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Guru vashist report

  1. 1. REPORT Optical Fibers in Communications [2011] SUBMITTED BY GURU VASHIST 0830531011 E.C. VII SEMARYABHATT COLLEGE OF ENGINEERING AND TECHNOLOGY
  2. 2. ABSTRACTCommunication is an important part of our daily life. Thecommunication process involves information generation,transmission, reception and interpretation. As needs forvarious types of communication such as voice, images, videoand data communications increase demands for largetransmission capacity also increase. This need for largecapacity has driven the rapid development of light wavetechnology; a worldwide industry has developed. An opticalor light wave communication system is a system that useslight waves as the carrier for transmission. An opticalcommunication system mainly involves three parts.Transmitter, receiver and channel. In optical communicationtransmitters are light sources, receivers are light detectors andthe channels are optical fibers. In optical communication thechannel i.e., optical fibers play an important role because itcarries the data from transmitter to the receiver. Hence, herewe shall discuss mainly about optical fibers.
  3. 3. INDEXS.NO. CONTENT1. History2. Introduction3. Fundamental of Optical Fiber4. Construction of Fibers5. Classification of Optical Fibers 5.1 Based on the materials used 5.2 Based on number of modes 5.3 Based on refractive index6. Modes And Propagation Of Light In Fibers7. Optical Fiber Cables8. Joint of Fiber9. Fiber Splices10. Fusion Splices11. Equipment Required for OFC Joint12. Electric Field With In Fiber Cladding13. Repeaters And Regenerators14. Light Sources15. Detecting the Signal16. Advantages Over Conventional Cables17. Application of the Optical Fiber Communication18. Features19. Essential Features of an Optical Fiber20. Drawbacks of Optical Fiber Communication21. Conclusions22. Bibliography
  4. 4. Optical Fibers in Communication1. HISTORY:-The use of visible optical carrier waves or light forcommunication has been common for many years. Simplesystems such as signal fires, reflecting mirrors and, morerecently signaling lamps have provided successful, if limited,information transfer. Moreover as early as 1880 AlexanderGraham Bell reported the transmission of speech using a lightBeam. The photo phone proposed by Bell just for years afterthe invention of the telephone modulated sunlight with adiaphragm giving speech transmission over a distance of200m. However, although some investigation of the opticalcommunication continued in the early part of the 20th centuryits use was limited to mobile, low capacity communicationLinks. This was due to both the lack of suitable light sourcesand the problem that light transmission in the atmosphere isrestricted to line of sight and severely affected bydisturbances such as rain, snow, fog dust and atmosphericturbulence. A renewed interest in optical communication wasstimulated in the early 1960s with the invention of the laser.This device provided a coherent light source, together with
  5. 5. the possibility of the modulation at high frequency. Theproposals for optical communication via optical fibersfabricated from glass to avoid degradation of the opticalsignal by the atmosphere were made almost simultaneously in1966 by Kao and Hock ham and Werts. Such systems wereviewed as a replacement for coaxial cable system; initially theoptical fibers exhibited very high attenuation and weretherefore not comparable with the coaxial cable they were toreplace. There were also problems involved in jointing thefiber cables in a satisfactory manner to achieve low loss andto enable the process to be performed relatively easily andrepeatedly in the field.In coaxial system the channel capacity is 300 to 10800 andthe disadvantages of the coaxial system are digging, electricaldisturbance, in winter cable contracts and breaks mutualinduction. The coaxial cable loss is 0.3db per every km.• In microwave system if we double the distance the loss willbe increased by 6db.• For the shorter distance the loss is higher.• In o.f.c. system Optical wire is small size, light weight, highstrength and flexibility. Its Transmission benefits includewide band width, low loss and low cost.
  6. 6. • They are suitable for both analog and digital transmission.• It is not suffered by digging, electrical interference etc.problems.2. Introduction:-Optical fibers are arguably one of the world’s most influentialscientific developments from the latter half of the 20thcentury. Normally we are unaware that we are using them,although many of us do frequently. The majority of telephonecalls and internet traffic at some stage in their journey will betransmitted along an optical fiber. Why has the developmentof fibers been given so much attention by the scientificcommunity when we have alternatives? The main reason isbandwidth – fibers can carry an extremely large amount ofinformation. More indirectly, many of the systems that weeither rely on or enjoy in everyday life such as banks,television and newspapers as (to name only a very limitedselection) are themselves dependent on communicationsystems that are dependent on optical fibers.3. Fundamentals of Fibers:-
  7. 7. The fundamental principle that makes optical fiberspossible is total internal reflection. This is described usingthe ray model of light as shown in figure 1.Figure 1 - Total Internal ReflectionFrom Snell’s Law we find that refraction (as shown by thedashed line) can only occur when the angle theta1 is largeenough. This implies that as the angle is reduced, there mustbe a point when the light ray is reflected, where theta1 =theta2. The angle where this happens is known as the criticalangle and is:4. CONSTRUCTION OF FIBERS:-In fibers, there are two significant sections – the core and thecladding. The core is part where the light rays travel and the
  8. 8. cladding is a similar material of slightly lower refractiveindex to cause total internal reflection. Usually both sectionsare fabricated from silica (glass). The light within the fiber isthen continuously totally internally reflected along thewaveguide. Figure 2: Structure of FiberWhen light enters the fiber we must also consider refraction atthe interface of the air and the fiber core. The difference inrefractive index causes refraction of the ray as it enters thefiber, allowing rays to enter the fiber at an angle greater thanthe angle allowed within the fiber as shown in the figure 3.
  9. 9. Figure 3 - Acceptance AngleThis acceptance angle, theta, is a crucial parameter for fiberand system designers. More widely recognized is theparameter NA (Numerical Aperture) that is given by thefollowing equation:5. CLASSIFICATION OF OPTICAL FIBERS:-Optical fibers are classified into three types based on thematerial used, number of modes and refractive index.5.1. Based on the materials used:-a. Glass fibers:
  10. 10. They have a glass core and glass cladding. The glass used inthe fiber is ultra pure, ultra transparent silicon dioxide (SiO2)or fused quartz. Impurities are purposely added to pure glassto achieve the desired refractive index.b. Plastic clad silica:This fiber has a glass core and plastic cladding. Thisperformance though not as good as all glass fibers, is quiterespectable.c. Plastic fibers:They have a plastic core and plastic cladding. These fibers areattractive in applications where high bandwidth and low lossare not a concern.5.2. Based on the number of modes:-a. Single Mode fiber:When a fiber wave-guide can support only the HE11 mode, itis referred to as a single mode wave-guide. In a step indexstructure this occurs w3hen the wave-guide is operating atv<2.4 where v is dimensionless number which relates thepropagating in the cladding. These single mode fibers have
  11. 11. small size and low dopant level (typically 0.3% to 0.4% indexelevation over the lading index.)In high silica fibers the wave-guide and the materialdispersion are often of opposite signs. This fact can be usedconveniently to achieve a single mode fiber of extremelylarge bandwidth. Reduced dopant level results in lowerattenuation than in multimode fibres. A single mode waveguide with its large and fully definable bandwidthcharacteristics is an obvious candidate for long distance, highcapacity transmission applications.b. Multimode fiber:It is a fiber in which more than one mode is propagating at thesystem operating wavelength. Multimode fiber system doesnot have the information carrying capacity of single modefibers. However they offer several advantages for specificsystems.The larger core diameters result in easier splicing of fibers.Given the larger cores, higher numerical apertures, andtypically shorter link distances, multimode systems can useless expensive light sources such as LED s . Multimode fibers
  12. 12. have numerical apertures that typically range from 0.2 to 0.29and have core size that range from 35 to100 micro-meters.5.3. Based on refractive index:-a. Step index fiber:The step index (SI) fiber consists of a central core whoserefractive index is n1, surrounded by a lading whoserefractive index is n2, lower than that of core. Because of an abrupt index change at the core claddinginterface such fibers are called step index fibers.b. Graded index fibers:The refractive index of the core in graded index fiber is notconstant, but decreases gradually from its maximum value n1to its minimum value n2 at the core-cladding interface. Theray velocity changes along the path because of variations inthe refractive index.The ray propagating along the fiber axis takes the shortestpath but travels most slowly, as the index is largest along thispath in medium of lower refractive index where they travel
  13. 13. faster. It is therefore possible for all rays to arrive together atthe fiber output by a suitable choice of refractive indexprofile.6. MODES AND PROPAGATION OF LIGHTIN FIBERS:-Also crucial to understanding fibers is the principle of modes.A more in-depth analysis of the propagation of light along anoptical fiber requires the light to be treated as anelectromagnetic wave (rather that as a ray). Figure 4 – ModesThe solid line is the lowest order mode shown on figure 4. Itis clear that according to the ray model the lowest order modewill travel down a given length of fiber quicker than the
  14. 14. others. The electromagnetic field model predicts the opposite– that the highest order mode will travel quicker. However, the overall effect is still the same – if asignal is sent down the fiber as several modes then as ittravels along the fibre the pulse will spread out, this can leadto the pulses merging and becoming indistinguishable. Figure 5: Propagation of light in fibersThe propagation of light is as shown in figure 5. When lightray enters the core with an angle strikes the surface ofcladding whose refractive index is less than that of core. Asthe incidence angle on surface of the cladding is greater thanor equal to critical angle total internal reflection takesplace. Hence the ray is reflected back into the core in theforward direction. This process continues until it reachesother end of the cable.
  15. 15. 7. OPTICAL FIBER CABLES:-When optical fibers are to be installed in a workingenvironment their mechanical properties are of primeimportance. In this respect the unprotected optical fiber hasseveral disadvantages with regard to its strength anddurability.Bare glass fibers are little and have small cross sectional areaswhich make them very susceptible to damage whenemploying normal transmission line handling procedures. It istherefore necessary to cover the fibers to improve their tensilestrength and to protect them against external influences.The functions of the optical cable may be summarized intofour main areas.These are as follows:-1. Fiber protection. The major function of the optical cable isto protect against fiber damage and breakage both duringinstallation and throughout the life of the fiber.
  16. 16. 2. Stability of the fiber transmission characteristics. Thecabled fiber must have good stable transmissioncharacteristics which are comparable with the uncabled fiber.Increases in optical attenuation due to cabling are quite usualand must be minimized within the cable design.3. Cable strength. Optical cables must have similarmechanical properties to electrical transmission cables inorder that they may be handled in the same manner. Thesemechanical properties include tension, torsion, compression,bending, squeezing and vibration. Hence the cable strengthmay be improved by incorporating a suitable strength memberand by giving the cable a properly designed thick outersheath..4. Identification and jointing of the fibers within the cable.This is especially important for cables including a largenumber of optical fibers. If the fibers are arranged in asuitable geometry it may be possible to use multiple jointingtechniques rather than jointing each fiber individually.8. JOINT OF FIBER:-Optical fiber links, in common with any line communicationsystem, have a requirement for both jointing and termination
  17. 17. of the transmission medium. The number of intermediate fiberconnections or joints is dependent upon the link length, thecontinuous length of the fiber cable that may be produced bythe preparation methods and the length of the fiber cable thatmay be practically installed as a continuous section on thelink. It is therefore apparent that fiber to fiber connection withlow loss and minimum distortion (i.e. modal noise) remainsan important aspect of optical fiber communication system.Before optical fibers splicing and joining are done certainpreparations are made with fiber or fiber cables as case maybe to achieve best results at the end surface. First of all theprotective plastic that covers the glass cladding is strippedfrom each fiber end, which is then cleaved with a special tool,producing a smooth and flat end.1. Fiber splices: these are semi permanent or permanent jointswhich find major use in most optical fiber telecommunicationsystem (analogous to electrical soldered joints).2. Demountable fiber connectors or simple connectors: theseare removable joints which allow easy, fast, manual coupling
  18. 18. and uncoupling of fibers (analogous to electrical plugs andsockets).The above fiber to fiber joints are designed ideally to coupleall the light propagating in one fiber into the adjoining fiber.By contrast fiber couplers are branching devices that split allthe light from main fiber into two or more fibers or,alternatively, couple a proportion of the light propagating inthe main fiber into main fiber.9. FIBER SPLICES:-A permanent joint formed between two individual opticalfibers in the field or factory is known as a fiber splice. Fibersplicing is frequently used to establish long haul optical fiberlinks where smaller fiber lengths need to be joined, and thereis no requirement for repeated connection and disconnection.Splices may be divided into two broad categories dependingupon the splicing technique utilized. These are fusion splicingor welding and mechanical splicing.Fusion splicing is accomplished by applying localized heating(e.g. by a flame or an electric are) at the interface betweentwo butted, prealigned fiber ends causing them to soften and
  19. 19. fuse. Mechanical splicing, in which the fibers are held inalignment by some mechanical means, may be achieved byvarious methods including the use of tubes around the fiberends (groove splices).A requirement with fibers intended for splicing is that theyhave smooth and square end faces. In general this endpreparation may be achieved using a suitable tool whichcleaves the fiber as illustrated.10. FUSION SPLICES:-The fusion splicing – of single fibers involves the heating ofthe two prepared fiber ends to their fusing point with theapplication of sufficient axial pressure between the twooptical fibers. It is therefore essential that the stripped (ofcabling and buffer coating) fiber ends are adequatelypositioned and aligned in order to achieve good continuity ofthe transmission medium at the junction point. Hence the fiberare usually positioned and clamped with the aid of aninspection microscope.Flame heating sources such as micro plasma torches (argonand hydrogen) and oxhydric micro burners (oxygen, hydrogen
  20. 20. and alcohol vapour) have been utilized with some success.However, the most widely used heating source is an electricarc. This technique offers advantages of consistent, easilycontrolled heat with adaptability for use under fieldconditions. A schematic diagram of the basic two fibers iswelded together. Shows a development of the basic are fusionprocess which involves the rounding of the fiber ends with alow energy discharge before pressing the fibers together andfusing with a stronger arc. This technique, known asperfusion, removes the requirement for fiber end preparationwhich has a distinct advantage in the field environment.A possible drawback with fusion splicing is that the heatnecessary to fuse the fibers may weaken the fiber in thevicinity of the splice. It has been found that even with carefulhandling; the tensile strength of the fused fiber may be as lowas 30 % of that of the uncoated fiber before fusion.11. EQUIPMENT REQUIRED FOR OFCJOINT:1) Optical fiber fusion splicer specification ( spicer machine )• AC input – 100 to 240v, frequency – 50/60Hz
  21. 21. • DC input 12v/A2) Fiber cutter• It converts irregular shaped fiber end into smooth & flat end.3) Chemicals used in OFC joint• HAXENE: To remove jelly from the fiber• ACETONE: For cleaning the OFC• ISO PROPENOT: For smoothness of optical glass.4) Sleeve: - To enclose fiber joint.5) Tool Kit6) Joint kit.• Joint encloser• Buffer• Adhesive tape.7) Generator /12V Battery8) Cotton clothes for fiber cleaning.
  22. 22. 12. ELECTRIC-FIELD WITH IN FIBERCLADDING:-One other significant point should be noted from theelectromagnetic field model. The model predicts that the EMfield does not suddenly drop to zero at the core-claddingboundary – it instead decays as negative exponential withinthe cladding as shown in the figure 6.This is crucial for various technologies relating to fibers.Figure 6 - The Electric Field within the Fiber CladdingThis method of signal transmission has benefits in terms ofsecurity – for the signal to be ‘tapped’ the fiber must bebroken (since effectively no energy escapes from the fiber)
  23. 23. and this can easily be detected (when no signal reaches theother end of the fiber!).This is one of the many advantages of the medium. Butmainly two factors, attenuation and dispersion of light, haveto be considered while transmitting the light over largedistances. We use repeaters and regenerators to reduce theattenuation and dispersion.13. REPEATERS AND REGENERATORS:-Optical repeaters are purely optical devices that are usedsimply to combat attenuation in the fiber; typically spans of80km upwards are now possible. The recent introduction ofsoliton transmission methods has increased the alloweddistance between repeaters and systems spanning 130kmwithout a repeater are now possible. Regenerators are devices consisting of both electronic and opticalcomponents to provide ‘3R’ regeneration – Retiming,Reshaping, Regeneration. Retiming and reshaping detect the
  24. 24. digital signal that will be distorted and noisy (partly due to theoptical repeaters), and recreate it as a clean signal as shown infigure 6 This clean signal is then regenerated (opticallyamplified) to be sent on. It should be noted that repeaters arepurely optical devices whereas regenerators require optical-to-electrical (O/E) conversion and electrical-to-optical (E/O)conversion. The ultimate aim of many fiber systemresearchers is to create a purely optical network withoutelectronics, which would maximize efficiency andperformance. Many aspects of such a system are in place, butsome still require the O/E and E/O conversion.Figure7 - A digital signal before (noisy and attenuated)and after regeneration
  25. 25. The most common optical amplifier currently in use is theEDFA (Erbium Doped Fiber Amplifier). These consist of acoil of fiber doped with the rare earth metal erbium. A laserdiode pumps the erbium atoms to a high-energy state; whenthe signal reaches the doped fiber the energy of the erbiumatoms is transferred to the signal, thus amplifying it.14. Light Sources:-Two types of light source are used with fibers, LEDs andLaser Diodes. LEDs can operate in the near infrared (the mainwavelengths used in fibers are 1300nm and 1550nm, alongwith 850nm for some applications); they can emit light at850nm and 1300nm. They also have the advantages of longlifetimes and being cheap. Unfortunately they are largecompared to the cross-section of a fiber and so a large amountof light is lost in the coupling of an LED with a fiber. Thisalso reduces the amount of modal control designers have overincident light. Laser diodes can be made to emit light at either1300nm or 1550 nm, and also over a small spectral width(unlike LEDs), which reduces chromatic dispersion. Theiremitting areas are extremely small and so the angle ofincidence of light on a fiber can be accurately controlled suchthat <5% of the possible modes within a multimode fiber will
  26. 26. be initially used. They are more efficient than LEDs in termsof coupling of light into the fiber, although they have shorterlifetimes than and are more expensive than LEDs.One crucial advantage of lasers over LEDs in today’s worldof digital communications is their high switching speed andsmall rise times, leading to increased bandwidth.15. Detecting the Signal:-The most efficient detectors are reverse-bias photo detectors.They essentially cause a current to flow when light is incidenton them. The choice of semiconductor that is used to fabricatethe detector is dependent on the wavelength sensitivity andthe responsivity that are required. Bandwidth considerationsare also important (determined by the rise time and fall timeof a detector); in detectors the fall time is often appreciablygreater than the rise time and so this must be used to calculatethe bandwidth of a detector.There are many further complications in detectors, includingnoise equivalent power that indicates how ‘clean’ a signalfrom a detector is. An analysis of how analogue and digital
  27. 27. signals are processed after the initial detector is alsointeresting.16. ADVANTAGES OVER CONVENTIONALCABLES:-a. Wide Bandwidth:Optical fibers offer greater bandwidth due to the use of lightas carrier. The frequency range used for glass fibercommunication extends from 2*e14Hz to 4*e14Hz. Henceoptical fibers are suitable for high speed, large capacitytelecommunication lines.b. Low Loss:In a coaxial cable attenuation increases with frequency. Thehigher the frequency of information signals the greater theloss, whereas in an optical fiber the attenuation is independentof frequency. They offer a loss of0.2 dBm/km, allowingrepeater separation upto 50Km or more.c. Freedom from electromagnetic interference:
  28. 28. Optical fibers are not affected by interference originatingfrom power cables, railways and radio waves. They do notlimit unwanted radiation and no cross talk between fibersexists. These fibers make an ideal transmission medium whenEMI (Electro Magnetic Immunity) is increased.d. Non conductivity:Optical fibers are non-conductive and are not effective bystrong electromagnetic interference such as lighting. Theseare usable in explosive environment.e. Small diameters and less weight:Even multi fiber optical cables have a small diameter and arelight weight, and flexible optical fiber cables permit effectiveutilization of speech and can also be applicable to longdistance use are easier to handle and install than conventionalcables.f. Security:Fiber optic is a highly source transmission medium. It doesnot radiate energy that can be received by a nearby antenna,and it is extremely difficult to tap a fiber and virtuallyimpossible to make the tap undetected.
  29. 29. g. Safety:Fibre is a dielectric and does not carry electricity. It presentsno sparks or fire hazards. It does not cause explosions, whichoccur due to faulty copper cable.17. APPLICATION OF THE OPTICAL FIBERCOMMUNICATION:-TRUNK NETWORKThe trunk or toll network is used for carrying telephone trafficbetween majorconurbations. Hence there is generally a requirement for theuse of transmission systemswhich have a high capacity in order to minimize costs percircuit. The transmissiondistance for trunk systems can very enormously from under20 km to over 300 km, andoccasionally to as much as 1000 km. Therefore transmissionsystems which exhibit lowattenuation and hence give a maximum distance ofunrepeatered operation are the mosteconomically viable. In this context optical fiber systems withtheir increased bandwidth
  30. 30. and repeater spacing offer a distinct advantage.JUNCTION NETWORK:The junction or interoffice network usually consists of routeswithin major conurbationsover distances of typically 5 to 20 km. However, thedistribution of distances betweenswitching centers (telephone exchanges ) or offices in thejunction network of large urbanareas varies considerably for various countries.MILITARY APPLICATION:In these applications, although economics are important, thereare usually other, possiblyoverriding, considerations such as size, weight, deployability,survivability (in bothconventional and nuclear attack and security. The specialattributes of optical fibercommunication system therefore often lend themselves tomilitary use.MOBILES:
  31. 31. One of the most promising areas of milita5ry application foroptical fiber communicationis within military mobiles such as aircraft, ships and tanks.The small size and weight ofoptical fibers provide and attractive solution to spaceproblems in these mobiles whichare increasingly equipped with sophisticated electronics. Alsothe wideband nature ofoptical fiber transmission will allow the multiplexing of anumber of signals on to acommon bus. Furthermore, the immunity of opticaltransmission to electromagneticinterference (EMI) in the often noisy environment of militarymobiles is a tremendousadvantage. This also applies to the immunity of optical fiberto lighting andelectromagnetic pulses (EMP) especially within avionics. Theelectrical isolation, andtherefore safety, aspect of optical fiber communication alsoproves invaluable in these
  32. 32. applications, allowing routing through both fuel tanks andmagazines.COMMUNICATION LINKS: The other major area for the application of optical fibercommunication in the militarysphere includes both short and long distance communicationlinks. Short distance opticalfiber systems may be utilized to connect closely spaced itemsof electronics equipment insuch areas as operations rooms and computer installations. Alarge number of this systemhave already been installed in military installations in theunited kingdom. These operateover distances from several centimeters to a few hundredmeters at transmission ratesbetween 50 bauds and 4.8 kbits-1. In addition a small numberof 7 MHz video linksoperating over distances of up to 10 m are in operation. Thereis also a requirement forlong distance communication between military installationswhich could benefit from the
  33. 33. use of optical fibers. In both these advantages may be gainedin terms of bandwidth,security and immunity to electrical interference and earth loopproblems overconventional copper systems.CIVIL APPLICATION:The introduction of optical fiber communication systems intothe public network hasstimulated investigation and application of these transmissiontechniques by public utilityorganizations which provide their own communicationfacilities over moderately longdistances. For example these transmission techniques may beutilized on the railways andalong pipe and electrical power lines.In these applications, although high capacity transmission isnot usually required, optical fibers may provide a relativelylow cost solution, also giving enhanced protection in harshenvironment, especially in relation to EMI and EMP.Experimental optical fiber communication systems have been
  34. 34. investigated within a number of organizations in Europe,North America and Japan. For instance, British Rail hassuccessfully demonstrated a 2 Mbits-1 system suspendedbetween the electrical power line gantries over a 6 km route inCheshire.Also, the major electric power companies have shown a greatdeal of interest with regard to the incorporation of opticalfibers within the metallic earth of overhead electric powerlines. fibers are now the standard.TELECOMMUNICATION:Optical point to point cable link between telephonesubstations.LOCAL AREA NETWORKS (LANs):Multimode fiber is commonly used as the "backbone" to carrysignals between the hubs of LANs from where copper coaxialcable takes the data to the desktop. Fiber links to the desktop,however, are also common.CABLE TV:As mentioned before domestic cable TV networks use opticalfiber because of its very low power consumption.
  35. 35. CCTV:Closed circuit television security systems use optical fiberbecause of its inherent security, as well as the otheradvantages mentioned above.18. FEATURE:-The fiber optics has become a preferred medium due to itssome important features like:• The bandwidth of the fiber and light beam is extremelywide. It is possible to handlesignals which turn on and off at gigabit per second rates (1gigabit, gbit =1000Mbitts).• The fiber itself is very thin and not expensive. The thinnessmeans that it is easy tohandle, and many fibers can be put in the trenches or narrowconduits.• The light signa-l is absolutely immune to electrical noisefrom any sources. Even if
  36. 36. there are sources of electrical noise directly touching thecable, the electric fields ofthe noise source cannot affect the light beam in the fiber.• The signal in the cable is secure from unauthorized listeners.It is relatively hard totap into the cable without being noticed, and the entire lightsignal is confined withinthe fiber. No light escapes to the outside where someone elsecould see it.• Since there is no electricity or electrical energy in the fiber,it can be run in hazardousatmospheres where the danger of explosion from spark mayexist. Also, the fiberitself is immune to many types of poisonous gases, chemicals,and water.19. ESSENTIAL FEATURES OF AN OPTICAL FIBER:-1. Optical fibers may be produced with good stabletransmission characteristics in longlengths at a minimum cost and with maximumreproducibility.
  37. 37. 2. A range of optical fiber types with regard to size, refractiveindices and indexprofiles, operating wavelengths, materials etc. be available inorder to fulfill manydifferent system applications.3. The fibers may be converted into practical cables whichcan be handled in a similarmanner to conventional electrical transmission cables withoutproblems associatedwith the degradation of their characteristics or damage.4. The fibers and fiber cables may be terminated andconnected together withoutexcessive practical difficulties and in ways which limit theeffect of this process onthe fiber transmission characteristics to keep them withinacceptable operating levels. It is important that these jointingtechniques may be
  38. 38. applied with ease in the field location where cable connectiontakes place.20. DRAWBACKS OF OPTICAL FIBERCOMMUNICATION:-The use of fibers for optical communication does have somedrawbacks in practice.Hence to provide a balance picture these disadvantages mustbe considered. They are• The fragility of the bare fibers;• The small size of fibers and cables which creates somedifficulties with splicing andforming connectors;• Some problems involved with forming low loss T- couplers;• Some doubts in relations to the long term reliability ofoptical fibers in the presenceof moisture;• An independent electrical power feed is required for anyelectronic repeaters;• New equipment and field practice are required;• Testing procedures tend to be more complex.
  39. 39. 21. Conclusions:-We are currently in the middle of a rapid increase in thedemand for data bandwidth across the Earth. For mostapplications optical fibers are the primary solution to thisproblem. They have potentially a very high bandwidth, withmany of the bandwidth limitations now being at thetransceivers rather than being an intrinsic property of the fiberallowing easy upgrading of systems without relaying cable. This is creating a surge in thedeployment of fiber both in backbones of networks and intopologically horizontal cabling, which inturn is supportingand propelling the industry into further research. With theadoption of new techniques such as DWDM, solitontransmission, and ultimately the purely optical network, wehave a medium that will satisfy our communication needs forthe foreseeable future.22. Bibliography:- • Optical Fibers And Sources For Communications
  40. 40. ---Adamsand Henning, • Principles Of Modern Optical Systems --- Andonovic and Uttamchandani • An Introduction to Optical Waveguides ---Adams, M. J.

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