1、 Chapter 6 Optical Sources and AmplifiersChapter 6 Optical Sources and Amplifiers6.1 Light-emitting Diodes6.2 Light-emitting Diode operating Characteristic6.3 Laser Principles6.4 Laser Diodes6.5 Laser-diode operating Characteristic6.7 Optical Amplifiers6.8 Fiber Lasers6.9 Vertical-Cavity Surface-emi
2、tting Laser DiodesLight sourceLight-emitting diodeLaser diodeModulation A Light-emitting Diodes is a pn-junction semiconductor that emits light when forward biased.Circuit6.1 Light-emitting DiodesIn the upper-energy band,called the conduction band,electrons not bound to individual atoms are free to
3、move.In the lower band,the valence band,unbound holes are free to move.Holes have a positive charge.6.1 Light-emitting DiodesTwo allowed bands of energies are separated by a forbidden region(a bandgap)whose width has energy Wg.6.1 Light-emitting DiodesIn a word,radiation from an LED is caused by the
4、 recombination of holes and electrons that are injected into the junction by a forward bias voltage.PNpn-junctionflash366.2 Light-emitting Diode operating characteristicmA0 50 100 1507654321mWThe optic power generated by an LED is linearly proportional to the forward driving current.Digital modulati
5、oncurrenttimeOutput powerinput currenttimeOptical powerThe diode is modulated by a current source,which simply turns the LED ON or OFF.Analog modulationAnalog modulation requires a dc bias to keep the total current in the forward direction at all times.Optical powertimetimecurrentAs we know,the opti
6、c spectrum of the source directly influences material and waveguide dispersion.Pulse spreading due to these causes increases linearly with source spectral width.LEDs operating in the region 0.8-0.9m generally has width of 20-50 nm,and LEDs emitting in the longer-wavelength region have widths of 50-1
7、00nm.6.2 Light-emitting Diode operating characteristic Coupling efficiency depends heavily on the radiation pattern of a emitter.-900 90BEAM ANGLEBEAM INTENSITYsurface-emitting LED Rays incident on a fiber,but outside its acceptance angle,will not be coupled.The acceptance angle for a fiber having N
8、A=0.24 is only 14,so a large amount of the power generated by a surface emitter will be rejected.-900 90BEAM ANGLEBEAM INTENSITYsurface-emitting LED Edge emitters concentrate their radiation somewhat more than surface devices,providing improved coupling efficiency.-90 -45 0 45 90120 30BEAM ANGLEBEAM
9、 INTENSITYPARALLEL PLANEPERPENDICULAR PLANEedge-emitting LEDFlash 386.3 Laser PrinciplesHere is a list of some characteristics that all lasers possess and that are important in their utilization:Pumping threshold The power input to a laser must be above a certain threshold level before the device wi
10、ll emit.Output spectrum The laser output power is not at a single frequency but is spread over a range of frequencies.1.Radiation pattern The range of angles over which a laser emits light depends on the size of the emitting area and on the modes of oscillation within the laser.uthe semiconductor la
11、ser diode uthe gas laseru the bulk Nd:YAGu the fiber lasercommon kinds of laserA laser is a high-frequency generator,or oscillator.For oscillations to occur,a system needs amplification,feedback,and a tuning mechanism for determining the frequency.energyEnergy is supplied from outside and atom enter
12、s excited state.ground stateexcited stateArriving photonPhoton arrives and interacts with excited atom.Arriving photonAtom emits additional photon and returns to the ground state.Arriving photonWhen a new photon is emitted it has identical wavelength,phase and direction characteristics as the exciti
13、ng photon.Population inversionThe number of atoms in the upper level exceeds those in the lower level.Population inversionThe number of photons will increase as they propagate.More photon will encounter upper level atoms(causing generation of additional)than will meet lower level atoms(which would a
14、bsorb them).A medium with population inversion has gain and behaves as an amplifier.6.4 Laser DiodesMETALLIZATIONn-AlGaAs,Wg=1.8eV CONFINEMENTn-AlGaAs,Wg=1.55eV ACTIVE LAYERn-AlGaAs,Wg=1.55eV CONFINEMENTGaAs SUBSTRATEP-GaAs,CONTACTSiO2,INSULATIONMETALLIZATIONSTRIPE CONTACT0.1-0.3m-1m-1m-1mThe struct
15、ure of an AlGaAs laser diodepowerConfinement LayerConfinement LayerActive LayerRefractive Index6.4 Laser DiodesMany laser diodes are edge emitters.Under forward bias,charges are injected into the active layer,causing the spontaneous emission of photons.Some of the injected charges are stimulated to
16、emit by other photons.If the current density is sufficiently high,then a large number of injected charges are available for stimulated recombination.The optic gain will be large.The threshold current is reached when the gain is large enough to offset the diode losses.At this point,laser oscillation
17、start.6.4 Laser DiodesGAIN OF THE AMPLIFYING MEDIUM819 820 821WAVELENGTH(nm)Output power of a laser diodeDiodes radiating a spectrum containing numerous longitudinal modes.6.5 Laser-diodes operating characteristicoperating characteristicoutput powerlinewidthtemperature sensitiveoperating characteris
18、tic (1)Output optic powerOPTICAL POWER(mW)CURRENT(mA)0 50 ITH 100 15054321Output optic power is plotted against forward input current.Digital modulation of a laser diodeTIMEiTIMEIdcCURRENTOPTICAL POWERisidcAnalog modulation of a laser diodeTIMEIdcCURRENTOPTICAL POWERisidcITHTIMEis6.5 Laser-diodes op
19、erating characteristicoperating characteristicoutput powerlinewidthtemperature sensitiveoperating characteristic(2)temperature sensitive CURRENT(mA)Output Power(mW)2040 600Laser diodes are much more temperature sensitive than are LEDsAs the temperature increases,the diodes gain decreases,and so more
20、 current is required before oscillation can begin-the threshold current becomes greater.(2)temperature sensitive CURRENT(mA)Output Power(mW)2040 600At a constant current,the output power of a laser diode will diminish if the temperature risesThere are two techniques for overcoming this problem:therm
21、oelectrically cooling the diode,and changing the bias current to compensate for changed threshold.(2)temperature sensitiveThe laser emission wavelength also depends upon the temperature.This effect arises from the dependence of the materials refractive index on temperature.6.5 Laser-diodes operating
22、 characteristicoperating characteristicoutput powerlinewidthtemperature sensitiveoperating characteristic(3)linewidth-2.5 -1.5 -0.5 0.5 1.5 2.5WAVELENGTH(nm)2.01.5 1.00.50.0INTENSITYLaser diodes typically possess linewidths of 1-5nm,considerably smaller than tho output spectra of LEDs.When the drive
23、 current is just a bit above the threshold,laser diodes produce multimode spectra(3)linewidth-2.5 -1.5 -0.5 0.5 1.5 2.5WAVELENGTH(nm)2.01.5 1.00.50.0INTENSITYWAVELENGTH(nm)2.01.5 1.00.50.0INTENSITY-2.5 -1.5 -0.5 0.5 1.5 2.5As the current increases,the total linewidth decreases,and the number of long
24、itudinal modes diminishes.At a sufficiently high current,the spectrum will contain just one mode.It is called single-longitudinal-mode laser.6.6 Narrow-spectral-width and Tunable laser diodes 6.6.1 Distributed-feedback laser Diode(DFB)The DFB laser diode is a single-longitudinal-mode laser diode.PnM
25、ETALIZED LAYERGRATINGACTIVE LAYERCLEAVED FACETOutput6.6.1 Distributed-feedback laser Diode(DFB)n0effnm202mOperating wavelength is determined from Braggs law DFB lasers have a number of unique properties arising from the grating structure.In addition to their narrow linewidths(typically 0.1-0.2 nm),w
26、hich make them attractive for long high-bandwidth transmission paths,they are less temperature dependent than are most conventional laser diodes.6.6.1 Distributed-feedback laser Diode(DFB)pngain phase BraggIG IP IB6.6.2 Tunable Laser DiodesThe gain current IG determines the amplification in the acti
27、ve region and the level of output laser power.pngain phase BraggIG IP IB6.6.2 Tunable Laser DiodesThe phase current IP controls the feedback from the Bragg reflection region.pngain phase BraggIG IP IB6.6.2 Tunable Laser DiodesThe current IB controls the Bragg wavelength by changing the temperature i
28、n the Bragg region.6.7 Optical Amplifiers Optical amplifiers will not solve the problem of reconstructing signal waveshapes,but they will allow extension of power-limited links.In other words,bandwidth-limited system will not be helped,but power-limited ones will.6.7 Optical AmplifiersSemiconductor
29、Optical Amplifier(SOA)Erbium-Doped Fiber Amplifier(EDFA)Erbium-Doped Waveguide Amplifier(EDWA)Fiber Raman Amplifier(FRA)6.7.1 Semiconductor Optical Amplifiers(SOA)MirrorInputOutputMirrorCurrent AR CoatInputOutputAR CoatFabry-Perot amplifierTraveling-wave amplifier6.7.1 Semiconductor Optical Amplifie
30、rs(SOA)SOA can be constructed by using stimulated emission,similar to laser.Achieving enough gain and doing so without adding too much noise has been a problem.The gain of SOA is polarization dependent.6.7.2 Erbium-Doped Fiber Optical Amplifier(EDFA)High gainWavelength of amplificationLarge bandwidt
31、hLow noiseEnergy states and transitionsErbium-doped glass fiber6.7.2 Erbium-Doped Fiber Optical Amplifier(EDFA)INPUT SIGNAL 1550nmWDMWDM980nm980nmPUMP-LASER DIODESINPUT SIGNAL 1550nmISOLATORISOLATORERBIUM-DOPED FIBER LOOPThe pumping light is absorbed by the erbium atoms,raising them to excited state
32、s and causing population inversion.6.7.2 Erbium-Doped Fiber Optical Amplifier(EDFA)INPUT SIGNAL 1550nmWDMWDM980nm980nmPUMP-LASER DIODESINPUT SIGNAL 1550nmISOLATORISOLATORERBIUM-DOPED FIBER LOOPThe excited erbium atoms are then stimulated to emit by the longer wavelength 1550nm photons,amplifying the
33、 signal.6.7.2 Erbium-Doped Fiber Optical Amplifier(EDFA)INPUT SIGNAL 1550nmWDMWDM980nm980nmPUMP-LASER DIODESINPUT SIGNAL 1550nmISOLATORISOLATORERBIUM-DOPED FIBER LOOPThe signal beam and the pumping beam from the left travel together down the fiber.The signal beam continually increases in strength wh
34、ile depleting the pump power.6.7.2 Erbium-Doped Fiber Optical Amplifier(EDFA)INPUT SIGNAL 1550nmWDMWDM980nm980nmPUMP-LASER DIODESINPUT SIGNAL 1550nmISOLATORISOLATORERBIUM-DOPED FIBER LOOPThe isolators are required to attenuate reflected waves(feedback),which would be amplified and could cause laser-
35、type oscillation.EDFA operating characteristicoperating characteristicoperating bandwidthgain saturationErbium-doped fiber lengthoperating characteristic(1)operating bandwidth15391569Operating bandwidth of more than 30nm are achievable,so a number of wavelength-division-multiplexing channels can be
36、amplified simutaneously.Dual-band amplifierL-band EBFAEDFA operating characteristicoperating characteristicoperating bandwidthgain saturationErbium-doped fiber lengthoperating characteristic(2)Erbium-doped fiber lengthThe Erbium-doped fiber lengths are typically a few tens of meters.The optimum leng
37、th depends on the amount of pump power available.(2)Erbium-doped fiber lengthThe pump power decreases as it travels down through the fiber,and eventually it becomes so weak that the gain reduced to zero,and the pumped fiber becomes absorbing rather than amplifying.EDFA operating characteristicoperat
38、ing characteristicoperating bandwidthgain saturationErbium-doped fiber lengthoperating characteristic(3)gain saturationgainPin (dBm)saturationSaturation is the decrease in gain that occurs when the amplified power reaches high levels.6.7.3 Erbium-Doped Waveguide Optical AmplifierWDMEDWLDINPUT SIGNAL
39、OUTPUT SIGNALThe waveguide is doped with erbium atoms.Integration is simpler,more economical,reduces size,reduces insertion losses.6.7.4 Raman Amplifier The EDFA provides significant amplification in the C-band.Amplifiers using stimulated Raman scattering have been developed for applications in othe
40、r bands.Development 6.7.4 Raman AmplifierSRS causes a new signal(a stokes wave)to be generated in the same direction as the pump wave down-shifted in frequency by 13.2THz provided that the pump signal is of sufficient strength.6.7.4 Raman AmplifierOptimal amplification occurs when the difference in
41、wavelength is around 13.2THz.The signal to be amplified must be lower in frequency(longer in wavelength)than the pump.6.7.4 Raman Amplifier6.7.4 Raman AmplifierWDMPUMP LASERINPUT SIGNALOUTPUT SIGNALOptical fiberISOLATORISOLATORRaman amplifier6.7.4 Raman Amplifier6.7.4 Raman Amplifierpulse amplitudeB
42、roadband Raman amplifierBroadband Raman amplifier6.7.5 Noise Figure/inoutSNFSN10logdBFFThe noise figure F is a measure of the noise characteristics of an amplifier.F gives an indication of the degradation in a signal owing to amplification.Amplification increases the signal power to a usable level,b
43、ut does degrade the information.It often expressed in decibels:6.7.5 Noise FigureSemiconductor Optical Amplifier(SOA):8dBErbium-Doped Fiber Amplifier(EDFA):6dBErbium-Doped Waveguide Amplifier(EDWA):5dBFiber Raman Amplifier(FRA):4.5dB6.7.5 Noise FigureOptical fiber powersignal powerASEnoiseOptical SN
44、R Number of amplifier6.7.6 Optical Amplifier ApplicationsTXAAARXFIBERFIBERLAUNCH AMPINLINE AMPPREAMPPOWER LEVEL6.8 Fiber Laser Laser diodes and light-emitting diodes dont couple the light they generate efficiently into fibers.This problem arises because of the different geometries of semiconductor s
45、ources and optical fibers.In addition,the radiation pattern of the source does not match the acceptance pattern of the fiber,and the emission pattern of a laser diode does not match the single-mode pattern of a single-mode fiber.Fiber amplifiers can solve this problem.A common one is Fabry-Perot res
46、onator,which consists of a pump,an amplifying section,and feedback in the form.Laser Diode Active FiberpLM1M2Mirror M1 transmits the pump wavelength p and reflects the laser Wavelength L,while mirror M2 istransmitting partially at wavelength L.6.8 Fiber LaserGRATINGGRATINGWDM980nmPUMP LASERERBIUM-DO
47、PED FIBER LOOPOutput Signal 1550nmErbium-doped fiber laser,the gratings act as partial mirrors at the laser-output wavelength.6.8 Fiber Laser6.9 Vertical-Cavity Surface-emitting Laser DiodesThis structure has several unique characteristics:l One is that the beam pattern is circular,the same shape as
48、 the fiber.This match improves the coupling efficiency.l VCSELs have short cavity lengths,which tend to decrease response times.This result is that VCSELs can be modulated at very high speeds.6.9 Vertical-Cavity Surface-emitting Laser DiodesMonolithic two-dimensional laser-diode arraysLight sourceLi
49、ght-emitting diodeLaser diodeLEDs are normally chosen for multimode SI links.GRIN fiber and an LED can combine to produce a system transmitting moderately high data rates over fairly long distances.Because of higher initial costs and increased circuit complexity,laser diodes are used only when necessary.The largest rate-length products are achieved when a single-mode laser diode is matched with a single-mode fiber and operated in the low-loss,longer-wavelength region such as the C or L bands(1530 to 1625nm)谢谢
