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1、Sharing Newsn nPhotonics spectra Photonics spectra n 2024/9/261 Chapter 3:Chapter 3:Optic Fibers BasicsChapter 4:Chapter 4:Optic Fibers Deeps Lecture 4Contents I. I.Attenuation II.II.Dispersion & bit rate2024/9/263Mechanisms of dispersionIntramodal dispersion & intermodal delayDispersion2024/9/264T
2、he importance of modesn nThe number of modes: n nNormalized frequency Normalized frequency parameter parameter V V n nProportional to Proportional to d d, , NANA, , and and 1/1/ n nFor a large V number ( For a large V number ( 20) 20) n nStep-index fiberStep-index fibern nGraded-index fiberGraded-in
3、dex fiberModes2024/9/265Graded-index fiber-the first solution to the modal-dispersionn nPrinciple Principle n nHow well a graded-index fiber reduce modal How well a graded-index fiber reduce modal dispersion?dispersion?n nGroup index of refraction: NGroup index of refraction: N1 1DispersionExample 3
4、.3.4 Problem: A graded-index fiber has N1=1.487, =1.71%. For a link 5km in length, computer pulse spreading due to modal dispersion and the maximum bit rate.Solution: (1) (2) pulse spreading per km length is:Conclusion: /8 of that of step-index fiber, that is, 2024/9/266A better solution - single mo
5、de (SM) fibern nThe structure of a SM fiberThe structure of a SM fibern nNote: Note: n n(1) N=V(1) N=V2 2/4 only good for V20./4 only good for V20.n n(2) for SM fiber: (2) for SM fiber: V V2.4052.405. .n nThe drawback of SM fiber:The drawback of SM fiber:n nHard and expensive to manufactureHard and
6、expensive to manufacturen nProne to macro- and micro-bending losses Prone to macro- and micro-bending losses Dispersion2024/9/267Mechanisms of dispersioncontdn nIntramodal (chromatic) dispersionn nFrom the finite spectral emission width of an From the finite spectral emission width of an optical sou
7、rceoptical sourcen nCharacterized by the Characterized by the rmsrms spectral width spectral width (fig.4-12 of Ref.4)(fig.4-12 of Ref.4)n nApprox. Approx. 5%5% of a central wavelength for LED; of a central wavelength for LED; 12nm12nm for LD for LDn n2 main causes in SMF: 2 main causes in SMF: mate
8、rial & waveguidematerial & waveguide dispersiondispersionDispersion2024/9/268Calculating pulse spreading caused by chromatic dispersionn nPulse spreading caused by chromatic dispersion:Pulse spreading caused by chromatic dispersion:n nD(D( ): ): the chromatic-dispersion parameter measured in the chr
9、omatic-dispersion parameter measured in ps/nmkmps/nmkmn nWhere SWhere S0 0 is the zero-dispersion slope in ps/(nm is the zero-dispersion slope in ps/(nm2 2km)km)n n 0 0 is the zero-dispersion wavelength, and is the zero-dispersion wavelength, and is the is the operating wavelengthoperating wavelengt
10、h. .Dispersion2024/9/269Total pulse spreadingn nTotal pulse spreading caused by modal & chromatic Total pulse spreading caused by modal & chromatic dispersiondispersionExample 3.3.5 Problem: What the chromatic dispersion for a graded-index fiber if S0=0.097ps/(nm2km), 0=1343nm and =1300nm?Solution:
11、Note: the minus sign “-” indicates that pulse spreading decreases as wavelength increases. for LED, =50nm, 2024/9/2610* Polarization mode dispersionDispersion2024/9/2611Bit rate & Bandwidthn nBit rate (or data rate)Bit rate (or data rate)n nDefinition: number of bits Definition: number of bits per s
12、econd per second overover one channelone channeln nDirect measurementDirect measurementn nBandwidth: Bandwidth: n nDefinition: the frequency range Definition: the frequency range signal can be signal can be transmitted w/o significant deteriorationtransmitted w/o significant deteriorationn nIn Hertz
13、In Hertzn nInformation-carrying-capacity char. for Information-carrying-capacity char. for analog sysanalog sys. . Can be transmitted2024/9/2612Read a data sheet-optical charc.n nGeneral sectionn nOptical characteristicsn nGeometric characteristicsn nEnvironmental spec.n nMechanical spec.n nOther ch
14、arac.2024/9/2613Reading a data sheet2024/9/2614Read a data sheet-Geo. Charc.2024/9/2615Read a data sheet-envirmental specifications2024/9/2616Table 3.1 Typical charac. Of MMFFiber typeFiber typeMaximum attenuation Maximum attenuation ( (dB/kmdB/km) )Minimum bandwidth Minimum bandwidth ( (MHzMHzkmkm)
15、 )at 850nmat 850nmat 1300nmat 1300nmat 850nmat 850nmat 1300nmat 1300nm62.5/12562.5/125 mm2.7102.7103.5 typical3.5 typical0.620.621.0 typical1.0 typical100400100400160 typical160 typical10015001001500500 typical500 typicalSource: Lightwave 1999 Worldwide Directory Of Fiber-optic Communications Produc
16、ts And Services, March 31, 1999, pp. 18-222024/9/2617Mechanisms of dispersioncontdn nOther factors:n nNonideal index profilesNonideal index profilesn nOptical power launching conditionsOptical power launching conditionsn nNonuniform mode attenuationNonuniform mode attenuationn nDispersion2024/9/2618
17、4.6 Dispersion in MMFn nTwo categories: n nIntermodal (modal) & Intermodal (modal) & intramodalintramodaln nWhat will be given in this section?n nTheoretical approachTheoretical approachn nKey resultsKey resultsn nPractical solutionsPractical solutionsDispersion in MMF2024/9/2619Review of dispersion
18、n nThe dependence of The dependence of n n on on through the medium.through the medium.n nDefinition 1Definition 1: : n nDispersion Dispersion changing the changing the v vn nDefinition 2:Definition 2:n nn: the measure of the bending of n: the measure of the bending of a beam within a mediuma beam w
19、ithin a mediumn nDispersion Dispersion different different (s) are (s) are bent at different angles. bent at different angles. Dispersion in MMF2024/9/2620Dispersion in optical fibersn nLight traveling at different v() , causes pulse spreadingn nTo bear in mind:n nDistinguish between Distinguish bet
20、ween 2 types2 types of dispersions: of dispersions: intermodal (modal) & intermodal (modal) & intramodalintramodal (chromatic) (chromatic)n nDispersion restricts Dispersion restricts bandwidth, or information bandwidth, or information carrying capacitycarrying capacityDispersion in MMF2024/9/2621Tot
21、al dispersion & pulse widthn nPulse widthPulse width: : n nfull width at half the maximum of the pulse power full width at half the maximum of the pulse power (FWHM)(FWHM)n nElectrical & optical bandwidthElectrical & optical bandwidthn nThe bandwidth of an optical fiberThe bandwidth of an optical fi
22、ber: the frequency range : the frequency range within which the output power of the information signal within which the output power of the information signal drops to half the input value.drops to half the input value.Dispersion in MMFAt the half power level2024/9/2622Pulse widthn nConclusion:n nOp
23、tical electrical bandwidths are different;Optical electrical bandwidths are different;n nE-bandwidth (system bandwidth): use the E-bandwidth (system bandwidth): use the current ratiocurrent ration nO-bandwidth: use the O-bandwidth: use the power ratiopower ration nIt is much more common toIt is much
24、 more common to measure measure, rather than , rather than calculate the bandwidth of multimode fiberscalculate the bandwidth of multimode fibersDispersion in MMF2024/9/2623Intermodal dispersion n nMechanism: Mechanism: n nDifferent modes Different modes different different group velocitiesgroup vel
25、ocitiesn nOutput-light pulse builds up Output-light pulse builds up through a merging of the through a merging of the individual mode pulses individual mode pulses n nIn Example 3.3.2, 86.57ns/kmIn Example 3.3.2, 86.57ns/kmn nManufacturer: 30ns/km? Manufacturer: 30ns/km? why?why?Dispersion in MMF202
26、4/9/2624Analysis of the formula n nAssumptions:Assumptions:n nAll modes are excited All modes are excited equallyequallyn nModes do not interact w/ one anotherModes do not interact w/ one anothern nAll modes travel at the same velocityAll modes travel at the same velocityn nn n does not depend on th
27、e does not depend on the n nThe fewer the modes, the less severe will be the spreading of The fewer the modes, the less severe will be the spreading of the output pulsethe output pulsen nFor Higher-order modes:For Higher-order modes:n nCarrying much less power, disappear first Carrying much less pow
28、er, disappear first negligible negligiblen nMode coupling Mode coupling the initially critical mode doesnt travel the longest the initially critical mode doesnt travel the longest distancedistancen nThe effective The effective RIRIn n( ( ) ) t the chromatic dispersionhe chromatic dispersionn nThe ac
29、tual pulse spread cannot be more than that calculated The actual pulse spread cannot be more than that calculated from the formula from the formula Dispersion in MMF1. All modes are excited;2. Each existing mode carries equal power;are not true2024/9/2625Chromatic dispersion (material-)n nTwo mechan
30、isms: Material dispersion & waveguide dispersionn nMaterial dispersion: Material dispersion: n nDue to the dispersive properties of the materialDue to the dispersive properties of the materialn nDepend on Depend on n(n( ) )n nBe proportional to the spectral width of the light sourceBe proportional t
31、o the spectral width of the light sourcen nEven SMF exhibits material dispersion Even SMF exhibits material dispersion Dispersion in MMF2024/9/2626Formula for material dispersionn nGroup delayGroup delayn nAssumption: all modes are excited Assumption: all modes are excited equallyequallyn nGroup (ti
32、me) delay per unit lengthGroup (time) delay per unit lengthn nBoth Both v vg g & & g g are wavelength dependent. fig. 4.22 are wavelength dependent. fig. 4.22 (p.128)(p.128)Dispersion in MMF2024/9/2627Formula for material dispersionn nExpand Expand g g into the Taylor series, linear approximation in
33、to the Taylor series, linear approximation used:used:n nD Dmatmat( ( ): material dispersion parameter/coefficient): material dispersion parameter/coefficientDispersion in MMF2024/9/2628n nExpression for material Expression for material dispersion:dispersion:n nFigure 4.23 (p.129)Figure 4.23 (p.129)n
34、 nMaking practical Making practical calculations:calculations:n nThe material-dispersion The material-dispersion parameter equivalent parameter equivalent to that of chromatic to that of chromatic dispersion in MMFdispersion in MMF2024/9/2629Waveguide dispersionn nPropagation constant dependent on wavelengthn nNegligible in MMF, but important in SMF2024/9/2630Bandwidth of MMFn nThe bandwidth:The bandwidth:n nIntermodal bandwidth:Intermodal bandwidth:n nFor SI fiber:For SI fiber:n nFor GI fiber: For GI fiber: n nMaterial bandwidth:Material bandwidth:2024/9/26312024/9/2632
