Fiber optic communications

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Class notes useful for Electronics students of SKU and RU and also ECE students of all the universities.

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Fiber optic communications

  1. 1. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.com FIBER OPTIC COMMUNICATIONSINTRODUCTION:Fiber-optic communication systems are lightwave systems that employ optical fibers forinformation transmission. Such systems have been deployed worldwide since 1980 and haverevolutionized the technology behind tele-communications .Optical communication systems usehigh carrier frequencies (~100 THz) in the visible or near-infrared region of the electromagneticspectrum. They are sometimes called light wave systems to distinguish them from microwavesystems, whose carrier frequency is typically smaller by five orders of magnitude(~1 GHz).The development of worldwide telephone networks during the twentieth century led to manyadvances in the design of electrical communication systems. The use of coaxial cables in place ofwire pairs increased system capacity considerably. The first coaxial-cable system, put intoservice in 1940, was a 3-MHz system capable of transmitting 300 voice channels or a singletelevision channel. The bandwidth of such systems is limited by the frequency-dependent cablelosses, which increase rapidly for frequencies beyond 10 MHz. This limitation led to thedevelopment of microwave communication systems in which an electromagnetic carrier wavewith frequencies in the range of 1–10 GHz is used to transmit the signal by using suitablemodulation technique.The first microwave system operating at the carrier frequency of 4 GHz was put into service in1948. Since then, both coaxial and microwave systems have evolved considerably and are able tooperate at bit rates ~100 Mb/s. The most advanced coaxial system was put into service in 1975and operated at a bit rate of 274 Mb/s. A severe drawback of such high-speed coaxial systems istheir small repeater spacing (~1 km), which makes the system relatively expensive to operate.Microwave communication systems generally allow for a larger repeater spacing, but their bitrate is also limited by the carrier frequency of such waves.The idea of using optical fibers for communication was suggested in 1966 , as they are capableof guiding the light in a manner similar to the guiding of electrons in copper wires. The mainproblem was the high losses of optical fibers. During the 1960s the fibre losses were of the orderof 1000 dB/km. A breakthrough occurred in1970 when fiber losses could be reduced to below20 dB/km in the wavelength region near 1 μm . At about the same time, GaAs semiconductor1
  2. 2. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comLasers, operating continuously at room temperature, were demonstrated. The simultaneousavailability of compact optical sources and a low-loss optical fibers led to a worldwide effort fordeveloping fiber-optic communication systems. Fig:Electromagnetic SpectrumEvolution of fiber optic system :The evolution of Fiber optic system can be divided into five generations in terms ofdevelopments and changesThe first generation of fiber optic systems operated near 0.8 μm and used GaAs semiconductorlasers. After several field trials during the period 1977–79, such systems became available2
  3. 3. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comcommercially in 1980 . They operated at a bit rate of 45 Mb/s and allowed repeater spacings ofup to 10 km. The larger repeater spacing compared with 1-km spacing of coaxial systems was animportant motivation for system designers because it decreased the installation and maintenancecosts associated with each repeater.The second generation of fiber-optic communication systems became available in the early1980s, but the bit rate of early systems was limited to below 100 Mb/s because of dispersion inmultimode fibers . This limitation was overcome by the use of single-mode fibers. By 1987,second-generation light-wave systems, operating at bit rates of up to 1.7 Gb / s with a repeaterspacing of about 50 km, were commercially available.The introduction of third-generation light wave systems operating at 1.55 μm was considerablydelayed by a large fiber dispersion near 1.55 μm. Conventional InGaAsP semiconductor laserscould not be used because of pulse spreading occurring as a result of simultaneous oscillation ofseveral longitudinal modes. The dispersion problem can be solved either by using dispersion-shifted fibers designed to have minimum dispersion near 1.55 μm or by limiting the laserspectrum to a single longitudinal mode. Third-generation light wave systems operating at 2.5Gb/s became available commercially in 1990. Such systems are capable of operating at a bit rateof up to 10 Gb/s .A drawback of third-generation 1.55-μm systems is that the signal isregenerated periodically by using electronic repeaters spaced apart typically by 60–70 km.The fourth generation of light wave systems makes use of optical amplification for increasingthe repeater spacing and of wavelength-division multiplexing (WDM) for increasing the bit rateIn most WDM systems, fiber losses are compensated periodically using Erbium-doped fiberamplifiers spaced 60–80 km apart. Such amplifiers were developed after 1985 and becameavailable commercially by 1990. The experimental results in 1991 showed the possibility of datatransmission over 21,000 km at 2.5 Gb/s, and over 14,300 km at 5 Gb/s . This performanceindicated that an amplifier-based, all-optical, submarine transmission system was feasible forintercontinental communication.The fifth generation of fiber-optic communication systems is concerned with extending thewavelength range over which a WDM system can operate simultaneously. The conventionalwavelength window, known as the C band, covers the wavelength range 1.53–1.57μm. It is beingextended on both the long- and short-wavelength sides, resulting in the L and S bands,respectively. The Raman amplification technique can be used for signals in all three wavelength3
  4. 4. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.combands. Moreover, a new kind of fiber, known as the dry fiber has been developed with theproperty that fiber losses are small over the entire wavelength region extending from 1.30 to 1.65μm . Availability of such fibers and new amplification schemes may lead to light wave systemswith thousands of WDM channels. The fifth-generation systems also attempt to increase the bitrate of each channel within the WDM signal. Starting in 2000, many experiments used channelsoperating at 40 Gb/s; migration toward 160 Gb/s is also likely in the future. Such systems requirean extremely careful management of fiber dispersion.Advantages of Optical Fibers :There are many advantages of optical fibers when compared to other methods.1. Long distance transmission : Optical fibers have low transmission losses when compared tocopper cables .So,data can be transmitted over longer distances and the number of repeatersrequired can be reduced.2. Infomation Capacity : Optical fibers have large information capacity ,because of their longerbandwidths .So,more information or data can be transmitted on a single fiber wire as comparedto a copper wire.This will reduce the cost.3.Small size and low weight : The dimensions of fiber cabels is relatively small as compared tocopper wires which are very bulky. This is very advantageous in some systems like aircraft,satellites , ships and in military applications where small light weight cables are preferred whencompared to copper cables.4.Immunity to Electrical Interference : As fiber cables are dielectric and non-matallic ,they areimmune to external electric disturbances and also not affected by electromagnetic interferenceor electric noise effects due to adjacent channels or near by electrical equipment. But, this isvery severe in copper cables.5. Enhanced Safety : As fiber cables do not have the problems of ground loops ,sparks and highvoltages unlike copper cables they offer a high degree of operational safety. The only limitationis ,care must be taken while handling LASER light to avoid possible damage to eyes.6. Signal Security : The fiber cable always guides the optical signal and hence there is a highdegree of data security from external disturbances.Where as in copper wires the electrical signalscan be easily tapped off.Basic optical laws and definitions :The phenomenon of total internal reflection, is responsible for guiding of light in optical fibers.4
  5. 5. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comA very important optical property associated with the material is its refractive index.Therefractive index of a material is defined as the ratio of velocity of light in free space to that in thematerial. The refractive index n =The value of n for free space or air is 1.00 and for water 1.33 and for silica glass 1.45-1.55andfor diamond 2.42Refraction and Reflection :The two important properties of light are Refraction and Reflection. When light travels from onemedium to another medium of different refractive indices, the ray bends at the interface of thetwo media. i.e there will be a change in the velocity of the light at the interface .This phenomenais known as Refraction of Light.Some time s depending on the refractive index of the secondmedium ,the light will retrace its path and come back into the same path.This phenomena iscalled Reflection.If the angle of incidence is Ф1 and angle of refraction is Ф2 and the refractive indices of the twomedia are n1 and n2 respectively ,the refraction relation is given by n1.Sin Ф1 = n2.Sin Ф2 . This law is called Snell’s law.5
  6. 6. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comThe angle between the incident ray and the normal drawn to the surface is known as angle ofincidence Ф1 . The angle between the refracted ray and the normal drawn to the interface isknown as angle of refraction Ф2.The incident ray , the normal to the interface and the reflected ray all lie in the same plane,whichis perpendicular to the interface plane between the two materials.This plane is called plane ofIncidence.As the angle of incidence Ф1 in an optically denser medium increases ,the refracted angle Ф2approaches .Beyond this angle there is no refraction possible.Hence the light ray totallyinternally reflected into the same medium.The angle incidence for which the angle of refractionis is known as the Critical angle (C) .When the incidence angle is higher than critical angle.the total internal reflection condition is satisfied.In such situation the light is totally reflectedback into the same medium (Glass) with no light escaping (from the glass).Optical fiber modes and configurations : An optical fiber consists of a cylindrical core ofsilica glass surrounded by a solid dielectric cladding whose refractive index is lower than thatof the core.Suppose the refractive index of core is n1 and that of the clad is n2 , it is rememberedthat always n2 < n1.The cladding reduces the scattering losses and also provides mechanicalstrength to the fiber and also protects the core from absorbing surface contaminants with which itcould come into contact.6
  7. 7. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comIn standard optical fibers the core material is a pure silica glass(SiO2) and is surrounded by aglass cladding .Higher –loss plastic –core fibers are also in use.In addition to this most of thefibers are encapsulated in an elastic ,absorption resistant plastic material.This plastic materialadds further strength to the fiber and mechanically isolates from geometrical irregularities,distortions or roughness of adjacent surfaces.Otherwise these irregularities cause scatteringlossesSo, an optical fiber is a wave guide that works at optical frequencies .This wave guide will be incylindrical form and the light energy propagates parallel to its axis.The propagation of the lightwaves through the fibers is decided by the structural characteristics .These structuralcharacteristics of the fiber decides the information carrying capacity and the response of thewave guide to the external perturbations.The propagation of the Light along the fiber cable axis is described in terms of a set of guidedelectromagnetic waves called the modes of the wave guide. These guided modes are also termedas bound or trapped modes of the wave guide. Each mode is a pattern of electric and magneticfield distributions that is repeated along the fiber at equal intervals. It is found that only certaindiscrete number of modes are capable of propagating along the guide.Types of FibersBased on the variations in the material composition(refractive index) of the core there are twotypes of Fibers. They are (i) Step Index fiber and (ii) Graded Index fiber .A step index fiber is one in which the refractive index of the core is uniform throughout andundergoes an abrupt change (or step) at the cladding boundary.A graded index fiber is one in which the refractive index of the core varies as a function of theradial distance from the centre of the core.These two types are explained in the diagram below.7
  8. 8. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comBoth the Step and Grdaed index fibers are classified into single mode and multi mode fibers.Asingle mode fiber supports only one mode of propagation and where as multimode fiberssupports many large number of modes as shown in figure below.8
  9. 9. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comFrom the above it is clear that the multimode fibers have larger core radii as compared to monomode fibers. So,it is easy to launch optical power into the fiber and also facilitates the couplingof similar fibers .Another advantage is that light can be launched into a multimode fiber using alight emitting diode source. Whereas the mono-mode fibers are excited using Laser diodes. TheLEDs have longer life than Laser diodes. Hence the multimode fibers have more applications.The disadvantage of multimode fibers is that they suffer from intermodal dispersion .i.e thepulse that is launched into the fiber will be distributed overall the modes and each mode maytravel with different velocity and arrive at the fiber end at a slightly different times. This can bereduced by using a graded index profile in the fiber core.Step-Index FibersLet us consider a step index fiber such that θi is the angle of incidence and θr is the angle ofrefraction.So,from Snell’s law n0 Sinθi = n1 Sinθrwhere n1 and n0 are the refractive indices of the fiber core and air, respectively.Suppose θc is thecritical angle we can write that Sin θc = n2/n1where n2 is the cladding index, the ray experiences total internal reflection at the core–claddinginterface.From the diagram it is clear that θr = (π/2 – θc) .So, we can write that noSin θi = n1sin θr = n1Cos θc = (n12 –n2 2 )1/29
  10. 10. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comHere noSin θi is known as Numerical Aperture (NA) of the fiber. This represents the lightgathering capacity of the optical fiber. So, NA = n1(2Δ)1/2 Here Δ = (n1 – n2)/n1The Δ is called fractional index change at the core-cladding interface.This Δ should be as largeas possible in order to couple maximum amount of light into the fiber. But this type of fiber hasthe limitation with multipath dispersion.Graded-Index Fibers : The refractive index of the core in graded-index fibers is not constantbut decreases gradually from its maximum value n1 at the core center to its minimum value n2 atthe core–cladding interface. Most graded-index fibers are designed to have a nearly quadraticdecrease and are analyzed by using α-profile, given bywhere a is the core radius. The parameter α determines the index profile. A step-index profile isapproached in the limit of large α. A parabolic-index fiber corresponds to α = 2.Similar to the case of step-index fibers, the path is longer for more oblique rays. However, theray velocity changes along the path because of variations in the refractive index. Morespecifically, the ray propagating along the fiber axis takes the shortest path but travels mostslowly as the index is largest along this path. Oblique rays have a large part of their path in amedium of lower refractive index, where they travel faster. It is therefore possible for all rays to10
  11. 11. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comarrive together at the fiber output by a suitable choice of the refractive-index profile. Hence thegraded index fiber will have less multi path dispersion.Semiconductor Optical Sources :The major light sources used for fiber optic communication applications are hetero junctionstructured semiconductor Laser diodes (Injection Laser Diodes) and Light emittingDiodes(LEDs).A hetero junction consists of two adjoining semiconductor materials withdifferent band gap energies. These devices are suitable for fiber transmission systems ,becausethey sufficient output power for a wide range of applications. Their optical power output can bedirectly modulated by varying the input current to the device. Also they have high efficiencywith compatible dimensional characteristics with those of the optical fiber.The LEDs and Laser diodes consists of a pn junction constructed by using a direct band gap III-V semiconducting materials .When this junction is forward biased ,electrons and holes areinjected into the p and n regions respectively .These injected minority charge carriers canrecombine either radiatively ( where a photon of energy hv is emitted) or non-radiatively (therecombination energy is dissipated in the form of heat). So, this pn junction is known as theactive or recombination region.The difference between LEDs and Laser diodes is that the optical output from an LED isincoherent, where as the optical output from the Laser diode is coherent. The LED is based onspontaneous emission and the Laser diode is based on Stimulated emission.In a coherent source the optical energy is produced in an optical resonant cavity and in anincoherent LED source, no optical cavity exists for wavelength selectivity and the outputradiation has a broad spectral width.Also the incoherent optical energy is emitted into ahemisphere according to a cosine power distribution and hence has a large beam divergence.Ingeneral LEDs are used with multimode fibers ,because only the incoherent optical power from anLED can only be coupled into a multimode fiber .And the Laser diodes are used for single modefibers.The semiconductor material used for the active layer of an optical source must have direct bandgap.Because only direct band gap material has high radiative recombination .In a direct band gapsemiconductor electrons and holes can recombine directly across the band gap without the need11
  12. 12. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comof a third particle to conserve momentum.The single element semiconductors are not direct bandgap materials. But most of the binary and terinary semiconductors can act as direct band gapmaterials.(For example III-V materials like GaP,InP).For operation in the 800 – 900 nm spectralrange the ternary semiconductor material Ga1-x Alx As is used.LIGHT EMITTING DIODES(LEDs) : The LEDs are used as optical sources where the bitrates less than 100 to 200 Mb/s are required ,with multimode fiber coupled optical power in thetens of microwatts.The LEDs require less complex drive circuitry than laser diode ,since nothermal or optical stabilization circuits are needed and LEDs can be fabricated at low costs.LED Structure : The LEDs used in fiber optic communication applications should have highradiance output ,fast emission response time and a high quantum efficiency. The emissionresponse time is the time delay between the application of a current pulse and the onset ofoptical emission. To achieve a high radiance and high quantum efficiency, the LED structuremust provide the stimulated optical emission to the active region of the pn junction whereradiative recombination takes place. Carrier confinement is used to achieve a high quantumefficiency.To achieve carrier and optical confinement LED configuration like double –hetero structure orhetero junction which consists of two different alloy layers on each side of the active region isimplemented.The LED structures can be classified as surface-emitting or edge-emitting, depending onwhether the LED emits light from a surface that is parallel to the junction plane or from the edgeof the junction region. Both types can be made using either a p–n homojunction or aheterostructure design in which the active region is surrounded by p- and n-type cladding layers.The heterostructure design leads to superior performance, as it provides a control over theemissive area and eliminates internal absorption because of the transparent cladding layers.In thesurface emitting configuration a well is etched through the substance of the device ,into whichthe fiber is then cemented in order to accept the emitted light.The circular active area is normally50µm in diameter and up to 2.5µm thick. The emission pattern is essentially isotropic with a1200 half power beam width. The surface emitter configuration is shown in figure below.12
  13. 13. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comThe edge emitter configuration is shown in figure below.It consists of an active junction region,which is the source of incoherent light and two guiding layers. Both these guiding layers have arefractive index which is lower than that of the active region but higher than the index of thesurrounding material. This structure forms a wave guide channel that directs the optical radiationtoward the fiber core. To match the typical fiber core diameters (50 -100 um) the contact stripesfor the edge emitter are 50 to 70 um wide. The edge emitter configuration is shown below.13
  14. 14. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comThe lengths of the active region usually range from 100 to 150 um and the emission pattern ofthe edge emitter is more directional than that of the surface emitter. In the plane parallel to thejunction where there is no wave guide effect, the emitted beam is lambertian(varying as cosθ)with a half power width of θ║ = 1200 .In the plane perpendicular to the junction the half powerbeam width is made as small as 25 to 350 by proper choice of waveguide thickness.SEMICONDUCTOR LASER DIODES : Semiconductor lasers emit light through stimulatedemission. Due to the fundamental differences between spontaneous and stimulated emission,they are capable of emitting high powers (~ 100 mW), and also emit coherent light. A relativelynarrow angular spread of the output beam compared with LEDs permits high coupling efficiency(~50%) into single-mode fibers. A relatively narrow spectral width of emitted light allowsoperation at high bit rates (~10 Gb/s), since fiber dispersion becomes less critical for such anoptical source. Furthermore, semiconductor lasers can be modulated directly at high frequencies(upto 25 GHz) because of a short recombination time associated with stimulated emission. Mostfiber-optic communication systems use semiconductor lasers as an optical source because oftheir superior performance compared with LEDs.Laser Diode Structures :The semiconductor laser diode consists of a thin active layer (thickness ~ 0.1 μm) sandwichedbetween p-type and n-type cladding layers of another semiconductor with a higher band gap. Theresulting p–n hetero-junction is forward-biased through metallic contacts. Such lasers are calledbroad-area semiconductor lasers since the current is injected over a relatively broad areacovering the entire width of the laser chip (~ 100 μm).14
  15. 15. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comThe laser light is emitted from the two cleaved facets in the form of an elliptic spot ofdimensions ~ 1×100 μm2. In the direction perpendicular to the junction plane, the spot size is ~ 1μm because of the heterostructure design of the laser. Here the active layer acts as a planarwaveguide because its refractive index is larger than that of the surrounding cladding layers (Δn≈ 0.3). Similar to the case of optical fibers, it supports a certain number of modes, known as thetransverse modes. In practice, the active layer is thin enough (~ 0.1 μm) that the planarwaveguide supports a single transverse mode. However, there is no such light-confinementmechanism in the lateral direction parallel to the junction plane. Consequently, the lightgenerated spreads over the entire width of the laser.In strongly index-guided semiconductor lasers, the active region of dimensions ~0.1×1 μm2 isburied on all sides by several layers of lower refractive index. For this reason, such lasers arecalled buried heterostructure (BH) lasers.LASER DIODE MODES AND THRESHOLD CONDITIONS : In a Laser diode ,a Fabry-Perot resonator cavity is formed with the help of two flat ,partially reflecting mirrors which aredirected to each other . The use of the mirrors is to provide a strong optical feedback in thelongitudinal direction which compensates for optical losses in the cavity.This Laser cavity canhave many resonant frequencies for which the gain is sufficient to overcome the losses. The sidesof the cavity are formed by polishing properly the edges so that unwanted emissions can bereduced.The light radiation within the cavity of the Laser diode sets up a pattern of electric and magneticfield lines called modes of the cavity.These modes are classified as two independent sets calledTransverse electric(TE) and Transverse Magnetic (TM) modes. Each of these modes can bedescribed in terms of longitudinal,lateral and transverse electromagnetic fields along the majoraxes of the cavity.The longitudinal modes are related to the length L of the cavity and anddetermine the principal structure of the frequency spectrum of the emitted optical radiation.Asthe length L is very larger than the Lasing wavelength(1um) many longitudinal modes can beformed.The lateral modes lie in the plane of the pn junction. These modes depend on the width of thecavity and side wall. It determines the shape of the lateral profile of the Laser beam. Thetransverse modes are associated with the electromagnetic field and beam profile in the direction15
  16. 16. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comperpendicular to the plane of the pn junction. These modes are very important as they largelydetermine the Laser characteristics like radiation pattern and the threshold current density.To determine the Lasing condition let us consider the EM wave propagating in the longitudinaldirection E(z, t) = I(z) e j(wt-βz)Where I(z) is the optical field intensity and w is the optical frequency in radians and β.is thepropagation constant. The lasing is the condition at which light amplification is possible in theLaser diode. The basic requirement is the population inversion.The optical amplification of theselected modes is provided by the feedback mechanism of the optical cavity.In the repeatedpasses between the two partially reflecting parallel mirrors ,a portion of the radiation associatedwith those modes having the highest optical gain coefficient is retained and further amplifiedduring each oscillation in the cavity.Lasing occurs when the gain of one or several guidedmodes is sufficient to exceed the optical loss during one round trip through the cavity .At thelasing threshold a steady state oscillation takes place and the magnitude and the phase of thereflected wave must be equal to the original wave. The condition for amplitude is I(2L) = I(0) and For phase e –jβL = 1The mode which satisfies the above condition reaches the threshold first.At one set of thiscondition all additional energy introduced into the Laser should enhance the growth of thisparticular mode.Fiber to Fiber joints :During the installation of fiber optic communication system ,it is alwaysimportant to interconnect the fibers with minimum losses.these interconnections or joints occurat the optical source ,at the photo detector and at intermediate points within a cable.There are twotypes of joints ,namely Splice and connector. The permanent bond between two fibers is calledsplice and demountable joint is called connector. The type of technique used for joining twofibers depends on whether a permanent bond or an easily demountable connection is required.The losses due to the joints depend on the parameters like input power distribution to the point,the length of the fiber between the optical source and the joint,the geometrical and wave guidecharacteristics of the two fiber ends at the joint and the fiber end face qualities.The optical power that can be coupled from one fiber to another is limited by the number ofmodes that can be transmitted in each fiber. For example, if a fiber in which 500 modes can16
  17. 17. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.compropagate is connected to a fiber in which only 400 modes can propagate ,then at most 80% ofthe optical power from the first fiber can be coupled to the second fiber.Mechanical alignment is a serious problem while joining two fibers because of theirmicroscopic size.Radiation losses occur due to the misalignment as the radiation cone of theemitting fiber does not match with the acceptance of the receiving fiber.The magnitude of theradiation loss depends on the degree of misalignment.There are three types of misalignments.Lateral misalignment , longitudinal misalignment and angular misalignment.Longitudinal separation occurs when the fibers have the same axis but have a gap between theirend faces.Angular misalignment occurs when the two axes form an angle so that the fiber endfaces are no longer parallel.Axial displacement (alos called lateral displacement) occurs whenthe axes of the two fibers are separated by a small distance .The most common misalignment thatoccurs in practice is axial displacement and it also causes large power loss.In addition to the mechanical misalignments ,differences in geometrical and waveguidecharacteristics of any two fibers being joined can also show effect on fiber couplig.Theseinclude variations in core diameter ,core area ellipticity ,Numerical aperture ,refractive indexprofile and core-cladding concentricity of each fiber.Fiber splicing Techniques: There are various fiber splicing techniques in use .The mostcommonly used are Fusion splice , V-groove ,tube mechanical splice ,elastic –tube splice andthe rotary splice.Fusion splices are made by thermally bonding two fiber ends together.In this method ,first thetwo fiber ends are pre-aligned and butted together .This is done under a microscope withmicromanupulators.The butt joint is then heated with an electric arc or laser pulse so that thefiber ends are momentarily melted and hence bounded permanently.This technique produce verylow splice losses of less than 0.06dB.In the V-groove splice technique ,the two fiber ends are first butted together in a V-shapedgroove and then bonded together with an adhesive or held in place by means of a cover plate.theV-shaped channel could be either a grooved silicon , plastic , ceramic or metal substrate.Thesplice loss in this method mainly depends on the fiber size and the eccentricity of the corerelative to the center of the core.17
  18. 18. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comThe elastic tube splice is a unique device that automatically performs lateral,longitudinal andangular alignment.It splices multimode fibers with losses in the same range as fusion splices,withre;latively less complexity and skill.This splice mechanism basically consists of an elastic tubewith a central hole .The diameter of the hole is slightly less than that of the fiber to bespliced.When the fiber is inserted ,it expands the hole diameter so that the elastic material exertsa symmetrical force on the fiber.This symmetric force allows an accurate and automaticalignment of the axes of the two joined fibers.A wide range of fiber diameters can be insertedinto the elastic tube.So,the fibers to be spliced need not have to be equal in diameter,becauseeach fiber moves into position independently reltive to the tube axis.OPTICAL FIBER CONNECTORS : Connectors are very important to connect two fiberswithout loss of the signal.There are different types of connectors available.They are screw-on,bayonet-mount and push-pull configurations.These include both single channel andmultichannel assemblies ,cable-to cable and cable to circuit card connections . The basiccoupling mechanism used in these connectors will be either the butt-joint or expanded beamtypes.But most of the connectors today are butt-joint type .These connectors employ ametal,ceramic or molded-plastic ferrule for each fiber and precision sleeve into which the ferrulefits.A good connector must have the following requirements.1.Low coupling losses:The connector assembly must maintain correct alignment so that losseswill be minimum.2.Interchangebility :Connectors from one manufacturer must be compatible with othermanufacturers.18
  19. 19. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.com3.Ease of Assembly: The installation of connector must be simple and it should not give troubleto the technitian.4.Low Environmental sensitivity: The connectors performance should not be affected byexrenal conditions like temperature ,dust and moisture etc.5.Low cost and reliable construction: The connector must be always reliable and must not bevery expensive.6.Ease of operation: The connection and unmounting must be simple and must be operated withbare hands with ease.PHOTO DETECTORS - PRINCIPLE:A photo detector senses the optical power falling upon it and converts this power into suitableelectric current. The photo detector must have the characteristics of high response or sensitivityto the incident radiation and sufficient bandwidth to handle desired data rate.The photo detectorshould also be insensitive to external temperature variations and other conditions. There arevarious types of photo detectors like photo multipliers, photo transistors and photo diodes ,pyroelectric detectors etc.But all these detectors do not meet the fiber optic communicationrequirements. Only photo diodes will be alone very useful for such applicationsThe photo detectors are used as optical receivers .The role of an optical reciver in a fiber opticcommunication system is to convert the optical signal back into electrical form and recover thedata transmitted through the light wave system. Its main component is a photo-detector thatconverts light into electricity by using the photoelectric effect. The requirements for a photo-detector are high sensitivity, fast response, low noise, low cost, and high reliability. Its sizeshould be compatible with the fiber-core size. These characteristics are best met by photo-detectors made of semiconductor materials.Principle :A reverse-biased p–n junction consists of a depletion region, that is essentially devoid of freecharge carriers and where a large built-in electric field opposes flow of electrons from the n-sideto the p-side (and of holes from p to n).When such a p–n junction is illuminated with light on oneside, say the p-side , electron–hole pairs are created due to absorption. Because of the largebuilt-in electric field, electrons and holes generated inside the depletion region accelerate inopposite directions and drift to the n- and p-sides, respectively. The resulting flow of current is19
  20. 20. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comproportional to the incident optical power. Thus a reverse-biased p–n junction acts as a photo-detector and is referred to as the p–n photodiode.The electron–hole pairs generated inside the depletion region experience a large electric field anddrift rapidly toward the p- or n-side, depending on the electric charge . The resulting current flowconstitutes the photodiode response to the incident optical power. The responsivity of aphotodiode is quite high (R~ 1 A/W) because of a high quantum efficiency.p-i-n PHOTO DETECTOR :The p-i-n photo diode is consists of p and n regions separated by a very lightly n-doped intrinsicregion.It is a very widely used semiconductor photo detector used in fiber optic receivers. Thetwo important characteristics of the PIN diode are the quantum efficiency and Responsivity.ThePIN detector circuit is shown in the diagram below. In normal operation ,a very large reversebias voltage is applied across the diode such that the intrinsic region is fully depleted of chargecarriers.i.e the intrinsic n and p carrier concentrations are negligibly small in comparison withthe impurity concentrations in this region.When a photon of energy greater than or equal to the band –gap energy of the semiconductorincidents on this,it will give up its energy and excite an electron from the valence band to theconduction band.this process generates free electron –hole pairs which are known as photocarriers. The design of the photo detector is such that these carriers are generated mainly in thedepletion region where most of the incident light is absorbed. The high electric field present in20
  21. 21. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comthe depletion region causes the carriers to separate and move across the reverse biasjunction.This gives rise to a current flow in the external circuit. This current is known as photocurrent.As the charge carriers flow through the material,some electron-hole pairs will recombine andhence disappear.On average the charge carriers move a distance Ln or Lp for electrons or holesrespectively. This distance is known as diffusion length.The time taken by a hole or electron torecombine is known as carrier life time and is denoted by tn and tp .The diffusion lengths andcarrier life times are related by Ln = (Dn Tn)1/2 and Lp = (DpTp)1/2where Dn and Dp are the electron and hole diffusion coefficients.In the photo diode operation it is clear that the optical absorption coefficient strongly dependson the wavelength for many semiconductor materials. So, a particular semiconductor materialcan only be used over a limited wave length range. This is the limitation in the photo diodeoperation.Also there is a limitation in the responsivity R of the p-i-n diode.Avalanche photodiodes - Structure of In GaAs APDs : To overcome the limitations of p-i-n diode’sresponsivity and to achieve larger responsivities this Avalanche Photo diode is used. This diodeconsists of an additional layer in which secondary electron–hole pairs are generated throughimpact ionization. So, the APDs multiply the photocurrent internally before it enters theamplifier circuitry.This carrier multiplication mechanism is called impact ionization. The newly21
  22. 22. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comcreated carriers also accelerated by the electric field and gain enough energy to cause furtherimpact ionization. This phenomena is called avalanche effect..An Avalanche Photodiode (APD) provides higher sensitivity than a standard photodiode. It isideal for extreme low-level light (LLL) detection and photon counting. Fabricated using Siliconor InGaAs materials, these devices provide detectivity from 400 nm - 1100 nm.Under reverse bias, a high electric field exists in the p-type layer sandwiched between i-type andn+- type layers. This layer is referred to as the multiplication layer, since secondary electron–hole pairs are generated here through impact ionization. The i-layer still acts as the depletionregion in which most of the incident photons are absorbed and primary electron–hole pairs aregenerated. Electrons generated in the i-region cross the gain region and generate secondaryelectron–hole pairs responsible for the current gain.The use of APDs instead of PIN photo detectors will result in improved sensitivity in manyapplications. In general, APDs are useful in applications where the noise of the amplifier is highi.e., much higher than the noise in the PIN photo detector. Thus, although an APD is alwaysnoisier than the equivalent PIN, improved signal-to-noise can be achieved in the system for APDgains up to the point where the noise of the APD is comparable to that of the amplifier. Structure of In GaAs APDs :22
  23. 23. Dr.Y.Narasimha Murthy ,Ph.D yayavaram@yahoo.comFor light wave systems operating in the wavelength range 1.3–1.6 μm, Ge or InGaAs APDs mustbe used. The improvement in sensitivity for such APDs is limited to a factor below 10 because of a relatively low APD gain (M ~ 10) that must be used to reduce the noise . The performance of InGaAs APDs can be improved through suitable design modifications to the basic APD structure. The structure of the In Ga As avalanche Photo Diode is shown in the figure below.The main reason for a relatively poor performance of InGaAs APDs is related to the comparablenumerical values of the impact-ionization coefficients αe and αh . As a result, the band width isconsiderably reduced, and the noise is also relatively high . Also, because of a relatively narrowband gap, InGaAs undergoes tunneling breakdown at electric fields of about 1×105 V/cm, avalue that is below the threshold for avalanche multiplication. This problem can be solved inhetero-structure APDs by using an InP layer for the gain region because quite high electric fields(> 5×10 5 V/cm) can exist in InP without tunneling breakdown. --------------xxxxxxxxxxxxxxx----------------The above class notes would have not been possible with out the help of the followingreferences. I owe very much to the following people.References: 1. Optical fiber communication-G.Keiser. 2. Fiber-Optic Communication Systems – Govind .p Agarwal23

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