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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 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:1.Pumping threshold The power input to a laser must be above a certain threshold level before the device
10、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 the laser.uthe semiconducto
11、r 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 is supplied from outside and atom e
12、nters 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 ex
13、citing 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 wou
14、ld 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,INSULATIONMETALLIZATIONSTRIPE CONTACT0.1-0.3m-1m-1m-1mThe st
15、ructure 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
16、 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 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 Laser-diodes operati
