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Fiber-Optic Communication ############################################################################### Fiber-Optic Communication From Wikipedia, the free 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 i
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  Fiber-Optic Communication###############################################################################Fiber-Optic CommunicationFrom Wikipedia, the free encyclopedia###############################################################################Fiber-optic communication is a method of transmitting information from one place toanother by sending pulses of light through an optical fiber. The light forms anelectromagnetic carrier wave that is modulated to carry information.First developed in the 1970s, optical fibers have largely replaced copper wirecommunications in core networks in the developed world.The process of communicating using fiber-optics involves the following basic steps:Creating the optical signal involving the use a transmitter, relaying the signalalong 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=============================================================================== Telecommunications companies to transmit telephone signals, Internet communication,and cable television signals.===============================================================================History===============================================================================Electrical systems were limited by their small repeater spacing (the distance asignal can propagate before attenuation requires the signal to be amplified), andthe bit rate of microwave systems was limited by their carrier frequency.In 1966 Kao and Hockham proposed optical fibers showed that the losses of 1000 db/kmin existing glass (compared to 5-10 db/km in coaxial cable) was due to contaminants,which could potentially be removed.Optical fiber was finally developed in 1970 by Corning Glass Works with attenuationlow enough for communication purposes (about 20dB/km), and at the same time GaAssemiconductor lasers were developed that were compact and therefore suitable forfiber-optic communication systems.This first generation (1975) operated at a bit rate of 45 Mbit/s with repeaterspacing of up to 10 km. On 22 April, 1977, General Telephone and Electronics (GTE)sent the first live telephone traffic through fiber optics, at 6 Mbit/s, in LongBeach, California.By the second generation (1981) the single-mode fiber was revealed to greatlyimprove system performance. By 1987, these systems were operating at bit rates ofup to 1.7 Gb/s with repeater spacing up to 50 km.Third-generation had loss of about 0.2 dB/km. These developments eventually allowedthe systems to operate commercially at 2.5 Gbit/s with repeater spacing in excess of100 km.The fourth generation used optical amplification to reduce the need for repeatersand wavelength-division multiplexing to increase fiber capacity (10 Tb/s werereached by 2001).The focus of development for the fifth generation is on extending the wavelengthrange. Other developments include the concept of optical solitons pulses thatpreserve their shape by counteracting the effects of dispersion with the nonlineareffects of the fiber by using pulses of a specific shape.===============================================================================Page 1  Fiber-Optic CommunicationTechnology===============================================================================-------------------------------------------------------------------------------Transmitters-------------------------------------------------------------------------------The most commonly-used optical transmitters are semiconductor devices such aslight-emitting diodes (LEDs) and laser diodes. The difference between LEDs and laserdiodes is that LEDs produce incoherent light (explosive wide spectrum), while laserdiodes produce coherent light (directional narrow spectrum).LEDs are suitable primarily for local-area-network applications with bit rates of10-100 Mbit/s and transmission distances of a few kilometers.A semiconductor laser emits light through stimulated emission rather thanspontaneous emission. Directionality allows more efficiency for single-mode fiber.The narrow spectral width allows for high bit rates due to reduction of chromaticdispersion. Can be modulated directly at high frequencies because of shortrecombination time.-------------------------------------------------------------------------------Fiber-------------------------------------------------------------------------------An optical fiber consists of a core, cladding, and a buffer (a protective outercoating), in which the cladding guides the light along the core by using the methodof total internal reflection.The core and the cladding are usually made of high-quality silica glass, orplastic. Connecting two optical fibers is done by fusion splicing or mechanicalsplicing and requires special skills and interconnection technology due to themicroscopic precision required to align the fiber cores.These fibers require less maintenance than common copper cables, once they aredeployed.Multi-mode optical fiber core (≥ 50 micrometres)Advantages:* Allows less precise, cheaper transmitters and receivers to connect to it as wellas cheaper connectors.Disadvantages:* Multimode distortion, which limits bandwidth and length* Expensive* Higher attenuationSingle-mode fiber core (<10 micrometres)Advantages:* Allows longer, higher-performance linksDisadvantages:* Requires more expensive components and interconnection methods-------------------------------------------------------------------------------Amplifiers-------------------------------------------------------------------------------Page 2  Fiber-Optic CommunicationThe transmission distance of a fiber-optic has been limited by fiber attenuation andby fiber distortion.* Opto-electronic repeaters: These repeaters convert the signal into an electricalsignal, and then use a transmitter to send the signal again with renewed intensity.But because of the high complexity with modern wavelength-division multiplexedsignals (needed to be installed about once every 20 km), the cost of these repeatersis very high.* Optical amplifiers: amplify the optical signal without converting the signal intothe electrical domain. Amplifiers have largely replaced repeaters in newinstallations.-------------------------------------------------------------------------------Receivers-------------------------------------------------------------------------------The main component is a photodetector, which converts light into electricity usingthe photoelectric effect.-------------------------------------------------------------------------------Wavelength-division multiplexing-------------------------------------------------------------------------------Is the practice of dividing the wavelength capacity of an optical fiber intomultiple channels in order to send more than one signal over the same fiber.This requires a WDM in the transmitting equipment and a WDD in the receivingequipment. Using WDM technology now commercially available, the bandwidth of afiber can be divided into as many as 80 channels to support a combined bit rate intothe range of terabits per second.-------------------------------------------------------------------------------Bandwidth-distance product-------------------------------------------------------------------------------Expressed in units of MHz×km. Is a trade off between the signal bandwidth and thedistance it can be carried. Today fiver-optics communication reach 14 Tbps over 160km speeds.-------------------------------------------------------------------------------Dispersion-------------------------------------------------------------------------------The maximum transmission distance is limited not by attenuation but by dispersion,or spreading of optical pulses as they travel along the fiber (i.e. errors caused bythe varying delay in arrival time between different components of a signal).Dispersion limits the bandwidth of the fiber because the spreading optical pulselimits the rate that pulses can follow one another on the fiber and still bedistinguishable at the receiver.* Intermodal dispersion: caused by the different axial speeds of differenttransverse modes, limits the performance of multi-mode fiber. Because single-modefiber supports only one transverse mode, intermodal dispersion is eliminated.* Chromatic dispersion: or group velocity dispersion (GVD) occurs mostly insingle-mode fiber because the refractive index of the glass varies slightlydepending on the wavelength of the light, which causes a short pulse of light tospread in time as a result of different frequency components of the pulse travellingat different velocities.* Polarization mode dispersion: or double refraction, occurs because slightimperfections or distortions in a fiber alter the propagation velocities of the twoPage 3  Fiber-Optic Communicationpolarizations (i.e. the ups and downs of the light-wave).-------------------------------------------------------------------------------Attenuation-------------------------------------------------------------------------------Fiber attenuation, which necessitates the use of amplification systems, is caused bya combination of material absorption, Rayleigh scattering, Mie scattering,connection losses, physical stresses to the fiber, microscopic fluctuations indensity, and imperfect splicing techniques.Material absorption is measured in dB/km and can increase due to impurities in thematerial used. Pure silica 0.03 dB/km and taday fiber causes 0.3 dB/km in contrastto the 1000 dB/km of the srcinals.-------------------------------------------------------------------------------Regeneration-------------------------------------------------------------------------------Recent advances in fiber and optical communications technology have reduced signaldegradation so far that regeneration of the optical signal is only needed overdistances of hundreds of kilometers.===============================================================================Comparison with electrical transmission===============================================================================Optical fiber is generally chosen for systems requiring higher bandwidth or spanninglonger distances than electrical cabling can accommodate.-------------------------------------------------------------------------------Optical Fiber-------------------------------------------------------------------------------* Exceptionally low loss* Long distances between amplifiers or repeaters* High data-carrying capacity 1:1000 compared to cable* No crosstalk* Immune to electromagnetic interference (EMI)* High electrical resistance* Lighter weight* Not electromagnetically radiating* Difficult to tap without disrupting the signal* No sparks* Much smaller cable size* Cables are available in 12 km lengths equals less splices (2 km - 4 km formulti-mode fiber cables)* Cables require in-line signal repeaters every couple of kilometers vs 100km withoptical-fiber with no active or passive processing-------------------------------------------------------------------------------Electrical Transmission-------------------------------------------------------------------------------* Lower material cost* Lower cost of transmitters and receivers* Capability to carry electrical power as well as signals* Ease of operating transducers in linear mode* Optical Fibers are more difficult and expensive to splice* Optical Fibers are susceptible to fiber fuse (an imperfection that can destroyseveral meters per second)Page 4
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