光纖通信 Optical Sources and Ampifiers

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

3、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 caus

4、ed 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.digital

5、 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 we kn

6、ow, 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 have

7、 widths of 50-100nm. 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

8、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 led edge emitters concentrate their radiation somewhat more than surface devices, providing improved coupling efficiency. -90 -45 0 45

9、 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 threshold

10、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 within

11、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.energyenergy i

12、s supplied from outside and atom enters 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

13、direction characteristics as the exciting 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

14、 will meet lower level atoms (which would absorb 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,insulationmetalliz

15、ationstripe 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 injected into the active layer, causing the spontaneous emission of photons

16、. 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. the threshold current is reached when the gain is large enough to offs

17、et 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 laser-diodes operating characteristicoperating characteristicoutput p

18、owerlinewidthtemperature 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 diodetimeitimeidccurrentoptical powerisidcanalog modulation of a laser diodetim

19、eidccurrentoptical powerisidcithtimeis6.5 laser-diodes operating 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

20、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)2040 600at a constant current, the output power of a laser diode will diminish if the temperature

21、 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 depends upon the temperature. this effect arises from the dependence of the

22、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)2.01.5 1.00.50.0intensitylaser diodes typically possess linewidths of 1-5

23、nm, 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 1.00.50.0intensitywavelength (nm)2.01.5 1.00.50.0intensity-2.5 -1.5 -0.5 0.5 1.5 2.5as

24、 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 narrow-spectral-width and tunable laser diodes 6.6.1 distributed-feedback l

25、aser diode (dfb)the dfb laser diode is a single-longitudinal-mode laser diode. pnmetalized 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 t

26、he 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 distributed-feedback laser diode (dfb)pngain phase braggig ip

27、 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 bragg reflection region.pngain phase braggig ip ib6.6.2 tunab

28、le laser diodesthe current ib controls the bragg wavelength by changing the temperature in 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-limite

29、d 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 amplifiers (soa)mirrorinputoutputmirrorcurrent ar coati

30、nputoutputar coatfabry-perot amplifiertraveling-wave amplifier6.7.1 semiconductor optical amplifiers (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 dependen

31、t.6.7.2 erbium-doped fiber optical amplifier (edfa)high gainwavelength of amplificationlarge bandwidthlow noiseenergy states and transitionserbium-doped glass fiber6.7.2 erbium-doped fiber optical amplifier (edfa)input signal 1550nmwdmwdm980nm980nmpump-laser diodesinput signal 1550nmisolatorisolator

32、erbium-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 1550nmwdmwdm980nm980nmpump-laser diodesinput signal 1550nmisolatorisolatorerbium-doped fiber loopth

33、e 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-laser diodesinput signal 1550nmisolatorisolatorerbium-doped fiber loopthe signal beam and the pu

34、mping 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 1550nmwdmwdm980nm980nmpump-laser diodesinput signal 1550nmisolatorisolatorerbium-doped fiber loopth

35、e isolators are required to attenuate reflected waves (feedback), which would be amplified and could cause laser-type oscillation. edfa operating characteristicoperating characteristicoperating bandwidthgain saturationerbium-doped fiber lengthoperating characteristic(1) operating bandwidth15391569op

36、erating 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 characteristicoperating characteristicoperating bandwidthgain saturationerbium-doped fiber lengthoperating characte

37、ristic(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 pump power decreases as it travels down through the fiber, and eventually it becomes so weak that th

38、e gain reduced to zero, and the pumped fiber becomes absorbing rather than amplifying. edfa operating characteristicoperating characteristicoperating bandwidthgain saturationerbium-doped fiber lengthoperating characteristic(3) gain saturationgainpin (dbm)saturationsaturation is the decrease in gain

39、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.integration is simpler, more economical, reduces size, reduces insertion losses.6.7.4 raman amplifier the edfa provides sig

40、nificant amplification in the c-band. amplifiers using stimulated raman scattering have been developed for applications in other 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

41、 provided that the pump signal is of sufficient strength.6.7.4 raman amplifieroptimal amplification occurs when the difference in 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

42、 laserinput signaloutput signaloptical fiberisolatorisolatorraman amplifier6.7.4 raman amplifier6.7.4 raman amplifierpulse amplitudebroadband raman amplifierbroadband raman amplifier6.7.5 noise figure/inoutsnfsn10logdbffthe noise figure f is a measure of the noise characteristics of an amplifier.f g

43、ives an indication of the degradation in a signal owing to amplification. amplification increases the signal power to a usable level, but does degrade the information. it often expressed in decibels:6.7.5 noise figuresemiconductor optical amplifier (soa): 8dberbium-doped fiber amplifier (edfa): 6dbe

44、rbium-doped waveguide amplifier (edwa): 5dbfiber raman amplifier (fra): 4.5db6.7.5 noise figureoptical fiber powersignal powerasenoiseoptical snr number of amplifier6.7.6 optical amplifier applicationstxaaarxfiberfiberlaunch ampinline amppreamppower level6.8 fiber laser laser diodes and light-emitti

45、ng diodes dont couple the light they generate efficiently into fibers. this problem arises because of the different geometries of 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-mod