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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 Direct Modulation of Laser DiodeDirect Modulation of Laser DiodeBias + DATABias + DATAIssues - Complex Dynamics YieldIssues - Complex Dynamics Yield External Modulation of Laser DiodeExternal Modulation of Laser DiodeBiasBiasBia
3、s + DATABias + DATAIssues - Additional ComponentIssues - Additional ComponentA 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
4、free to 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
5、 caused by the 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.Di
6、gital modulationcurrenttimeOutput 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
7、we know, the optic 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
8、 have widths of 50-100nm. 6.2 Light-emitting Diode operating characteristicCoupling efficiency depends heavily on the radiation pattern of a emitter. -900 90BEAM ANGLEBEAM INTENSITYsurface-emitting LEDRays incident on a fiber, but outside its acceptance angle, will not be coupled. The acceptance ang
9、le for a fiber having NA=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 LEDEdge emitters concentrate their radiation somewhat more than surface devices, providing improved coupling efficiency. -90 -45 0
10、45 90120 30BEAM ANGLEBEAM 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:1. Pumping threshold The power input to a laser must be above a certain threshol
11、d level before the device will emit. 2.Output spectrum The laser output power is not at a single frequency but is spread over a range of frequencies. 3. 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 withi
12、n the laser. uthe semiconductor laser 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.Light amplif
13、ication by stimulated emission of radiation laserstimulated emissionenergyEnergy is supplied from outside and atom enters excited state.E E1 1ground stateE E2 2excited statestimulated emissionE E2 2E E1 1h h Arriving photonPhoton arrives and interacts with excited atom.stimulated emissionE E2 2E E1
14、1h h Arriving photonAtom emits additional photon and returns to the ground state.h h h h stimulated emissionE E2 2E E1 1h h Arriving photonWhen a new photon is emitted it has identical wavelength, phase and direction characteristics as the exciting photon.h h h h stimulated emissionPopulation invers
15、ionThe 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 absorb them).A medium wi
16、th population inversion has gain and behaves as an amplifier.energylasinglasingMM1 1MM2 2mirrormirrorPartial Partial mirrormirrorlasermirrormirrorPartial mirrorPartial mirroroscillationLaser output6.4 Laser DiodesMETALLIZATIONn-AlGaAs, Wg=1.8eV CONFINEMENTn-AlGaAs, Wg=1.55eV ACTIVE LAYERn-AlGaAs, Wg
17、=1.55eV CONFINEMENTGaAs SUBSTRATEP-GaAs,CONTACTSiO2,INSULATIONMETALLIZATIONSTRIPE CONTACT0.1-0.3m-1m-1m-1mThe structure of an AlGaAs laser diodepowerConfinement LayerConfinement LayerActive LayerRefractive Index6.4 Laser DiodesMany laser diodes are edge emitters.Under forward bias, charges are injec
18、ted into the active layer, causing the spontaneous emission of photons. Some of the injected charges are stimulated to 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.
19、The threshold current is reached when the gain is large enough to offset the diode losses. At this point, laser oscillation 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
20、 Laser-diodes operating characteristicoperating characteristicoutput powerlinewidthtemperature sensitiveoperating characteristic (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 diodeTIME
21、iTIMEIdcCURRENTOPTICAL POWERisidcAnalog modulation of a laser diodeTIMEIdcCURRENTOPTICAL POWERisidcITHTIMEis6.5 Laser-diodes operating characteristicoperating characteristicoutput powerlinewidthtemperature sensitiveoperating characteristic(2) temperature sensitive CURRENT (mA)Output Power(mW)20 4060
22、0Laser diodes are much more temperature sensitive than are LEDsAs the temperature increases, the diodes gain decreases, and so more current is required before oscillation can begin-the threshold current becomes greater.(2) temperature sensitive CURRENT (mA)Output Power(mW)20 40600At a constant curre
23、nt, the output power of a laser diode will diminish if the temperature risesThere are two techniques for overcoming this problem: thermoelectrically cooling the diode, and changing the bias current to compensate for changed threshold.(2) temperature sensitiveThe laser emission wavelength also depend
24、s upon the temperature. This effect arises from the dependence of the materials refractive index on temperature.6.5 Laser-diodes operating characteristicoperating characteristicoutput powerlinewidthtemperature sensitiveoperating characteristic(3) linewidth-2.5 -1.5 -0.5 0.5 1.5 2.5WAVELENGTH (nm)2.0
25、1.5 1.00.50.0INTENSITYLaser diodes typically possess linewidths of 1-5nm, considerably smaller than tho output spectra of LEDs.When the drive 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 1.00.50.0INTENSI
26、TYWAVELENGTH (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 longitudinal modes diminishes. At a sufficiently high current, the spectrum will contain just one mode. It is called single-longitudinal-mode laser.6.6 Narro
27、w-spectral-width and Tunable laser diodes6.6.1 Distributed-feedback laser Diode (DFB)The DFB laser diode is a single-longitudinal-mode laser diode. PnMETALIZED LAYERGRATINGACTIVE LAYERCLEAVED FACETOutput6.6.1 Distributed-feedback laser Diode (DFB)Operating wavelength is determined from Braggs lawDFB
28、 lasers have a number of unique properties arising from the grating structure. In addition to their narrow linewidths (typically 0.1-0.2 nm ), which make them attractive for long high-bandwidth transmission paths, they are less temperature dependent than are most conventional laser diodes.6.6.1 Dist
29、ributed-feedback laser Diode (DFB)pngain phase BraggIG IP IB6.6.2 Tunable Laser DiodesThe gain current IG determines the amplification in the active 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 Brag
30、g reflection region.pngain phase BraggIG IP IB6.6.2 Tunable Laser DiodesThe current IB controls the Bragg wavelength by changing the temperature in the Bragg region.6.7 Optical AmplifiersOptical amplifiers will not solve the problem of reconstructing signal waveshapes, but they will allow extension
31、of power-limited links. In other words, bandwidth-limited system will not be helped, but power-limited ones will.6.7 Optical AmplifiersSemiconductor Optical Amplifier (SOA)Erbium-Doped Fiber Amplifier (EDFA)Erbium-Doped Waveguide Amplifier (EDWA)Fiber Raman Amplifier (FRA)6.7.1 Semiconductor Optical
32、 Amplifiers (SOA)MirrorInputOutputMirrorCurrent AR CoatInputOutputAR CoatFabry-Perot amplifierTraveling-wave amplifier6.7.1 Semiconductor Optical Amplifiers (SOA)RR1 1RR2 2I ISOASOASOA can be constructed by using stimulated emission, similar to laser.Achieving enough gain and doing so without adding
33、 too much noise has been a problem.The gain of SOA is polarization dependent.SOA ProductInput signalInput signal1530nm-1570nm1530nm-1570nmAmplified output Amplified output signalsignalPower laser Power laser (Pump)(Pump)980nm or 980nm or 1480nm1480nmFiber containing Fiber containing erbium erbium do
34、pantdopant6.7.2 Erbium-Doped Fiber Optical Amplifier (EDFA)High gainWavelength of amplificationLarge bandwidthLow noiseEnergy states and transitionsenergyenergy1550nm1550nmemissionemissionWW2 2980nm980nm1480nm1480nmWW3 31550nm1550nmWW1 1Erbium-doped glass fiber6.7.2 Erbium-Doped Fiber Optical Amplif
35、ier (EDFA)INPUT SIGNAL 1550nmWDMWDM980nm980nmPUMP-LASER DIODESINPUT SIGNAL 1550nmISOLATORISOLATORERBIUM-DOPED FIBER LOOPThe pumping light is absorbed by the erbium atoms, raising them to excited states and causing population inversion. 6.7.2 Erbium-Doped Fiber Optical Amplifier (EDFA)INPUT SIGNAL 15
36、50nmWDMWDM980nm980nmPUMP-LASER DIODESINPUT SIGNAL 1550nmISOLATORISOLATORERBIUM-DOPED FIBER LOOPThe excited erbium atoms are then stimulated to emit by the longer wavelength 1550nm photons, amplifying the signal. 6.7.2 Erbium-Doped Fiber Optical Amplifier (EDFA)INPUT SIGNAL 1550nmWDMWDM980nm980nmPUMP
37、-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 while depleting the pump power. 6.7.2 Erbium-Doped Fiber Optical Amplifier (EDFA)INPUT SIGNAL 15
38、50nmWDMWDM980nm980nmPUMP-LASER DIODESINPUT SIGNAL 1550nmISOLATORISOLATORERBIUM-DOPED FIBER LOOPThe isolators are required to attenuate reflected waves (feedback), which would be amplified and could cause laser-type oscillation. EDFA operating characteristicoperating characteristicoperating bandwidth
39、gain 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 amplified simutaneously.Dual-band amplifierL-band EBFAEDFA operating characteristicoper
40、ating 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 length depends on the amount of pump power available.(2) Erbium-doped fiber lengthThe pum
41、p 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 characteristicoperating characteristicoperating bandwidthgain saturationErbium-doped fiber lengthoper
42、ating characteristic(3) gain saturation3dB3dBP Pout,satout,satgainPin (dBm)saturationSaturation is the decrease in gain that occurs when the amplified power reaches high levels.6.7.3 Erbium-Doped Waveguide Optical AmplifierWDMEDWLDINPUT SIGNALOUTPUT SIGNALThe waveguide is doped with erbium atoms.Int
43、egration is simpler, more economical, reduces size, reduces insertion losses.6.7.4 Raman AmplifierThe EDFA provides significant amplification in the C-band.Amplifiers using stimulated Raman scattering have been developed for applications in other bands. Development 6.7.4 Raman Amplifierfiberfiber(a)
44、 No pump(a) No pumppower(dBpower(dB) )1550nm1550nmpower(dBpower(dB) )1550nm1550nm1450nm1450nmfiberfiber(b) With pump(b) With pump1550nm1550nm1450nm1450nm1550nm1550nmSRS 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 pro
45、vided that the pump signal is of sufficient strength.6.7.4 Raman Amplifierpower(dBpower(dB) )1550nm1550nm1450nm1450nmfiberfiber1450nm1450nm1550nm1550nmOptimal amplification occurs when the difference in wavelength is around 13.2THz. The signal to be amplified must be lower in frequency (longer in wa
46、velength) than the pump.6.7.4 Raman Amplifier6.7.4 Raman AmplifierWDMPUMP LASERINPUT SIGNALOUTPUT SIGNALOptical fiberISOLATORISOLATORRaman amplifier6.7.4 Raman Amplifierz zEDFAEDFANonlinear EffectsNonlinear EffectsNoise HighNoise HighRARAHigh pump powerHigh pump power6.7.4 Raman Amplifierpulse ampli
47、tudeBroadband Raman amplifierBroadband Raman amplifierUltraflat amplifier6.7.5 Noise FigureThe 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 le
48、vel, but 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 powerASE
49、noiseOptical SNR Number of amplifier6.7.6 Optical Amplifier ApplicationsTXAAARXFIBERFIBERLAUNCH AMPINLINE AMPPREAMPPOWER LEVEL6.8 Fiber LaserLaser diodes and light-emitting diodes dont couple the light they generate efficiently into fibers. This problem arises because of the different geometries of
50、semiconductor sources 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
51、 is Fabry-Perot resonator, 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 LaserGRATINGGRATINGWDM
52、980nmPUMP LASERERBIUM-DOPED 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 c
53、ircular, the same shape as 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 las
54、er-diode arraysLight sourceLight-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)
