1、Fiber-Optic Communication TechnologyChapter 3 Optical Transmitters2023-2-3OE,HUST2Chapter 3.Optical TransmittersnIntroductionnBasic conceptsnSemiconductor lasers(Laser Diode)nLaser CharacteristicsnLight-Emitting Diodes(LED)nTransmitter Design2023-2-3OE,HUST3Optical transmitter：光发射机LED:发光二极管LD：激光二极管S
2、pontaneous emission：自发辐射Stimulated emission：受激发射Stimulated absorption：受激吸收Boltzman statistics：玻尔兹曼统计分布Thermal equilibrium：热平衡Spectral density：光谱密度Population inversion：粒子数反转Fermi-Dirac distribution：费米狄拉克分布Conduction band：导带Valence band：价带Forward-biased：正向偏置Junction：结Fermi level：费米能级Bandgap：带隙Heavy do
3、ping：重掺杂Homojunction：同质结Heterojunction：异质结Double heterostructure：双异质结Electron-hole recombination：电子空穴复合Cladding layer：包层Auger recombination：俄歇复合Kinetic energy：动能Nonradiative recombination：非辐射复合Surface recombination：表面复合Internal quantum efficiency：内量子效率Direct bandgap：直接带隙Indirect bandgap：非直接带隙Carrier
4、 lifetime：载流子寿命Lattice constant：晶格常数Ternary and quaternary compound：三元系和四元系化合物Substrate:衬底LPE：液相外延VPE：汽相外延MBE：分子束外延MOCVD：改进的化学汽相沉积MQW:多量子阱2023-2-3OE,HUST4Electron-hole pairs 电子空穴对External quantum efficiency 外量子效率Fresnel transmissivity 菲涅耳透射率Lambertian source 朗伯光源Power-conversion efficiency 功率转换效率Wal
5、l-plug efficiency 电光转换效率Responsivity 响应度Rate equation 速率方程Surface-emitting 表面发射Beam divergence 光束发散Edge-emitting 边发射Resonant cavity 谐振腔Gain coefficient 增益系数Differential gain 微分增益Laser threshold 激光阈值Threshold current 阈值电流Group index 群折射率External cavity 外腔Broad area 宽面Stripe geometry 条形Diffusion 扩散Ind
6、ex-guided 折射率导引Ridge waveguide laser 脊波导激光器Buried heterostructure 掩埋异质结Lateral 侧向Transverse 横向SLM:Single Longitudinal mode单纵模MSR:Mode suppression ratio 模式抑制比DFB:Distributed Feedback 分布式反馈Bragg diffraction 布拉格衍射Bragg condition 布拉格条件DBR:distributed Bragg reflector 分布式布拉格反射器Phase-shifted DFB laser 相移DF
7、B激光器Gain coupled 增益耦合Coupled cavity 耦合腔2023-2-3OE,HUST5Superstructure grating 超结构光栅VCSEL:vertical cavity surface-emitting lasers 垂直腔表面发射激光器Photon lifetime 光子寿命Spontaneous-emission factor 自发辐射因子Characteristics temperature 特征温度Slope efficiency 斜率效率Differential quantum efficiency 微分量子效率Linewidth enhanc
8、ement factor 线宽加强因子2023-2-3OE,HUST63.1 Introduction3.1.1 Components of Optical TransmittersBinary to singleCoding/line codingModulatorOptical SourceDriving CircuitPCMChannel couplerOptical signal output2023-2-3OE,HUST7Biased current Modulation currentLD(10Gb/s)modulatorLD/LEDModulation currentBiased
9、 current(2.5Gb/s)(a)Direct Modulation(b)External Modulation2023-2-3OE,HUST81.stability:power&wavelength2.reliability:25 years(Pout to Pout/2)3.small emissive area compatible with fiber core dimensions4.right wavelength range 0.85 m:GaAlAs/GaAs 1.31 m,1.55 m:InP/InGaAsP5.narrow linewidth dispersion,p
10、hase noise6.direct modulation!?7.high efficiency&low threshold:MQW-LD,Ith 10mA3.1.2 Requirements for Optical SourceMQW DFB LD2023-2-3OE,HUST9Chapter 3.Optical TransmittersnIntroductionnBasic conceptsnSemiconductor lasers(Laser Diodes)nLaser Characteristics nLight-Emitting Diodes(LED)nTransmitter Des
11、ign2023-2-3OE,HUST103.2.1 Three fundamental transition processes1.Spontaneous Emission LED2.Stimulated Emission LD,SOA3.Stimulated Absorption PIN/APD3.2 Basic ConceptsLight Emission2023-2-3OE,HUST113.2.2 Emission and Absorption RatesE2N2N1E1spon2stim2abs1RA NRB NRB Nhh:spectral density of the electr
12、omagnetic energyphIn thermal equilibrium,according to Boltzmann Statistics:kB:Boltzmann ConstantT:Absolute Temperature21/exp(/)exp(/)gBBNNEk Thv k T221phphANBNB N/(/)exp(/)1phBA BBBhv k T338/exp(/)1phBhvchv k TAccording to Plancks formula:2023-2-3OE,HUST1233(8/)(Einsteinscoefficients)AhvcBBB 1.Bk Th
13、2.1eV,25meVBh k T visible or near-infrared region,room temperature-1stimspon/exp(/)-11BRRhv k T1,1sponsponstimabsRRRR ,thermal sourcesN2N1,RstimRabs(population inversion)thermal equilibrium laser operation?Operation condition for laser:External pumping source is needed:injection current,pumping ligh
14、t etc.2023-2-3OE,HUST13n原子是由原子核和绕原子核旋转的电子组成。最里层的电子距原子核最近，受原子核束缚最强，能量最低（包括电子的动能和势能）。越外层的电子受原子束缚越弱，能量越高；n电子只能处于特定的能级之上；n能级图用一系列高低不同的水平横线来表示电子所能取的确定能量；n原子中的电子通过和外界交换能量的方式发生能级的跃迁热跃迁和光跃迁。Energy bands in semiconductor conduction band&valence band2023-2-3OE,HUST14n实际物体是由大量原子构成的，每一原子的电子特别是外层电子除受本身原子的势场作用外，还
15、受到相邻原子的作用。n半导体材料中原子在共价键的作用下形成紧密相间、周期排列的晶格结构。电子能级受晶格作用发生分裂而形成能带；Si2023-2-3OE,HUST15u价带价带：由共价键束缚的价电子所占据的能带为价带；u导带导带：由自由电子占据的能带为导带，导带位于价带之上；u禁带：禁带：导带和价带之间被宽度为Eg的带隙分开，称为禁带；n绝缘体绝缘体：Eg 7eV，电子不容易跃迁到导带；半导体半导体：Eg1eV，电子容易跃迁到导带；导体导体：Eg0eV，没有带隙。2023-2-3OE,HUST16Energy bands in semiconductor recombination betwee
16、n electrons and holesThe occupation probability for electrons in the conduction and valence bands is given by the Fermi-Dirac distributions:122111()1 exp()/()1 exp()/fcBfvBcvfEEEk TfEEEk TEfc,Efv are the Fermi levels in conductionband and valence band respectively()1/2()1/2cfcvfvfEfE2023-2-3OE,HUST1
17、712212()(,)()1()CsponcvcvERA E EfEfEdE21EE 3/21/223(2)()2rcvgmE:joint density of states,which describe the number of states per unit volume per unit energy rangeEg:bandgapmr:reduced massmc,mv:effective masses of electrons&holes in conduction and valence bands,respectivelycv2023-2-3OE,HUST18122121212
18、2()(,)()1()()(,)()1()CCstimcvcvphEabsvccvphERB E EfEfEdERB E EfEfEdE population-inversion condition:stimabsRR2121()()cvfcfvgfEfEEEEEEfcfvEE in thermal equilibrium:pumping energy into semiconductor by injecting current fcfvEETo get laser output,2023-2-3OE,HUST191.Type of semiconductor Intrinsic semic
19、onductor:undoped,Fermi level is lying in the middle of the bandgap n-type semiconductor:Fermi level moves toward the conduction band as the dopant concentration increases p-type semiconductor:Fermi level moves toward the valence band as the dopant concentration increases3.2.3 p-n junctions2023-2-3OE
20、,HUST20 n-typeIntrinsic p-typeforward biased p-type semiconductor&n-type semiconductor2023-2-3OE,HUST21(a)in thermal equilibrium(b)under forward biased2.p-n junctionsunder forward biased:built-in electric field is reduceddiffusion of electrons and holes across the junctionelectrons and holes are pre
21、sent simultaneously in depletion regiongenerate light through spontaneous emission or stimulated emissionin thermal equilibrium:the Fermi level must be continuous across the pn junctionachieved through diffusion of electrons and holes across the junction.2023-2-3OE,HUST22 Homojunction:equal bandgaps
22、 nthe same semiconductor materialnwide region for electron-hole recombinationndifficult to realize high carrier densities Heterojunction:different bandgaps Double-heterojunction:sandwiching a thin layer between the p-type and n-type layers such that the bandgap of the sandwiches layer is smaller tha
23、n the layer surrounding it.12ggEE12ggEE4.Homojunction&heterojunction2023-2-3OE,HUST232023-2-3OE,HUST24nActive layer:light is generated inside it as a result of electron-hole recombinationnhigher density of carriers higher index waveguide(1D)nHeterojunction:confinement of carriers&optical field n0.85
24、m:cladding/active:GaAlAs/GaAs 1.31m,1.55m:cladding/active:InP/InGaAsP2023-2-3OE,HUST251.electron-hole recombination3.2.4 Nonradiative Recombinationlightsponteneous emissionradiative recombinationstimulated emissionnokinetnradiative recombinaticenergyhionof electron or holeate Trap of defectsSurface
25、recombinationAugerNonradiative recombination2023-2-3OE,HUST262.internal quantum efficiencyintrrrrtotrrnrRRRRRRrr:radiative recombination rate Rnr:nonradiative recombination rate Rtot:total recombination rate :recombination time /rrrrRN/nrnrRNintnrrrnr Nonradiative recombination,especially Auger reco
26、mbination(temperature dependent)is harmful to devices!intoutTPInrR positive feedback 2023-2-3OE,HUST27E0kint50%rrnrE0kk1k2int1nrrr(1)direct-bandgap(GaAs,InP)(2)indirect-bandgap(Si,Ge)3.carrier lifetimeA:defects&traps 23/spnrcRRNANBNCNB:spontaneous radiation C:Auger c2023-2-3OE,HUST28 Quality of the
27、heterojunction interface depends on the lattice constant of the two materials.(matching!)gE(x)=x(ev)3.2.5 Semiconductor Materialsternary compoundg(J)hcEg1.24E(ev)x1-x0.85:GaAsAl GasGaAlm 2023-2-3OE,HUST291-xxy1-y1.31,1.55:InPInGas PInGaPAsm quaternary compound0.85m:GaAlAs/GaAs(cladding/active)1.31m,
28、1.55m:InP/InGaAsP(cladding/active)2gE(y)=1.35-0.72y+0.12y,x/y=0.45(0y1)2023-2-3OE,HUST302023-2-3OE,HUST31Chapter 3.Optical TransmittersnIntroductionnBasic conceptsnSemiconductor lasers(Laser Diodes)nLaser Characteristics nLight-Emitting Diodes(LED)nTransmitter Design2023-2-3OE,HUST323.3 Semiconducto
29、r lasers(Laser Diodes)nAdvantages of stimulated emission compared with spontaneous emission of semiconductor materialsemitting high power (to 100mW)narrow angular spreadnarrow spectral widthdirect modulation at high frequency(to 10GHz,because is small)c2023-2-3OE,HUST33Components of Semiconductor La
30、sersGaAlAs/GaAsgain medium optical gain semiconductor materialInP/InGaAsPcleaved facetresonant cavity optical feedback built-in grating(DFB/DBR)pump source population inversion injection current2023-2-3OE,HUST34z=0z=LInjection currentGain mediumResonant cavityResonant cavityModel of laser2023-2-3OE,
31、HUST353.3.1 Optical GainnPeak gain of medium:when:differential gain(gain cross section):injection carrier density:transparent carrier density :threshold carrier density()()pgTgNNN,0TnetNNgNT is equal to Nth?gTthNNN2023-2-3OE,HUST36Figure 3.9:(a)Gain spectrum of a 1.3-m InGaAsP laser at several carri
32、er densities N.(b)Variation of peak gain gp with N.The dashed line shows the quality of a linear fit in the high gain region.Blue or red shifting of peak wavelength when injected current increases?2023-2-3OE,HUST373.3.2 Feedback and Laser ThresholdnFeedbackR1R2n0=1n2201201nocoating:1nnnRRnnn2023-2-3
33、OE,HUST38nThreshold 0int120 exp()exp(-)exp(2)EgLa LikLR RE1/21/20int2int10exp(1/2)exp(1/2)()exp(1/2)exp(1/2)()exp(2)EgLa L RgLa L RikLE2023-2-3OE,HUST39nAmplitude conditionnPhase conditioncavmiraaaRRLagint21int)1ln(2122 /2mkLmvvmcnL2cnLspacing of oscillating frequencyoscillating frequencythreshold g
34、ainMLM2023-2-3OE,HUST403.3.3 LD StructuresnBroad-area LDFigure 3.12:A broad-area semiconductor laser.The active layer(hatched region)is sandwiched between p-type and n-type cladding layers of a higher-bandgap material.light-confinement mechanism in the direction perpendicular to the junction plane i
35、ntroduced by double heterostructure XYdistribution in near field 2023-2-3OE,HUST41no such light-confinement mechanism in the lateral direction parallel to the junction plane.the light generated spreads over the entire width of the laser.relatively high threshold current and a spatial pattern that is
36、 highly elliptical and that changes in an uncontrollable manner with the current.How about spatial mode in waveguide and distribution in far field?2023-2-3OE,HUST42n Gain-guided semiconductor lasersFigure 3.13:Cross section of two stripe-geometry laser structures used to design gain-guided semicondu
37、ctor lasers and referred to as(a)oxide stripe and(b)junction stripe.Stripe lasersXY2023-2-3OE,HUST43solve the light-confinement problem by limiting current injection over a narrow stripe.the spot size is still not stable as the laser power is increased.Injection current induced index variety!2023-2-
38、3OE,HUST44n Index-guided semiconductor lasersFigure 3.14:Cross section of two index-guided semiconductor lasers:(a)ridge-waveguide structure for weak index guiding;(b)etched-mesa buried heterostructure for strong index guiding.XY2023-2-3OE,HUST45Multi-Quantum-Well LDn 有源区厚度薄110nmn 周期结构，将窄带隙的很薄的有源区夹在
39、宽带隙 的半导体材料之间，形成势能阱n 多个势能阱-多量子阱（MQW）2023-2-3OE,HUST46homojunctionDouble heterostructureStripe geometryMulti-quantum-well Relatively stronger confinement of injected carriers and output photons,thus lower threshold current,and higher slope efficiency!2023-2-3OE,HUST473.3.4 Control of Longitudinal Mode
40、s/mmsmSMSRPPSide Mode Suppression Ratio(SMSR):10()10log ()()mmsmPmWSMSRdBPmWorMLMLossSLM2023-2-3OE,HUST48Distributed Feedback(DFB)Lasers(/2)Bmn 相位光栅在波导中产生折射率的周期性变化，使正反向传播的行波产生耦合。当光波长满足布拉格条件时，耦合达到最大。在布拉格条件下，某一入射波长几乎被全反射，光栅起到了对波长选择性反射的作用。光栅周期满足：2023-2-3OE,HUST49Coupled-cavity laser Figure 3.18:Coupled
41、-cavity laser structures(a)external-cavity laser;(b)cleaved-coupled cavity laser;(c)multisection DBR laser.2023-2-3OE,HUST50 增益介质反射镜准直透镜透镜光纤增透膜滤光片高反膜cExternal cavity laser2023-2-3OE,HUST51Sampled Grating DBR LaserDBR:distributed Bragg reflector2023-2-3OE,HUST52Cleaved-coupled cavity laser2023-2-3OE,
42、HUST53VCSEL2023-2-3OE,HUST54思考题1.现有半导体激光器的F-P谐振腔，长度为400m，材料折射率为3.5，谐振腔两端面一端镀有增反射膜，反射率为90，另一端没有镀膜。现有半导体激光器工作在1550nm附近，要求谐振腔谐振的阈值增益系数小于75cm1，请问：如何选择半导体材料和组分？谐振腔内部损耗系数应满足什么条件？2023-2-3OE,HUST55Chapter 3.Optical TransmittersnIntroductionnBasic conceptsnSemiconductor lasers(Laser Diodes)nLaser Characteris
43、ticsnLight-Emitting Diodes(LED)nTransmitter Design2023-2-3OE,HUST563.4 Laser Characteristics 3.4.1 CW CharacteristicsFor a SLM laser,the rate equations:sppcdPPGPRdtdNINGPdtqP,N:number of photons&carriers spspRn G2spn()()/gNTgTNggGgGNNgNNVGV Net rate of stimulated emissionoptical gain:1int()pgcavgmir
44、 g:peak gain of material :gain cross section,or differential gain.gPhoton lifetime:2023-2-3OE,HUST57Threshold of current&carrier00sppcdPPGPRdtdNINGPdtqassuming 011()spthTRNpththTccNpNNGqNqINGFor I Ith(R1=R2)int(/)()()12()2pthintmirethmiregmirPq IIPIIqPP CW operation:0,0dPdNdtdt2023-2-3OE,HUST58nThre
45、shold of P-I curvesththIIIISpontaneous emissionStimulated emissionI0:constantT0:characteristic temperatureGaAs:T0=120K,InGaAsP:T0=50 70K00()exp()thIT I T/TP-I curvesintnreITRPnBending of P-I curves Rnr:mainly depending on Auger recombination in InGaAsP LDs Solution:built-in thermoelectric cooler is
46、used to deal with temperature sensitivities of InGaAsP LDs2023-2-3OE,HUST59EfficienciesInternal quantum efficiency:ninti t,100%()thphotonsthroughinjected electstronsfor IISlope efficiency:2eddPhdIqDifferential quantum efficiency:intintmirdmir External quantum efficiency:2/2/eeextPqPphotonemission ra
47、teelectroninjection rateI qI1/,dthextdIII wall-plug efficiency:000 (),gtogtotextextexttEfEqVqorVqV2023-2-3OE,HUST603.4.2 Small-Signal Modulation()sin()(,:phase delay)()sin()bmmmmmbmmmP tPptN tNnt()()bmpI tII ftnsmall-signal modulation:,()sin()bthmbthpmIIIIIftt22()()(0)()()mmRRmmRmRRmRpHpii 1/21/2322
48、333()(),()244NbNRdBbthRRPG PGfIIq RR:frequency of :damping rate of rrelaxation oscillatioelaxation oscillnsationsnFrequency response2023-2-3OE,HUST61Figure 3.21:Modulation response of a laser as a function of modulation frequency at several bias levels.Modulation bandwidthn the efficiency is reduced
49、 when the modulation frequency exceeds R by a large amount.2023-2-3OE,HUST623.4.3 Large-Signal ModulationExternal modulation for high speed transmission!nFrequency chirp leading edge:mode frequency shifts toward the blue sidetrailing edge:mode frequency shifts toward the red side :amplitude-phase co
50、upling parameter,ex.bulk material:48;MQW:3cINn11()2cNTpdGNNdt()()()I ttt 2023-2-3OE,HUST63nElectro-optical Delay&Relaxation Oscillation Pre-biased to reduce delay time!请参见江剑平编著的半导体激光器2023-2-3OE,HUST64nPattern effect TBIBdTP“11”“11”当电光延迟时间与电调制速率对应的的码元持续时间相近时，会使“0”码后的第一个“1”码脉冲宽度变窄，幅度变小，严重时使单个“1”码丢失，这种
