光通信技術課件lecture

1、Department of Optical EngineeringA Review check listTransmitterThree partsLight sourceCoupling opticsDriving electronicsLED basic propertiesP-N junctionInternal quantum efficiencyRadiative/nonradiative binationLambertian sourceSpontaneous emissionRise/fall timeSLED v.s. ELEDLD basic properties Struc

2、ture Optical characteristicsPower v.s. driving currentSpectral linewidthWavelength v.s. temperatureWavelength v.s. driving currentDFB characteristics structure spectral linewidthVCSEL basic properties Structure Advantages Optical characteristicsDepartment of Optical EngineeringFermi-Dirac distributi

3、onFermi-Dirac distribution f(E)=1/1+exp(E-Ef)/kBT) - exp(-E/ kBT) when E kBT, EfDepartment of Optical EngineeringEnergy level distribution for P and N type semiconductors before contactDepartment of Optical EngineeringP-n in equilibriumIn equillibrium, the Fermi energy are the same for both sidesb/c

4、 same distribution: fA (E)= fB (E) -EfA= EfBDepartment of Optical EngineeringForward biased: P-N no longer in equilibriumForward bias reduces potential barrier, vice versaIn forward bias, bination es easierOptical properties depends on Eg and Fermi levelsNo BiasDepartment of Optical EngineeringDoubl

5、e HeterostructureActive region 0.1-0.01 mConfining layers 2 mContact layers 3 mDifferent bandgapsNonbiased: up to Ef are filledForward biased: below Efc fillledAbove Efv vacantTwo exits:Transition (high probability)Climbing over barrierPopulation inversion: electrons - voids ( holes)Department of Op

6、tical EngineeringEfficiency of a laser diodeInput-output Pout=EphotonNleak/esc/t; Ephoton=hc/Nleak=ext(IFt/e) (radiated photons v.s. injected electrons)Pout= ext(IF/e )Ephoton ext=(e/Eg)P/ I =(e/Eg)S (electrons - radiated photons)S(W/A)=(Ephoton/e) ext (Eg/e) ext Three types of efficiencyP=Pout/Pdc=

7、P/(IFEg/e)+IF2R) (power in - power out)int (meaning?), ext,Department of Optical EngineeringA review of concepts: transmitterDepartment of Optical EngineeringVCSEL (vertical-cavity surface emitting lasers)Hottest area of transmitter!Short cavity; 2m, adjacent longitudinal mode spacing=72 nmnaturally

8、 single-mode operationSmall footprint; be able to packaged into dense arraysSmall active region - high current density - low power consumptionHigh switch time?Department of Optical EngineeringTunable VCSELDepartment of Optical EngineeringTunable VCSELDepartment of Optical EngineeringRadiation patter

9、nsActive region 1x10 x100 mElliptical cone 10 x30Longitudinal modes 2L=Ntransverse modes TEMxxDepartment of Optical EngineeringBlock diagram of a transmitter module6 func. blocks:Data conversionLaser driverBias/operation and modulation controlTemperature controlLDCoupling opticsDepartment of Optical

10、 EngineeringData conversion unitFunctionsEncodingParallel to serial conversionReshapingEncodingAll data transmissions are governed by clock signalNRZ ( non-return-to-zero) format disadvantage: it carries a dc component -generating heat, no infoManchest code - no dc component disadvantage: double ban

11、dwidthRZ (return-to-zero) codeDepartment of Optical EngineeringModulationInternal and external modulationInternal/direct modulationBandwidth restricted by the LD relaxation time (relaxation osc. Freq. r=MP/ph)Chirp: peak radiation freq. changes with driving currentSerious problem for DFB LDHarder a

12、laser diode is driven, the faster its response (p.397, Fig.10.16 b).External modulationMach-Zehnder modulator (LiNbO3)Electroabsorption modulator (semiconductor based) Department of Optical EngineeringDirect modulationDelay timeThere is a delay time before a LD starts to lase after the modulation pu

13、lse has been applied td=lnIp/(Ip+Ib-Ith), : bination time, Ip : amplitude of modulation current, Ib:dc bias currentChirpDefinition: deviation of freq. from its center freq.Order of magnitude: 0.1-1 GHz/mAMechanism: carrier population change/refractive index changeLinewidth broadeningDirect Modulatio

14、nDepartment of Optical EngineeringLimit of Direct Modulation FrequencyDepartment of Optical EngineeringDepartment of Optical EngineeringMach-Zehnder modulatorLiNbO3 basedElectric field induced refractive index changeInterfering at coupling pointDepartment of Optical EngineeringElectroabsorptive modu

15、latorBandgap of waveguide material decrease as voltage appliedCutoff wavelength increasec1/EgMay be integrated into one substrate with DFB -monolithically integrated chipKey problem: chirpDepartment of Optical EngineeringAbsorption v.s. VoltageDepartment of Optical EngineeringChirp v.s. ModulationLo

16、ss Modulator: Reason for ChirpKramers-Krnig relationsReal part and imaginary part of refractive index are relatedWhen loss is increased, n n changes instantaneous phase change: chirpDepartment of Optical EngineeringDepartment of Optical EngineeringNoisePhase fluctuationsLinewidth broadeningIntensity

17、 fluctuationsRIN ( relative intensity noise) RIN(dB/Hz)=10logPN2 / P 2BWCausesAmplified spontaneous emissionBackreflectionDepends on (p.400)1) relaxation osc. Freq. : increases with r then decreases with r2) laser power: P-3 at low power, P-1 at high powerDepartment of Optical EngineeringCoupling opticsButt ( direct ) coupling=Pinfiber/Po=TNA2Single-mode 10-20%, multimode: 50-70%Lens-coupling80%Lens set to match beam to a fiber coreMachined fiber tipDepartment of Optical Engineering1.6m DFB LD Module (1)Anritsu Corp.Department of Optical Engineering1.6m DFB LD Module (2)Three st