光纤通信.docx

1、光纤通信Fiber-optic communication光纤通信Fiber-optic communication is a method of transmitting information from oneplace to another by sending light through an optical fiber. The light forms anelectromagnetic carrier wave that is modulated to carry information. First developedin the 1970s, fiber-optic commu

2、nication systems have revolutionized thetelecommunications industry and played a major role in the advent of the InformationAge. Because of its advantages over electrical transmission, the use of optical fiberhas largely replaced copper wire communications in core networks in the developedworld.The

3、process of communicating using fiber-optics involves the following basicsteps: Creating the optical signal using a transmitter, relaying the signal along thefiber, ensuring that the signal does not become too distorted or weak, and receivingthe optical signal and converting it into an electrical sig

4、nal.ApplicationsOptical fiber is used by many telecommunications companies to transmittelephone signals, Internet communication, and cable television signals. Due to muchlower attenuation and interference, optical fiber has large advantages over existingcopper wire in long-distance and high-demand a

5、pplications. However, infrastructuredevelopment within cities was relatively difficult and time-consuming, and fiber-opticsystems were complex and expensive to install and operate. Due to these difficulties,fiber-optic communication systems have primarily been installed in long-distanceapplications,

6、 where they can be used to their full transmission capacity, offsetting theincreased cost. Since the year 2000, the prices for fiber-optic communications havedropped considerably. The price for rolling out fiber to the home has currentlybecome more cost-effective than that of rolling out a copper ba

7、sed network. Priceshave dropped to $850 per subscriber in the US and lower in countries like TheNetherlands, 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 transoceanicf

8、iber communication lines. By 2002, an intercontinental network of 250,000 km ofsubmarine communications cable with a capacity of 2.56 Tb/s was completed, andalthough specific network capacities are privileged information, telecommunicationsinvestment reports indicate that network capacity has increa

9、sed dramatically since2002.HistoryThe need for reliable long-distance communication systems has existed sinceantiquity. Over time, the sophistication of these systems has gradually improved, fromsmoke signals to telegraphs and finally to the first coaxial cable, put into service in1940. As these com

10、munication systems improved, certain fundamental limitationspresented themselves. Electrical systems were limited by their small repeater spacing(the distance a signal can propagate before attenuation requires the signal to be2amplified), and the bit rate of microwave systems was limited by their ca

11、rrierfrequency. In the second half of the twentieth century, it was realized that an opticalcarrier of information would have a significant advantage over the existing electricaland microwave carrier signals.In 1966 Kao and Hockham proposed optical fibres at STC Laboratories (STL),Harlow, when they

12、showed that the losses of 1000 db/km in existing glass (comparedto 5-10 db/km in coaxial cable) was due to contaminants, which could potentially beremoved.1The development of lasers in the 1960s solved the first problem of a light source;further development of high-quality optical fiber was needed a

13、s a solution to thesecond. Optical fiber was finally developed in 1970 by Corning Glass Works withattenuation low enough for communication purposes (about 20dB/km), and at thesame time GaAs semiconductor lasers were developed that were compact andtherefore suitable for fiber-optic communication syst

14、ems.After a period of intensive research from 1975 to 1980, the first commercialfiber-optic communication system was developed, which operated at a wavelengtharound 0.8 m and used GaAs semiconductor lasers. This first generation systemoperated at a bit rate of 45 Mbit/s with repeater spacing of up t

15、o 10 km.On 22 April, 1977, General Telephone and Electronics sent the first live telephonetraffic through fiber optics, at 6 Mbit/s, in Long Beach, California.The second generation of fiber-optic communication was developed forcommercial use in the early 1980s, operated at 1.3 m, and used InGaAsPsem

16、iconductor lasers. Although these systems were initially limited by dispersion, in1981 the single-mode fiber was revealed to greatly improve system performance. By1987, these systems were operating at bit rates of up to 1.7 Gb/s with repeater spacingup to 50 km.The first transatlantic telephone cabl

17、e to use optical fiber was TAT-8, based onDesurvire optimized laser amplification technology. It went into operation in 1988.TAT-8 was developed as the first undersea fiber optic link between the United Statesand Europe. TAT-8 is more than 3,000 nautical miles (5,600 km) in length and wasthe first t

18、ransatlantic cable to use optical fibers. It was designed to handle a mix ofinformation. When inaugurated, it had an estimated lifetime in excess of 20 years.TAT-8 was the first of a new class of cables, even though it had already been used inlong-distance land and short-distance undersea operations

19、. Its installation waspreceded by extensive deep-water experiments and trials conducted in the early 1980sto demonstrate the projects feasibility.Third-generation fiber-optic systems operated at 1.55 m and had loss of about0.2 dB/km. They achieved this despite earlier difficulties with pulse-spreadi

20、ng at thatwavelength using conventional InGaAsP semiconductor lasers. Scientists overcamethis difficulty by using dispersion-shifted fibers designed to have minimal dispersionat 1.55 m or by limiting the laser spectrum to a single longitudinal mode. Thesedevelopments eventually allowed 3rd generatio

21、n systems to operate commercially at2.5 Gbit/s with repeater spacing in excess of 100 km.3The fourth generation of fiber-optic communication systems used opticalamplification to reduce the need for repeaters and wavelength-division multiplexingto increase fiber capacity. These two improvements cause

22、d a revolution that resultedin the doubling of system capacity every 6 months starting in 1992 until a bit rate of10 Tb/s was reached by 2001. Recently, bit-rates of up to 14 Tbit/s have been reachedover a single 160 km line using optical amplifiers.The focus of development for the fifth generation

23、of fiber-optic communicationsis on extending the wavelength range over which a WDM system can operate. Theconventional wavelength window, known as the C band, covers the wavelength range1.53-1.57 m, and the new dry fiber has a low-loss window promising an extension ofthat range to 1.30 to 1.65 m. Ot

24、her developments include the concept of opticalsolitons,” pulses that preserve their shape by counteracting the effects of dispersionwith the nonlinear effects of the fiber by using pulses of a specific shape.TechnologyModern fiber-optic communication systems generally include an opticaltransmitter

25、to convert an electrical signal into an optical signal to send into the opticalfiber, a cable containing bundles of multiple optical fibers that is routed throughunderground conduits and buildings, multiple kinds of amplifiers, and an opticalreceiver to recover the signal as an electrical signal. Th

26、e information transmitted istypically digital information generated by computers, telephone systems, and cabletelevision companies.TransmittersThe most commonly-used optical transmitters are semiconductor devices such aslight-emitting diodes (LEDs) and laser diodes. The difference between LEDs andla

27、ser diodes is that LEDs produce incoherent light, while laser diodes producecoherent light. For use in optical communications, semiconductor optical transmittersmust be designed to be compact, efficient, and reliable, while operating in an optimalwavelength range, and directly modulated at high freq

28、uencies.In its simplest form, an LED is a forward-biased p-n junction, emitting lightthrough spontaneous emission, a phenomenon referred to as electroluminescence. Theemitted light is incoherent with a relatively wide spectral width of 30-60 nm. LEDlight transmission is also inefficient, with only a

29、bout 1 % of input power, or about 100microwatts, eventually converted into launched power which has been coupled intothe optical fiber. However, due to their relatively simple design, LEDs are very usefulfor low-cost applications.Communications LEDs are most commonly made from gallium arsenidephosph

30、ide (GaAsP) or gallium arsenide (GaAs). Because GaAsP LEDs operate at alonger 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 ofLEDs causes higher fiber dispersion, considerably limiting their bit

31、 rate-distanceproduct (a common measure of usefulness). LEDs are suitable primarily forlocal-area-network applications with bit rates of 10-100 Mbit/s and transmission4distances of a few kilometers. LEDs have also been developed that use severalquantum wells to emit light at different wavelengths ov

32、er a broad spectrum, and arecurrently in use for local-area WDM networks.A semiconductor laser emits light through stimulated emission rather thanspontaneous emission, which results in high output power (100 mW) as well as otherbenefits related to the nature of coherent light. The output of a laser is relativelydirectional, allowing high coupling efficiency (50 %) into single-mode fiber. Thenarrow spectra