 Tbit/s.
Because the effect of dispersion increases with the length of the fiber, a fiber transmission system is often characterized by itsbandwidth–distance product, usually expressed in units ofMHz·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 over which 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.
Usingwavelength-division multiplexing, each fiber can carry many independent channels, each using a different wavelength of light. The net data rate (data rate without overhead bytes) per fiber is the per-channel data rate reduced by theforward error correction (FEC) overhead, multiplied by the number of channels (usually up to eighty in commercialdense WDM systems as of 2008[update]).[needs update]
The following summarizes research using standard telecoms-grade single-mode, single-solid-core fiber cables.
| Year | Organization | Aggregate speed | Bandwidth | Spectral efficiency,(bit/s)/Hz | WDM channels | Per-channel speed | Distance |
|---|---|---|---|---|---|---|---|
| 2009 | Alcatel-Lucent[41] | 15.5 Tbit/s | 155 | 100 Gbit/s | 7000 km | ||
| 2010 | NTT[42] | 69.1 Tbit/s | 432 | 171 Gbit/s | 240 km | ||
| 2011 | NEC[43] | 101.7 Tbit/s | 370 | 273 Gbit/s | 165 km | ||
| 2011 | KIT[44][45] | 26 Tbit/s | 336[A] | 77 Gbit/s | 50 km | ||
| 2016 | BT &Huawei[46] | 5.6 Tbit/s | 28 | 200 Gbit/s | ~140 km? | ||
| 2016 | Nokia Bell Labs,Deutsche Telekom &Technical University of Munich[47][48] | 1 Tbit/s | 5–6.75 | 4 | 250 Gbit/s | 419–951 km | |
| 2016 | Nokia-Alcatel-Lucent[49] | 65 Tbit/s | 6600 km | ||||
| 2017 | BT &Huawei[50] | 11.2 Tbit/s | 6.25 | 28 | 400 Gbit/s | 250 km | |
| 2020 | RMIT, Monash & Swinburne Universities[51][52] | 39.0–40.1 Tbit/s | ~4 THz | 10.4 (10.1–10.4) | 160[A] | 244 Gbit/s | 76.6 km |
| 2020 | UCL[53] | 178.08 Tbit/s | 16.83 THz | 10.8 | 660 (S, C, L bands) | 270 Gbit/s | 40 km |
| 2023 | NICT[54] | 301 Tbit/s | 27.8 THz | 10.8 | 1097 (E, S, C, L bands) | 250–300 Gbit/s | 50–150 km |
| 2024 | NICT[55] | 402 Tbit/s | 37.6 THz | 10.7 | 1505 (O, E, S, C, L, U bands) | 170–320 Gbit/s | 50 km |
The following table summarizes results achieved using specialized multicore or multimode fiber.
| Year | Organization | Aggregate speed | Per core speed | Bandwidth | Spectral efficiency, (bit/s)/Hz | No. of propagation modes | No. of cores | WDM channels (per core) | Per channel speed | Distance |
|---|---|---|---|---|---|---|---|---|---|---|
| 2011 | NICT[43] | 109.2 Tbit/s | 15.6 Tbit/s | 7 | ||||||
| 2012 | NEC,Corning[56] | 1.05 Pbit/s | 87.5 Tbit/s | 12 | 52.4 km | |||||
| 2013 | University of Southampton[57] | 73.7 Tbit/s | 73.7 Tbit/s | 1 (hollow) | 3 × 96 (mode DM)[58] | 256 Gbit/s | 310 m | |||
| 2014 | Technical University of Denmark[59] | 43 Tbit/s | 6.14 Tbit/s | 7 | 1045 km | |||||
| 2014 | Eindhoven University of Technology (TU/e) andUniversity of Central Florida (CREOL)[60] | 255 Tbit/s | 36.4 Tbit/s | 7 | 50 | ~728 Gbit/s | 1 km | |||
| 2015 | NICT,Sumitomo Electric andRAM Photonics[61] | 2.15 Pbit/s | 97.7 Tbit/s | 22 | 402 (C, L bands) | 243 Gbit/s | 31 km | |||
| 2017 | NTT[62] | 1 Pbit/s | 31.25 Tbit/s | single-mode | 32 | 46 | 680 Gbit/s | 205.6 km | ||
| 2017 | KDDI Research andSumitomo Electric[63] | 10.16 Pbit/s | 535 Tbit/s | 6-mode | 19 | 739 (C, L bands) | 120 Gbit/s | 11.3 km | ||
| 2018 | NICT[64] | 159 Tbit/s | 159 Tbit/s | tri-mode | 1 | 348 | 414 Gbit/s | 1045 km | ||
| 2020 | NICT[65] | 10.66 Pbit/s | 280.5 Tbit/s | 9.2 THz | 30.5 | tri-mode | 38 | 368 (C, L bands) | 762 Gbit/s | 13 km |
| 2021 | NICT[66] | 319 Tbit/s | 79.8 Tbit/s | single-mode | 4 | 552 (S, C, L bands) | 144.5 Gbit/s | 3001 km (69.8 km) | ||
| 2022 | NICT[67][68][69] | 1.02 Pbit/s | 255 Tbit/s | 4 | 801 (S, C, L bands) | 51.7 km | ||||
| 2022[A] | Technical University of Denmark[70][71] | 1.84 Pbit/s | 49.7 Tbit/s | 37 | 223 | 223 Gbit/s | 7.9 km | |||
| 2022 | NICT[72][73][74] | 1.53 Pbit/s | 1.53 Pbit/s | 4.6 THz | 332 | 55 (110-MIMO multiplexer) | 1 | 184 (C-band) | 1.03 Tbit/s | 25.9 km |
| 2023 | NICT[75] | 22.9 Pbit/s | 603 Tbit/s | 18.8 THz | 32 | tri-mode | 38 | 750 (S, C, L bands) | 803.5 Gbit/s | 13 km |
Research fromDTU,Fujikura andNTT is notable in that the team was able to reduce the power consumption of the optics to around 5% compared with more mainstream techniques, which could lead to a new generation of very power-efficient optic components.
| Year | Organization | Effective speed | No. of Propagation Modes | No. of cores | WDM channels (per core) | Per channel speed | Distance |
|---|---|---|---|---|---|---|---|
| 2018 | Hao Hu, et al. (DTU, Fujikura & NTT)[76] | 768 Tbit/s (661 Tbit/s) | Single-mode | 30 | 80 | 320 Gbit/s |
Research conducted by the RMIT University, Melbourne, Australia, have developed a nanophotonic device that carries data on light waves that have been twisted into a spiral form and achieved a 100-fold increase in current attainable fiber optic speeds.[77]The technique is known as orbital angular momentum (OAM). The nanophotonic device uses ultra-thin sheets to measure a fraction of a millimeter of twisted light. Nano-electronic device is embedded within a connector smaller than the size of a USB connector and may be fitted at the end of an optical fiber cable.[78]
For modern glass optical fiber, the maximum transmission distance is limited not by direct material absorption but bydispersion, the spreading of optical pulses as they travel along the fiber. Dispersion limits the bandwidth of the fiber because the spreading optical pulse limits the rate which pulses can follow one another on the fiber and still be distinguishable at the receiver. Dispersion in optical fibers is caused by a variety of factors.
Intermodal dispersion, caused by the different axial speeds of differenttransverse modes, limits the performance ofmulti-mode fiber. Because single-mode fiber supports only one transverse mode, intermodal dispersion is eliminated.
In single-mode fiber performance is primarily limited bychromatic dispersion, which occurs because the index of the glass varies slightly depending on the wavelength of the light, and, due to modulation, light from optical transmitters necessarily occupies a (narrow) range of wavelengths.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 calledbirefringence and can be counteracted bypolarization-maintaining optical fiber.
Some dispersion, notably chromatic dispersion, can be removed by adispersion 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.
Fiber attenuation is caused by a combination ofmaterial absorption,Rayleigh scattering,Mie scattering, and losses inconnectors. Material absorption for pure silica is only around 0.03 dB/km. Impurities in early optical fibers caused attenuation of about 1000 dB/km. Modern fiber has attenuation around 0.3 dB/km. Other forms of attenuation are caused by physical stresses to the fiber, microscopic fluctuations in density, and imperfectsplicing techniques.[79]
Each effect that contributes to attenuation and dispersion depends on the optical wavelength. There are wavelength bands (or windows) where these effects are weakest, and these are the most favorable for transmission. These windows have been standardized.[80]
| Band | Description | Wavelength range |
|---|---|---|
| O band | Original | 1260–1360 nm |
| E band | Extended | 1360–1460 nm |
| S band | Short wavelengths | 1460–1530 nm |
| C band | Conventional (erbium window) | 1530–1565 nm |
| L band | Long wavelengths | 1565–1625 nm |
| U band | Ultralong wavelengths | 1625–1675 nm |
Note that this table shows that current technology has managed to bridge the E and S windows that were originally disjoint.
Historically, there was a window of wavelengths shorter than 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 address this.
When a communications link must span a larger distance than existing fiber-optic technology is capable of, the signal must beregenerated at intermediate points in the link byoptical communications repeaters. 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 to the point thatregeneration 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; andsolitons, which use nonlinear effects in the fiber to enable dispersion-free propagation over long distances.
Although fiber-optic systems excel in high-bandwidth applications, thelast mile problem remains unsolved asfiber to the premises has experienced slow uptake. However,fiber to the home (FTTH) deployment has accelerated. In Japan, for instanceEPON has largely replaced DSL as a broadband Internet source. The largest FTTH deployments are in Japan, South Korea, and China. Singapore started implementation of their all-fiber 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 85% nationwide.[needs update]
In the US,Verizon Communications provides a FTTH service calledFiOS to selected high-average-revenue-per-user markets within its existing territory. The other major survivingincumbent local exchange carrier,AT&T, uses afiber to the node (FTTN) service calledU-verse with twisted-pair to the home. Their MSO competitors employ FTTN with coax usinghybrid fiber-coaxial networks. 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 isEthernet passive optical network (EPON). In Europe, and among telcos in the United States, ATM-basedBroadband PON (BPON) andGigabit PON (GPON) had roots in theFull Service Access Network (FSAN) and ITU-T standards organizations under their control.
The choice between optical fiber and electrical (orcopper) transmission for a particular system is made based on a number of trade-offs. Optical fiber is generally chosen for systems requiring higherbandwidth, operating in harsh environments or spanning longer distances than electrical cabling can accommodate.
The main benefits of fiber are its exceptionally low loss (allowing long distances between repeaters), its absence of ground currents and otherparasite 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 nocrosstalk, in contrast to some types of electricaltransmission lines. Fiber can be installed in areas with highelectromagnetic interference (EMI), such as alongsidepower 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 kilometers (62 mi), with no active or passive processing.
Optical fibers are more difficult and expensive to splice than electrical conductors. And at higher powers, optical fibers are susceptible tofiber fuse, resulting in catastrophic destruction of the fiber core and damage to transmission components.[81]
In short-distance and relatively low-bandwidth applications, electrical transmission is often preferred because of its lower cost. Optical communication is not common in short box-to-box,backplane, or chip-to-chip applications.
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 using the same equipment that is used to install copper and coaxial cables, with some modifications due to the small size and limited allowable pull tension and bend radius of optical cables.
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. TheInternational Telecommunication Union publishes several standards related to the characteristics and performance of fibers themselves, including
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 fordigital audio cable usingplastic optical fiber to connect digital sources to digitalreceivers.
An optical fiber will break if it is bent too sharply
Optical sensors are advantageous in hazardous environments because there are no sparks when a fiber breaks or its cover is worn.
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