S-",. J ohn. M.. 19 !>1 -Oc>l ical 'betEl itll,ogrl phy: p.l nc:kJdes incil lelectrical permittivity, of free space (go), relati ve (1: ,)solid acceptance anglequantum efficiency (optical detector)angular coupling efficiency (tibet joint)couplingefficiency (optical source to filler)differential external quantum etTlCiency (optiee.1 source)external power efficiency (optical source)internal quantum efficiency (optical source)lateral coupling efficiency (fi ber joint)overall power conversion efficiency (optical ' Duree)total external quantum efficiency (opticaI IDuree),lGLOSSARYOF SYMBOLS AND ABBREVIATIONS9 angle, fiber acceptance angle (0,,), Bragg diffraction angle (8. )!I. acoustic wavelength, period for perturbations in a fiber'" cutoff period for perturbat ions in a fiberA optical wavelength~ long wavelength cutoff (photodiode)A" wavelength at which first order dispersion is zero~ magnetic permeability, relative permeabilityw,). permeability of freespace linear retardation~ angle, critical angle (4'0)V scalar quantity representing E or Hfield0) angular frequency. of the subcarrier waveforminanalogtransmission(CIl(), of the mooulating signal in analog transmi ssion (ro",)lIl" spot size of the fundamer.tal modeV vector operator, Laplacian operator (V1)A-D analog to digital CMOS complementary metala.c. alternating current oxide siliconAGC automat ic gain CNR carrier to noise ratiocontrol CPU cent-at processing unitAM amplitude modula Lion CSP channelled subsrrateAPD avalanche photodiode planar (injection laser)ASK amplitude shift keying CW continuous wave orBER bit error rate operanonBH buried heterostruct ure D-A digital to analog(injection laser) dB decibelBOD bistable opt ical device D- IM direct intensity mcdula-CAM computer aided manu- tionfacture DBF distributed feedback (inCATV common antenna televi- ject jon laser)"onDBR distributed Bragg renee-CCTV erose circuit tele\o; sion tor (injection laser)CDH constricted doobic de. direct currentheterojunction (injectiQn DH double heterostructure or11m) heterojunction (injectionoodld I'ftII'k IIvtnlon Iller or LED)xvIIIGLOSSARY OF SYMBOLS AND ABBREVIATIONSplasma-activatedchemical vapor deposi -tionplano-convexwaveguide(injection laser)probability d.:nsity func -t ionpulse frequencymod ula-tionp-i-n photcdiode fol-lowed by a field effecttransist orphase modula tionpulse position modulationphase shift keyi ngPost , Telegraph and Tele-communicationspulse width modulationreach-through avalanchephotodioderadio frequency inter-ferencerootmea n sq uarerelaxat ion oscillationret urn to zerosurface acoustic wavespace division multi- su per high frequencyseparated mulliclad layer(injection laser,signal to noise ratiotime division mult iplexingtransverse electrictransverse electromag-net ictransverse junction stripe(inject ion laser)transverse magnetictrlogicultra high frequencyvapor axial depositionvoltage co ntrolled oscil-latorveryhigh frequencyvapor phase epitaxywavelength division multi-plelinaWentzel, Kramer" Bril-louin (&rI . ly.i. niql,lr) for ....dad fiberwldthnd ,witch pomtZfnlat dioc1tPCVDPCWPDFPFMPI N-FETWKBTJSPMPPMPSKP1Tem,RORZSAWSOMPWMRAPDUHFVADVCORF'VHFVPEWDMSNRroMTETEMnlTILSHFSMLWPSZDdouble sideba nd (a mpl i-tude modulation)tr aditionalmode designa-tionelectromagnetic iet er-terenceelectromagnetic pulseerror functioncomplementary errorfunctionfreq uency divisionmulti-plexingfield effect transistorfrequency modulationfrequency shift keyingfull width ha lf powerhigh density bi polartraditionalmode designa-tionhelium-neon (laser)high frequency .high vol tageintermediate freq uencyinjection laser diodeintensity modulationintegrated opticsinput/out putintersymbol interferencelocal area net",-or"li ght emitting diodelarge optical(in-jection laser)li nearly polarized (modenotation)liquid phase epitaxymodifiedchemical vapordepositionmet al Schot tky fieldelTeet transistormetal lntegrated-seml-conductor field effecttransi st orneodyrninm-dopedyttrium- aluminum-garnet (laser)nonreturn to zerooptical time domainreflectometrvoutside va por phaseoxidationpul se a mplitude modula -

pulse code modulationplutic-cl&d silica (fibetj___ ':L , :::...;-_.1MISFETNd :YAGNRZOTDROVPOFETFMFSKFWItPHDBHEFDMMESFETEMIPAMDSBEHLPHe- NeHFHVIFILD1M10I/ OlSILANLEDLOCLPEMCVDPCMPCSEMP,nerfc1IntroductionCommunicationmaybe broadlydefinedasthetransfer of informationfromone point to When the information is to be conveyed over anydistance a. communication system is usuallyrequi red. Wit hin a commcnicntion,yst ernthe informationtransfer is frequentlyachievedhy.s upen mposingormodulat ingthe informationonto anelectromagnet ic wave whichacts 3!> acarrier fortheinformationsignal. This modulated carrier is then tr ansmitted tothe required dest ination where it is received.and the original information signal"15 obtained by demodulat ionl Sophisticated techniques have been developed forthis process usingelectromagnetic carrier wavesoper anog at radio frequeocics .>u well asmicrowaveandmilli meter. wavefrequencies, However, ' communi-cation' may also be achievedusing an electromagnetic carrier which is selected(romtheoptical range01frequencies. ! Gto' HISTORICALDEVELOPMENTTheuseofvi sibleoptical carrierwaves orlight for communicat ionhasbeen-. common for many years. Simple systems such as signal fires, ref1 ccting' mirrors and, more r ecently, signall ing lamps have provided successful, if'limited, informationtransfer. Moreover. asearlyas 1880AlexanderGra hamBell reported the transmissionof speech using a light beam[Ref. I I. Thephctophoneproposedby Bell j ust fouryearsafter theinventionof thetele-phonemodulated sunlight witha diaphragm giving speech tr ansmission {!Vcradlttance of 200 m. However. although some investigat ion of optical cum-municationcontinuedin theearlypanof the20t h Century[Refs, 2 nnd 31it susewas limited to mobile. low capacity communication huh. Thiswas due toboththe lackof suitable light sourcesand theproblemthat light transmissionIntheatmosphereis restrictedtoline of sightandseverelyaffectedby di st ur-ances such as rain, snow, fog. dustand atmospheric turbulence, Neverthelesslower frequency and hence longer wavelengthelec tromagnetic waves" (i.e.radioandmicrowave)provedsuitablecerriersfor informationtransferin thelJMpropq.tlon0( elecuolTlJ,l r:etie in freesoece,.... \elength }..the; \'IlaDItr of IfJhtInI 'I'IClI lIme of theI inhem or"=df.:. '. ". - .r2 OPTICALFIBERCOMMUNICATIONS: PRINCIPLESANDPRACTICEII)I~,IIatmosphere, being farless affectedbytheseatmosphericconditions. Depend-ing Oll their wavelengths these electromagnetic carriers can be transmitted overconsiderabledistances but arelimitedintheamount of infor mationtheycanconveybytheirfrequencies (i.e. theinformation.carrying capacity is directlyrel atedto the bandwidth or frequency extent of the modulat ed car rier. which isgenerallylimited10 afi xedfractionofthecarrier frequency). Intheory, thegreaterthe car rierFrequency, thelarger theavailable transmissionbandwidthandthus theinformation-carrying capacity of the communication system. Forthi s reason radi o communication was devel oped to higher frequencies (i.e.VHF a nd UHF) leadingtot hc introduction of t he even higher frequencymicrowave a nd. latt erly, millimeter wave transmission. The relative frequenciesa ndwa velengths of theset ypes of electromagnetic waveCAn be observedfromt he electromagneticspectru mshowninFig. 1.1. Inth is cont extit mayalsobenoted t hai communication at optical frequencies offers an increase in t hepotent ial usablebandwidt h by a factor of around 10" over high frequencymicrowavetransmis sion. Anadditiona l benefit of theuscofhighcarrier fre-quen cies i ~ the generalabili ty of the communic at ionsystem to concentratet hea va ilable power withinthe transmutedelectromagnet ic wa ve. thus givinganimproved systemperformance[ Ref 4 1.Arenewed interest inoptical communication was stimulated intheearl yJ960swit ht heinventionof the laserIRef. 51. This deviceprovidedapower fulcoherent light sourcetogether withthepossibi lit yofmod ulationa t highfre--q uency. Inadditionthe10v. beam divergence of the laser made enhancedfreespace opt ical t ransmission a pract ical possibili ty. However, t he previouslyment ioned co nstraints of light transmiss ion in the atmosphere tended torestri ct thesesystemstoshort distancea pphca nons. Nevert heless, despitetheproblems some modest free space opt ical communication links ha ve bee nimplement edfora pplicationvsuchas the linkingofatelevisioncamera toabase vehicle and for da ta links of a fe whundred me ters betweenbuil dings.Thereisalso some interestin opt ical communication between satellites in outerspace usingsimilar techniques IRef. 6J.Although the use of laser for freespace optical communicat ion provedsomewhatlimited. t heinvention of thelaserinst iga tedatremendous researcheffort in t he st udy of optical components 10 achieve reliable info rmationt ra nsfer usinga lightwavecarrier. TIleproposalsfor opticalcommunicationvia dielectric waveguides or optical fibers fabricated fromglass were madea lmostsimultaneously in1966byKaoandHockham (Ref. 7J andWerts l Ref8J toavoiddegradationoftheopt ical signal bythe atmosphere. Suchsystemswere viewed as a repl acement for coaxial c able or ca rrier t ransmi ssionsystems. Initia ll y the optical fiber s exhibited very high attenuation (i.e.1000dB km")andwere therefore not comparable witht he coaxial cables t heyweretorepl ace (i.e. 5-1 0 dB km" ). There we re also serious problems involvedwit h joint ingt hefiber cablesinasatisfactorymannertoachievelow1055andtoena bletheprocess tobeperformedrelativel yea!>ilyandrepeatedlyintheINTRODUCTION,>~!0e ,5!,,>!v.. ....:l0!;,,-. EveE00,;, a-0,~00e, 0-.,00e~ .", >1c~1e1!"~0 0"L '-,'" ;;" 55I~ rI-'b,f'-0i0~,,s,0~ ;,,,00ic,,E ,, ,0- ,>,0~ ,,~ ~>0>H> 0e1i E,IE-a -a,, -.6U-0,,,I, - 00-~" ,, - equat ion.Furt hermore, practi cal parabulic refractiveindexpr ofile core fi bersexhibit atruncatedpa rabolicdist ribut ionwhichmerges intoaconstant refract ive indexat the claddi ng. Hence Eq.(2.92) is not exact for real fibers.Equat ion (2.92) docs, however, a llowus to consider themode number pla nespannedhy the radial andaz imuthal mode number s mandI. This plane isdispl a yed in Fig. 2.29 whereeach modeofthe fiber described by a pair ofmodenumbersis representedasapointint heplane. Themodenumber planecontains guided, leak y and radiati on modes. The mode boundary whichseparatesthe guidedmodes from t he lea ky and radi ation modes is indicated bythesolidline inFig. 2.29. Itdepicts a constant value of pfollowingEq. (2.92)a ndoccurswhen p= n1k. Therefore. all t he pointsint hemodenumber planelyi ng belowthe line p=nJ kare associated wit hgulded modes whereastheregionabovethi slineis occupiedby leakyandradiati onmodes. Theconcept.5(2.95)(2.96)(2.93)," MM " bun,l "l' ,M=4f (l) dlOPTICALFIBERWAVEGUIDES An uas low asLOmW insingl e mode fibers. Nevertheless. this is still ahigh p ower level foropti cal communications and may beeasilyavoided. Brillouinand Ramanscattering are not usually observed in multimode fibers because their relativelylargecorediametersmakethethresholdopticalpower levels extremelyhigh.Moreover it shouldbe notedthat the thresholdoptical powersforboth thesescatteringmechanisms maybeincreasedbysuitableadjustment or theotherparametersinEqs. (3.6)and(J .7). Inthiscontext. operationat thelongestpossible wavelength is advantageous although this may be offset by t hereducedfi ber attenuati on(fromRaylei ghscatteringandmaterialabsorption)normallyobtained.3.8 FIBERBENDLOSSOptic. 1fiberssuffer radiationlosses at bends or curves 00theirpaths. This isdueto theeoerlY in tbe evanescent field atthe bend exceedingthevelocity ofTil eth resholdopt ical power f orst imulat edRamal) scatt eringmClYbeobtJ i.wdf romEq. (3. 71, where:PR. 5.9 )( 10-ldl}.q.cIB'" 5.9 x 10-2X62X1 3 x 0 .5A IOfl9 si ngle mode oplic1l l l ibe . has a... enenceuco of 0 .5 ri b km- ' wh.moperi'll ;"'Il .1'a wa....ele" 9ttl of 1.3IJ.ml iber corediam" t ",r is 6IIJTl and tile1lr",.",,;.1J'''' ''';PD"f-- - --"'...."'""-- - ---so'"'""' OC ',."""' ''''o', "", ';--;';:--!;,--7'0-;'1 0 U 1.4 1 .."", , 1, ,,.,,,1, I"",,)11' '''';0].....-mP"J'I!l",n, ''''1;00 "",,-I,_., ..., )' TIl . rml pul.. brOl de'llngIn t erlTlS of t hemet e' ;11dispers ion paramet er followinglX. mpl. 3.8 II S1 hrtf't by:Hence.t h'" m1 SpulsebroadentrH;j per Io;;rometer due t omat erial dispersion:. lig'l ti nOl,ll tCllheml,l lt imode step inde.. fiber consisti ng of. nlelI.11lU1i. or r K t . ~ u . r NnlOt lonwith\I n;! lrea.ss OPTICALFIBER COMMUNICATIONS: PRINCI PLESANDPRACTICEThemeanvalue Al l for t heunit inputpulse of Fig_3. 10 is zero, and assum-ingt hisismaintained for the output pulse, then fromEqs. (3.29) and(3.31):cr. =M] = J oftheopticalpower isconcentratedinthefirst half of theinterval. Hencethermspulse broadening is a useful parameter for assessment of intermoda l dispersionin munimode graded index fibers. It may be shown [Ref. 35 1that the rms pulsebroadeningofanearparabolicindexprofilegradedindex fiber0"1 isreducedcomparedtothesimilar broadeningforthe correspondingstep index fiber0 ,(l,e. with thesamerelative refractiveindexdifference)following:gradingas wasdiscussedinSection2.5. However, followingEq. (2.40) thelocal groupvelocityisinverselyproportional tothelocal refractiveindex andthereforethelonger sinusoidal pat hs are compensated for byhigher speeds inthelower index medium away fromtheaxis. Hence there is an equali zation ofthe tra nsmission limesofthevarious trajectories towards the transmissiontimeof theaxial raywhichtravelsexclusively in the high indexregionatthecoreaxis. andattheslowest speed .Asthesevariousraypathsmaybecon-sidered to represent thedifferent modes propagating inthe liber, then thegradedprofilereduces thedisparityinthemodetransit times.Thedramaticimprovement inmultimodetiber bandwidthac hievedwithaparabolic ornearparabolic refracti ve indexprofile is highlighted by considera-tion of thereduceddelay differencebetween thefastest and slowest modes forthis gr aded index fiber o T ~ . Using a ray theoryapproac h the delay difference isgivenby(Ref. 331:92 OPTICALFIBER COMMUNICATIONS:PRINCIPLESANDPRACTICE{l"" ofIRefs. 35. 361:12,1.

sisgivenbycombining Eqs. (3.26) and(3.39)as [Refs. 27. 361:Lf/1t12".Example 3 .9(3.40)(3.4 1)IIijComparet hermspulsebrolIdenir'lgper ki lomeler d ue10 interrnodal disilersionforthe mutlimode stepinde>! f ibe' 01 example3 ,8 wilhthecorresponding rmspulsebroadeningl oranoptimumnearPdrabolicp rofil egrarl fldindel-oTTRANSMI SSIONCHARACTERISTI CSOF OPTI CALFIBERS100 OPTICAL FIBERCOMMUNICATIONS: PRINCIPLESANDPRACTICEwithregardtoconnectordesign lRef. 491inordertoreducetheshift inspecklepatterninducedbymechanical vibrationandfiber misalignment.Hence, modalnoise maybe prevented onan optical fiberlink throughsuitablechoice of thesystemcomponents. However, thismaynot alwaysbepossibleandthencertain levels of modalnoisemustbetolerated.Thistendstobe thecase onhigh qualityanalog optical fiber links where multimcde injection lasersare frequently used. Analog transmissionis also more susceptibletomodalnoise due to the higher optical power levels required at the receiver whenquantumnoise effects are considered (see Section 9.2.5). Therefore it isimportantthatmodalnoiseis taken intoaccountwithinthe designconsidera-tions for these systems.3.12 POLARIZATIONCylindrical optical fibers do not generally maintain the polarization state of thelight input formore thana few meters,andhenceformost applicationsinvol-ving optical fiber transmissionsome form of intensitymodulation(see Section7.5) of theoptical sourceisutilized. Theoptical signal isthusdetectedbyaphotodiodewhich is insensitivetooptical polarizationor phaseofthelightwavewithin the fiber. Nevertheless. recentlysystems and applications havebeeninvestigated 1Ref. 521 (see Section10.8) which do require the polarizationstates ofthe input light to bemaintainedover considerable distances, andfibers havebeendesignedforthispurpose.Thesefibersaresinglemode, andthe maintenance of the polarization state is described in terms of aphenomenonknown as modal birefringence.3.12.1 Modal BirefringenceSinglemodefiberswith nominal circular symmetryaboutthecoreaxisallowthe propagation of twonearlydegenerate modes with orthogonal polarizations.They are therefore bimodal supporting HEll and HEYI modes where theprincipal axesxand yaredeterminedbythesymmetryelements of thefibercross section. Thus the Fiber behaves as a birefringent mediumdue to thedifference in the effective refractive indices and hence phase velocities for thesetwo orthogonally polarized modes. The modes therefore have differentpropagationconstants ~ x and ~ y whicharedictatedbythe anisotropy of thefiber cross section. When the fiber cross sectionisindependentofthefiberlength Lin the z direction then the modal birefringence BFfor the fiber is givenby [Ref. 53],(3.47)It may be noted that Eq. (3.51) may be obtaineddirectly fromEq. (3.48)where:101(3.52)(3.50)(3.51)(3.49)(3.48)TRANSMISSIONCHARACTERISTICSOFOPTICALFIBERSwherecisthevelocityoflight ina vacuumand 6A. isthesourcelincwutth.However, when phase coherence is maintained (i.e. over the coherencelength) Eq. 3.48leadstoa polarizationstatewhichisgenerallyelliptical butwhich varies periodically along the fiber. This situation is illustrated inFig. 3.16(a) [Ref 531wherethe incident linear polarizationwhich is at 45withrespect tothe xaxisbecomescircular polarizationat 41...,...n12, and linearagainatc1l = n. The process continues through another circular polarization atcJi = 3nl2before returningto the initial linear polarization at c1l = In. ThecharacteristiclengthLscorrespondingtothis process is knownasthe beatlength. It isgivenby:Substitutingfor SF fromEq. (3.47) gives:2.L. ~ -;;c-"'-;,-,( ~ . - ~ . )Typical single mode fibers are found to have beat lengths of a fewcentimeters [Ref.55], and the effect maybe observed directlywithin a Fiber viaRay1eiih scattering with useofa suitablevisible source(e.g. He-Ne laser)[Ref. :561. It appears BS a series of bright and dark bands with a periodwhere A. is theoptical wavelength. Light polarizedalongoneof theprincipalaxes will retain itspolarization for all L.The difference in phase velocities causes the fiberto exhibita linear retarda-tion4I(z) which depends onthe fiberlengthLin thezdirectionand is given byIRef. 531:assuming that the phase coherence of the two mode components is maintained.Thephasecoherenceof the twomode components is achieved when the delaybetween thetwotransit times is less thanthe coherence time of thesource. Asindicated in Section 3.11 the coherencetimefor the sourceisequal tothereciprocal oftheuncorrelated sourcefrequency width(lIof).It maybe shown[Ref54J that birefringentcoherenceis maintainedoveralengthoffiber Lbc- (i.e. coherencelength) when:102 OPTICALFIBERCOMMUNICATIONS: PRINCIPLESANDPRACTICEFig.3.16 An illustrationof the beat lengthin asinglemode optical fiber [Ref. 531: {a) thepolarization states against illizI; Iblthe light intensity distribution over the beatlength within the fiber,corresponding tothebeat lengthasshown in Fig. 3.16(b). The modal birefrin-gence B1may bedeterminedfromtheseobservationsofbeat length.Example 3.11Thebeatlengthinasinglemodeopticalfiber is9 ernwhenlight fromaninjectionlaser witha spectral linewidthof1 nmand a peak wavelengthof 0.9urn is launchedintoit. Determinethemodal birefringenceandestimate the coherencelengthinthissituation. In addition, calculate the difference between the propagationconstants forthe twoorthogonal modesandcheckthe result,Solution: Tofind the modal birefringence Eq. {3.501 maybe used where:A 0.9x10-"SF~ ~ = 1x10-'LB0.09KnowingSF' Eq. (3.491 may be used to obtainthe coherence length:1.20.81 X 10-12Lbe~ - - 'C', = 81 mBFIiJ.. 10'x10 gThedifferencebetweenthepropagationconstantforthetwoorthogonal modesmay be obtainedfromEq, {3.51) where,271 21113.-13y=- ~--= 69.8La 0.09The result may be CheckedbyUIln; Eq, 13.47) whirl:_..... _.._ ~ ~Hence, for a beat lengthof 0.7mm:103(3.53) 69.8211BF211X 10-0801,3x10-'BF==1.63x10----llExample 3.12TRANSMISSIONCHARACTERISTICSOFOPTICALFIBERS1.3xBF = =1,86xlO-30.7X ).

B,However, the cross-polarizingeffect maybeminimizedwhentheperiodofthe perturbations is less than a cutoff period A..: (around 1 mm). Hencepolarization-maintainingfibers maybedesignedbyeither:This typifiesa high birefringencefiber.For ebeat lengthof 80 m:whIchIndlCltll 8' lowbirefringencefiber.Twopolarization-maintainingfibersoperatingatawavelength of1,3 limhavebeatlengthsof 0.7mmand80 m. Determinethemodal birefringencein eJchcaseandcomment on the results.Solution: Using Eq.the modal birefringence isgiverl by:(a) High(large)birefringence: themaximizationof themodal birefringence,whichfollowingEq. (3.50), maybeachievedbyreducing thebeat lengthLHtoaround 1 mmor less; or(b) Low(small)birefringence: theminimizationofthepolarizationcouplingperturbationswith aperiod of A. Thismaybeachieved by increasingA..:givingalargebeat lengthof around50 mor more..In anonperfect tiber various perturbationsalongthefiber lengthsuchasstrainorvariations in thefiber geometryandcomposition lead to coupling ofenergyfromone polarization to theother.Theseperturbationsare difficult toeradicateas theymay easily occur in thefiber manufactureandcabling. Theenergy transfer isat a maximumwhentheperturbations haveaperiodA,correspondingtothebeat length, anddefinedby[Ref. 52J:104 OPTICALFIBERCOMMUNICATIONS: PRINCIPLESANOPRACTICETechniques are being developed in order to produce both high and lowbirefringencefibersin-ordertofacilitatecoherentopticalfiber communicationsystems. Fibersmaybe madehighlybirefringent by deliberately inducing largeasymmetric radial stress. This may be achieved through thermal stress byusing materials with widelydifferent expansion coefficients coupled withanasymmetrical elliptical structure. A linear polarization state has beenmaintained overa kilometer of fiber withanextinction ratio of 30 dB using thistechnique !Ref. 571. Further investigation of the optimal cross sectiongeometryfor highbirefringencehas suggested [Ref. 57] fiber corecrosssec-tions shaped as a bowtie (bowtie fiber).To design lowbirefringence fibers it is necessary to reduce the possibleperturbations within the fiber during manufacture. Therefore extreme caremust bc taken whenjacketing and winding these fibers in order to reducebandsor twiststhat maycontribute tobirefringence. It is also necessary to usematerials which minimize the thermal effects that maycreate birefringence.Onetechnique usedtominimizethetemperaturedependence of birefringencewhichhas provedsuccessful istospinthefiber preformduringmanufacture[Ref 581. This method, which reduces thelinearretardationwithinthefiber,has producedfibers with nobirefringent properties and variations inoutputpolarizationresult onlyfromfiber packaging. However, evenwiththeselowbirefringence spun fibers some formof polarization controller IRef. 591 isnecessary to stabilize the polarization state within the fiber.PROBLEMS3.1 Themeanoptical power launched into anoptical fiber linkis 1.5 mW and thetiber has an attenuation of 0.5 dB kmI. Determine the maximumpossible linklength without repeaters(assuminglosslcssconnectors) whenthe minimummeanoptical power level requiredat thedetector is211W.3.2 The numerical input/output meanoptical power ratio in a I kmlengthofoptical fiber isfound tobe2.5. Calculatethereceivedmeanoptical powerwhen a mean optical power of I mWis launched into a 5 km length of the tiber(assumingnojointsor connectors).3.3 A15 kmoptical fiber linkuses fiber witha loss of1.5 dB kmI. Thetiber isjointedeverykilometer withconnectorswhich given anattenuationof 0.8 dBeach. Determinetheminimummeanoptical powerwhichmust be launchedinto the fiber in orderto maintain a meanoptical power level of OJjJW at thedetector.3.4 Discuss absorption losses in optical fibers comparing and contrastingtheintrinsicandextrinsicabsorptionmechanisms.3.9 Explain what ismeant bythe critical bendingradius for anoptical fiber.Asinglemodestepindexfiberhasacritical bendingradius of 2mmwhenilluminated with light at a wavelength of 1.30 urn. Calculate therelative refrac-tive index differencefor the fiber.3.10 Agraded index fiber has a refractive indexat thecoreaxis or 1.46withacladding refractive index of 1.45. The criticalradius of curvature whichallowslargebendingtosses tooccur is S4 urn whenthefiber is transmittinglight of aparticular wavelength. Determine the wavelengthofthetransmitted light.3.6 A K20-Si02glass core optical fiber has an attenuation resulting fromRayleigh scattering 01'0.46 dB kmI at a wavelength of I urn. The glasshas anestimated fictive temperature of 758K, isothermal compressibility of8.4 x 10-11m' N-1, andaphotoelastic coefficient of 0.245. Determine fromtheoretical considerations the refractiveindex ofthc glass.'053.5 Brieflydescribelinear scatteringlosses inoptical fibers withregard to:(a) Rayleigh scattering;(b) Miescattering.The photoclastic coefficient and the refractive index for silica arc 0.286and 1.46 respectively. Silica has an isothermal compressibility of7 x 10-11m' N-1andanestimated fictivetemperature of 1400 K. Determinethe theoretical attenuation indecibels per kilometer due tothefundamentalRayleigh scattering in silica at optical wavelengths of 0.85 and 1.55 lim.Boltzmann's constant is 1.381x10-23JK.3.7 Compare stimulated Brillouin and stimulated Raman scattering in opticalfibers, andindicatethe wayin which they may be avoidedin optical fiber com-munications.The thresholdopticalpowers for stimulatedBrillouinandRamanscatteringinalong8IJ.mcorediametersinglemodefiberarefoundtobe190 mWand1.70 Wrespectivelywhenusinganinjectionlasersourcewitha bandwidthofI GHz.Calculate theoperatingwavelength of thelaserandtheattenuationindecibels per kilometer ofthe fiber at this wavelength.TRANSMISSIONCHARACTERISTICSOFOPTICAL FIBERS3.8 The thresholdoptical powerforstimulatedBrillouin scatteringat a wavelengthof 0.85IJ.minalongsinglemodefiber usinganinjection lasersourcewithabandwidth ofSOO MHzis127 mW.Thefiberhas an attenuation of2 dB km-Iat this wavelength. Determine the threshold optical power for stimulatedRamanscattering withinthe fiber at a wavelength01'0.911n1 assuming the fiberattenuationis reduced to I.S dB km I at this wavelength.3.' 1 (a)A multimode stepindexfiber gives a total pulse broadening or 95 nsover a5 kmlength. Estimate thebandwidth-lengthproduct forthefiberwhena nonreturntozerodigital codeis used.(b) A single mode step index fiber has a bandwidth-length product of10 GHzkm, Estimatethermspulse broadeningover a40 kmdigital opticallinkwithout repeatersconslsttng of thefiber. andusing areturntozero code.106 OPTICALFIBERCOMMUNICATIONS: PRINCIPLESANDPRACTICE3.12 An8kmoptical fiberlink wi thout repeaters uses multimode graded index fiberwhich has a bandwklth-lengrh product of -4 00MHz km. Est ima te:(al thetotal pulse broadeni ngonthelink:(bl thermspulse broadeningon thelink.It may be assu med that areturnto zerocodeisused.3.13 Brieflyexpl ainthereasons for pulsebr oadening duetomaterial dt-oer sioninoptical fibers .Thegroupdelay T ~ in an optical fiber is givenby:T ~ . :(n,-",",)" dJ.where c is me velocity of light in a vacuum, n , is rhe core refracti ve index a nd). isthewa velength of thetransmit tedlight. Dcnveanexpressionfor the rmspulsebroadening due10 mater ial dispersioninanoptical fiber anddefine themat erial dispcrslunparameter.Thematerial dispersionparameterfora gla ss tiberis 20 psnm " ! km" at awavelengthof I.S urn. Est imatethepulsebroadeningduetomaterial disper-sionwithin thefiber whenlight is launched fromaninj ection la ser source withapeakwa velengthof 1.5J1D1 andanrms spectra l widthof2 nmintoa30kmlengthofthe tiber.3.14 The mater ial di spersion in an optical fiber ddined by 1d2" lId). 21is 4. 0x10-2IJ1T1 2. Estimatethepulsebroadeningper kilometer due10mal erial dis-persionwit hin the fiberwhen it is illumi natedwithall LEOsource with apea kwavcjength of 0.911JT1 anda nrmsspectral widthof45 nrn.3.15 Describet hemechanismof inrermodaldispersioninaroultimodestepindexfiber.Show that the total broadening of a light pulse oT, due to int ermodaldisper-sionin a mu ltimodc step indextibe r may be given by :whereLis the fibcr lengt h, NA is thenumer ical apert ure of thefil:>er , n I is thecore refractive indexand ciii the velocityoflight in a vac uum.Amuhimodestepindexfiber hasa numerical ape rtureof 0.2a ndacorerefractiveindex of 1.47. Estimatetheba ndwidth- lengthproduct for thefiberassumingonly intermodal dispersion anda ret urn tozerocode when:(a) thereis no modecouplingbetweent he guided modes;(b) mode coupling betweentheguided modesgives acharacteristiclengthequivalent to0.6of the act ual fiber length.3 .16 Using the rela tionfor (iT, gi\'enin problerr. 3.15. derive anekpressionforthe rms pulse broadeningdue to lntermodal di spersionina mufl imodestepinde xfiber. Compare thisexpressionwithasimil ar expressionwhichmaybeobt ainedfor anopti mumnear parabolic profLie I radedindex. fiber.3.21 Describe thephenomenon of modalnoise inoptical fi bers andsuggest how itmaybe evolced.3 .20 Discuss dispersionmechanisms withregard10singlemodelibers indicatingthe dominating effect s. Hence describe howint ramodal disper sion maybeminimized withinthesingle-mode region.3 .23 Asingle mode fibermaintain s birefrin gent coherenceovera length of 100 kmwhenit is illuminatedwithaninjection laser source with a spectrallinewidth of1.5 nmIlItda peak. wa velength of1.32 urn. Esti ma te the beat length withi n thefiber andco mment on the !"euIL101 TRANSMISSIONCHARACTERISTICSOFOPTICALFIBERS3.22 Expl ai nwhat is meant by:(a) modal birefringence;(b) thebeat length;in single modefiber s.Thedifferencebetweenthepropagationconst antsfor theIWOonhogonalmodes in a single mode fiber is 250. It is illuminated with light of peakwavelength 1.55 umfrom aninjection laser so urce..... itha spcctr allinewldth of0.8nm. E."timatethecoherencelengthwithinthefiber .3 .18 Amulttmode, optimu m near par abolic profile graded index fiber has a materialdispersi on par amet er of 30 ps nmI km-'when used witha goodLEDso urceof r rns spectral width25 nm. T he fiberhacanumericalaperture or O.4andacoreaxisrefr activeindex of 1.48. Estimatethe tot al rms pulse broadeni ng perkilomtter withinthe fiber assuming waveguidedispersion 10benegligible.Henceest imatethebandwidth-lengthproduct for thefiber .3 .17 An 11kmoptical fiber linkconsistingof optimumnea r parabolic pror:J egraded inde xfi berexhibits rms intermodal pulse broadening of 346 ps overitslength. If the fiber has arelativerefract ive index differenceof 1.5%, estimateIhe core SKis refractive index. Hence determine thenumerical aperture for thefiber .Estimatethebandwidth-lengt hproductfor thestep index fiberspecificdinproblem3. 15 eonr.idcringthcrmspulr.ebroadening. due tointerrnodaldisper-sion within the fiber andcomment ontheresult. Indicate the poMibkimprove-ment in the bandwidrtr- Icngrh product when an optimumnear pa raboli..:profile graded index fibe r with the same relative refract ive index difference andcore axis refractive index is used. In both ca ses assume only intermodal disper-~ i O D .....ithinthefiber and the U ~ ofa ret urn to zerocode.3 .19 Amultimoot stepindelt fiberhas arelativerefractiveindex differenceof 1%anda core refractive index of 1.46. The maximum optical band....-idth that maybe obtained witha pan icul arsourceona 4.5 kmlinkis 3.1 MHz.(3) Determine the r ms pul se broadeni ng per kilometer resulting fromintramodal di5 1, 1978.56 A.Papp a ndH. Harms, -polar ia at ion optics of inde x- gradientopncal waveguidefi bers'. Appl.Opt . 14. pp. 2406-24 11, 1975.57 A. J. Ba rlow. D. N. Payne. M. P. varnhamand R. D. Birch, ' Polarisationc haracteristi cs of fi bres for coherent detect ion systems', l EE Colloq. onCo herence inOpt. Fibre S)'f. t., London, 25th Ma y 191( 2,58 D. N. Payne, A. J. Barlow and J. J. Ramskov Hansen, ' Development of lowandhigh birefri ngence optica l fi bres' . I EEEJ. Quant umElettron.; QE- 18(4), pp.477-487, 1982.59 R. Ulrich,'Pol arisat ionstabilisationonsingl e-modefi bre' , Appt , Phys. Leu., 35,pp. 840-842, 1979.60 M. J. Adams, D. N. PayneandC. M. Ragdale,' Birefringenceinopt ical fibreswithelliptica l cross-section', Electron. Len.,pp, 298- 299, 1979.61 T. Kat suyama, H. Matsu mu raand T. Suganuma. ' Low lou single-polarisationfibres', Electron. Lett" 17(1 3), pp. 473-474, 1981. ',I11041 !,42!43I 44,454.\47I484"504Optical Fibers, CablesandConnections4.1 INTRODUCTIONOptical fiber waveguidesand theirtransmission characteristics have been con-sideredinsomedetail inChapters2and3. However, we haveyettodiscussthe practical considerations and problems associated with the production,applicationandinstallation of opticalfiberswithina line transmission system.Thesefactors arcofparamount importanceifoptical fiber communicationsystemsarctobe consideredasviablereplacements forconventional metallicline communication systems.Optical fiber communicationis of little use if themany advantagesof optical fiber transmissionlinesoutlinedinthe previouschapters maynot beapplied inpracticeinthe telecommunicationsnetworkwithout severedegradationoftheir performance.It isthereforeessential that:(a) Optical fibers may be produced with good stable transmission charac-teristics inlong lengths at a minimumcost and with maximumrepro-ducibility.(b) Arangeof optical fiber types withregardtosize,refractiveindicesandindexprofiles, operatingwavelengths, materials, etc., beavailablein ordertofulfill manydifferent systemapplications.(c) Thefibersmaybe convertedinto practical cables which canbe handledina similar manner toconventional electrical transmission cables withoutproblems associated with the degradation of their characteristics ordamage.(d) The fibers and fiber cables may be terminated and connected together{jointed) without excessive practical difficulties and in ways whichlimit theeffect of thisprocesson thefiber transmission characteristics tokeep themwithinacceptableoperatinglevels. It is important that these jointing tech-niquesmaybeappliedwitheasein thefield locations wherecable connec-tion takesplace.In this chapter wetherefore pull together the various practical elementsalSociatedwithoptical fiber communications. Hencethevariousmethods forpreparina optical fibers (both liquidand vapor phase) withcharacteristics suit"ablefor telecommunicationsapplicationsareoutlinedinSections4.2to4.4.112 OPTICAL FIBER COMMUNICATIONS : PRINCIPLESANDPRACTICEThisisfollowed inSection-t.5withconsideration of commercially availablefibers describing in general termsboth the types and their characteri..tics. Therequirement .. for optical fi bercablinginrelat ionto fi berprotectionarethendiscussedin Section4.6 prior 10considerat ion of cable design in Section 4,7.InSection4.8we deal withthelossesincurred when optical fi bersare con-nected together. This discussion provides a basis for considerationof thetechniques employed for jointingoptical fi bers. Permanent fiberjoints (orsplices) a rcthendealt withinSection4.9priortodiscussionof thevarioustypesof demountablefi berconnector in Sections4. 10 to4. 12.4.2 PREPARATIONOFOPTICALFIBERSFrom t he considerations of optical waveguidmg of Chapter 2 it is clear that avariationof refractiveindex inside the optical fiber (i.e. between t hecore andthecladding) is afundamental necessityinthefabrication offi bersforlighttransmission. Henceat least twodifferent materials which are transparent tolight over the operating wavelength range (0.8-1.61Ufl ) arc required. Inpractice these materials rnust exhibit relatively lowoptical attenuationandthey must therefore have lowintrinsic absorptionandscatteringlosses. Anumberof organic andinorganicinsulatingsubstances meet theseconditionsinthevisibleandncar infrared regionsofthespectr um.However. inordertoavoidscatteringlossesin excess of thefundament alintrinsic losses. scattering centers such as bubbles. strains and grainboundaries must be eradicated. This tends to limit the choice of suitablematerials for thefabricationofoptical fi berstoeither glasses(or glass-likematerials) and monocrystallinestructures(certainplastics).It isalsouseful, andinthecase of gradedindexfi bersessential, that therefracti ve index of the material maybe varied by suitable doping with anothercompatible material. Hencethese twomaterials..houldhave mutual solubilityoverarelatively wide range of concentrations. This is.onlyachieved in glassesorglass-like materials. andtherefore monocrystalline materials areunsuitablefor the fabricationofgraded index fi bers. but maybeusedfor stepindexfi bers. However, it isapparent thatglassesexhibit thebest overall materialcharacteristicsforuse inthefabricationof lowlossoptical fibers. Theyarctherefore used almost exclusively inthe preparationof fibers for tel ecom-municationsapplications. Plast iccl adIReI'. 1]andall plastic fibers fi ndsomeusein short-haul, lowbandwidthapplications.In this section the discussion will therefore be confi ned to the preparation ofglass fi bers. This isa twostageprocessinwhichinitiall ythepureglassisproducedand convertedintoaform (rod or preform) suitable for making thefiber. Adrawingor pullingtechniqueis then employedtoacquiretheendproduct. The methods of preparing the extremely pure optical glasses aeneraJlyfall intotwomajor categories whichare:113 (" 1'\"'_- lIn,.,-1 - - 1-- -1..... ..." ,.. ..., G' lItI1l. ldnglurn. e- fot t!'leproduction hlgil c.riW gl..,n4).w.< i oldThe first stage in thi s process is the preparation of ul tra pure material powderswhich are usuall yoxides or carbonates of the requiredconstit uents. TheseincludeoxidessuchasSiO GeO" 8201andA20J andcarbonates such as4.3 LIQUIDPHASE(M ELTI NG) TECHNIQUES(a) convent ionalglassrefi ning techniques inwhichtheglas!> isprocessedinthe molten Mate (melt ing methods producinga mulncomponent glassstructure:(b) vapor phase depositionmethodsproducings ilica-richglasses which havemeltingtemperatures that are too high toallowtheconventional meltprocess.These processes. with theirrespective drawing techniques. are describedin thefollowingsections.OPTICALFIBERS. CABLESANDCONNECTI ONS114 OPTICALFIBERCOMMUNICATIONS: PRINCIPLESANDPRACTICE

(n",Ne----------------'. Fig. 4.2 High purityglassmeltingusingaradiofrequencyrooucuon tumace[Refs. 6-8].Na, CO" K2CO" CaCD) and BaCD.] which will decompose into oxidesduringtheglass melting.Veryhighinitialpurityis essential andpurificationaccountsfor alargeproportion of thematerial cost: neverthelessthesecom-poundsarecommercially availablewithtotal transitionmetal contentsbelow20parts in109andbelow1 part in109forsome specific impuritiesIRef. 21-Thepurificationmaythereforeinvolvecombinedtechniquesof fine filtrationand coprecipitation, followedby solvent extraction before recrystallization andfinal dryinginavacuumtoremoveanyresidual OHions (Ref. 31.Thenext stageistomelt these highpurity, powdered, lowmeltingpointglassmaterials to formahomogeneous,bubble-freemulticomponent glass. Arefractive index variationmaybe achievedby eithera change in the composi-tionof thevariousconstituents orby ion exchange whenthematerialsarein4.3.1 Fiber DrawingThe traditional technique for producing fine optical fi ber waveguides is tomakea preformusing therodin tubeprocess. Arodof core glassis insertedintoa lubeofcladdingglass and thepreformisdrawnina vertical mufflefurnace as illustrated in Fig.4.3 IRef.91. This technique is useful for theproductionof stepindexfi berswithlarge coreandcladding d iameterswherethe achievement of low attenuation is notcritical as there is a danger of incl ud-ing bubbles andparticulate mailer at thecore-claddi nginterface.Another techniquewhichi!i alsosuitablefor theproductionof largecorediameterstepindexfibers. andreduces thecore-cladding interfacepr oblems,is called the stratified melt precess. This process, developedby Pil kingtonLabor atories[Ref. 10J, involves pour ing a layer of cladding glass over the coreglassin a platinumcrucible asshown inFig. 4.4[Ref. I ll . Abait glass rod isdipped into the molten combinat ionandslowly withdrawn giving a compositecere-cladpreformwhich may bethendrawninto afiber.Subsequent development inthe drawing of o pticalfi bers (especially gradedlndell) producedbyliquidphasetechniqueshas concent ratedonthe doublecruciblemethod. Inthis methodthecoreandcladdingglassinthefonn ofIIp&rate rods Is fedi;ntotwoconcentricplatinumcrucibles as ill ust ratedin115themoltenphase. Themelting of these multicomponent glass systems occursat relat ivelylow temperaturesbetween 900and1300 Candmay take placein asilicacrucible as shown inFig. 4. 1 [Ref. 4]. However. conta mination canariseduringmeltingfromseveral sourcesincludingthe furnace environmentand the crucible. Both fusedsilicaand platinum crucibles have been used withsome success although an increase in impurity content was observed when themelt washeldina plat inumcrucibleat hightemperatures over longperiodsIRef. 51.Silica crucibles can give dissolution into the melt which may introduceinhomogeneities into the glass especially at high meltingtemperat ures. Atechniquefor avoidingthis involves melting[heglass directlyintoa radio-frequency (RF approximately 5 MHz) induction furnace whilecoolingthesilica by gas orwater flow as shown inFig. 4.2 IRefs. 6-81. The materials arepreheatedtoaround1000ClCwheretheyexhibit sufficient ionicconducti vityto enable coupling betweent he melt and the RF fi eld. The melt is alsoprotectedfrom anyimpurities in the crucible by athin layer of solidifi edpureglasswhichformsdue tothetemperat ure difference between the melt andthecooled silicacrucible.Inbothtechniquestheglass ishomogenizedanddr iedbybubblingpuregases throughthemelt. whil st protectingagai nst anyairbornedust particleseither originating in the melt furnace or present as atmospheric conta mination.Art erthemelt hasbeensuita blyprocessed. it is cooled andformedinto longrods(cane)ofmulticomponent glass.OPTICALFIBERS,CABLESANDCONNECTIONS116 OPTICAL FIBERCOMMUNICATIONS: PRINCIPLES AND PRACTICE [I I ,II

Hi""Flg.4.3 Opti cal fiber fromaoretc-ml Ref. 9].Fig. 4.5IRef. 41. Tbeassemblyis usuallylocated inamuffl e furnace capableof heating the crucible co ntents toa temperature of between 800 and 1200 C.Thecrucibleshavenozzlesintheirbases fromwhichthecladfiberis drawndirectlyfromthemelt asshowninFig. 4.5. Indexgradi ngmaybe achievedthrough thediffusion of mobile ions across the core-claddinginterfacewithinthemoltenglass. It ispossible toachieve a reasonable refractive: index profileviathis diffusionprocess, although dueto lackof precise control it isnotpossible to obtain the optimum near paraboli c profil e which yields thet,p"""rm,/rfig. 4.4 ' ''''''\>IeThesU"lI t ifi ttdme1t process (gl.. onglasstechnique' forprOClt.ICl nll ;rU' rod.or preform. [Ref. 11I.---=:.-117_ _- ....d ....mi nimumpulse dispersion (see Section 3.9.2). Hence graded index fi bersproduced by this technique arc substantially lessdi spersivethanstepindexfi bers. but do not have the bandwidth-length prod ucts ofoptimumprofi lefi bers. Pulse dispersionof1- 6 ns kmJ [Refs, 12. 13J is quite typical.depend-ingonthe material systemused.Some of thematerial systems usedinthefabricationof multicornponentglassstepindex and gradedindex fi bers aregiveninTable4.1.Usingveryhighpuritymeltingtechniquesandthedouble crucible drawingmethod, step index and graded index fibers with attenuations as lowas3.4 dB krrr ' [Ref. 14) and 1. 1dB km' " [Ref 2J respectively have beenproduced. However. such lowlosses cannot beconsistentlyobtainedusingliquid phase techniques and typical lo sses for multicomponent glass fi bersprepared continuously bythese methods are between 5 and 10 d Bkm1 .Therefore, liquid phuc techniques have the inherent disadvantage of obtainingOPTICALFIBERS, CABLESANDCONNECTIONSFiG.4.5 Thedoublecrucible met hul: of. - ..... _ 1'-..,sheat hfollowedby armoringconsistingofcorrugatedsteel tapewithlongi-tudinaloverlap. Asecondpolyethylene jacketactsas an cuter cable sheathgivinj;t the cable an overall diameter of around 2.5 em. The use of thealuminumtubealsoallows the cabletobe operatedunderpressurized condi-Homwhichgives theaddit ionaladvantagesof:(a) analarmintheeventofsheathperforation;(b) sheathfault location;(c) theexclusionor reductionof water ingresset a sheat hfault.Trials of various optical fiber cable designs have taken place t hroughout theworld sinceJ977 with little indication of failure due to the possible degradationFig.4.24 bamplesofmull ifiber cable deslg'l [Ref. 521: fa) SillCOl ' 18fiber ductcable ;fbI s seccr 49tiber ...nit cable......... A a.ntnlCotpalltlonmultifibercable IRel. 641. ,.,144 OPTICALFIBERCOMMUNICATIONS: PRINCIPLESANDPRACTICEI,,mechanisms. It is thereforesuggested[Ref.51 jthat thereis apossibility thatcurrent commercial optical fiber cables manufactured for telecommunicationspurposeshavebeen'over-engineered' andarethusworkingverysuccessfully.Henceit islikelythatthefuturedevelopment of optical fibercableswill con-centrate on simplerdesignswhichwillbring bothproductionand cost benefitsas optical fiber systems are utilized more fully in telecommunication networks.4.8 OPTICALFIBERCONNECTIONOptical fiberlinks, incommonwithanyline communicationsystem, havearequirement for both jointing and termination of the transmission medium. Thenumber of intermediate fiberconnections or joints is dependent uponthe linklength(betweenrepeaters), thecontinuous lengthof fiber cablethat maybeproducedby thepreparationmethods outlinedinSections 4.2--4.4, andthelength of fiber cable that maybe practically or conveniently installedas a con-tinuous section on the link. Current practice allows single lengths of fiber cableof around1 kmtobeinstalled. However, itis anticipated[Ref. 56]thatthiswillbe increased to several kilometers, especially for submarine systems wherecontinuous cable laying presentsfewer problems.Repeater spacing on optical fiber telecommunication links is a continuouslyincreasingparameter withcurrently installeddigital systems operatingoverspacingsofupto 30kmtogether withthe prospect ofrepeaterspacingsofmanytensandevenover 100kilometers forthe longwavelength single modesystemsof the near future. (For example, 100 km operationwithout repeaterswas achieved by British Telecominthe laboratory(uncabledfiber) at thebeginningof 1982 witha 140 Mbit s 1 singlemode systemoperatingat awavelengthof 1.55 11m. Inthis casethefiber producedby aMCVDprocesswas jointed (spliced)at 6 kmintervals.) It is thereforeapparent that fiber-fiberconnections with low loss and minimum distortion (e.g. modal noise with multi-modefibers)is of increasingimportancewithinoptical fibercommunicationsin order to sustain the repeater spacings required for developing systems.However, inthis context optical fiber jointinghastoacertainextent laggedbehindthetechnologiesassociatedwiththeothercomponents of optical fibercommunication systems (fiber, sources, detectors, etc.). Neverthelessin recentyears there has been an increasing interest in this topic and significantadvances have been made. Therefore inthis and thefollowingsections wereviewthetheoretical and practical aspects offiber-fiber connections withregardto both multimode and single mode systems. Fiber termination tosources and detectors-is not considered as the important aspects of these topicsarediscussedinthechapterscoveringsources anddetectors(Chapters 6, 7and 8). Neverthelessthediscussiononfiber jointing is relevant toboth sourceand detector coupling, as many manufacturers supplytheseelectro-opticaldevicesalreadyterminatedtoafiberopticpigtail in ordertofacilitatedirect__ r connectiontoanoptical tiber link,J where r is the fraction of theli ght reflected at a single interface, "Iis the refrac-tive index of t hefi ber core and n is t he refractive index of themedium betweenthetwojointed fi bers(i.c. for air n =1). Howeverinordertodeterminetheamount of light reflected at a fi ber joint, Fresnel refl ection at both fiberinterfaces must be taken into account . The loss indecibels due to Fresnelreflectionat a singleinterfaceisgivenby :145(4.13)(4. 12)r ~ (n,-n )'n, + nLoss-,... = - 10loglo( I - r)OPTICAL FIBERS, CABLESANDCONNECTIONSBeforewe considerfi ber-fiber connection in furtherdet ail it isnecessary toindicate the two maj or ca tegories of fiberjoint c urrentl y bot h in use a nddevelopment. Thesea re:(a) Fiber splices : these are semipermanent or permanent j oints which findmajoruseinmost optical fi bertelecommuni cationsystems(analogoustoelectrical solderedjoints),(b) Demountablefiberconnectors orsimplyconnectors : t heseareremovablejoints which alloweasyfast manual coupling and unco upling of fiber s(a nalogous toelectrical plugs andsockets).Amajorconsiderationwithall types of fiber-fiberconnectionist heopticallosse ncounteredat the interface. Even whenthe twojointedfiberendsaresmooth and perpendicular to the fiber axes, and the two fi ber axes are perfectlyaligned, a small proportion of the li ght may be reflected back into thetransmitting fiber causing att enuation at the joint. Thi s phenomenon, knownasFresnel reflection, is associated with the step changes in refract ive index at thej oi ntedinterface (i.e. glass- air -glass). Themagnitude of this partial reflectionof the light transmitted through the interface may be estimat ed using theclassical Fresnel formul afor lightofnormal incidenceandisgivenby [Ref.l7):Henceusingt herelationships giveninBqs. (4.12)and(4.1 3) itispossibletodetermine the optical attenuat ion due toFresnel reflection at a fiber-fiber j oint.Jt isapparent that Fresnel refl ectionmaygiveasignificant lossat afiberjoint evenwhenall other aspect s of theconnectionare ideal. However, theeffect of Fresnel reflectionata fiber-fiber connectioncan be reduced to a verylowlevel throughtheuseofanindexmatchingfluidin t hegapbetweenthejointed fibers. Whentheindexmatchingfl uidhas the same refractive index asthefiber core, losses due to Fresnel reflectionare int heoryer adi cated.Unfortunately Fres nel refl ection is only one possible source of optical loss at Aber joinL Apotentially greatersource of lossat atiber-fiber connection is" Ililolidbym!I&llpmenlof thetwo jointedfibers. Inordertoappreciatethe' ~ _ ....e tue:en' DC vuioulJ connection techniques itisuseful, n-.. l.lll: dmll,146 OPTICALFIBERCOMMUNICATIONS: PRINCIPLES ANDPRACTICEExample4.2An optical fiber has acore refractive indexof 15. Twolengths of the fiber withsmoothandperpendicular {to thecoreaxes] endfacesarebuttedtogether. Assum-ingthefiber axesareperfectlyaligned, calculatetheoptical lossindecibelsatthejoint {duetoFresnel reflection) whenthereis asmallairgapbetweenthefiber endtecaa.Solution: The magnitude of the Fresnel reflection at the fiber-air interface isgiven by Eq. (4,12) where:("' -" )'n, + n( 1.5- 10)'1.5+1.0

2.50,04Thevalueobtained for r corresponds toa reflectionof 4% of thetransmittedlight atthesingleinterface. Further, theoptical loss in decibelsat thesingleinterfacemaybe obtained using Eq. {4.13) where:Los5fre. = -1010910 {1 - r) = -10IOg10 0.96=0.18dBAsimilarmaybe performedfor theotherinterface(air-fiber). Howeverfromconsiderations of symmetry it is clear that the optical loss at the secondinterface is also0.18dB.Hencethe total lossduetoFresnel reflection at thefiber joint isapproximately0.36dB.4.8.1 FiberAlignment andJoint LossAnydeviations inthegeometrical andoptical parametersof thetwoopticalfibers which are jointed will affect the optical attenuation (insertion loss)through the connection. It isnot possible withinany particular connectiontechnique toallowforall thesevariations. Hence there are inherent connectionproblems whenjointingfibers with, for instance:(a) different coreand/or claddingdiameters;(b) different numerical apertures and/or relativerefractiveindexdifferences;(c) different refractive indexprofiles;(d) fiber faults (coreellipticity, coreconcentricity, etc.).Thebest resultsare therefore achieved with compatible (same) fibers which aremanufacturedtothelowesttolerance. Inthis case thereis still theproblem ofthe quality of the fiber alignment provided by the jointing mechanism.Examples of possiblemisalignment betweencoupledcompatible optical fibersare illustratedinFig. 4.26[Ref. 581. It isapparent that misalignment may_'.,f "'.... j - ...,...rlg.4.28 Thet r seepoS$iblet ' I P e ~ of misalignment whichmay c ccur when jo' ming com-patibleopt ical fibers IRef. 58J; (a)longitudinal misalignment; (bI lateralmis-alignment; Ie! angular misalignme nt .147..,occur in three dimensions, theseparationbetween the fibers (longitudinal mis-alignment), the offsetperpendiculartothefibercore3llCS (lateral/radial/axialmisalignment)andtheangle betweenthecoreaxes(angular misalignment).Optical losses resulting from these three types of misalignment depend uponthe fiber type, coredi ameter andthe distribution of the optical power betweenthepropagatingmodes. Examplesof themeasuredoptical lossesduetothevarious typesof misalignment are shown in Fig. 4.27. Figure 4.27(a) [Ref 581shows the attenuation characteristic for both longitudinal andlateralmisalign-ment of a 50 um core diameter graded index fiber. It may be observed that thelateral misalignment givess ignilkantlygreater losses per unit displacementthan the longitudinal misalignment. For instance in this case a lateral displace-ment of10 urngives about I dBinsertionloss whereasa similar longitudinaldisplacement gi v'es an insertion loss of around 0. 1 dB. Figure 4.27(b) lRef. 591shows theattenuation characteristic for the angularmisalignment of two mul-timodestepindexfi bers withnumericalapertures of 0.22and0.3. Aninser -tionloss of around1 dB is obtai nedwith angular misalignment of 40and5 0for the0.22 NAand0.3 NA fibers respectively. IImayalsobeobservedinFig. 4.27(b) thattheeffect of anindexmat chingfluid in the fibergapcausesincreased losses with angular misalignment. Therefore it is clear that relativelysmall levels of lateral and/or angular misalignment can cause significantattenuationat a fiber joint. This is especiallythe case for smal l core diameter(less than 150 urn) fibers which are currentlyemployedfor most telecom-municationpurposes.Theoretical andexperiment al studies of fiber misalignment in optical fiberconnections [Refs. 60- 721 allowapproximate determination of the lossesencounteredwiththevariousmisalignments of different fi bertypes . Wecon-elderhere someof theexpressions usedtocalculate losses due tolateral andInlularmisalignment of optical fiber joints. Longitudinalmisalignment is notdl.cunedindetail 8!1 it tends tobethe least important effect andmaybelarlof)' avoU!td Ie Abet cceeeeooe. Also there issome disalreemenl over theMlp1tudI ol-tbt-lOlili chat to lonaftudlnaJ mluJi,nment whenit is calculated. "OPTICALFIBERS. CABLES ANDCONNECTIONS148 OPTICALFIBERCOMMUNICATIONS: PRINCIPLES ANDPRACTICEIn,mion1o," (1 unolil2.03 4 5 bInde, ","clL,"g"l' Air l'l'P = (J,227 g 9 10Fig.4.27 Insertion loss characteristicsfor jointed optical fibers with various types ofmisalignment: (a) insertion lossdue tolateral andlongitudinal misalignmentfor a 50IJ.m core diameter graded indexfiber. reproduced with permissionfromP. Mossman, TheRadioand Electron. Eng., 51. p. 333, 1981: (bl inser-tionlossduetoangular misalignment forjointsintwomultlmodestepindexfiberswithnumerical aperturesof 0.22and0.3. ReproducedwithpermissionfromC. P. SandbankIedl, Opticaf Fiber CommunicationSystems, JohnWiley&Sons, 1980.theoreticallybetweenMiyazaki et al. [Ref. 61J andTsuchiya et al. lRef. 62].Both groups of workers claimgood agreement with experimental results whichis perhaps understandablewhenconsidering thenumber of variables involvedin themeasurement. However, it is worth noting that the lower losses predictedby Tsuchiya et at. agree more closely with a third group of researchers[Ref. 63]. Alsoall groupspredict higher losses for fibers with larger numericalwherenlisthecorerefractiveindex,nistherefractiveindex of themediumbetween the fibers, y is the lateral olTset of the fibercore axes, and a is the fibercore radius. The lateral misalignment loss in decibels may be determined using:apertures whichisconsistent with intuitiveconsiderations (i.e. thelarger thenumerical aperture, the greater the spread of the output light and thehigher theoptical loss at a longitudinallymisalignedjoint).Theoretical expressions for thedeterminationof lateral and angular mis-alignment lossesareby no meansdefinitivealthoughinall casestheyclaimreasonable agreement with experimental results. However, experimental resultsfromdifferent sources tend to vary (especially for angular misalignment losses)duetodifficulties of measurement. It isthereforenotimpliedthat theexpres-sions given in thetext are necessarilythe most accurate, as at present thechoiceappears somewhat arbitrary.Lateral misalignment reduces the overlap region between the two fibercores. Assuminguniformexcitationofall theoptical modes inamultimodestepindexfiber theoverlappedareabetweenbothfiber coresapproximatelygivesthelateral couplingefficiencyTlI'I' Hence thelateralcouplingefficiencyfor two similar step index fibers maybe written as [Ref. 62J:The predictedlossesobtainedusingtheformulaegiveninEqs. (4.14) and(4.15) are generally slightly higher than the measured values due to theassumption that all modesareequally excited. This assumption is onlycorrectfor certaincases of optical fibertransmission. Alsocertainauthors [Refs. 61and7l]assumeindex matching andhence noFresnelreflectionwhichmakesthe firstterm in Eq. (4.14)equal to unity (as nl/n= 1). This may be validif thetwofiberendsareassumedtobeinclosecontact (i.e. noair gapinbetween)andgives lower predictedlosses. Nevertheless, bearingin mindthesepossibleinconsistencies, useful estimates for the attenuation due to lateral misalignmentofmultimodestepindex fibers maybe obtained.Lateral misalignment loss in multimodegraded index fibers assuming auniform distribution of optical power throughout all guided modes wascalculatedbyGloge[Ref. 65]. He estimatedthat the lateralmisalignment losswasdependent ontherefractiveindexgradient (l for smalllateral offset andmay be obtainedfrom:149(4.16)(4.14)(4.15) Losslat= -10loglo T\lat dB_ 16(n,/n)' I { _,(y) (Y) [ (Y)']'}Tllat- 2cos - - - 1--(I + (n l/n))4rt 2a a 2ae)whlrethe Iltltll cOUpllnl efficiencywasgivenby:OPTICALFIBERS, CABLESANDCONNECTIONS150 OPTICAL FIBER COMMUNICATIONS: PRINCIPLESAND PRACTICE(4. 17)Hence Eq. (4. 17) may beutilized toobtainthe lateral misalignment lossindecibels . With a parabolic refract ive index profile where a= 2, Eq. (4.16)grves:(4. 18)A furtherestimateinclud ing the leakymodes, gavea revisedexpression forthe lateralmisalignment loss given in Eq. (4.17) of O.7S(yl a). Thisanalysis wasalso extended to stepindex fibers (where a= co) and gave lateral misalignmentlosses of O.64(y/a) andO.5(y/ a) for thecases of guidedmodesonly andbothguidedplus leakymodes respectively.Example4.3AstepirnJell f iber hasacoreref ract i veindell of 1.5anda0C\f 8diamet er 01 SO10m.The fi ber IS jolnl ed w it ha lilt !!' '' 1 mi5alignment between t he core .. xes of 5I' m.Esti mat et heinsertio n10S5at t he joint duetot neI,n eral misalignmenTassumi ngaunilormdislfibut ion of power between all gu;dedmodeswhen'(a) t here isasmall air g ap at ttl ejoint;(bl thejoint isconsi dered i n d e ~ mat ctled ,S olution ' (a) Th e coupl ing efficiency for a mult imode st ep i ndex f iber withuniformill uminat ionof all propli gat ingmodes isgi venby ECl. 14,141 as:16(1. 5121~tI +l, 5 j4 l':=0.29312(1.471) - 0.210 991; 1- 0.804Thei nsertion lossduetol at eral misalignment isgivenbyECl ' (4 . 151 wtle'eLosSj ., = - 10 log, o '1')1 ., = - 10 10910 0,804= 0 ,95dBHence assumi ng asmal l airgapat t he joint theinsert ionlossis approxi mately1 dBwhenth e lat eral offset is 10"'of t hO!! fi ber diameter.(bl W hel'l til ej oint is considered inde x mat chedu.e. 1'10 ai r gapl t hecoupti ngefficiency maybeagainobt ai ned f romEq. 4,14where:.t;= 0.64 = 0.64 (: 5 ) =0. 128151Loss,= - 10IOg l O 0 .872= 0.59dB= 03 161211.471) _O.2!O.99IJ}= 0.8 72Therefore the insertion10$$is :Agrl ded Indexfibera cerebcnc refractive indexprofile(a =2) anda coredllmeter of 5011m. ESl i mil t e the lnsertlon loss due t o a 3urn lateral mi sali gnment atI flblrJoi nt whenthereI, index matchingand(.1 Iherl !I unifor mill umination01guidedmodesonly:fbI there rl uniforml ilumlnal ionot all guidedandleakymodes.J oIIJrkJn: I_I AnufllnV uniform II!urnll'\l tion ofmod" ooly. the misal ign-ment lOll mlf beOOt_lned \IIin; Eq. 14.181, whereOPTICALFIBERS. CA8LES ANDCONNECTIONSWith index matchingthe insertion loss at the joint in exa mple 4.3 is reducedto approximately 0.36 dB. It may be noted that the difference between the lossesobtainedinparts(a) a nd(b) correspondstotheoptical lossdueto Fresnelref lectionat t he similar fiber-air-fiber interfacedeterminedin exa mple4.2.The result may bechecked usingthe formulae derived byGloge for amuhimode step index Fiber where the lateral misalignment loss assuminguniformillumination ofall guided modes isobtainedusing:Hencet helateral couplingefficiencyis givenby Eq. (4. 17) as:111"l =I- 0.128 =0.872Again using Eq. (4. 15), the insertion loss due to the lateral misalignmentassumingindexmatchingIS:LosslM= - 10loglo0.872= 0.59 d BHence using the expressionderived byGlogeweobeaint hesamevalueorapproximately0.6 dB fer theinsertionloss.....i ththeinhere nt assumptionthatthere is no change in refractiveindex at thejoint int erface. Although thisest imateorinsertionloss maybe showntoagree with certain experimentalres ults IRef. 6 Il a value of around I dBinsertion loss for a )0%lateraldisplacement wi th regard to t he core diameter (as esti mated in example 4.3(a)is moreusuallyfoundtobethecasewithmuhimodestep indexfi bers(Refs.59. 72 and731. Furtheritisgenerally acceptedthat t helateral offsetmustbekeptbelow5% of thefi bercore diameter in ordertoreduceinsert ion loss at aj oint tobelow0.5 dB IRef. 721.,152. - - -OPTICALFIBERCOMMUNICATIONS: PRINCIPLESANDPRACTICEi , = 0.85 e)= 0 8S( 235 ) = 0. 102H UIcouplingi, givenbV Eq, (4.17) as :11 ,,! "':" 1- L , = 1- 0 . 10 2 = 0898Hence t ne inserti n loss duetoth e l at eral misa lignment is g;ve" by Eq. 14 .151.where:.. = - 10 109100.898 = 0 47d Blbl Whenassumingt he uniformillumi nation01 bot hguidedlind leakymodesGloge"s f ormulabecomes:i t =0 75 (: ) =O.7S( :5 )=0,090Thereforethecouplingeffici ency is111_1 = 1- 0. 0 90 _= 0,91 0and t he insertion lossdue 10lat eral misalignment is:""- 10 log10 09 10 = 0.41dBItmay be notedbyobservingFig. 4.27(a) which showsthe measured lateralmisal ignmentloss fora50J.U1l diameter gradedindextiber t hat t he losses pre--dicted above are very pessimistic (the loss for 3tJ.I11 offsetshown in Fig. 4.27(.1)is less than0.2 d B). Amodelwhichisfoundto predictinsertionloss duetolateral misalignment in graded index fi be rs with greater accuracy wasproposed byMiller and Mettler {Ref. 661. Int his model they assumed thepowerdist ributionat the fiber output tobe of aGaussian form. Unfortunatelythe analysis is too detailed for this text as it involves integration usingnumerical techniques. Wethereforelimit estimatesofinsert ionlossesduetolat eral misalignment inmultimodegradedindexfibers tot heuseof Gloge'sformula.Angul armisalignment lossesat joint s in multimode step index fibers maybepredictedwith reasonableaccuracyusing an expressionfor the angular coupl-ingefficiency givenby (Ref. 621:(4.19)where9istheangulardi splacement inradiansand.6. is therelative refractiveindexdifferencefor t hefiber. Theinsertionlossduetoangular misal ignmentmaybe obtainedfromtheangularcoupling efficiencyinthesamemannerasthe lat eral misalignment loss following:(4.20)Loss.... - - 10loalO1'1u,. .Hence153. :ll/180 ]110.4[1 - 5ll/180 ]II 0.2TJang ~ 0.926[1::= 0.862= 0.79716(148)2TJang ~ 4(1+1.481For the0.4 NAfiber:For the0.2 NAfiber.Loss. ng= -10109'0 TJono ., -10log10 0.7970.98dBThe insertion lossdue to the angular misalignment may be obtained fromEq.{4.20), where:Thenumerical apertureis related totherelativerefractiveindex difference followingEq. (2.10) where:Thetneerncn lossduetothe angular misalignment is therefore:L O S S . ~ g =-1010910 0.862_0.64dBTwo multi mode step index fibers have numerical apertures of 0.2 arid 04respectively, aridbothhavethesamecorerefractiveindexwhichis148. Estimatetheinsertionloss at a joint ineachfibercausedby a5angular misalignment of thefiber core axes. It may be assumed that the mediumbetween lhe fibers is air.Solution: The angular cOlJpling efficiency isgivenby Eq. {4. 191as16(n,lnI 2[ n ]TJol1g ~ 4 1 ---(1 + In!/n)) llNAExample4.5The formulae giveninEqs. (4.19) and(4.20) predict that thesmaller thevalues of d the larger the insertion loss duetoangular misalignment. Thisappears intuitively correct as small values of dimply small numericalaperturefiberswhichwill be moreaffected by angular misalignment. It is confirmed bythemeasurements shown in Fig. 4.27(b) anddemonstratedinexample4.5.OPTICALFIBERS, CABLESANDCONNECTIONS154 OPTICALFI BER COMMUNICATIONS:PRINCIPLES ANDPRACTICEHenceit may be noted fromexample4. 5 that t hei nsertionloss duetoangular misalignment is reduced by using fi bers with large numericala pertures. This is t he opposite trend to the increasing insertion loss withnumerical apert ure for fiber longit udinal misalignment at ajoint.Misalignment losses at connections in single mode fi bers have beent heoreticallyconsideredbyMarcuseIRef. 681and Gambling1'1 aL[Refs.69and 701. Thetheoretical anal ysis whichwas instigated byMarcuscisbasedupontheGaussianor near Gaussian shape of the modes propagating in singlemode fi bers regardless ofthe fi ber type (i.e. step index or graded index).f urther development of this t heory by Gambling 1'1 at. IRef. 70Jgavesimplified formulae for boththelateral and angular misalignment losses atjoi ntsin single modefi bers.In theabsence of angularmisal ignment Gamblingel ttl. calculatedt hat t heloss Tjdueto lateral. offset y was givenby:(Y ) '1', = 2.17 000 dB(4.21)whererooisthespot sizeofthefundamental mode.Thespot sizeisusuallydefi nedas t he widthtoIl l' intensity of theLPOImode, orin termsof t hespotsizeof anincident Gaussianbeamwhichgivesmaximum launching effi ciencyIRef. 141. However, the spot size for the LPOImode (corresponds toHE mode)maybeobtainedfromtheempirical formula IRefs. 68and 69 1:(4.22)1.62V- ' 5+ 2.88V-6)21,, (0"- .6"-5_+--"'-----;;:;-_ _ ~~ = o -where ">0 ist hespot sizein 11m. a isthefiber coreradiusand Visthenor-malizedfrequencyfor the fi ber. Alternat ivelytheinsertionlossT. causedbyan angularmi salignment alinradians) ata j ointin asingle mode fi bermaybegivenby:T.= 2. 17 ( eroonIV) ' dBaNA(4.23)where 11 1 is thefi bercore refract iveindex and NA is t henumerical aperture oft hefibe r.It must benoted that theformulaegiveninEqs.(4.21) and(4. 23)assume that the spotsizes of t he modes in the two coupledfibers are thesame.Gamblinges al.I Ref. 70J also deri....ed a somewhatcomplicated formula whichgavea good approximationfor thecombinedlossesdueto bot hlateral andangular misal ignment at a fiberjoint. However theyindicate that for smalltotal losses (less than 0.75 d B) a reasonable approximationis obtained bysimplycombiningBqs. (4.2 1) and(4.23).= 0,49dBHence the total insertion loss is1552'(In/180)X3.12Xl.46X2.4)4x 0.1= 2.1710.65 +1,62(2.41-15 + 2.88(2.41-51~4----:cc-----The lossdue to angular misalignment may be obtainedfromEq. l4.23) where:= 0.22dBrou = aThe loss due tothe lateral offset is given by Eq. 14.21)as:TT ~ 7, +T=0.22+0.490.71 dB71=2.17(-,-)2 =2.17(_'_)'(l)o 3.12Exsmpls4.6Asingle mode fiber has the followingparameters:normalized frequency(VI = 2.40core refractive index(n1) = 1.46core diameter (2a1 = 8).l.mnumerical aperture (NA) = 0.1Estimate the total insertion loss of a fiber joint with a lateral misalignment of 1umandan angular misalignment of 1.Solution: Initially it is necessarytodetermine thespot sizein the fiber. Thismaybe obtainedfromEq. (4,221 where:10.65+1,62V15+2,88V6)Inthisexample the loss due to angularmisalignment is significantly largerthanthat duetolateral misalignment. However. aside fromtheactual mag-nitudes of the respective misalignments, theinsertion losses incurred arealsostrongly dependent upon the normalized frequency of the fiber. This isespeciallythecasewithangular misalignment at asingle modefiber jointwhereinsertion losses of less than 0.3 dBmaybe obtained when theangularOPTICALFIBERS, CABLESANDCONNECTIONS168 OPTICALFI BERCOMMUNICATIONS: PRINCIPLESANO PRACTICEmisal ignment is 1D withfi bersofap propriate Vvalue. Neverthelessforlowloss singlemodefiber joints it is importa nt that a ngular alignment isbetterthan 1.Wehave considered in some detail the optical attenuation a t fiber-fiber con-nections. However we have not yet discussed the di stortionofthetransmitt edsignal at afiber joint. Althoughworkin t hisarea is initsinfancy,increasedinterest hasbeengenerat edwiththe use of highl ycoherent sources(inj ectionlasers)andverylow d ispersionfi bers.It isapparent that fibercon-nections st rongly affec t the signal t ransmission causing modal noise (seeSection 3. 11) and nonlinear distortion (Ref. 76] when a coherent light source isutilizedwit ha multimode fi ber. Alsoit hasbeenreported IRef. 77) t hat thetransmissionof a connectionin a coherent multimode systemis e xtremelywavelengt h-dependent exhibit ing a possible 10%change in the transmittedoptical wa velengt hfor averysmall change(0.001 nm)in thelaser emissionwavel ength. Neverthel ess it has been found that these problems may bereduced by theuseofsinglemodeoptical fiber (Ref. 761.F urt hermore theabove modal effe cts become negligible whenan incoherentsource(light emittingdiode) is used with multimode fi ber. However, inthisins tance there is often some mode conversion at the fi ber joint which can maketheconnectioneffectivelyact asamodemixer orfilter [Ref. 781. Indicationsaretnat t his phenomenon whichhas been investigated(Ref. 791with regard tofiber splices, is more pronounced with fusionsplices than wit h mecha nicalsplices, both of whi charedescri bed inSection4.9.4, 9 FIBERSPLICESApermanent joint formedbetweentwoindividual optical fibersint he field orfactoryis knownas afi bersplice. Fibersplicing is frequentlyused toestablishlong-haul opticalfiber linkswheresmaller fiber lengthsneed tobe joined, andt here is no requirement for repeated connection and disconnect ion. Splices maybedivided intotwo broad categoriesdepending uponthe splicingtechniqueutilized. Theseare fusionsplici ngor welding and mechanical splicing.Fusionsplicing is accomplished by applying localized heating (e.g. by anameoranelectrica rc)at theinterfacebetweent wobutted, prealignedfiberendscausing themtosoftenandfuse, Mechanicalsplicing, inwhicht he fibersa reheldin alignment bysome mechanical means, may be achievedbyvariousmethods including the useof t ubes around thefiber ends (tu be splices) orv-grcovesintowhich the butted fi bersareplaced(groove splices). All thesetechniques seektooptimize thespliceperformance(i.e. red ucet he insertionlossat the joint) through bothfiber endpreparationandalignmentof the twoj ointedfibers. Typicala veragespliceinsertionlossesformultimodefibers areinthe range 0.1-0.2 dBIRef. 81) which is generallyabetter performance thanthat e xhibited by demountable connections (see Sections 4.10-4.12). It may beFig .4..28 Opl' calfi ber end prepSl allOfl : t he principle of scribe end bre ak cutt ing [Ref. 821.4.9.1 FusionSplice.Thefusionsplicingofsinglefiber sinvolvestheheatingof thetwopreparedfi berendstotheir fusingpoint with theapplicationof sufficientaxial pressurebetweenthetwooptical fi bers. It is thereforeessential that thestripped(ofcablingandbuffer coating) tiber ends areadequately positioned and aligned inordertoac hievegoodcont inuity of thetransmi ssionmediumatthe junctionpoint .Hencethe fibersareusuallypositionedandclamped with theaid of anInspectionmicroscope.Flameheatingsourcessuchasmicroplasmatorches (argonandhydrogen)IDd olhydricnucroburnetl (oxYlen,hydrogenandalcohol vapor)havebeenutillzld wilh tome I\KlCtIi (Ret'. 841. However, the monwidely usedheating157~ " " ~ . I # i"i'i. ..,,..... ~ prop...tioo'VOPTICALFIBERS. CABLESANDCONNECTIONSnotedthatthe insertionlossesof fiber splicesaregenerallymuchless than thepossible Fresnel reflectionloss at abuttedfiber-fiberjoi nt. Thi sisbecausethereis nolarge stepchangeinrefractiveindexwiththefusion spli ceas itformsacontinuousfi ber connection, andsome method of index matching (e.g.a fluid] tends tobeutilizedwithmechanical splices. However, fiber spli cing(especially fusion splicing) is at present a somewhat difficult process toperformina fieldenvironment and suffers frompract ical problems in thedevelopment offield-usable tools.A requirement withfibers intended for splicing is that they have smooth andsquare end faces. In general this endpreparationmay be achieved using a suit-abletool whichcleaves the fi ber as illustrated in Fig. 4.28[Ref. 82). Thi sprocess is often referred to as scri be and break or score and break as itinvolvesthescoring of the fibersurfaceunder tension withacutting 1001(e.g.sapphire. diamond, tungsten carbide blade). Thesurface scoring creates failureas the fiber istensioned and a clean, reasonably square fi ber end can beproduced.Figure 4.28 illustrat es this process withthefi ber tensi oned around acurved mandrel. However. straight pull, scribe and break tools are alsoutilized, whicharguablygivebetter results [ Ref. 831.158OPTICALFIBER COMMUNICATIONS: PRINCIPLES ANDPRACTICEsourceisan electricarc. This technique offer s advantages of consistent. easilycont rolledheat withadaptabilityfor useunderfieldconditions. Aschematicdiagr amof thebasicarcfusionmethodisgiveninFig. 4.29(a)IRefs. 8 Jand85 1ill ustrating how the two fibers are welded together. Figure 4.29(b) (Rer. 731shows a development of the basicarc fusion process which involves the round-ingof thefiber endswithalowenergydischargebefore pressingthe fi berstogether andfusingwithastrongerarc. Thistechnique. knownas prefusion,removes the requirement for fiber end preparation which has distinctI. )oFig. 4.29",Elect ri c arc: fuSi onsplici ng : 1. 1anexampleof fusi onloFcingIDl)trl tu. [R. f .81 . 00851: lb)sc:t1emat lcil luetr ationof t ~ 1 pefu. lon n" ethod for ICIMltl lyspl 'cirog optic, t fl berl lRef. 731...--OPTICAL FIBERS, CABLESANDCONNEc n ONS 159Hb""'"\! (I1.1,(I!'"j,,II'"I I,)4.9.2 Mech.nlcel Splice.Anumber of mechanicaltechniques forsplicing individualopticalfi bershavebeen developed. A common method involves the use of an accuratel yproduced rliidaHiRmenl lube intowhichtheprepared fi ber ends are per-manentlybonded. TN. ll1ultubcsplice isillustrated in Fia. 4.31(a} [Ref 94)and may udlhl a..... or otr&m!c: =apiUary withaninner diameter j ustlarae. .advantage in thefieldenvironment.It hasbeen utilizedwith multimode fibersgivingaverage splicelossesof0.09 dB IRef. 86J.Fusionsplicing of singlemode fi bers withtypical core diameters between 3and 10 umpresents problems of more critical fi ber alignment (i.e. lateraloffsets of less than I urnare required for lowloss joints). However, spliceinsertion losses below0.3 dB may be achieved due to a self-alignmentphenomenonwhichpartiallycompensate"for anylateral offset.Self-alignment. illustratedinFig. 4.30IRefs. 85. 87and881. iscausedbysurfacetensioneffectsbetweenthetwofiber endsduringfusing. Arecentlyreported [Ref. 89J fi eld trial of single mode fiber fusion spli cing over a 31.6 kmli nkgavemeanspli ce insertionlosses of0.18and0.12 dBat wavelengths of1.3and 1.55 urn respectively.Apossible drawback withfus ionsplicing is thaitheheat necessary tofusethe fibersmay weakenthe fiber in the vicinity of the splice. Ithas been foundthat even with careful handling. the tensile strength ofthe fusedfi ber may be aslowas 3Q9(, ofthat of t heuncoated fi ber beforefusionIRef. 911. Thefiberfracture generall y occursintheheat-affectedzone adj acent to the fused joint.Thereducedtensilestrengthisattributed[Refs. 91 and921tothe combinedeffectsof surface damagecausedbyhandling. s urfacedefect growthduringhealing andinducedresidential stressesduetochangesin chemical composi-tion. It isthereforenecessarythat thecompletedspliceispackagedsoastoreduce tensileloadinguponthefiber inthe vicinityof thesplice.Fig. 4.30 Self- ali gnment phenomenon which takes otace d u r i n ~ fusion splicing.talbetc refusion: Ib} d u r i n ~ fusion; (c)after fusionIRefs. B5. 67andB8J.160 OPTICALFIBERCOMMUNICATIONS: PRINCIPLESANDPRACTICEOpti",1fiilerSquareero "'diun "pilla,)'"(.) (Il)Fig. 4.31 Techniquesfor tubesplicing .ot optical fibers: lal snugtubesplice[Ref.941:(b) loose tube splice utilizingsquarecross sectioncapillary[Ref. 96J.enough toaccept theoptical fibers. Transparent adhesive, (e.g. epoxyresin) isinjectedthroughatransverseborein thecapillarytogive mechanical sealingandindexmatchingof the splice. Averageinsertionlossesas lowas 0.1dBhavebeenobtainedIRef. 95] withmultimodegradedindexandsinglemodefibers usingceramic capillaries. However,in general, snugtubesplicesexhibitproblems withcapillarytolerance requirements.A mechanical splicingtechniquewhichavoids the criticaltolerance require-ments of thesnug tube splice is shown in Fig. 4.31(b) IRef. 961. This loose tubesplice usesan oversized square section metal tube which easilyaccepts the pre-paredfiber ends. Transparent adhesiveis first insertedintothetubefollowedby the fibers. The splice is self-aligningwhen thefibers are curvedin the sameplane, forcing the fiber ends simultaneously into the same corner of the tube, asindicatedinFig. 4.31(b). Meanspliceinsertionlosses of 0.073dBhave beenachieved[Refs. 88and97]using multimode gradedindex fibers withthe loosetubeapproach.Analternativemethodofobtaininga tight fittingspliceisbyuseofthecollapsedsleevesplicingtechniquewhichisillustratedinFig. 4.32[Ref. 981.Cullap",d ,h",,'"~ c : : = = { " ' ~ .(oj OpticalflberFlg.4.32 Th. eeuee..d 11vt.pllcln" technique {Ref, B8],OPTICALFIBERS. CABLESANDCONNECTIONS'B',I,.,Tl,Fik".. V1ll' """,-d ..........ThismethodutilizesaPyre's glasssleeve whichhas a lower melt ing pointthanthe fibers to bejointed. When thesleeve isheated toits softeningpoint itcollapses due to surface tension, eventually forming a solid rod . Figure 4_32(a)showsapartiallycollapsedPyrexsleeve formedby local heating of the sleeve.With t he collapsed sleeve splicing technique the glass sleeve is collapsedarou nd one of the preparedliberends to forma tight lilti ng socket as shown inFig. 4.J Ub). Thesecondfiber isthen insertedintothe socket andthewholeassemblyisbondedwith epoxyresinasillust rat edinFig.4.J2(c). Henceanindexmatchedsplice is created. This technique is useful in the splicing of twofibers withdi fferent diamet ers. Inthiscasethe sleeveiscollapsedover thelarger diameterfi berbeforetheinsertionof the second. smaller di ameter fiber.The collapsingis then continuedtoformasocketof anappropriatesizeforaclose fi t to the smaller fiber. Coll apsed sleeve splices are generally protected byenclosureinametal ferrule. Theyhaveexhibited insertion losses in theranged B IRef. 591 whenusingmul timodegradedindex fiberswith avera getosses of0.5 dBinthefield.Othercommon mechanicalsplicing techniques involve theuse of grooves tosecurethe fibers co be j ointed. A simple methodutilizes aV-groove intowhichthe two prepared Fiber ends are pressed. The v -groove splice which isillustratedinFig. 4.33(a)[Ref 991givesalignment of thepreparedfiberendsthroughinsertion in thegroove. Thespli ce is made permanent by securing thefiber sinthev -grcove withepoxyresin.Jigs for producingv -groovesplicesI -iII162 OPTICALFIBERCOMMUNICATIONS: PRINCIPLES ANDPRACTICE,,,__------------------c.,--/ ------/(yliIlJri,,1 ;J"",.,Fig.4.34 The Scrinqroove" splice [Ref. 101J: la) expanded view of the splice;Ibl schematic cross sectionof thesplice.haveproved quite successful, givingjoint insertion losses ofaround 0.1 dB[Ref. 82].V-groove splices formedbysandwichingthe buttedfiberendsbetweenaV-grooveglasssubstrateanda flat glassplateasshownin Fig. 4.33(b) have alsoprovedvery successful inthelaboratory. Splice insertion losses of less than0.01dBwhencouplingsingle mode fibers have been reported[Ref. 100]usingthistechnique. However, reservationsareexpressedregardingthe field imple-mentationofthese splices with respect tomanufacturedfiber geometry, andhousingofthe splice in order to avoid additional losses due to local fiberbending.Aslightly morecomplexgroove splice knownas the Springroovewspliceutilizesabracketcontainingtwocylindricalpins whichserveasanalignmentguide forthe twopreparedfiberends. The cylindrical pin diameter is chosen toallowthe fibers to protrude above the cylinders as shown in Fig. 4.34(a)IRef. 101]. Anelastic element (aspring) is used to press the fibers intoagrooveandmaintain thefiber endalignmentasillustratedinFig. 4.34(b). Thecompleteassemblyissecuredusingadrop of epoxyresin. Meanspliceinser-tion losses of0.05 dB[Ref. 88) have beenobtainedusingmultimodegradedindexfibers withtheSpringroove'"splice. Asimilarmechanical splicingtech-niqueisillustratedinFig. 4.35 [Ref. 941. Inthis casethespringisreplaced

-----=:::Cl'lindrical pin'Hoot ,11I';llbhk-- - ---

-- - - - --,Fig. 4.36 Theprecision pinsplice [Ref. 94J.I,OPTICALFIBERS, CABLESANDCONNECTIONS 163byathird cylindrical pinandthewhole as sembly isheldin placewith aheatshrinkablesleeve. Thi sprecision pinsplice hasgivenmean insertionlosses ofaround 0.2 dB IRef. 881with multimode fibers.,Multlpl . fi ber IPllclng of f be, ribbon ClIble using a Qroon alignm6'f'11 techniQlleI.... 1021./.-,Kul>i"t ~ u " l4.9.3 MultipleSplice.Multiplesimultaneous splicinghasmainlybeenattemptedusingmechanicalsplicing methods. Groove splicing techniques have been utilized for thesimultaneous splicingofanarrayoffiberswithinanatribboncable.Figure4.36 IRef. 102] shows a groove splice for a five fiberribbon cable. IIutilizes agroovedmetal substratewiththegroovespacing equal 10thespacing of thefi bersin the array. The plastic coating is removed from the fi ber ends and theyare preparedusing asuitable scribeandbreaktool. Then thetwo ribbon endsare placed into the grooves, the Fibers are pressed together and hel d in positionwith arubber sheet and8. cover plate. Finallyepoxy resi n is added toprovideindexmatching aswell assecuringa permanentsplice. A12fiberversion ofthis splice using aninjectionmoulded plasticsubstrate gave an average splicelossof 0.2 dBIRef.97J.A ribbonsplice using etchedsilicon chips is shown in Fig. 4.37 [Ref. 103].Similar chips maybe utilized to form a12x12 array of 12 ribbon cables eachcontaining 12fibers IRef. 821. Thewholeassemblyisclampedtogether toforma singlemultiplesplice.Multiple fiber splicing of a circular cross section cable has alsobernachieved. A splice or this type is shown in Fig. 4.38 1Ref. 1041 and consists ofmatched sets of precisionmoulded circular coll ars (one shown) which containaseries of groo....es around the circumference. Thefi bers are inserted intotheprecisiongrooves andbondedwithadhesive. They are then cut. polishedandcovered withindexmatchingmaterial at thejointingends. beforethe twocollars are brought together andfastenedwith semicylindrical shells and pins.j!I164 OPTICALFIBERCOMMUNICATIONS: PRINCIPLESAND PRAcnCEFig. 4.37 Mul t iple fi bersplicing of Slackedribboncables usingprecision mutn-v-qroovesilicon chips[Ret 1031.I.,,.Fig.4.38 Cross sect ion of the stet -core cable for mult i ple fi ber mechanical So' icing[Ref .l 04J.Whenthetwogroovedcollars areproduced fromasinglepiece of alumina,averagespli ceinsertionlossesof 0.3 dB[Ref 88J have beenobtainedwithmultimodestepindexfibers.4.10 FIBERCONNECTORSDemountable fiber connectors aremore difficult toachieve thanoptical tibetsplices. This isbecause theymustmaintainsimilar tolerance requirementsto4.11 BUTTJOINTEDCONNECTORSButtjointedconnectorsarethemostwidelyusedconnectortypeandasub-Itantial number have been reported. In this section we review some of the morecommonbutt J o i n t e d ~ connector designswhichhavebeen developedprimarily(a) the fiber termination whichprotects andlocates thefiber ends;(b) the fiber endalignment toprovideoptimumoptical coupling;(c) the outer shell whichmaintains theconnectionand thefiber alignment,protects the fiber ends from the environment and provides adequatestrengthat thejoint..,,'.165The useof anindexmatchingmaterial inthe connector betweenthetwojointed fibers can assist theconnector design in two ways. It increases the lighttransmission through the connection whilst keeping dust and dirt from betweenthefibers. However, thisdesignaspect isnot alwayspractical withdemount-ableconnectors, especiallywherefluidsareconcerned. Apartfromproblemsofsealing and replacement whenthejoint is disconnectedandreconnected,liquidsin thisinstancemay havea detrimental affect, attracting dustanddirtto the connection.There area large number of demountable single fiber connectors, both com-merciallyavailableandunder development, whichhave insertionlosses in therange 0.2-3 dB. Fiber connectorsmaybe separated into two broad categories:butt jointed connectors andexpandedbeamconnectors. Butt jointed ccn-nectorsrelyuponalignment of the twoprepared fiber ends in closeproximity(butted) toeachother sothat thefiber core axescoincide. Expandedbeamconnectorsutilize interposed optics at the joint (i.c. lenses or tapers) in order toexpandthebeamfromthe transmittingfiber endbefore reducingitagain toasize compatiblewiththe receivingfiberend.splices in order to couple light between fibers efficiently, but they mustaccomplish it in a removable fashion. Also the connector design must allow forrepeatedconnectionand disconnection without problemsof fiber alignmentwhichmayleadto degradationin theperformance of the transmissionline atthe joint. Hence tooperate satisfactorilythe demountable connector mustprovide reproducibleaccuratealignment oftheoptical fibers.In order to maintain an optimumperformance the connector must alsoprotect the fiber ends fromdamage whichmayoccur due to handling (connec-tion and disconnection), must be insensitive to environmental factors (e.g.moistureanddust)andmustcope with tensile loadon the cable. Additionally,the connector should ideally be a low cost component which can be fitted withrelative ease. Hence optical fiber connectorsmaybe consideredinthreemajorareas, whichare:OPTICALFIBERS. CABLESANDCONNECTIONS166 OPTICALFIBERCOMMUNICATIONS: PRINCIPLESANDPRACTICEfor uscwithmultimode fibers. Nevertheless in certain cases as indicated in thetext, similar designs havebeen usedsuccessfullywithsingle mode fibers.4.11.1 FerruleConnectorThebasic ferrule connector (sometimes referredtoasa concentric sleeve con-nector), which is perhaps the simplest optical fiber connector design. isillustratedinFig.4.39(a) [Ref. 58]. Thetwofibers tobeconnectedarcper-manentlybonded(withepoxyresin) inmetal plugs knownasferruleswhichhave anaccurately drilled central hole in their end faceswhere the stripped (ofbuffer coating) fiber islocated. Within the connector the two ferrules areplacedinanalignment sleeve which, using accuratelymachinedcomponents,allows thefiberends tobebutt jointed. Theferrules areheldinplacevia aretainingmechanismwhichin theexampleshowninFig. 4.39(a) is aspring.It is essential with this type of connector that the fiber end facesare smoothand square(i.e. perpendicular tothefiberaxis). This maybeachievedwithvaryingsuccessbyeither:(a) cleaving thefiber before insertion intotheferrule;(b) inserting and bondingbefore cleaving the fiber close to the ferrule end face;('on""wr

Forru!,

,Icc"R'laining '{,)PI"", ,'o,t"'forfih,',Staink" ""el'"'[errukIIk,.I,,", """,IiiLIlTIIIVatchj,wd

Fig. 4.39 Ferrule connectors: lal structure of a baSIC ferrule connector [Ref. 58);lb) structureof awatchjewel connector ferrule [Ref. 59].4.11.2 Blconical Connector.... ...-0 CrollHetlon ofIll. blconlCi I connector[Refs. B1I nd 105J./,.7P....t;onpinSlittod ,Icc",ho,,,ingSlit!ed ,keYCAxi,] comp"",;on ,p'h,gCOUpli"! nn! pipeAd,p101Fig. 4.41 Theceramiccapillary connectorshowingtheferruleplugand the adaptor intowhichtwo plugsare located [Refs. 107 and 108).4.11.4 DoubleEccentricConnectorThe double eccentric connector does not relyonaconcentric fixed sleeveapproachbut is adjustable, allowingclose alignment ofthefiber axes. Themechanism, whichis showninFig. 4.42[Refs. 58and62], consistsof twoeccentriccylinderswithintheouterplug.Itmaybe observedfromFig. 4.42that the optical fiber is mounted eccentrically within the inner cylinder.Therefore when the two connector halves are mated it is always possiblethroughrotationof themechanismtomakethefiber core axescoincide.Thisoperationisperformedonbothplugsusing either an inspectionmicroscope ora peak optical adjustment. Themechanisms are thenlockedto give permanentalignment.Thisconnector typehasexhibitedmeaninsertion lossesof 0.48 dBwithmultimodegradedindexfibers: useofindexmatchingfluidwithintheconnectorhasreducedtheselosses to 0,2 dB. The double eccentric connector-16.I. 1-/7 i ,-, , ,, ,f-- - 1.1--.-,;,,,."~ "~ , ,.." /; If-""1',".-, I / )'IFil>orT",oS optical fibre'. E/trull . LeU.. 13(24), pp-755-756. 1977.U G. A. C. M. Spicnngs, T. P. M. Meeuwsen, 1-'. Meyer, P. J. W. Severin andC. M. G. Jochem. ' Sornc aspects ,)( thepreparation of a lka li lime gennosilicateopti cal fibres" Phys, Chem, Gton es(G B). 2 1(1), PI'.30-3 1, 1980.18 U. Lydtinand F. Meyer . ' Review of techniques applied in optica l fibreprepara-tion', ActaElectron.;22(31. pp. 225- 235, 1979.l' J. F. Hyde, USPatent 2272342. 1942.18 F. P. Kapron, D. B. Ked and R. D. Mau rer, ' Radiation losses in opticalwaveguides', Appl. Phy s. Len; 10, pp. 423-425, 1970.1. B. Bendowand S. S. Mitra. FiberOptics. PlenumPress, 1979.20 D. B. Keck and R. Bouillie, ' Measurement s on high-bandwidth opticalwaveguides', Optic