optical fiber communication
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CHAPTER 1INTRODUCTION 1.1 Optical fiberFiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances. Fiber-optic communicationis a method of transmitting information from one place to another by sending pulses oflightthrough an optical fiber. The light forms anelectromagneticcarrier wavethat ismodulatedto carry information. First developed in the 1970s, fiber-opticcommunication systemshave revolutionized thetelecommunicationsindustry and have played a major role in the advent of the Information Age. Because of itsadvantages over electrical transmission, optical fibers have largely replaced copper wire communications in core networksin thedeveloped world. Optical fiber is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Researchers atBell Labshave reached internet speeds of over 100petabitsper second using fiber-optic communication.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 anelectrical signal.
1.2 History of optical fiberPrior to the introduction of optical fiber the information was transmitted through electromagnetic wavesFiber optics really developed in 1950s with the work of Hopkins and Narendra Singh Kapany in UK and Van Heel in HollandActual invention took place in 1960 after the development of semiconductor laser and LED.In 1880Alexander Graham Belland his assistantCharles Sumner Taintercreated a very early precursor to fiber-optic communications, thePhotophone, at Bell's newly establishedVolta LaboratoryinWashington, D.C.Bell considered it his most important invention. The device allowed for thetransmissionof sound on a beam of light. On June 3, 1880, Bell conducted the world's first wirelesstelephonetransmission 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 1966Charles K. KaoandGeorge Hockhamproposed optical fibers at STC Laboratories (STL) atHarlow, 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 byCorning Glass Works, with attenuation low enough for communication purposes (about 20dB/km), and at the same time GaAssemiconductor laserswere 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 0.8m and used GaAs semiconductor lasers. This first-generation system operated at a bit rate of 45Mbpswith repeater spacing of up to 10km. 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 first wide area network fibre optic cable system in the world seems to have been installed by Rediffusion in Hastings, East Sussex, UK in 1978. The cables were placed in ducting throughout the town, and had over 1000 subscribers. They were used at that time for the transmission of television channels,not available because of local reception problems. The system is still in place, but disused. The second generation of fiber-optic communication was developed for commercial use in the early 1980s, operated at 1.3m, and used InGaAsP semiconductor lasers. These early systems were initially limited by multimode fiber dispersion, and in 1981 thesingle-mode fiberwas 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 1.7Gb/s with repeater spacing up to 50km.The firsttransatlantic telephone cableto use optical fiber wasTAT-8, based on Desurvire optimized laser amplification technology. It went into operation in 1988.Third-generation fiber-optic systems operated at 1.55m and had losses of about 0.2dB/km. This development was spurred by the discovery ofIndium gallium arsenideand the development of the Indium Gallium Arsenide photodiode by Pearsall. Engineers overcame earlier difficulties withpulse-spreadingat that wavelength using conventional InGaAsP semiconductor lasers. Scientists overcame this difficulty by usingdispersion-shifted fibersdesigned to have minimal dispersion at 1.55m or by limiting the laser spectrum to a singlelongitudinal mode. These developments eventually allowed third-generation systems to operate commercially at 2.5 Gbit/s with repeater spacing in excess of 100km.The fourth generation of fiber-optic communication systems usedoptical amplificationto reduce the need for repeaters andwavelength-division multiplexingto increasedata 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 10Tb/s was reached by 2001. In 2006 a bit-rate of 14 Tbit/s was reached over a single 160km line using optical amplifiers.The focus of development for the fifth generation of fiber-optic communications is on extending the wavelength range over which aWDMsystem can operate. The conventional wavelength window, known as the C band, covers the wavelength range 1.53-1.57m, anddry fiberhas a low-loss window promising an extension of that range to 1.30-1.65m. Other developments include the concept of "optical solitons, " pulses that preserve their shape by counteracting the effects of dispersion with thenonlinear effectsof 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 protocoldata traffic was increasing exponentially, at a faster rate than integrated circuit complexity had increased underMoore's Law. From the bust of the dot-com bubble through 2006, however, the main trend in the industry has beenconsolidationof firms andoffshoringof manufacturing to reduce costs. Companies such asVerizonandAT&Thave taken advantage of fiber-optic communications to deliver a variety of high-throughput data and broadband services to consumers' homes.
1.3 Parts of Optical Fiber Core Cladding Buffer Coating
Fig.1.2.Parts of optical fiber
CHAPTER 2How Does an Optical Fiber Transmit Light?
Fig.2.1 Transmission of light in O.F.C.
2.1 Physics of total internal reflection The angle of the light is always greater than the critical angle Cladding does not absorb any light from the core The extent that the signal degrades depends upon the purity of the glass and the wavelength of the transmitted light
Fig.2.2 Total internal reflection in OFC
2.2 Fiber Optics Data Links
Fig.2.3 Fiber Optic Link
3.1 Introduction to SplicingSplices are permanent connection between two fibres. The splicing involves cutting of the edges of the two fibres to be spliced. This cut has to be carefully made to have a smooth surface and is generally achieved by a special cutting tool. The two ends, thus, prepared are then brought together and made to butt against each other. The fibres are then fixed permanently and reinforced. The fixing process can be achieved in a number of ways. It could be mechanically fixed permanently through uses of epoxies or through fusion. There are two types of fiber splicing Mechanical splicing and Fusion splicing.
Mechanical splicing doesnt physically fuse two optical fibers together, rather two fibers are held butt-to-butt inside a sleeve with some mechanical mechanism. You will get worse insertion loss and back reflection in mechanical splices than in fusion splices (the second type we are introducing below).Mechanical splicing is mostly used for emergency repairs and fiber testing. You can check out some mechanical splice products here.
Figure 3.1: Mechanical splice
The second type splicing is called fusion splicing. In fusion splicing, two fibers are literally welded (fused) together by an electric arc. Fusion splicing is the most widely used method of splicing as it provides for the lowest insertion loss and virtually no back reflection.
Fusion splicing provides the most reliable joint between two fibers. Fusion splicing is done by an automatic machine called fusion splicer (fusion splicing machines).
Figure 3.2: Fusion splice
3.2 Splice LossesSplice losses can be divided into two categories as shown in Table. Extrinsic and intrinsic splice loss factors
3.3 Splicing Methods The following three types are widely used: 1. Adhesive bonding or Glue splicing. 2. Mechanical splicing. 3. Fusion splicing. 1. Adhesive Bonding or Glue Splicing
This is the oldest splicing technique used in fibre splicing. After fibre end preparation, it is axially aligned in a precision Vgroove. Cylindrical rods or other kind of reference surfaces are used for alignment. During the alignment of fibre end, a small amount of adhesive or glue of same