UNIT OVERVIEW OF OPTICAL p> FIBER COMMUNICATION SIA GROUP PART-A SHORT QUESTIONS WITH SOLUTIONS Q1. Define optical fiber communication and list its elements. Ans: The process af exchanging information from known as optial fiber eominuieation 1 destination by transmitting light pulses over an optical fiber is “The various clements of optical fiber communication areas follows, Eloetricaltransiittr Optical soure Optical fiber eable Optical detector Electrical receiver @2,__List three generations of optical fibers, Ans: The optical fiber generations were mainly divided based on operating wavelength as show in below table: ration Year| Winele T] Fist generation gaiem | 1975 ¥50- nm 2 eration system |] 1979-84 | 1310 aim ‘Thitd geucration system _| 1985-95 | 1550 ama Table G3.” List different optical spectral bands. Ans: The various optical spectral hands a 1, Original (Q) baad 1260 naa to 1360 nen 2 Extended (F) bund — 1360 rm to 1460 3. Show 4S) bana — 1460 1m to 1534 am 4. Conventional (C) band — 1S30.am 10 1565 nm, 5. Long L) band — 1365 am 1625 an 6___Ules-omg (U band — 1625 nm 1675 rm. G4. Draw the structure of optical fiber. Ans: The structural arranzesoent of fiber optic cable is ilustratd in figure Figure SPECTROM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS Si GROUP > Scanned with CamScannerAns: the necessity of cladding In optical Model pers, a8) Cladding is an external layer that covers the fiber and protects from extemal disturbances. 4 26. Ans: Iris necessary in optical fers to, Reduce loss of light Reduce scattering light Provide protection forthe fhe rom surface confinement Add mechanical siength to fer What is Snell's law? (Mode! Papers, a} | Nov/Dec. (R13), ON9)) ‘The Snell's law states the behaviour of light ray, when it strikes the interface of two mecia having different refractive indexes According to Snell's lw, i ‘Where. nae reactive indexes of iyo media's andy, 21 a are the angle oFncldenee and angle of refvaction spectively. rom equation (1), From the above expression, itis elewr that the ratio of relative index of two media is fnverssly proportional to the sine of refiactive and incicent angles. 7. Define total internal reflection and mention conditions for the same. od! Paper, 20) Whe the angle of incidence is sreater than the ertical angle, the light rays reflect into the denser medium. This phe Condition a8. total internal recetion, for Total Internal Reflection “The ight ray must pas ly fom denser a rarer median. The angle of incidence (0) shove have greater valve 8s ‘compared to eritieal angle (0) Define the terms, Acceptance Angle 2. Numerical aperture Phase velocity 4. Group velocity. ‘Ans: OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] Accept Angle The maximum angleat which the ‘on fiber to propagate down the fiber is known as Acceptance angle”. The maximum aeeeptance angle ‘ea he derived from the numerical aperta [Namevial apertus, NA ~ sin 0,1, A o,-sn’ (av) B= sin (Way Namerical Aperture The numerical aperture is figure of merit, which isused toexplain the angles associated wit light props fiberopties. The sinc of halF-angke of acceptauce angles ferredas ‘numerical aperture’ Usually for optical lens, nurnrical aperture ranges fom 0.3 t0 1.3 NA=are sin yn? 03 Where, 1, Refiactive inde of eore 1a, Refinctive index of cladding Phase Velocity The speed at which energy in a particular mode travels along the fiber with constant phase is known as phase velocity. Mathematically itis expressed as, v5 ~ Group Velocity The speed at which energy in a p travels along the fiber is known as grup velocity “Mathematieally it is expressed as fo ~ 3B ‘Astep index fiber has a normalized frequency V Ans: Nate: += 26.6 at 1300 nm wavelength. Ifthe core radius Is 25 nm, find the numerical aperture, NowiDec17, (R19). 16) Jn the fon question, 25 jum misprint as 25 nn, Given ta Fora sip index Ger, Nonmatized frequeney. = 26.6 Wavelength, = 1300 nm = 1300 10m Core radius, S10 Numerical aperture, N=? ‘The relation between Pnuner and numer aperture is expressed as, oy wo Look for the SIA GROUP Loco > on the TITLE COVER before you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 1.3 (On substituting comesponding values in equation (I), we denedSatir" 266- na 1300 «107 266~ 120.83 NA 266, 2 Mae -029 D088 [= Numerical aperture, N= 0220] 10. Give the relation between numerical aperture of skew rays and meridional rays, Model Papers ota) for) What are skew rays? ov ibae 6 (9) 040) ‘Refer Only Skew rays) Ans: Skew Rays The Fays that are wot confined and follow te angular heaical pal alo ie ibe are huown as show rays The Numerical Apsiture (1) of skew rays is given by Na =n, sin0,, cose Whe n,~ Uy Refractive inde of air 8 Masimum input angle or acceptance single fr skew rays 1r Angle between the eore radius und projection of skew mays in we dimensions Mevidional Rays The rays that ane confined to the plane containing eore axis are known as meridional rays, The Numerical Aperture (NA) oF meridional tuys is wiven hy. Na~n, sn, Where, 9, 1s Refiactve index of air 0 Accepiauce angle for meridional rays, O11. Differentiate between skew rays and meridional rays. ness May Jue, (08),a18) the following are the differences hetween skew rays and meridignal rays Show Rays Veridional Rave To] Skow rays travel along dhe fiber | Moridional ray travel trough fiber via wal intemal reflections 2. | His contined wa single plane 2. | tris confined to the meridian planes of an optical fiber 3. | tis very dificult to truck a skew ray. 3, | tis easy to trick a meridional ray 12. Classify optical fibers based on the refractive index profile Ans: The clasilication of optical fibers hase om the refractive inex ofthe wore is, Step-indes Fiber Instep-indes optical fiber, the refractive inde ofthe core is uniform and undergo step change at he boundary of ead 2 Graded lea Fiber 1 ido, the relative index ofthe eore varies with the radial distance fiom the center o fib Tn graded-indes opt SPECTRUM @LLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP {J Scanned with CamScanner1 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] G13. Define Single Mode Fiber (SMF) and Multimode Fiber (MMF). Ans: Single Mode Fiber (SME) “The single mode fiber is defined as an eptical ber, whiel is de red For the transmission of single ray of Tight as caver distances ‘This is used forthe transmission of signals over ln Multimode Fiber (MMP) The multimode fiber is an optical flber which is designed for the transmission of muliple rays of Tight and is used Fr signal transmisscn over short distances such as within a building oe campus, The mahimad inks eam suppor the data rates of LW Mbps to L0.Glyps. 214, List the advantages of SMFs over MIMFS. Ans: the advantages ofsinele mode fers (SM) over multimode ibers (MES) are Lov fibration 3. Simple parameter dependence 4. Low dispersion than in MME S. High bandwith 6.__ SMF cam integrate ensly with other devies to Function in inte 15. White short notes on the terms, (i) Cutoff wave length (i) Mode field diameter. Ans: 6) Cater Wavelength The wavelength above which a particular Ober becomes single-moded is known as cutoff wavelenath, Fora single-mode fibers, cutoff wa Jength is given by, F, —Curoft normalized frequency’ oF V-number @ Radivs of core | Retiative index of core A- Relative index ditferenee Using the relation cut-off wavelength cam also be expressed as, ra he v For step inde fiber, F- 2.408, b 2405 Look for the SUA GROUP Loco {J} on the TITLE COVER bofore you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 1s Moacr Mode Field Diameter (MPD) isa fundamental parameter used for characterizing the properties of single-mode fiber It ‘canbe determined from the mode-fild distribution 0 fundamental mode in the bot Hs defined as the diameter of the actual guide! optical power distibution (which is wesely one sierou ewer tau the diameter of liber core) ‘GAG. Why do we prefer step index single mode fiber for long distance communication? Ans: (Mode Papers, 0) | NowiD4e-17, (R19, aN) In stop inde single mode fihers last ght with lange handwichh is the source of eommyinicaton at dhe transmitter interface, I allows lone distance transmission potentially near about 70 kn a high specs. Moreover. the transmission las nd the dispersion'degradation of te signal is negligible Thuis the step index singke mode fibers are preferred for long distance communication, GAT. What are the modes supported by an optical fiber? Ans: Mayne, 08,0 Optical fibers support three types of made propagation, They ae Guided modes Raiation modes Leaky mode (Q18. What is mode coupling? Ans: (Mode Paper ara) | NoriDee12. (20), ae) The process of coupling ihe energy fiom one mode to other is known as mode coupling. Due to the mode coupling. average propagation delay becomes less and intermodal distortion reduces, for lng fiber ength's the effet of mode coupling on pulse istortion is significant Q19. State the advantages of light wave communication system over EM wave system. Ans: The advanta sof light wave comnnnication system over EM wave system are, 1. Low transmission los. 2 Wide bandwidth, 3. Absence of electromagnetic interference, 4 Opti S. Less erosstalh, since the upticaleuble is not w umd fiber arco small size Hight in weigh. sive medium, 6. Optical fibers in light wave communication do not radiate energy resulting in privacy and security of signals Require less ost to design a light wave system than electromagnetic wave system. G20. Mention the applications of optical fibers. Ans: The applications of optical fibers ar, 1. Opical fibers can he ase asa mdm in (clecommunication und carmpuler networking. Due to Hesibilily 2, These cut be used in emote sensing 3. ‘These can he used to transmit power using a photo voltaic cell to convert light into electricity. 4, These cu be used as lilt pales in medlcal applications, 5. Theses used to oute the sunlight from root tothe other pats ofthe building 6. These are also used in imaging opties. Q21, What are glass fibers and give their classification. Ans: Glass smixtue of selenide, sulfides and mal exides. It can be olassitied iat, 1. Halide Gilass Fibers 2. Active Glass Fibors 3. Chhnlgenide Glass Fibers, [SPECTROM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP > Scanned with CamScannerESSAY QUESTIONS WITH SOLUTIONS 41.1 HISTORICAL DEVELOPMENT 22. Briefly explain historical development of optical fiber communication. Ans: Historically, development of optical fbr communication took place in subsequent phases withthe advent in the enhanee- ‘ment of technology. The various stages that heen abserved during the development oF optical fiber communication ae, 6 1 Frequency In early days, optical communication was achieved by employing visible optical carrier waves light for transfer of inirmation. Investigations held in tne begining of 20° century reveal that oplieal eormmusieatin is restrivted only to jor reasons bend sul estiti mobile i. to the communication links of low-eapweity. The se (i) Lack of suitable light sources Gi) Line-oF sigh ransmission oF Bight in camosphere Gil) Severe impact of atmospheric disturbances such os snows. dust, fag, rin, ee Use of lower frequeney and Io pact of wtmospherie disturhanee, Ths iniurn provides suitable carriers for tuasfee of information, The amount of information ansferred by these electromagnetic Waves depends on the bandwich (or extent of frequency) provided for the maduloted carior Increase inthe egueney of eartet signal incase the bundwiah of teasinssion whieh ita inereasss the capacity of information inthe communisation system. Duc to this reason, nigh frequencies such as VILF and UMP are used for developing radio communication. This inturn paved the way forthe introduction af even higher frequency micronvave and millimetor wave transmission, Thus, the ase of optical fequcneies for comamunication provides inereased hunwidch (ic, by um order of 4") ‘wlin compared to microwave comtnieation, These ish frequency cicviers also offer inapeoved system performance because fot the conccrtzaion of available power within the lransmiticd clectramagnelie was tie waves (such as radio and microwave) ean reduce the re below represents the relative Fequencies aad wav ele ws oF the electromagnetic spectra, Figure: Elotromagnotie Spectrum Indicating Be 31 of Optical Fibor Communic “The invention oP haer in carly 19606, triggered a renewed interest in optical communication, The additional bens of laser sucks powesful coherent light, modulation at hizh frequeney and low beau divergence provided the improved fice space optical transmission, Despite its advantages, lasers restricted the communication 9 shore-distance applications only Some ofthe important applications include, (9) Linking of television camera to a base vehicle (ii) Linking of data hetween building separate at afew hundred meters (iii) Communication berween satellites in outer space 3. Diclectrie Waveguides 1h, 1966, Kao and Hlockham simultoneously introduced select of sigaals by atmosplirs. Use ofthese optical sompoacats reduco ic waveguides or glass optical fibers to avoid the degralation 1 signal oss 1 below $ dBkm within 10 yea Look for the SIA GROUP LOGO Gp on the TITLE COVER before you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 17 X__ Semiconductor Optical Sources and Detectors As the optical fisquencies have very small wavelengths, a a camponents. A stccesstl optical her system is implemented in size with optical fibers, namely w technology has been adopted to develop the optical hy designing and tahricating devices which are commpatihle (3) Semieanductor optical sures injection lasers andl LEDS Gi) Detectors - pholudindes and phot-transistors, Performance of apical fibers can be improve! and extended over a specific region by employing different semicondoctar alloys. 5. Photonie Bandgaps Phoomie ban 1 are developed in erystels or oplieal fiers wo propagate light darough thei. For instance, « photonic ir holes. A single mode of ied into such holey shout the communication process Ils, these subsequent develapment in the improventent of optical fiber communication helped in ataining successfil transmission of message signals througout the system, 1.2 THE GENERAL SYSTEM, ADVANTAGES OF OPTICAL FIBER COMMUNICATIONS 023. Draw the block diagram of an optical transmission link and explain the function of each element. crystal her has microstruenured regular lasice cavainin Fibers aad thas transi one posse tho Ans: (Optical fiber communieation isthe process of exchnging information from souree to destination hy transiting light pulses over an opical fiber. In other words, an eptica fiber communication system employs light wave technology to transmit ‘ala ovr Ger cahle, hy converting eletronic signals o light pulses, Figure belo ilusiates the block disgramn a an pti ‘Aber communication syste, Optical nr Freie |_ [Opie i Taal Tlecnead detestor [>| receiver Destination Figure 1 can be observed from figure that aa optical fiber communication system besides “information source" aad ‘Destination’, consists of \coteieal transmitter “The message signals ubiained liom the information source are coaverted into electrical signals, Optieal Source Fiber optical conumunication systems employ optical sources for eletical-optical conversion. a oer wards, electieal sigma {ciher analog or digital) sxe convert into eosesponuting optical signals using apical sources. The most oman sources of optical signals ar light esting ods (LEDs) sob state Taser diode and semiconductor sr Optical Fiber Cable (Optical fiber cable ats a a transiting medion between the transmitter and receiver. Ihe eable has one or more glass fibers which provide special protection For optical signals optical detector Optical detcctors anc used for detecting optical signals and then converting these signals into cetrical signals. This ‘demodulation of optical signals cau be carried out using ploto diodes placto-transsiors or photoconductors. Elctrieal receiver “The electrical eosiver ascepts the clectrial signals fram the optical detector an! transmits it towards the destination end ‘SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP Scanned with CamScanner18 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] Q24. Explain the function of each block with a help of neat block diagram of adigital optical fiber communication system. Ans: Digital Optical Fiber Link ‘A divi Optical fiber link employs (techniques for Wansmiting and receiving dat Figure below illistates the block diagram of a digital optical fiber link. ‘information ‘Laser “ " a4 Tnpliier ib Lf Devoder nice utp aro Figure: Black Diagram of Digital Optical Fiber Link Itcan be observed from figure that digital optical iher link consists of Digital Information Source A digital intimation sonee produces messsge bits in eligi fom atthe int of the hee Fink Encoder The digital input is suitably encoded atthe encoder for optical uausmission. Laser Drive Cireuit se. laser Drive cirewits are generally employed for converting information signa imo suitable modulated signal. n this ea drive circuit dircetly movllates the igensiy ofthe laser with the ence digital inpat, Thus, digital optical signal ispredaced. Laser “The digital optical signal obtained fiom the drive circuit is convert int Light signal using a semiconductor laser source Cable An optical her eable acts a8 a transmitting medium or optical channel berween teansentter and receiver. Avalanche Photo Diode (APD) Ahi into ete ly sensitive sermiconductor device namely Avalanche Photo Dinde (APD) isused to conver the received light signals eal signals Amplifier and Equalizer jn_and minimize the Inter Symbol Interference (ISI) respectively, This minimization in noise bandwidth reduces the possibility of distortion inthe received signal Amplifier ane equalizer (or filter) section is emplayed 19 provide Decoder The final Block othe digital optical fiber Fink decodes the received signal in order to obtain the original digital message signal ‘Thus, a igital output is prociced a the receives terminal, Q25. Describe the basic block diagram of a optical communication system and explain how it conventional co-axial cable communication system. iffers from Ans: (Wode| Papers Q2 | Nov.Oae-17, (R13, G2) [Block Diagram of Optical Communciation System For answer refer Unit Q23, Look for the SIA GROUP Loco JB on the TITLE COVER before you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 19 Ditterenees herwvew Optical Fibers nnd Conventional Co-anial cables Op Conusial Cable L | Less weight 1 | teavier chan optical bers. 2. | Immune RF intesforomecic.,nocatcrmal | 2. | Enteral shiclding required for shielding is required. reducing RF interference. 3. | Low loss of 0.2 alvkm 4. | A considerable loss of Sdbskm, 4. | Elcccally isolate 4. | Teispnoe to electrical hazards, S. | Large bandwith ofubout LO" to LOY Hz, | S. | Bordwidih of $00 MtLz are obvained, 6. | Large spacing between repeater, 6. | Spacing distance between repeters is ess 3. | Misher dara rates oFabour tea bits 1. | Data ares of meza bits per second. per second, 8. | Secured signal 8. | Signal can be tapped easily 9. | Noerass tlk 9] Steans are whe devived esterally at the roveiver to roduve eno tlk w.] expe 10.| Comparatively less expensive. Q26. List out the advantages and disadvantages of optical fiber communications. Ans: Advantages of Optical Fiber Communication 1. The carrier fraqusney of optical Bhors is the ninge of 1! to 10! Hz, Duc to this frequency ns lange potential undid for transmission, Ln etter worl optical fibers have (Optical fibers provide high degree of signal security. This is primarily due tote fact dha dhe light (or energy) present ‘within the fbers does not radiate significantly and na inter can derest te message signal being transite oop se information - carry 3. Optieal fibers have smal size and low weight, The diameter ofthese Gets is comparable tw the diameter of haan hair Because of its small dimensions, protective coatings around fibers do not considerably increase size and weight AL Optical fibers are more immune to interference and cross tlk. As optical fibers are usually made up of tielectric materials it tects message signals tom Radio-Frequency Interference RFI electromagnetic Imerference FMI) and Fleecromasnetic puis (EMP) Thus, even an electrically noisy environment aloes not eet the inoemation being trast in dhe fiber ale, Moreover, optiel bers are als immiane to strikes and erosstalk 5. Fabrication of optical fibers from glass or plastic polymers offers an advantage of electrical isolation, which helps in ‘avoiiing difficulties scl as earth Koop und inerfee problems, 6 Optical bers are less eapensive as ihas low manufacturing, hauling and maintenance cos 7. Transmission of message signal through optical fers suiTes very low allesation, Thus, optical fiber communication ‘exhibits To teas ls, 8, Dus tthe low-loss property af optical fiber cables, the eommunicution process minimizes the ncod fir intermediate epeairs and line amplifies. Thus, dhe systema ell is impeoved because ofthe less aumnber of optical repeaters and amplifier. Moreover, this also provides ease in maintenance of optical fiber comwinication system, 9, Optical fiber cables are rigid crmpuact and Rexible in nature. These fihers are manufactured with very high tensile strengths ng thus offer easy transportation. installation and storage facility Disadvantages of Optical Fiber Communica 1, Atteruation offered by the optical fibers depends upon the material by whieh itis made Complex electronic transmitter and receiver. ley is eoguited The coupling of optical fibers is ditieul. 4. Skilled labours are required to maintain the optical Fler eommasieation Scparated power supply i required for electron repeaters a flere staes. ‘SPECTROM ALLAN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GRour QD Scanned with CamScanner1.10 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] (Q27. Compare the advantages and disadvantages of guided optical communication lines with that of microwave systems. Ans: ‘The advantages of optical communivation system over microwave system ane mentioned below cms Microwave Systems 1] ses lass optical fibers or plastic optical ers for [ 1. | Uses eovanial cable or microwave wanexuldes for transmission transmission, 2. | Low weight, hence large wansmission distance or | 2. | Heavier than optical fibers samme weight of microwave link 3. | Lanse bandwidh orange 10"to 10" Hz 3. | Bandwidth is lesser in she range of HIS t0 LOH 4. | Electrically isolated, hence no shielding is required. | 4. | Prone to electrical disturbanees and henoe shiclding for roducing RP interference. 5. | Low loss of 0.2 dBvkm. considerable loss of § Bik, 6, | Lange spacing between repeaters about Hin 3040 km. | 6. | Spacing distance between repeaters is hess, is suitable only tor shoct distance if waveguides are used 7. | Because large bandwidth, higher data rate ofthe — | 7. | Data rates of mega bits per second can be obiained bits per second 8, } Message security is obtained, 8. | Signal ean be tapped easily 9, | No cross talk, hence many fiber communication 9, | 1eshielding isnot dane properly, cross talk is introduced channels can be packed inside one single cable, The disadvantages of optical communication system over microwave system are mentioned below: Opal Communication Systems Microwave Systeme 1, | Expensive ansmntier and receiver 1, ] Simple and less expensive tansmitier and receiver 2. | Diicut coupling 2.| Easy coupling Power transmission depends upon the quanta — | 3. | Output power is inetly coupled tothe transmission clficieney of light source (LED or LASER). 4 | Unable w excite the yorminal device directly 4. | Able io ope Q28. With respect to bandwidth explain how optical fibers are advantageous over coaxial cables. Ans: ‘The optical carrier frequency in the range of 10" to 10! Fiz (In genera. isthe hear infrared around 10" Tz 10 10" G2) obtains far greater potential bandwidth compared to that of metallic cable systems (.e.. coaxial cable bandwidth upto 500 MEIz) or even millimeter wave radio systems (ie.. 700 MIZ2), The available bandwidth (othe fiber system is not completely used but modulation without interfering the electronic repeaters is possible al several siga hertz over s hundred oF kins and hundreds of mega her over throe hundred kins. Thos it eam be inferred that the information carrying eapacity of the optical fiber systems is most superior than the best copper cable systems, The transmission distance inthe wideband! ancl coaxial systems, is confined only to a few kms at handswicihs lover one hundred mega hertz because of the losses comprised The gal, The enhancement in the uilization of bundwid is obtained by the wavelength division meter optical frequency that is an extension of the fer bandwidth is limited by the w ‘optical particu: larly with dense packing of optical wavelengths (or fine frequeney spacing) In wavelength division multiptexin optical signals are transmitted cach at different centre wavelengths, in parallel, on the same Fibers. This method provides huge potentia! for the information carrying capacity to a optical fiber whase magnitude is very high that ean be achieved by using copper cables or wideband radio systems, Look for the SIA GROUP Loco {J} on the TITLE COVER beiore you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) Nevnalzad Apt (V) OrPowerih) wa. Ans: 10 om) gf) ° Plog! BW Frequony {optical andi a A What are the applications of optical fi ‘communication? The applications of optical fher cammunication ar “The fibon bbenvceu remote stations, are use for point-to-point commanication Due to farge infor con carrying capacity, the fibers are used asa channel in local area networks, lane etic, hospitals, oels, baaks and their branches, Fibercopties ate used in contral and instrumentation fields ()_- Measurement of temperate, presse, elvettie field electron density ee i) Detect mctear radiation in medical diagnosis (Such as endoseopes) ware extensively used Electrica isolation propesty of optical fibers ake suitable for communication in electrically hazardous environments suc as short eres und abrasions Optica fibers are extensively use in miliary applications and defence services for data transmission because of their high degree of signal security Optical Liber cables also find applications in exe television, ships submarine cables and space vehicles. The other engineering apications of optical ber include soeurity and alum systems, industrial automation and process contol 1.3 OPTICAL FIBER WAVEGUIDES — 30. Ans: INTRODUCTION, RAY THEORY TRANSMISSION, TOTAL INTERNAL REFLECTION, ACCEPTANCE ANGLE, NUMERICAL APERTURE, SKEW RAYS Explain the structure of optical fib Model Papers 03) Thestruciure optical her vonsists of Following pur, Core Chadsing ‘Sheatls or ur (Coating) Lai The structural arrangement of Aber optic cable is ire below represented in ig Sheath Cladding es Butter Figure: Fier Optic Cable Core The core is eylindrical dicletric ro, which doesn't eonduet, I ix made of glass through whieh dhe propagation of light takes place Cladding The core is surrounded by cladding, which has fess refiaetive index than that of core. The cladding is mode of citer glass or plastic, The Functions of eladéing are as follows, 6) Reduces loss of light Gi) Reduces scattering loss il) Provides protection for the ier from surface coutausaaats Gv) Provides mechanical strength to fiber 3. Sheath or butier The cladding is enclosed by a material called sheath to provide extra protection, Iti also called bute or which protets the fiher fram physical damag Tealso prevents abrasions scattering losses caused by rrierobends. The reffactive index of sheath is made bese than tat cladding (Q34. Explain the ray theory of the optical fiber with, the help of a neat sketch, Ans: Ray optiesilsivates dhe light propagation mechanism hy means of rays. Ray «pic oF geometric otic representation is the simplest way shen the wavelength ofthe ineidene light is less than the size ofthe eore such as multimode hers, The ry optics i deseribod for ideal multimode stp index wptial ‘cane, The illustation is simplified by considering the say ofty congruence path that ais as @ mods, Skew says and meridional rays ace the types of rays that propagate throught the fiber. Meridional rays are restricted only toa plane of core axis, hoc they cat he tracked casly, Mevidional nysanecateporized in wo classes. They a 1 Bound rays 2. Unbound rays SPECTROM ALLAN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP > Scanned with CamScanner112 Tihs aps that Ber Wappen iy core fied Wael ay the fiber core axis are knosen as oud rays. The wy thal tet refracted out of the fiber eore into cladding are known | unbounded rays Skew rays ae not limite to @ single plane a follow Use eliel path as shown ia figure (2). Ray path projected to ihe en! fae Ry pat Fier Figure 1}: Skow Rays in Step Sine, the sew rays ae aot restricted a particle plane they eannor be tracked easly, Henee, analysis of meridional rays is enough For obtaining the picture ray propagation in a optical ocr eable In ray optics, most ofthe trapped skew rays inthe fiber aro considered as leaky rays. Hones, they are not included im the ray theory. bor a step-indes fiber the meridional rays are represented as shown in igure (2) m,__ Clndding Refiacted ray Chacing Figur (2: Meridional Rays in Stop index Fb The light from the medium having refractive index * incidents om the fer wore axis alan angle," enters the fiber core, This light ray hits the eore-cladding interface at 9 narmal angle 7. The meridional rays propagates in a zig-eag path along the fiber when che light strikes the interface at an angle seh that it reflects toaly into the some media, The minimum angle 4, at which she meridional rays lundergocs total internal reflection (TTR) is. given by, sing =f 4 from sl’ as o the ry is incident on eore-cladding interface with an angle less thon munirrum angle, then iti rfiacted into the cladding as shows in figure @). OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] Lig tmerface Figure a) By applying snl’ lato the aifiher core interface, the maximum entrance angle, cas be nated as, sin (®,,..) =a, sin (0) =, 608 nasi” ” Psi +801 On bsg oston (1 in tion (2, meus wintaon atl ssio0, aye & the incident angle, is smaller than aeceprance angle, thon rays undergo Total Tntemal Relleetion (TTR), otherwise it gets lost in the cladding, quation (3) referred as the Numerical Aperture (NA) cof sip index bse for meridional ays Hence, NA =H sin. Whew, A cei lading inde difference Q32, Describe the quantum nature of light. Explain basic optical laws in detail with rel ‘mathematical expressions. Ans Quantum Nature of Light Aceoriing ts Max Planck's quantum thenry of radiation, isalseayseminad or absorbed in ‘quanta oF photons re uitsealled Look for the SLA GROUP Loco {J} on the TITLE COVER before you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) TS tindicates that optical radiation as particle as well as wave properties, which is known as dl natare of igh. The particle nature arises from the observation that light cmergy’is always emitted or absorbed in discret unis ie photons. And the experiments showed the relatiouship between the encray Band the frequency’ the photon is given by. Poh Where b Plank's constant = 6.625 6 1S And this frequency cam he measured by consid wave nature of light. Whos light energy falls on an stom, s photon may strike wn electron and thereby results inthe transfer of eneray’ fran [Photon to electron, The electron takes that much energy whieh ior cary level so itmay take a past iWeequires to jump toi orall ofthe photon energy. Figure 2: Emission of Photon the Initial and Final Psion ofthe Electron (0 and (a Aer some time this excited electron may jump back to its original place by the emission of a photon of the sume absorbed! ener The angle between the incdeat ray and the interface bevwecn dhe wo mediums is qual to Uhe angle between dhe reflected ray ad the interface as shown in figure G3), Figure) Smells Law Consider ay falling on the interface between two media ss shown inthe figure (4 1.13 Figure a) tthe hending of the reflected ray towards the denser medium resus cause of the difference in the sped li inthe wor materials, The relationship atthe interface is given by. 1 ia = in And is known as Snell's aw when a the less denser medium. if tke angle of incidence is inereased vw a puticular angle (called extcal angle) Uae refiasted ray hecovnes parallel wo rhe interface. When the incident farther, no : iicrnalrelTection as shown inf et ray From more denser medium move to cof the wave ielracted x this phenomenon isle total wen ars Q33. Whats the concept of total internal reflection? Explain with a suitable optical cable setup. Ans: Nodel Papers. 03 ‘Total Internal Reflection When the angle ol incidence is grouter than the eiical ‘angle, the light rays reflet into de denser medivaa. This phe- nomenon is known as tral ternal reflection, Conditions for Total [nternal Reflection 1. The lshiray mst pss only fom denser to racer medi, 2. Theangle of incidence (8, should have greater value as compared 1 cateal angle Whom a light ray’ is incident on the imine between ‘wo diseetries of eliferemt refiaetive indices. part of the ray is reflected ack into the first medium while the rest is bent (or retuted) in he second medium, When the refuctive index of first mesma (is preater than the refractive index af second rdiuen (1) then retraction ooeurs. This fraction is such hal tcumgle between the incdeat angle ws sbowa i ure (1). ‘SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS, SIA GROUP Scanned with CamScanner14 tomy Meu Mtn signteve \ ie Low ive Ratt , Figro (1) Frain Snell law 4, sin, =n, sab, a) Ineanbe observed from figure (I) hat ane of refaetion (0.)is greater than the angle of ineidenee(0,). Ths is primarily ‘due tothe fact that, > 1, Whenever the aaele of elmetion becomes 90° the reffacted ray become parallel te interface between dielectries, This represents the limiting case of refiaction wherein of incidence is always less than 9°, Suh incident angle ut the dielectric intefuoy is known as erie angle (4) shown in igure Q), Figure 2: Aimiting Case of Retraction From equation (1) the value of ete angles piven as, sino," 2) W the angle of incidenee is further inereased than the tical angle, us ight is eonpletely reflccted back ito te sane diclectric mediuin with greater efficieney. Such phenomenon of complete reflection of light is termed 9s fatal internal Figure (3) illustrates he diagrammatic representation ofthe total ema reflection OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] Figure (a) Total Lnternal Rellectiou in Optical Fiber Figure (4) illustrates the diagrammatic representation of total internal reflection aloug the optical fib AWA Figure (a) 034, Using ray theory transmission approach, explain the following, 4. Reflection Refractive index Snell's law Critical angle Acceptance angle Numerical aperture. (on) Explain about numerical aperture in the fiber with a neat diagram, NoeiDee-16 (R43, G2) ‘efor Only Numerical Aperture) Ans: 1 Retest Relletin is a phenomenon in which light ray reflects buck shun ic invidents on the houndary bulween to iedia, Look for the SIA GROUP LoGo J on the TITLE COVER before you buy Scanned with CamScannerero we UNIT-1 (Overview of Optical Fiber Communication) % Retractive tm nis defined as the ratio of solovty oP Tight ina rarer media 0 velocity of fightin a denser mean, Velocity of fight in rurer mei ‘Velocity of light in denser medium ‘Whco light lls on the surface ofa medium, dae angle made by he incident ray with norm wo the surCice is called angle of incidence (0) and the angel made with aommalthroueh whieh the Hebi relincted (bent is called angle of retetion (7, Reactive ilex (2) ean also be defined as the ratio of sine of angle of incidence [sin f ta sine ufangle of refiaction (snr) and given by sine Nocal Inciden ray Reflects ry suitace Figure (1 Reflection and Refraction of Light Ray 3. Snell's Law “The Suoll's law states the behaviour of light ray, whew it suikes the interface of two media having dfleeeat reactive indexes, According to Snell's law, [sim = asin) in Where and ae efactive indexes oftiwo media'sand n,> 1,4) and @arethe angle of incidence and angle ofetzaction respectively From equ cry ‘From the above expression, it is clear that uke sia of rliactive index of two anedia is iaversely proportional to te sue cof rfftctive and incident angles, 4. Critical Angle \Whon igh ray is incident on the interes hetwoen two dicleeries of eilfering reftsetive inces, pant othe ray is re= flected into the first medium and the rest fs bent or reacted in the second medium, Ip, i the refiactive index ofthe fist meinaand isthe retatv ines of he sean mute reo hex place wn > ay tow i the are) Loviaksny |, Seeley \ tiasnien Qf, atta ae is | leRetocton Teale Ratacton whore > 4, Figur 2 [SPECTROM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP > Scanned with CamScanner1.16 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] The honing or efaction af he Tight at the terface fs duc tothe difference in the speed of light n bwo materials that have different refractive indices, Inis observed from figure 2a) that 4 i the angle of incidence, 0, isthe angle between incident light and normal st the surface ofthe interface. 6, is the angle of retraction ic. it isan angle berween refracted light and normal draven at surface of interface in the lower reactive index mediums, The relatioaship ude interface is known os "Snell's law” of rettion, I gives the telationship between the angle of incidenes and angle of refraction, According to Sills a 1 sin, ~n, sin, 5 sino, sing 1 From wavelength, = “> itis clear that angle of incidence is always smaller than angle of reftaction, The angle of r refraction is always greater hovause the reactive inden iis greater than a, When the angle 4. reaches 91 the related rays become parle to the interfce between the dielectrics, Ite ineident angle is increased. « point will be reached where the refiaete ay is parle! the ylass surfave, This point is known as eitical angle aPlaeidenes The ritcal angle and reacted ray that become parallel to the interface re shown in the figure 2(b), 5. Acceptance Angle The maximum angle at which the light ray may ineident ov fher wy propagate down the her iy known as “Aecyplance angle”. The maximum acceptance angle ean he derived from the mamerical aperture equation 35, Numericl aperture NA=sin 0, =m VER 0,~sin* nV] o.=sin () sin (NA) 6, Numerical Aperture figure of merit, whieh is sed to exploin the angles associate wih light propagation infer “The numerical aperture is le is reerre as "Numerical Aperture’ [i lasted in fsure (3). ‘plies. The sine oF hal-angle oF acceptance ‘Mathematically. mmacrical aperture is expressed as, NA = sin O. Figura (3) From Snell’ law sin 8, ~ sin 6 Heere,9,=0, and, 99° Then, sin 8. 2, sing. = =n (90? 0) Fram A ARC, 5 80 9,180" 8-18) 90-0, 0-908) * 4 ® Applying Stl’ lat sir imeave LesinO=sin0, (2 = fara sino sind. a sing, ~ 2 Look for the SIA GROUP LOGO Gj on the TITLE COVER beiore you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 1.17 From figonomenic ientigy Ts sin 8, + e03"0,— sind ae [From equations (2) and (3)] Wheres and 1, ue reiactive indices of core aud cladding respectively {Q35. Discuss in brief about ‘rays’ in optical fiber communication. Ans: Rays: In an optical fer, the sight Fines that represent path or direction of the i ‘Types of Rays: The (i) Meridianal rays uh is known as "Rays", 1p are classified ino fv types namely. (ii) Skew rays (@)— Meridional Rays: The rays thet are confined tothe plane containing cose axis (Le. the axis of symmetry of Aber) are known as meridional rays Alternatively, ig total internal reflection, ional rays are the rays that pass through Such rays follow casy to track path, as dhey travel along the fiber ini fer under plane as shown in figure (1). Figure (1: Merional Rays Furthermore, meridional niys are eategarired into tw rays namely, (a) Bounded rays (h) —Univounded The ays thal get rapped in ths cove while tavelliag slong the fibctanis are huowa as bowled rays, la contra, ths ays that get refracted out of he ere ais are termed as unbounded rays. Figure (2) illustrates the d tie representation of Founded and Figure (2: Bounded and Unbounded Rays (i) Skew Rays: A ray that follows angular helisal path along the fiber is known us skew ray. These rays are not eomined track as they propagate alone the Hider sic representation of skew ray in an optical bc. losin sad ae more dfeul (0) Crain view (Ske ay down he ber Figure (3: Propogetion of Skew Roys SPECTROM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP 2 Scanned with CamScanner1.18 ‘G36. Calculate the numerical aperture ofa step Index fiber having n, = 1.48 and n, = 1.46. What is the ‘maximum entrance angle 3, for this fiber of the ‘outer modium is the air with n = 1. Ans: Nov 48,091) that, Fora step index fiber, (Core reactive index, n, ~ LAS Chalding vlractive indes, 1, — LAG Numerical aperture. NA, Maximum enigance angle, 8, = Tike expression for numerical apertve is expressed a, NA- yay? (146) = 058s = 02s “NA = 0.2425] “Maximum enirunce angle is expressed as, b,-sin (NAY ~ sin 0.2425) Ho) aa Q37. The core of an optical fiber is made of glass of refractive index 1.55 and in clad with another glass of refractive index 1.48. Determine, () Numerical aperture (li) Acceptance angle (iil) Critical angle, Ans: Given tha, For an optical ib. Core refractive index, = 1.55 (Cladking relative inden. n,— LAS (Numerical aperture, NA ii) Aceeplance angle, =? iy Criical angle, = (@—Numcrical aperture of a given optical fiber ean be calculated by using the following relation as NA~ (n? 03)!" ~ 88-148) = 2120) 0.46 NA= 0.46) OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] Ti) Acceptance angle of a given optical Tiber ean he csleulated by using the following relation us 0, = Sir (NAY ~ sin (0.46), 2739 Critical angle of a given optical fher cam be vafeulated by using the following elation, vfs) ~sin 40.958) (G38. Atypical refractive index difference for an optical fiber designed for long distance transmission is 1%. Estimate the numerical aperture and the solid acceptance angle in air for the fiber when the core index is 1.49, Also calculate the critical angle at the core cladding interface within the fiber. Ans: NoniDee-16, (R12), 0210) Given that. orn optical ber, Relative eftactive index difference, A Core index, 9, = 1.49 Numerical yperure, N=? Solid acespuanee angle, 9, =? Citieal angle, 0, = 2 Numetival ypesture of an optical ber ean be valeulated Dy sing the following relation, Nan BE => NA=149¥2x0.01 -021 [-NA=O3IT Solidaeceplanceangle an optical iberin aris obtained 8, rain, HINA) CoNt=sin 8) nAO.211¥ =o. i ES Look for the SLA GROUP Loco {Jp on the TITLE COVER before you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 1.19 CCriscalanglearthe core—cladding interface ofan apical ) (i) The expression for mulipalh dispersion per unit length fiber is given by. is given by, ma — a8 6 ) Oe simi ay fe a= ” ‘ sn 0, sin (1-001) =F Snthe ae = sin (099) (On substituting corresponding values in the sboweequse dis tion, we get, 0.034 ous SCT) 310" 99.7810" @39. The speed of light in vacuum and in the core ITD of silicon fiber is 3 * 10" mis and 2 * 10° mis respectively. When the fiber is placed in alr the etitical angle the core cladding interface is 75° | Gao, Tha volocity of light in tha core ofa stop Indox calculate the fibers 2.01 10* ms", and the critical angle at (i) Numerical aperture of the fiber the core-cladding interface is 80". Determine the )) Multipath time dispersion per unit length. sumerlat aperture ar the soseptance anal for : fiber in ar, assuming it has a core diameter a Pete mat) suitable for consideration by ray analysis. The Te mon the velocity of light in a vacuum is 2.998 10°ms"'. NowDec.7 (R13), 040) 1x10 seein | Spout of ight in vacua, © = 3 = LO ms Speed of lial in the core = 2» 108 ws Given that Critical angle, 0, ~ 7 lan step indes fiber, Li) Numerical aperture ofthe fiber, N=? Velocity oF light, y= 201 10° m’s (ii) Multipath time dispersion per unit longth, Dit=? ,o hi inion per un engi, Cal ange a coreg iter, 0, = 80° ©) Teeqrenin mmc perie of rire Velocity of light in vaccum, ¢~ 2.998 = 10" mvs NAS mya o ‘Numerical aperture, NA~? Where ‘ te — Retiuctive index of core eceptance angle, 3 — Indes diference Te exesson fr numerical pst give Speed of yk vein KAS itn a Ret " Speed of light in core Where, san 1, Cove reactive index ' dao® 1, ~ Cladding refractive index, Ss nats e The expression for eritical angle is given as Indes rence, a= = = 2 o-sn [2] “The expression fir velocity a light fs given a, ve oO Lsin59 Gr 0, On substituting comesponding values in equation (3), sas a ‘On substituting equations (2) and (3) in eq wees, _ “ 201 =o = 2998210" NA- 15300 7 = 1.59026 eo =038 Sor FNA=09) =1491 ‘SPECTROM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP QD Scanned with CamScanner1.20 ‘Ou sobsthuiag the values of and 9, ka equation 2) weget, wos (i) tt = sin 80" TasT = = 1491 sin 80" n= L468 (On substituting the values of 9 sa yiLa9b? (1.468)" 0260 ‘Namricil Aperune. Nd = 0.260) Acceptance ung, ,~ sin (Nl) sin(0.20) 507 ‘Acceptance angle 1.4 CYLINDRICAL FIBERS - MODES, V- NUMBER, MODE COUPLING, STEP INDEX FIBERS, GRADED INDEX FIBERS Q41. Discuss the mode theory of circular waveguide. Ans: Node! Paper, a3) Ciecular waveguide maintains @ uniform circular cross section along its length. Its also known as cylindrical waveguide isthe possible selinioa of Uke wave equation r9p- resenting the distribution of feld that propagates in an opal fiber ‘The mode of save propagation of aptical ber is obtained by solving the maxwell equations related to the field, The analysis the houndary comditions at the interlace jeomponent (thatthe tangential component af electric component is 72r0) is earied out by converting the field equations into eylindtieal coordinate system lo gencral while solving the msawell equations, wo modes known as Transverse Electric (TE) and Transverse Magnetic exists. But in optial fiber, a dhied mode known, as “hybrid modes” arises due to the coupling hetween the cleetrie and magnetic ficld components by the boundary conditions. OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] Tighrid movies are assigned as FIV or HF mode. TF, mode is the dominant mode in a eireular wa The subscript refers the possible modes of propagation af the ‘optical field. TM,, mode is called eiscular magnetic wave and TE,, mods is called circular electric wave because thsi respective field lines form concentric circles euide The sirmplifcd analysis is carried out by considering the assumption that the differcnee of efruetive indies wf cone nd lading is very small, 9,“ L, This approximation uses ‘only four hybrid comport, The ied eomponms ar called linsar polarised modes aud ace represeuted as LP, ere, j and nv are the integers representing the mode solution. Inthis linear polarization scheme, fy the lwest-ondor modes, 17, is obtained from H1S,_ made and L,, isobtained fiom TE. TM,, and #6, rds 42. Define and explain the linear polarized modes in optical fiber. Ans: ysis of modes in a fiber ‘san be obtained by using “weakly guiding fiber approxinia- tion’. The prineiple assumption of this apprasimarton is that the difersnes of reiractive indices of core and eluding is very stall ie. "9, 021 lo this approximation, the eleciromagnerie field pat tems aud propagation constants ef TE,,. TM, HE, modes is similar corresponding For example, HE, and HE, , are degenerated TE, ,.TE, and HE, are de; The linearly polarized modes have only polacizarion inthe plane transverse 1 the fiber axis ie erated no longitu components, The lowest order LP,, mode is referred as Fundamental mode and eorresponds to two polarization of the HE., mode. The first higher-order mods, the LP, mode, consists of IE.,, polarized modes of IE,, TM,, and TE,, Analysis hy Equations. Atm <<. AP For the simpler aaalysis, of modes (Only four field components are considered Certain coordinates are use The eigen yalue equation for transverse component ven by, Bis ky diam eny=(Bey (Le Lsingheapprosinstion £ = £) ion reduces, Btheabove equa sone ee(he by w Using the recurrence r ‘oF equations For equation (1) a ions for J and fy Wo sets oblained wit positive and Look for the SIA GROUP Loco Gp on the TILE COVER before you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 1.21 The postive sigh equation corresponds 19 FH modes J, Quad oa) Wht) * es, ta The negative sign equation corresponds to ITE mades saa), hou) _ . uF sa) * of, soa) o wo), Koo) cea Patten TE atl TM mods AUP Hes TE. Trausverse electric ode IM. Transverse magnetic mode HIE Hybrid mode A parameter ‘jis defined 05, [2 for Teac ns v1 for Emodes [et tor tnemodes “The equations (1), (2) vad (3) caa be writien ina combine form usinu '/ = dy sua _ ak; (000) Hina) ~ Klemp # Equation (4) shows that the modes are characterized by a common set of */ and “m’ satisfy the same equation ere “nis the radial order ancl *s isthe equal eircunnferential order. It meons that these modes are degenerated ie they have same value af *p “These degencrated moules ste called lineal Polarized Modes (LP) and areassignod as ZP,, modes despite their TM, TE, EH or HE field configuration, All linearly polarized saodes exhibit similar intensity patterns in transverse as TE, TM. and hybrid modes Lr, _ Lee PAL LP Mode Exact Mode Number of Total Number of Modes Designation Degencrate Modes LP, HE, Le, Fy TM, HE 4 Lp. TE, FH, 4 Lr He, 1, BE, FH 4 LP, Ty TM HE, 4 Table Table represents the composition of lower order lineatly polarized modes. Useful Features of LP-Mode 1. The ability of ready virualization of any mode 2. Adainment af four discrete mode patterns fram a single LP, mode SPECTROM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS Sin GROUP > Scanned with CamScanner1.22 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] (G43. Explain the field distribution in a symmetrical waveguide and different types of modes in it the overview ofthe mods is examined by considering the electric field distributions of several low-order moxlal fields in the phinar diclecsie slab waveguide Exponential f J tecay Chaddineny Harmonie Coren vation Txponentiad des Coange,| Figur: Eletrc Field Distributions for Several ofthe Lawer-Order Guided Wodes ina Symmetrical Slab Waves The cove of diloctric slab has a refiuetive index 9, an its surtounded by cladding that has a relative index »,. The ‘mid fields in the figure ae the solutions of max-well uations of skib waveguide. Is order ofa mode i equa to the number of field zeros across the guide Mode is also rslated to the aggle thatthe ray cougrucnce of this mode makes with axis of fiber. The order of the mode becomes higher. When tho angle is sleeper. The itd vary hannonicaly ie, sinusoidally in the guiding region of reltuctive inde andi decays exponentially ouside the region clad ‘rom the figure, itean be scon that, apart ftom the finite mumber of guided modes, some ofthe untrapped radiation modes centered! partially into cladding, These radiation medesare also solutions of sume boundary value prolom, hence these are kro a lacing motes. Movle coupling occurs, 1, When the cladding modes and high-order cove modes propagate along the fiber. 2. ‘The electic fields of guided modes of ore extends partially into cladding There is loss of power frum the core modes duc to diffasion of posser between core and cladding Feesides the hound an refracted modes, there is othr moves called I rates their power in the core during propagation aud this attemuates, ‘This power rdiation out of the waveguide comes from 4 quantum mechanical phenomenon galled “tunnel effet The mode is sud (o be guided us the below condition is saistied for the propagation factor nko fin Where, Refractive inden of core in, Refractive indes of lading ‘ a The “eutolf eondition” f= 1.6 is the houndary between truly guided modes and leaky modes. Took for the SIA GROUP Loco Gj on the TITLE COVER before you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 1.23 {Q44. Distinguish between leaky modes and guided modes in optical fiber transmission, Ans: “The differences between leaky and guided modes in optical fiber transmission are mentioned below: Leaky jided Modes 4s. Ans: “The modes which propagate inside the eore wth wave propagation constant () less than nd ben Whew, 2n,— Reftactive index of cladding Wave prop bare called leaky modes. These modes ate confined partially tothe eore, at 7era at core-claddling boundary, instead they ‘These are high loss modes. In this mode, E-lilds and H-lickds exhibit ‘exponential devay inthe cone medium. These mexles exhiit low as we proceed slong. the length of bse, Altera specific distanes (Lea few centimeter) these modes will disap cat Differentiate rays and modes. Tiae muds which propagate inside the eore with sn (i Hes between on fare called guide! mowed, wave propaga ud fie. k= [ Whats 1, Refractive index of core i penetrate partially ino the adding Jectrc field components of guided! waves are Thaese ate Low Hoss (i los Loss) modes, In this mous, E-fields and Helles vary harmonically in core and decay exponcatially inchadding Tike penetration of fick into cladding nercases a8 we proceed fram low order modes to highsr torder modes These modes exis till he end ofthe fiber and constitutes major power pat in recived signal The differences hotween rays snd modes are mentioned below Rays Modes 10. "Rays are di tal Hines that represent pall or direction ofthe light Fields of mays are firmed From light propagation. Ray cesplain various propa ‘ical bers, vilized 40 ory’ has eon success satin plienomens into Ray theory cannot be considered fara types of optival bers Total reflection takes plac Autenuativn is unit, Losses dypend om power less fun the source, 110° tbeintnon othe noma aye en oF, Bendling losses depend on curvature of rays, ays propagate with different ray paths and J independent of refractive inex profile in the optical fiber core, Mod isa stable propagation sate in an optical iber (or) bundle of light rays eateing the fiber at same angle Fields of mod Mode theory becomes quite complicated or has ho been suecessillyulilized are formed hy larly of ras Mode theory can he considered fir all types of optical bers Partial reflcction takes plas Atteauation [i cladding layer] is oss Losses are high bseaus li as well as ber ca Kak ot om cadding .¥ is associated with al rays traveling at 0,22 ((9+0/20,) cadins IFhigh order modes leak out ofthe core, bending losies umber of modes propagated in fiber dspend an fibers Numerical Aperture (NA), coceaamter and seavelensth of ight ‘SPECTROM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS, SIA GROUP 22 Scanned with CamScanner4 ‘Q46. What is Vnumber in connection fiber? What is its significance. opt Ans: amber is an important parumoter that determines the nuraber of mods suppasted by a fiber. The mathematical expression fr ealeuating F-ber ven by 1 8 ytd v= 70a a Where, ‘2 Radivs ofthe fiber core 2. Wavelength NA — Numerical sperture Faquation (1) is 2 dimensionless quantity and numerical value indicating aunber of modes. Tnasigpinds ibe. the number of modes propagating aby. any | Moroover the for single-mode props he fiber avis is nmust he Less wherein only one ray travels alo than 2.405 ie, P<2ans For multi-mode optical fiber, the F-number is significantly urcater than 2:4. This F-number determines the number modes supped bythe fiber and is piven by. fiber is tused f indicat fractional power aw in the core and clan Number of moiies supported by an opie Ie Mathemaicaly Bigs 4 Pyar ®) Wire Fy Power flow ia cladding P Total power in fiver Mf Number males Tican be osserve fem equation (3) that power fle in Jai num 47. Compute the V-number and number of modes supported by a fiber with n, = 1.55, n, = 150, core radius is 25 um and operating wavelength 4s 1300 nm, Ans: Gi Foran optical fiber. = 25 «10m AL COMMUNICATIONS [JNTU-HYDERABAD] 300 nm = 1300 10% m Number af moules ="? 1° number can be ealeulated by using the tolling relation, yp 2 (ver =n 7 sin 1300 «10% I one = 1208 (Vu 4717 Lean The number of modes, the fiber ean support is given by, re 12s ‘Therefore, dhe approximate numberof mades supported by the given fbr is L112 =m Q48. Determine the normalized frequency at 0.82 jim wavelength for a step index fiber having 25 jim core radius, n,= 1.48 and n, = 1.48. How ‘many modes propagate in this fiber at 0.82 jim? What percentage of optical fiber power flows in cladding? (Mode Paper 2b | NowDec-17, (R12), a2) Ans: Foran optical fier. Core refiactive index, — LA Cladding telmative index, 0, = 1.46 Wavelength, 082 jm Core rds, a= 28 Number of modes propagating. =") Parver flows in cladding, Pg? ‘The expression for modes of propagation is given by, Na~ Vint Look for the SMA GROUP Loco {J on the TITLE COVER before you buy Scanned with CamScannerOb UNIT-1 (Overview of Optical Fiber Communication) 1.25 ‘Om substicuting the corresponding values in above 082-10 s10-" : Fe 8256" ag? ae 2x 25x10" Smee 0.24 - 4597 ORD Ihe respootive value in equation (1), 8 wy A@ 26 Muss @ eh fiber al OR? pm TOSS modes propagate ir thew ‘The percentage ofpower ow inclaldingcan be obtained | profile by using, = £10861! Fem qution 2 = 0.0109 P, = [200 BasDDP% [Ry =e ain Around 4 percent ofthe power lows int 249. Explain with necessary diagrams the different types of fiber structures, (ode Papert aaa) | Dee 14. (R09, aH) | 2 for) Explain about cylindrical fibers with neat | profile diagrams. Ans: Optical Fiber A diclvetric waveguide that operates at optical Frequencies i known as optical fiber. Ie is generally available fn eplindrieal form, Ihe optical fibers are broadly classified into, 1. Step index fiber Grae inde fer + Step indes fiber is Further divided in rode step inex fiber cy Multi ode sep inde fiber is shown inf ‘Single rads stop index Heri ret ne(), wena pra Figure 1: Single Mode Step Index Fiver In single meste step index ier, ‘The typical dimiension of eo is § 2 men and clakling is 125 mam (b) The refsctive index ofthe core is uniform aad at the cladding boundary irundergoes a step change, fc) ‘There's only one made of propagation ‘The multimode step inex fiber with refiactive index is show in figure 2), Figura (2: Multimode Stop Index Fiber Ti multimue stp ind fs (a) Hundreds of modes are present. (5) Thelypieal dimension of eoreis $0 wo 2001mimand clakling is 125 to 400m, © ‘can be transmitted dincelly into fer using LED. Graded lndes Fiber: Graded index fiber also coatains single mode and multimode, “The multimode grad inde ber with refractive index is shown in figure (3), Figure 3: Multimode Graded Index Fiber In graded index fiber, ()Therefiuctive indes ofthe core ismade w varyas 8 fimetion of radial distance taken from the coacr of the fiber Gi) The dimsnsion of its core is $0 1 1000 mm and lacking is 125 to 140 sa, ‘When compaced to single mode, multimode has drawhack that it suffers frm intermale clspersion ‘SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS, SIA GROUP 2b Scanned with CamScanner1.26 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] {Q50. Distinguish the step Index fibers and graded Index fibers. (Step index and graded index fibers. {ii) Multimode step index and single mode step index fiber. (on) rentiate between step index and graded index fibers. saison, (3), (Refer Only Step fadex and Graded Index Fiber) Ans: (Step Index and Graded Index Fibers Thy differences between step index and graded index fibers are mentioned in table (1. Slep fades Fiber Graded laden Fiber Tras the same rofactive mic all pois | 1] The core has a conceal vary whieh forms a parabole 1] thea ng relractive TOex, 2,] The eore diameter is almast double the diameter ofa | 2.| The care diameter is half w the diameter of GI fiber for snultimode Gt ibe rmuttinwd, Propagation of signals is by meridional ray, 3.| Propagation ofsignas is by helical rays. 4,] Signals distortion is high due to intemnodal dispersion. | 4,| Signal distortion i low due to the absence of internodal Alispersion 5,| undid is lower: 5.| Higher hand ih 6,| Ugh attenuation of signals 6.] Attenuation of signal is low 2.] Large numerical aperture 7.| Small numeric aperture Table it) (i) Multimode Step Index an Mode Step Index Fiber The differen ul single moe step index flbes are shown in table (2), Multimode Step Tndex Fiber Single Mode or Monomode Step Index Fiber T.] Many tajectories of ight propagate in he Aber T ] Only one rajectory or one mode of Tight ray is alowed ‘to propo to dhe roesiving end in single made Sliber The diameter ofsingle muude SI Her is small wallow fe mode to propagate 3.| The diameter of eone range upto SO ji 3, | The wore diam 2.| They haw ris lesser than 19 im. 4.| There is. lorge difference herween core refiative | 4, | There isa small diference herween the core Indes and cladding refractive ines. relfactive index and cladding refetiveindes, 5.] The strength of signal during propagation weakens |S, | Tis use for long ‘or attenuates more in ultimode step index fiber lence, its uss for short distance communication, 6.| Fabrication cost is very Tess. 6.| Fabrication costs very high, Table (2) G54. Compare single mode fibers and graded index fibers, List the characteristics of materials suitable for ‘optical fibers. Ans: [Wedel Paper @3() | Nov/Dee-17, 13), 2a) Index and Graded Index Fiber For anwwver refer Unit, Q50, Topie: Step Trdex and Ciraded Tad Fiber Characteristics Required for Optical Fiber M thin and flexi Ata specific wavelength, the material must he transparent to guide the optical signals eleetively srenges betwen St They must be lon They must have low scattering losses 1s} allenuation, absorption snd dispersion of apical sl 1 3 4 They must oer ‘The reitutive indices of eore and cladding matrials must he physically compatible to provided total intemal relleetion efficiently Took for ha SIA GROUP LOGO Gp on the TITLE COVER below you buy Scanned with CamScanner veUNIT-1 (Overview of Optical Fiber Communication) 1.27 (Q52. Explain step index fiber structure in detail. Ans: An optical fiber withthe combination of the core of constant refractive ind,” and cladding of slihily lower refiaetive index ‘is called stp index fee interface hevefore it scaled as step inde fiber. There are The sep index fiber makes a step cing atthe eace-claddin reo types of step inde fibers. They are 1. Sinplemode step inde fiber and 2. Multimode step index ib ‘The general refruetive index profit fr step index for may be defined as, my reatCore) MV Tn p= a (Cladding Figura (Step index bors Multimode Step Index Fiber ‘The multimode stop index hers shown i igure(1), The eore diameter of multimuke stop index beri 0 ram oF 2 «hue ta sshich, te propagation of many miades withia the her core is possible, ie (2) Shows a snultimade signal which consists of 3 males ic, lawer eeder mode, middle order mode aad b crder moe, Ax shox in the figure, nw order mode travels leg the ass and mide order mode eflsets rie athe nerfs. ‘Whereas higher order mode sefloots many times aesoss ie fiber, which vesulis pulse broadeaiag aud cispersion, Because of ese drawbacks multimode step index fiber i used rarely in telecommunication a RRA Aawondernee Miler nae Figura (2: Configuration of Multimode Step index Fiber lex Fiber Singlemode Step In this made, the pulse dispersion effect i minimized andl accepts only the lowest er mosle which makes i to be wsed in ong distance systems, These kindof bers ae known as singe gp inde ib 59, Explain in detall the graded index fiers with neat diagrams. Tov bee8 9) 288) © Explain about graded index fibers with diagrams. Ans: In graded indes fibers, refuctive index is noe constant but decreases with radial distance. {usually varies fiom amosinaam value ot, atthe Gber ass toa minima valu of, beyond the eoee rau (a) inthe fiber cladding, This variation of retiactive index is matnematically expressed 3s, nsfor)m[L_ 2A] at cladding (2 0) SPECTRUM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP {> fie ateme tro) mv) Scanned with CamScanner1.28 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] A Relative refractive index dilference Profile parameter 2a, Refractive indes of core 2n.— Refractve index of cladding Cove radius, Depending on the values aft, the possible relzaetive index profiles are estes 1. Whemar~ |, the refiaetive index profile has triangular shape. 2 When =2, the proile has parsbolie profile 3. Whea a =e, the profile is constant 1 (I) illustrates the diggessamatiereprescttation of possible fiber reuetive index profiles nin) Refine index 7 2 Racal stance () Figure (1: tractive Index Profiles ane graded ines Gers. Figure (2) lusts the diaurarnmatie Tia nerional rays follow curve pas when moving thro Fepresentation of graded index fiber and its structure Refractive inde Clavine ure (2k Structure of Graded Index Profiles The number of modes supported by a graded ines. fiber given by wate )(5) Mate goede hers deg ess ind dipersin when omprd 0 mine ep in her. Thi is primarily duc 1 the varying refractive index profile. Thus, graded inde fibers offer many alsa fibers. Q54, Compare the fiber structure and NA in step index and graded index fibers. Ans: Fiber Structure A ‘fiber consists ofa single sols dielectric eylinder of rac “a and reftactive inde, called as core of te fiber. The core is surrounded by a solid dielecric cladding with refractive index 7, that is less than m,. he variation of material composition af core give se 19 the nwo commonly used fiber types (0) the refnetive ides ofthe core is uniform throughout and undergoes an sbrupt change atthe cladding boundary thea sic a fiber i called step index fiber (i) Wihe core reftactive index wradually varies slong the radial distance from the centre ofthe fiber and becomes equal to the ‘active index ofthe cladding atthe boundary, then sul fiber i ele! praed-index Sher Look for the SUA GROUP Loco {J on the TITLE COVER before you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 1.29 The Profle Trex Pra By > Claciing ow la: Single Mode Stepinex Fiber (5): Multimode Stepinex Fiber + 0 {¢}: Multimode Graded-Index Fiber Figure 1) The stependex and pradedsindes fibers are Further divided ino single mode and multimode fibers. The core radius siagle rmaxl fiher is very small hence only one made of propagation is possible apd laser diode is required to launch the light beam i the fiber. Multimode fibers has larger core rains an hence supports many hundrads of mods of propagation. Due te laryer core radius, an LED is sulicient to launch he lig fibers suffer from Intermodel dispersion, 1fcam int fiber making ithe expensive than single mode fers. But multimode Nu eal Aperture ‘There are 190 types of nays that ean propagate through fiber, thay re meridional ys ar skew rays, Merion res ae confined tothe midian planes of fber which contains core avis whereas skew rays ane nor confined toa single phan at instead tendo Follow a hell path al considered g the fibcr. To obtain the gcncral condition of ray propagation theougl fiber mericlions 1, Steprindes ‘Consider a step-indes fiber with core radius “e” andl refractive index a, and with a cladding of rtiactve index which is lower than 7, then we have nna) ‘Where, "A" is eal the eore-claddn indss mat an index difference, when alight ray enters the Bhor care fun a medium of reactive (und strikes the coreslad ourdary ata normal angle 0 such tha it results in oul internal reflection, wen by stills aw Then the angle shoul! wot he less than 0, 80> im by applying Snell's law 10 ait-fiber face boundary and using equation (1) it can be related to maximum entrance angle von by. Where, 6, > “Therefire for so Indes fiber numerical aperture is given by NA=nsind,, ‘SPECTROM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP 2D Scanned with CamScannerRefructed Ray 4 a Core ™ Cladding Figure 2) Graded-Indes Fiber Fora graded indes fiber the refractive index difference Ais given by Ais upprosimately equal in boul sep-indes fiber und grade inde fiber Numerical aperture of graded ines fiheris a Function of | Position across, the ease en face whereas, NA is steprindex is constant across the core. The light incident onthe fiber core at position ill propagate through Gher only iF iy with in the local numerical aperture athe ier a hat postion given by” eat} Naw lu" xa ongl=(riah frre Where, iy the radial distance from the centered the fiber “a” is the radius of core «cis dincasionless parameter cling the shape of inde pofilcan N/N0 is Anal Nusnzieal Apseure defined as, NAMO)= O° O)—H Bur. 0) Naoy= nV28 ‘Therefare Numerical Aperture of graded index fiber Acercanes from NA(O) lo zero as °F moves from exslreto core cladding boundary Le. ateenireNA is equal to that of sep index and gradually reduces util it bocomes zero a the eoreladding boundary AL COMMUNICATIONS [JNTU-HYDERABAD] 41.5 SINGLE MODE FIBERS - CUT-OFF WAVELENGTH, MODE FIELD DIAMETER, EFFECTIVE REFRACTIVE INDEX Q55. Explain in detail about single mode fibers with diagrams. Ans: Tina single mode fiber, she efractive index be, This isthe basis of weukly Pye igs Hehe Heyy He Veet ied fiber approximation. (He, Hi tig) Figg Fagg ad. for, and Ty modes Vl joe H V1 jor H, modes snes (i) Baek LP, mode is derived from an M.,, made (il) Lach.2P, modes derived trom Zi,» Fy,,and Hs, sn0de, (ili) Bach 2 mode (r> 2) is derived from an Hf, and Lin, mode The electromagnetic fiold foe a given mode is not ‘confined within the fiber core, Bu itchanges froma an oscillating Form in the ore meade to an exponential decay in the cladding The elecivomaznetic field fora given mode does not wote zeroat thecore cladding interface. Electromagnetic energy ofa guided made is caried partially inthe core and partially incladcing, Away fiom its cutof frequency, the guided mode energy ach concentrated in the core. As the cutoff is approached, Jd patterns furher into the cladding perventage of the eooney propagates in the cladding. AL he ‘uta the fic is purely conecntraed inside the core and the Figure: Single Mede Stop indox Fiber The ypical dimension of core ig 810 12 kml clalding is 125 pm Jn step index fiber, the refiaetive index of the care is uniform snd at the cladding boundary, it undergoes a step change. In single mode step index fiber, there i only one mode ‘oF propagation, Look for the SMA GROUP Loco {J on the TITLE COVER before you buy Scanned with CamScannerUNIT-1. (Overview of Optical Fiber Communication) 1.31 {Q56. Write short note on Modal Birefringence and Mode Field Diameter. Ans: In single mode fiber, there exis twa independent degenerate moles, They are characterized by twa parameters, 1. Modal Bi 2. Movde File Diameter Loom Birefringence sal polarizations i allowed in single mode fibers ul HE, The propagation of two approximately degenerate moves with ott sre asis, Thus, these are referred as bimodal supported HE* using the syrametry with nominal circular symmetry about the rele! ‘modes, Here, the super soripts x and y denotes the principle aves and Liber eros section, The difezcace ix the elTective eliaetve indices and phase velocities far these rthogoaally polarized roves makes the fiber to funetion as a birefringent medium, The independency of filer eros section with the fiber Fengtt in the 2-lirection yields the expression for meskl birefringence B, 23, Where, B, Propagation constant forthe mode * B, — Propagation constant forthe mode“) Optial wavelonatl, Fiber biroti also defined as the dillerence between the effective relative indives of polarized waves i. Bonn, 2 “The polirization ofthe fundamental HE, mode in 4 sinale mode fiber is as shown in figure below. Ay y Horizontal mo Figure Horizontal and vertical modes ate excited when Light is launched into the fiber and thete exists phase delay between the modes as they propagate wlong the fiber. Ie phase diferenee is an integral multiple of he dhe length of fiber over whit birefringent coherence is maintained is called Fiber eat length, It is expressed as, 2. Mode Field Diameter (MID) ‘Mode Field Diameter (MFD) is «fundamental paramete be determined Irom the mode-Geld distribation of Fundamental mode in the fbst es defines usc for characterizing the properties of single-mode fiber. Tt can asthe diame af the weal ded optical power distribution (which is nearly one micron Jaeger than the diameter of fiber cote). Th parameter of single-mode for is faetion ofthe Following se parameters (@) Radius of the core (a) (b) Wavelength ofthe optical source 2) (6) Refractive index profile of the fiber (0) ‘SPECTROM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP @D. Scanned with CamScanner1.32 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] Te below Wheres The diy Teseniarion oT distabution of Hehe an Fiber indicating mode Weld ameter lading Figure: Distribution of 1! Mode Field Diameter Mathematically. Moxke Fick Diameter (MFD) is expressed a 22.60) MED ~ uF 0 Where. 297, Spot size (Full wid of farfetd eistebution) d—Fiber core ameter PVenumber MEDs can he me ted by estimating fare intensity lstrbation 4) af Berea ie, MID —21F,~ [ Bem ‘Where, £1r)= Ene!" — Rains 5 — Field at ero radius, “The size of x maxle field diameter (MD) helps in estimating the performance af the fibers W'the MED oa single mode fiber is large, then these are mare prone to fiber hending. On the other hand, single move fibers with small MFDs exhibit high coupling losses a earncetions, 57. Give an account of cutoff wavelength for single mode and multi mode fibers and discuss its dependence ‘on fiber length. Ans: ‘Single mocle region and rwiti-mode regions are distinguished using eut-ofT wavel sep inde fibers. Only LP, mods is available at this wavelength “The wave guide disporsions and the common materia! dispersion fr three different single mods bors is shown in oO. Tina eo Danes be : SS Wve ie = sie! | Dope Sar Tay tT wwnsbaph Look for the SIA GROUP Loco {J on the TITLE COVER before you buy Scanned with CamScannerUNIT-1 (Overview of Optical Fiber Communication) 1.33 Ther The cut-off wavelengil is delermined by ws with a single 1 em radius loop. Aight having bine width net move than [0 am sinstalle inte the ber, ia order to excite both the modes LP,, and LP., unity (2) measured and atthe same wavelength P42) is abso measurcd, The logarithmic rato is ube by ws RG2)~ 1 log | POD PA) har which power of higher-order LP, mode is dropped to 0.1 dB with respect The maximum wavele to Taadameatal LP, power is kiowa as ellective cul-oll wavelength jie, for MU) 0.1 dB ‘The resultant total disporsions for the modes shown in re (1) s shown in Bigute (2), nom opt Tins Winekagh > Figure 2} The mulimede fiber allows more than ous mods of transmission, This transmission acenrs until che tronsmission| ‘wavelunggh sles than the cut wavelength (2), The ype of Aispession that eeu in is imtrmastal disp sion where the wavelen; ‘The average ofthe refractive index over the mem Lough which the fundamental mede propagates is kuowi as effective refractive inex The rate of change of phase of LP, made propagating Where 2, = Wavelength ofthe LP,, mod Ejeutive rftactve inde o'single made bri given by ny Bik Hnce, the effective avtiacive inden ol made A, Guadaaacatal Ton constant iv Fingle mode Ber varie = OS _ 3m = 2072.82 = 2873, [Number oF Q65. Find the cutoff wavelength for a step index fiber having core refractive index of 1.48, radius of core is 4.8 jim and relative index difference Is 0.25%, Given that, For scp index fiber Cove refiactive index, 2, = 148 Radius of core, a =4.8 jim Relative index difference, A= 0.2 025 100 A= 0.0025 Considering, a single mode optical fiber, By substituting all the comesponding values in equation (weet Dame 4.810% 1L48(2 «0,0025)!7 at a wavelength of 1.3 im have beat lengths of 0.7 mm and 80 m. Determine the modal bi- refringence in each case and comment on the results. Given thot Fiber operating wavelength, 21.3 mmm Beat kenuth, Beat length. Ls, Modal i 0.7 mm 80mm nee. B=? The expression for bireffingence is given hy. B= ta For heat length, Ly =07 mm 2 _ isd ® a0 1.86 » UD 5, ~ 1.86 10° indicates that fiber is high nee fiber, For hoat length J, — 80 min Lato g,-% = 13a" Ly Sox107 = 163 ™ Ui B, ~ 1.63 * 10° indicates that the fiber is low birelringence fer. 1.6 _ FIBER MATERIALS - GLASS, HALIDE, ACTIVE GLASS, CHALGENIDE GLASS, PLASTIC OPTICAL FIBERS Q67. List the requirements that must be satisfied by materials used to manufacture optical fibers. An Fiber Materials Most of the fibers are made up of glass consisting of either Silica (Si0,) oF Silicate, High-loss glass fibers are used a sas for short-imnsmission distances an low-loss glass ers are sed forlong distance applications. Plastic fibers are kss ened ssersio "aor _ se 4, | Meaofthetigheratenaton tan hs hers 2.403 ~ 2.403 Glass Fi The gas fhersare made fom oxides. The mastconion = 13110 oxide i silica whose reffative index is 1438 at $30 am. To at different nao fbr, the dapams such as GeO, P.O, ae 1B. inl t silica, GeO, and P,O, ineoases the relative inden 1000 whereas fluorine or B,O, decreases the refractive index, Few fiber compasitions ae given below as follows, 1313 10" 107 1313 «10 pgm d y 1. GeO, SiO, Core: SiO, Cladding P,0,—Si0, Core; SiO, Clulding SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS. SIA GROUP 3 Scanned with CamScanner1.38 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] The composed of pure silica is refered to as silica alas, silica of fused silica, Some desirable properties of silica are, ingle raw material Tor sia Iwan The g 1. Resistance to deformation at temperature as bigh as 1000 2. High sesistance to breakage from thermal shock. 3. Good chemical durability. 4. Mish anspareney in both the visible sud infrared Basie Requirementsand Considerations in Fiber Fabrication 1. Optical fibers should aye masimmum reproducibility. 2 Fihers shoul be fabricated with good sable ranseission| chavacteristies Le. the fiber should have invari ‘ransnission characterises in long lengths. Dilferent size, rettactive indes ond refractive index. profile, operat available to meet different system applications wayclengls material, Fiber must be 4. Thefihers must he flexible to convert ino practical eables without any degradation of their characteristics, Fibers must be fabricate! in such a way that fling (splicing) ofthe fiber should not alee it chusacteritiey und the fibers may be terminated or ccannected together with less practical dtficulies, iher Fabrication in a Two: we Process 1. Initially glass is produced and then converted into preform or rod 2 Pulling or drawing othe preform into fiber Q68. Write in detail about glass fibers. Ans: Glass Fiber Glass liber is anistuce of sfenides, slides and mtal ides. Ir ean he classified into, 1. Halide Glass Fibers 2. Active Glass Fibers CChalgenide Glass Fibers Glass ig made of pure SiO, which efaetve index 1458 at $30 nim, The reactive inden of SIO, caa be ineeeased oF decreased by adding various exides is known as dopants The oxides GeO, oF P.O, inereases the refractive index and B,O, decreases the reffactive index of SiO, combinations are, ( Gee,-SiO, Core; SiO, cladaing Gi) P.O, -Si0,, Core: $10, cladding (il) S10, Core: BLO, - SiO, cladding (ix) GeO, BO, - SiO, Core: B.O, ~ SiO. cladding From absve, the refactive index of core fs maximum ‘compared tw the cladding 1 Halide Glass Fibers Halide glass fibers contains fuorine,chlorins, bromine ad foi, Tlie most common Halide glass iberis heavy “metal oorde glass ruses ZrF, asa major component. This fluoride glass isnown by the name ZBLAN. since 1's constituents are /1F,, BaF, Lak, AIF, and Nak The percentages of these elements to form ZBLAN Muoride las is shown as fallow, Materia Molecular percentage ae, Bat Lak AF, Nu These materials adds up tomake the eore ofa plss fiber. By replacing 21, by Hak, che lower refnerive index glass is cobiained The it ichsan ofthese plasesis 01 100,01 dB, 2. Active Glass Fibers Active glass fers are formed by adding erhivin and neodymium 0 the glass fibers. The above material performs amplification a attenuation 3. Chalgenide Glass Fibers CChalgcnide glass fibers are discovered in order make tse of the nonlincar propeitics of glass ibs, Ir contains ether “S", "Se" or "Te", because they are highly nonlinear and it also contains one element from PT, CP Br, "Ca Ba’ oS? The mostly used ehalzenide plas is AS.S,, AS, S..Se isused wy make the eore und AS.S, it used to make the cladd anurerial oe glass ibe. The iasertion Loss is arouud | dB Q69. Explain about “chalgenide glass fibers”. Ans The optical fier in which hoth the core and cadding lasses are macs up of chalcogen elements as major constituents isealled chalgenice glass Her These glass hers must be made up of atleast one chaleagen (Group IV elements le 8, Se. Te an other elements such as PI, Cl, Br. Cd et Characteristics 1. These glasses support 1300 telecommnication and lo 1500 am wavelengths ia cavelength applications «hig refractive index plete infrared to tte spectrum, Look for the SLA GROUP Loco Gp on the THLE GOVER beter you bay Scanned with CamScannerUNIT-1. (Overview of Optical Fiber Communication) 1.39 X__These glasses have poor quantum eficieney, The typical value of quantum efieency ranges fom 3 —5%, ss fiburs have small absorption cross scetion, 6. un low photon frequeney and hence, they can he used for laser transition in near ancl mide HR region, 7. Single mode chalgenide fber core is made oF AS,, Sq, Se, and the eadding is made up of A 8, “These glasses exhibit loss of arcund 1004 dl8km typically 9. Chulgenide elass has feaures of high optical, wonelinearity aul long length of interaction, 10. These properties help in creating optical ampliits, fiber lasers oprical switches hy cholsenice glass G70. Write short notes on plastic optical fibers, Ans: Plastic optical fiers ar the fibers which are made up of plastic material The core of this fiber is made up af Poly Methyl Methaerylate (PMMA) of Per Flourmated Polymer (PEP), Plastic optical Gibers offer move attcnvatioa than glass fiber and is used for short clstance applicatious. These fibers are tough andl durable due tothe presence of plastic material. The modulus of ths plastic material is two orders of magnitue lower than that of silica and even | nim diameter graded index plastic optical fber can be installed in conventional fiber cable tones he diameter of the core oF these hers are 10-20 ines larger than that of saezificiag coupllug efficiencies, So we eaa use inexpeasive connector moulding technology. ass fiber which reduees the connector losses without splices and transceivers made up of plastie injection- Graded inde plastic eptival fiber is in great demand in customer premises tv deliver hiul-speed services duc wo its hight bani “The three hey ftons tobe satisfied by the ma rial used for designing an optical fiber ae 1. Flexibility to mould itt.any size, shape ete 2. should wuld the ight efficiently 3. Availabilty of compatible materials for eare and cladding manutctane The optical fibers made of plastic do satisfies the ahove three conditions The plastic optical hers are used for high speed services. The high bandwidth graded index Polymer (plastic) Optical Fibers (POF) provide high spoed data services. The POF contains polymethyl methacrylate or & perlucrinatd polyence as its core. Thus, these fibers are named as PMMA POF and PEP POF, They are used because oftheir durability and toushness that withstands in abusive environments connector tolerance is increased in POF's (keeping coupling efficiencies constant) hecause of their larger core diameters tha silica Is easy 1o manufacture POF eennecters than silica, The plastic i 8 cheaper mnteial and abundant, [lence, components needed for POF's like spies, connectors can also be manufactured whenever required. The rmuin disadvantage of POF's is they have more losses than silica opel fiber ‘The PMMA has core rains 0.2 mm and eladling 14 650 mim wav length, Hopes dis 0,5 mn sith NA~ (0.25, This exhibits a stenation of [50 dB tevoveru bandwidih «2.5 Ghis ver 100 m. Thisean be used for. because a low speed they’offer higher attenuation, “The PFPOF has core radia of 0.0625 -0.15 mm and eladding radius of 0.125 -0.3 mm with NA~ 0.20. Ie exhibit 60— S143. Attenuation por km at 650-31 nm. Halso operates on same bandwidh as PMMA. Hence, this ean be use fr medias speed applicarions also os atenustion is considerably less thaa PMMA. Kora same speed! PPP has 80 dB less atenvtion thus PMMA, Hence. iis most preferable, ly high spood application G71. What are the mechanical properties of optical fibers? Explain. Ans: Mechanical Properties of Fibers The fundamental mechanival properties oF the fibers are Siren State Fa Dynan tai These properties play a vital role when the fibers are nse as medium For transmission of da system, The fibers should be eapable of hearing stresses and strwins induced during the cabling pr SPECTROM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP (> wwoptical communication Scanned with CamScanner1.40 OPTICAL COMMUNICATIONS [JNTU-HYDERABAD] T Seegih The srength ofthe fers is confined de ta the moto cracks or stresses at shir surfaces. Wh re Crack ip Si ackio (aun) Exered (0) her one Figure: Hypothetical Model of Micro Crack The ‘igure represents the hyposherical model of micro crack, also known as Griffith miro crack. The shape ofthe mero crack is lips! The sn wth ofthe her erack is given by: K-WKo Where, 1K ~ Sess intensity factor ranges from 0.6 190.9 MN Y— Micro erack geometry constant As the fiber consists of many erucks of dilferent sizes distributed randomly, ils fracture strength must be represented statistically, The commutative peobability 0 fiber failure is piven by. fio.Ly-1 eo) wn Where, £- Length of fiber a — Sires level No) — Total erucks per unit long und ean be expressed by using Weibull distbution as ‘oY via A 2 iting equation 2) in equation (1), ‘Whe, fal mance constants wit inet stent cistibaton, By sub vat) 1-ea) The commutative probbility of fiber failure is given by, Fiat. 2. Statfe Fatigue ‘Under the humid conditions and tensile stress the wradual increases inthe wniero cracks ofa glass is called “static causes fiber failure even at very low sires levels ‘The chemical erosion due to the water molecules al the miro erack tip also inereuses stale fatigue, particularly when the fiber is kept under pressure, To avoid tbe cracks duc to chemival crosiva, coatings are applied iraiately alice Uieir facture 3 tamie Fatigue Te suring effects on an optical fiber when itis installed in duct causes the fiber to experioace repeaied sites, ds 10 which cracks appear. This phenomena ix eallad dynamic fatigue “The fher cam be moe eclableand immune to dynamic Fatigue hy of proof testing, Here, fibers are expos tt tensile {oad nueh ercater tan the expect! lad a fiber may esperisuee during manulsetuing, installation and service, The bers which do not pass this test are discarded. Aer completion oF proof test, the failure probability (,) fer is expressed as. Look for the SIA GROUP Loco {J on the TITLE COVER before you buy Scanned with CamScanner