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1、Chapter 6 Optical Sources and Amplifiers,Chapter 6 Optical Sources and Amplifiers,6.1 Light-emitting Diodes 6.2 Light-emitting Diode operating Characteristic 6.3 Laser Principles 6.4 Laser Diodes 6.5 Laser-diode operating Characteristic 6.7 Optical Amplifiers 6.8 Fiber Lasers 6.9 Vertical-Cavity Sur
2、face-emitting Laser Diodes,Light source,Modulation,A Light-emitting Diodes is a pn-junction semiconductor that emits light when forward biased.,Circuit,6.1 Light-emitting Diodes,In the upper-energy band, called the conduction band, electrons not bound to individual atoms are free to move.,In the low
3、er band, the valence band, unbound holes are free to move. Holes have a positive charge.,6.1 Light-emitting Diodes,Two allowed bands of energies are separated by a forbidden region (a bandgap) whose width has energy Wg.,6.1 Light-emitting Diodes,In a word, radiation from an LED is caused by the reco
4、mbination of holes and electrons that are injected into the junction by a forward bias voltage.,pn-junction,flash36,6.2 Light-emitting Diode operating characteristic,mA,The optic power generated by an LED is linearly proportional to the forward driving current.,Digital modulation,current,time,Output
5、 power,input current,time,Optical power,The diode is modulated by a current source, which simply turns the LED ON or OFF.,Analog modulation,Analog modulation requires a dc bias to keep the total current in the forward direction at all times.,Optical power,time,time,current,As we know, the optic spec
6、trum 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 50-100n
7、m.,6.2 Light-emitting Diode operating characteristic,Coupling efficiency depends heavily on the radiation pattern of a emitter.,-90 0 90,BEAM ANGLE,BEAM INTENSITY,surface-emitting LED,Rays incident on a fiber, but outside its acceptance angle, will not be coupled. The acceptance angle for a fiber ha
8、ving NA=0.24 is only 14,so a large amount of the power generated by a surface emitter will be rejected.,-90 0 90,BEAM ANGLE,BEAM INTENSITY,surface-emitting LED,Edge emitters concentrate their radiation somewhat more than surface devices, providing improved coupling efficiency.,-90 -45 0 45 90,120 ,3
9、0,BEAM ANGLE,BEAM INTENSITY,PARALLEL PLANE,PERPENDICULAR PLANE,edge-emitting LED,Flash 38,6.3 Laser Principles,Here 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 le
10、vel before the device will emit. Output spectrum The laser output power is not at a single frequency but is spread over a range of frequencies. 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 las
11、er.,the semiconductor laser diode the gas laser the bulk Nd: YAG the fiber laser,common kinds of laser,A 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 amplification by
12、 stimulated emission of radiation,laser,stimulated emission,Energy is supplied from outside and atom enters excited state.,stimulated emission,Photon arrives and interacts with excited atom.,stimulated emission,Atom emits additional photon and returns to the ground state.,stimulated emission,When a
13、new photon is emitted it has identical wavelength, phase and direction characteristics as the exciting photon.,stimulated emission,Population inversion The number of atoms in the upper level exceeds those in the lower level.,Population inversion,laser,mirror,Partial mirror,oscillation,Laser output,6
14、.4 Laser Diodes,The structure of an AlGaAs laser diode,6.4 Laser Diodes,Many 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 emit by other photons. If the curre
15、nt 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 Diodes,Outpu
16、t power of a laser diode,Diodes radiating a spectrum containing numerous longitudinal modes.,6.5 Laser-diodes operating characteristic,operating characteristic,output power,linewidth,temperature sensitive,operating characteristic,(1) Output optic power,Output optic power is plotted against forward i
17、nput current.,Digital modulation of a laser diode,Analog modulation of a laser diode,6.5 Laser-diodes operating characteristic,operating characteristic,output power,linewidth,temperature sensitive,operating characteristic,(2) temperature sensitive,(2) temperature sensitive,(2) temperature sensitive,
18、6.5 Laser-diodes operating characteristic,operating characteristic,output power,linewidth,temperature sensitive,operating characteristic,(3) linewidth,(3) linewidth,6.6 Narrow-spectral-width and Tunable laser diodes,6.6.1 Distributed-feedback laser Diode (DFB) The DFB laser diode is a single-longitu
19、dinal-mode laser diode.,6.6.1 Distributed-feedback laser Diode (DFB),Operating 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 ), which make them attractive for lon
20、g high-bandwidth transmission paths, they are less temperature dependent than are most conventional laser diodes.,6.6.1 Distributed-feedback laser Diode (DFB),6.6.2 Tunable Laser Diodes,The gain current IG determines the amplification in the active region and the level of output laser power.,6.6.2 T
21、unable Laser Diodes,The phase current IP controls the feedback from the Bragg reflection region.,6.6.2 Tunable Laser Diodes,The 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 reconstruc
22、ting 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 Amplifiers,Semiconductor Optical Amplifier (SOA),Erbium-Doped Fiber Amplifier (EDFA),Erbium-Doped Waveguide Amplifier (E
23、DWA),Fiber Raman Amplifier (FRA),6.7.1 Semiconductor Optical Amplifiers (SOA),6.7.1 Semiconductor Optical Amplifiers (SOA),SOA Product,6.7.2 Erbium-Doped Fiber Optical Amplifier (EDFA),High gain,Wavelength of amplification,Large bandwidth,Low noise,Energy states and transitions,6.7.2 Erbium-Doped Fi
24、ber Optical Amplifier (EDFA),The 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),6.7.2 Erbium-Doped Fiber Optical Amplifier (EDFA),6.7.2 Erbium-Doped Fiber Optical Amplifier (EDFA),EDFA
25、operating characteristic,operating characteristic,operating bandwidth,gain saturation,Erbium-doped fiber length,operating characteristic,(1) operating bandwidth,Operating bandwidth of more than 30nm are achievable, so a number of wavelength-division-multiplexing channels can be amplified simutaneous
26、ly.,Dual-band amplifier,EDFA operating characteristic,operating characteristic,operating bandwidth,gain saturation,Erbium-doped fiber length,operating characteristic,(2) Erbium-doped fiber length,(2) Erbium-doped fiber length,EDFA operating characteristic,operating characteristic,operating bandwidth
27、,gain saturation,Erbium-doped fiber length,operating characteristic,(3) gain saturation,gain,Pin (dBm),Saturation is the decrease in gain that occurs when the amplified power reaches high levels.,6.7.3 Erbium-Doped Waveguide Optical Amplifier,The waveguide is doped with erbium atoms.,Integration is
28、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 other bands. Development,6.7.4 Raman Amplifier,fiber,(a) No pump,SRS
29、 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 Amplifier,Optimal amplification occurs when the difference in wavelength is around 13.2THz. The signal
30、 to be amplified must be lower in frequency (longer in wavelength) than the pump.,6.7.4 Raman Amplifier,6.7.4 Raman Amplifier,6.7.4 Raman Amplifier,z,6.7.4 Raman Amplifier,pulse amplitude,Broadband Raman amplifier,Broadband Raman amplifier,Ultraflat amplifier,6.7.5 Noise Figure,The noise figure F is
31、 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, but does degrade the information. It often expressed in decibels:,6.7.5 Noise Figure,Semiconductor Optical
32、 Amplifier (SOA): 8dB,Erbium-Doped Fiber Amplifier (EDFA): 6dB,Erbium-Doped Waveguide Amplifier (EDWA): 5dB,Fiber Raman Amplifier (FRA): 4.5dB,6.7.5 Noise Figure,Optical fiber,power,signal power,ASE noise,Optical SNR,Number of amplifier,6.7.6 Optical Amplifier Applications,6.8 Fiber Laser,Laser diod
33、es and light-emitting 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
34、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 resonator, which consists of a pump, an amplifying section, and feedback in the form.,Mirror M1 transmits the pump wavelength p and re
35、flects the laser Wavelength L, while mirror M2 is transmitting partially at wavelength L.,6.8 Fiber Laser,Erbium-doped fiber laser, the gratings act as partial mirrors at the laser-output wavelength.,6.8 Fiber Laser,6.9 Vertical-Cavity Surface-emitting Laser Diodes,This structure has several unique
36、characteristics: One is that the beam pattern is circular, the same shape as the fiber. This match improves the coupling efficiency. 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-e
37、mitting Laser Diodes,Monolithic two-dimensional laser-diode arrays,Light source,LEDs 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),