1EE290C - Spring 2004Advanced Topics in Circuit DesignHigh-Speed Electrical InterfacesLecture 27Case StudiesOptical LinksBorivoje NikolicApril 27, 2004.2AnnouncementsQuiz this Thursday:~2 questions, 30 minutesNo lectures next weekProject discussion/help on Tu, May 4, 4-5:30Project presentationsTu, May 11, 2:30-5:30 in BWRC23OutlineProperties of fiber opticsMaterial: B. Razavi, “Design of Integrated Circuits for Optical Communications,” McGraw-Hill 2003.Illustrations are from this bookRamaswami, Sivarajan, “Optical networks,” 2nded, Morgan-Kaufmann 2002.4Optical Communications1840s – Babinetdemonstrated ‘bending’ of lightInitially through water, later through quatrz rods/fibers1954 – van Heel et al, fiber optic image transmissionGenerated endoscopy applications1950-1960 –discovery of lasers 1966 – Ko/Hockem – fiber-optic communications1970 - Maueret al, fiber with 20dB/km loss1977 – AT&T deploys first fiber-optic telephone lines35Fiber-Optic CommunicationsPros:Enormous freqency range (~10’s of THz)Large bandwidth (50GHz)Low loss (<2dB/km)Small diameter and light weightNo EMIIssuesClock and data recovery issuesDispersionOptical/electrical interfaces6Network Evolution47Multi-Mode FiberExample8Copyright © The McGraw-Hill Companies, Inc. Fiber Optic CommunicationsSimple optical system+ driver and amplifier+ MUX and DMUX.59Modified transmitterCopyright © The McGraw-Hill Companies, Inc.Modified SystemModified receiver10Copyright © The McGraw-Hill Companies, Inc.Complete Optical System611LasersSemiconductor lasersIII-V semiconductor is the gain medium within the positive feedback systemFiber lasersErbium-doped fiber is the gain medium12Copyright © The McGraw-Hill Companies, Inc.Laser DiodesTypical laser diode characteristic Modulatedinput and output waveformsMonochromatic lightEmits light when I > ITH713Laser PropertiesEfficiencyQuantum efficiency: #photons/ #electronsSpectral purityReal lasersThreshold currentITHshould be lowVoltage dropLaser diodes need 2-3V swingsSwitching speedsOutput powerLifetime14Stimulated emissionCopyright © The McGraw-Hill Companies, Inc. Operation of a LaserOptical oscillatorsEnergy bands Excitation of an electronSpontaneous emission815Copyright © The McGraw-Hill Companies, Inc.Fabry-Perot LaserSimplified operationFacetsGain medium (cavity)Transmitted waves add in phaseGain + reflectivity requirement to oscillate (lasing threshold)16Copyright © The McGraw-Hill Companies, Inc. Laser StructuresSimple laser diodeDouble heterostructureHigher-order modes917Multiple-Longitudinal ModesConditions:Wavelength has to be within the bandwidth of the mediumCavity length ~ N λ/2Multiple wavelengths satisfy the conditionMulti-longitudinal mode (MLM) laser~10nm spectral widths18Copyright © The McGraw-Hill Companies, Inc.Resonant StructureResonant structure using periodic variation of refractive indexRealization using a gratingDistributed feedback laser1019DF LasersReflectors are distributed along the cavity30dB suppression of non-dominant modesCan add external filtering (external cavity)EA: Electronic absorptionLN: LiNbO320Copyright © The McGraw-Hill Companies, Inc. VCSELsVertical cavity surface emitting lasersIf the cavity is small enough (c/2nl), there will be room for only one mode within the gain band widthThin layer can be deposited in semiconductor technology1121Copyright © The McGraw-Hill Companies, Inc. Turn-on Delay and VariationRandomness in turn-on processSpontaneous emission22Copyright © The McGraw-Hill Companies, Inc. Relaxation Oscillation“Ringing” in the time domain – exchange of energy between photons and electrons1223Copyright © The McGraw-Hill Companies, Inc. Extinction RatioEX = 10log(PH/PL)24Copyright © The McGraw-Hill Companies, Inc. Temperature and Aging Drift1325Copyright © The McGraw-Hill Companies, Inc. External ModulationMach-Zehnder modulator26Optical Sources~401.5DFB-LD + LN~201.5DFB-LD + EASingle mode~101.3/1.5DFB-LDMLM~51.3/1.5FP-LD~10.85- 1.3VCSELLow cost~1~1.3LEDFeatureSpeed [Gb/s]λ [µm]Source1427Copyright © The McGraw-Hill Companies, Inc. Optical FibersSingle-Mode Fiber28Copyright © The McGraw-Hill Companies, Inc. SM Fiber Loss Profile1529Types of FiberLow costeasy alignment<3000.65Multi-mode plastic (980um core)Long distance0.5-0.21.3 – 1.5Single-ModeGlass (8-10um core)Easy alignment3.5-1.50.85- 1.3Multi-ModeGlass (62.5um – 50um core)FeatureLoss [dB/km]λ [um]Type30Fiber DispersionDifferent frequencies (wavelengths) travel with different velocities Chromatic dispersionShape of the transmitted pulse changesPulse spreads - ISI1631Copyright © The McGraw-Hill Companies, Inc.Chromatic DispersionIndex –change in delay per km of fiber for 1nm in change in the wavelength32Polarization Mode DispersionDifferent propagation velocities for different polarizations1733Copyright © The McGraw-Hill Companies, Inc. PhotodiodesGeneration of an electron-hole pair by means of a photon34(a)Copyright © The McGraw-Hill Companies, Inc. Photodiode CharacteristicPhotodiode characteristic, current conduction by electrons and
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