2021, 26, OE of HUST, Optical Transmitters, Introduction Basic concepts

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  • 1.2021/9/26 OE of HUST 1 Chapter 3. Optical Transmitters Introduction Basic concepts Light-Emitting Diodes (LED) Semiconductor lasers (Laser Diode) Laser Characteristics Transmitter Design
  • 2.2021/9/26 OE of HUST 2 3.1 Introduction 3.1.1 Components of Optical Transmitters Binary to single Coding/line coding Modulator Optical Source Driving Circuit PCM Channel coupler Optical signal output
  • 3.2021/9/26 OE of HUST 3 Biased current Modulation current (≥10Gb/s) Modulation current Biased current (≤2.5Gb/s) (a) Direct Modulation (b) External Modulation
  • 4.2021/9/26 OE of HUST 4 1. stability : power & wavelength 2. reliability > 25 years (Pout to Pout /2) 3. small emissive area compatible with fiber core dimensions 4. right wavelength range 0.85 µm : GaAlAs/GaAs 1.31 µm, 1.55 µm : InP/InGaAsP 5. narrow linewidth → Dispersion 6. direct modulation 7. high efficiency: MQW-LD Ith ~ 10 mA 3.1.2 Requirements for Optical Source
  • 5.2021/9/26 OE of HUST 5 MQW DFB LD
  • 6.2021/9/26 OE of HUST 6 Chapter 3. Optical Transmitters Introduction Basic concepts Light-Emitting Diodes (LED) Semiconductor lasers (Laser Diodes) Laser Characteristics Transmitter Design
  • 7.2021/9/26 OE of HUST 7 3.2.1 Three fundamental transition processes 1. Spontaneous Emission → LED 2. Stimulated Emission → LD, SOA 3. Stimulated Absorption → PIN / APD 3.2 Basic Concepts Light Emission
  • 8.2021/9/26 OE of HUST 8 3.2.2 Emission and Absorption Rates E2 N2 N1 E1 spectral density of the electromagnetic energy In thermal equilibrium : kB: Boltzmann Constant T: Absolute Temperature According to Boltzmann Statistics :
  • 9.2021/9/26 OE of HUST 9 In the visible or near-infrared region at room temperature thermal equilibrium laser operation N2>N1, Rstim>Rabs (population inversion) Conclusion:
  • 10.2021/9/26 OE of HUST 10 Energy bands in semiconductor
  • 11.2021/9/26 OE of HUST 11 The occupation probability for electrons in the conduction and valence bands is given by the Fermi-Dirac distributions: Efc, Efv are the Fermi levels in conduction band and valence band respectively
  • 12.2021/9/26 OE of HUST 12 joint density of states : the number of states per unit volume per unit energy range Eg: bandgap mr: reduced mass mc, mv: effective masses of electrons & holes in conduction and valence bands, respectively
  • 13.2021/9/26 OE of HUST 13 Population-inversion condition: in thermal equilibrium: pumping energy into the semiconductor from an external energy source a forward-biased p-n junction
  • 14.2021/9/26 OE of HUST 14 1. Type of semiconductor Intrinsic semiconductor: undoped, Fermi level, lying in the middle of the bandgap n-type semiconductor: Fermi level moves toward the conduction band as the dopant concentration increases p-type semiconductor: Fermi level moves toward the valence band as the dopant concentration increases 3.2.3 p-n junctions
  • 15.2021/9/26 OE of HUST 15 n-type Intrinsic p-type forward biased
  • 16.2021/9/26 OE of HUST 16 Forward biased p-n junctions (a) in thermal equilibrium (b) under forward biased 2. under forward biased: built-in electric field is reduced diffusion of electrons and holes across the junction generate light through spontaneous emission or stimulated emission
  • 17.2021/9/26 OE of HUST 17 P N
  • 18.2021/9/26 OE of HUST 18 3. (1) Homojunction: the same semiconductor material wide region for electron-hole recombination difficult to realize high carrier densities (2) Heterojunction: different bandgaps (3) Double-heterojunction: sandwiching a thin layer between the p-type and n-type layers such that the bandgap of the sandwiches layer is smaller than the layer surrounding it. Homojunction & heterojunction
  • 19.2021/9/26 OE of HUST 19 Double-heterojunction
  • 20.2021/9/26 OE of HUST 20
  • 21.2021/9/26 OE of HUST 21 Active layer: light is generated inside it as a result of electron-hole recombination higher density of carriers →higher index → waveguide (1D) Heterojunction: confinement of charge carriers & the optical field 0.85µm: cladding/active: GaAlAs/GaAs 1.31µm, 1.55µm: cladding/active: InP/InGaAsP
  • 22.2021/9/26 OE of HUST 22 1. electron-hole recombination 3.2.4 Nonradiative Recombination Trap of defects Surface recombination Auger Nonradiative recombination
  • 23.2021/9/26 OE of HUST 23 2. internal quantum efficiency Rrr : radiative recombination rate Rnr : nonradiative recombination rate Rtot : totale recombination rate τ : recombination time
  • 24.2021/9/26 OE of HUST 24 Nonradiative recombination is harmful to devices! positive feed back E 0 E 0 k1 k2 (1) direct-bandgap (GaAs, InP) (2) indirect-bandgap (Si, Ge)
  • 25.2021/9/26 OE of HUST 25 3. carrier lifetime A : defects & traps B : spontaneous radiative C : Auger
  • 26.2021/9/26 OE of HUST 26 Quality of the heterojunction interface depends on the lattice constant of the two materials. (matching !) 3.2.5 Semiconductor Materials ternary compound quaternary compound 0.85µm: GaAlAs/GaAs (cladding/active) 1.31µm, 1.55µm: InP/InGaAsP (cladding/active)
  • 27.2021/9/26 OE of HUST 27
  • 28.2021/9/26 OE of HUST 28 Chapter 3. Optical Transmitters Introduction Basic concepts Light-Emitting Diodes (LED) Semiconductor lasers (Laser Diodes) Laser Characteristics Transmitter Design
  • 29.2021/9/26 OE of HUST 29 3.3.1 Power-Current Characteristics 3.3 Light-Emitting Diodes (LEDs) a forward-biased p-n junction → spontaneous emission → LED
  • 30.2021/9/26 OE of HUST 30
  • 31.2021/9/26 OE of HUST 31 Power-conversion efficiency (wall-plug efficiency)
  • 32.2021/9/26 OE of HUST 32 P-I Curve Responsivity: (1) responsivity remains constant when I is small (2) bending of P-I curve: (3) no threshold
  • 33.2021/9/26 OE of HUST 33 3.3.2 LED Spectrum an approximate expression: LEDs are suitable for LAN with low bit rate & short distance !
  • 34.2021/9/26 OE of HUST 34 Figure 3.7: (b) spectrum of the emitted light for a typical 1.3-μm LED. 超宽带光源 白光LED
  • 35.2021/9/26 OE of HUST 35
  • 36.2021/9/26 OE of HUST 36 3.3.3 Modulation Response Rate equation: : injection carrier : carrier of recombination (nonradiative, spontaneous emission) Sinusoidal modulation: Ib : bias current Im : modulation current ωm : modulation frequency
  • 37.2021/9/26 OE of HUST 37 Since modulated power is related to linearly
  • 38.2021/9/26 OE of HUST 38 3.3.4 LED Structures (a) (b) (a) surface-emitting LED (b) edge-emitting LED
  • 39.2021/9/26 OE of HUST 39 思考题 1. 以下论述正确的是：（ ） A、非辐射复合会影响发光器件的发光效率； B、正向偏置的PN结中导带和价带的准费米能级趋于一致； C、半导体材料要发光，必须实现粒子数的反转； D、LED中最初的光子来源于内部的自发辐射； E、电子与空穴复合不一定产生光子； F、双异质结结构提高了半导体光源的量子效率； G、工作于1.55m处的半导体光源有源层材料为InP ； H、温度升高发光器件的发光效率会下降； I、间接带隙半导体材料中非辐射复合效率高于辐射复合效率， 不适合用作光源材料。
  • 40.2021/9/26 OE of HUST 40 Chapter 3. Optical Transmitters Introduction Basic concepts Light-Emitting Diodes (LED) Semiconductor lasers (Laser Diodes) Laser Characteristics Transmitter Design
  • 41.2021/9/26 OE of HUST 41 3.4 Semiconductor lasers (Laser Diodes) Advantages of stimulated emission compared with spontaneous emission of semiconductor materials emitting high power (to 100mW) narrow angular spread narrow spectral width direct modulation at high frequency (to 10GHz, because is small)
  • 42.2021/9/26 OE of HUST 42 Components of Semiconductor Lasers
  • 43.2021/9/26 OE of HUST 43 z=0 z=L Injection current Gain medium Resonant cavity Resonant cavity
  • 44.2021/9/26 OE of HUST 44 3.4.1 Optical Gain Peak gain of medium: when : differential gain (gain cross section) : injection carrier density : transparent carrier density : threshold carrier density NT is equal to Nth ?
  • 45.2021/9/26 OE of HUST 45 Figure 3.9: (a) Gain spectrum of a 1.3-μm InGaAsP laser at several carrier densities N. (b) Variation of peak gain gp with N. The dashed line shows the quality of a linear fit in the high gain region. Blue or red shifting of peak wavelength vs. injected current ?
  • 46.2021/9/26 OE of HUST 46 3.4.2 Feedback and Laser Threshold Feedback R1 R2 n0=1 n
  • 47.2021/9/26 OE of HUST 47 Threshold
  • 48.2021/9/26 OE of HUST 48 Amplitude condition Phase condition spacing of oscillating frequency oscillating frequency threshold gain
  • 49.2021/9/26 OE of HUST 49 MLM Loss SLM
  • 50.2021/9/26 OE of HUST 50 3.4.3 LD Structures Broad-area LD Figure 3.12: A broad-area semiconductor laser. The active layer (hatched region) is sandwiched between p-type and n-type cladding layers of a higher-bandgap material. light-confinement mechanism in the direction perpendicular to the junction plane introduced by double heterostructure
  • 51.2021/9/26 OE of HUST 51 no such light-confinement mechanism in the lateral direction parallel to the junction plane. the light generated spreads over the entire width of the laser. relatively high threshold current and a spatial pattern that is highly elliptical and that changes in an uncontrollable manner with the current.
  • 52.2021/9/26 OE of HUST 52 Gain-guided semiconductor lasers Figure 3.13: Cross section of two stripe-geometry laser structures used to design gain-guided semiconductor lasers and referred to as (a) oxide stripe and (b) junction stripe. Stripe lasers
  • 53.2021/9/26 OE of HUST 53 solve the light-confinement problem by limiting current injection over a narrow stripe. the spot size is not stable as the laser power is increased
  • 54.2021/9/26 OE of HUST 54 Index-guided semiconductor lasers Figure 3.14: Cross section of two index-guided semiconductor lasers: (a) ridge-waveguide structurefor weak index guiding; (b) etched-mesa buried heterostructure for strong index guiding.
  • 55.2021/9/26 OE of HUST 55 Multi-quantum-well LD 有源区厚度薄1~10nm 周期结构，将窄带隙的很薄的有源区夹在宽带隙 的半导体材料之间，形成势能阱 多个势能阱--多量子阱（MQW）
  • 56.2021/9/26 OE of HUST 56 homojunction Double heterostructure Stripe geometry Multi-quantum-well Relatively stronger confinement of injected carriers and output photons, thus lower threshold current, and higher slope efficiency!
  • 57.2021/9/26 OE of HUST 57 课堂作业 1. 现有F-P谐振腔的LD，长度为400m，有源区材料折射率为3.5，谐振腔两端面一端镀有增反射膜，反射率为90％，另一端为自然解理面。若LD工作在850nm波长附近，要求其阈值增益系数小于75cm-1，问： (1)如何选择半导体材料和组分？ (2)材料的内部吸收损耗为多少（单位cm-1）？ (3)该LD输出纵模频率间隔为多大（单位GHz） ？
  • 58.2021/9/26 OE of HUST 58 3.4.4 Control of Longitudinal Modes MLM Side Mode Suppression Ratio (SMSR): Loss SLM Pmm Psm or
  • 59.2021/9/26 OE of HUST 59 Distributed Feedback Lasers 相位光栅在波导中产生折射率的周期性变化，使正反向传播的行波产生耦合。当光波长满足布拉格条件时，耦合达到最大 在布拉格条件下，入射波几乎被全反射，起到反射镜作用（对波长有选择性的反射镜） 光栅周期满足：
  • 60.2021/9/26 OE of HUST 60 Coupled-cavity laser Figure 3.18: Coupled-cavity laser structures (a) external-cavity laser; (b) cleaved-coupled cavity laser; (c) multisection DBR laser.
  • 61.2021/9/26 OE of HUST 61 增益介质 反射镜 准直透镜 透镜光纤 增透膜 滤光片 高反膜
  • 62.2021/9/26 OE of HUST 62 Sampled Grating DBR Laser
  • 63.2021/9/26 OE of HUST 63 VCSEL
  • 64.2021/9/26 OE of HUST 64 Chapter 3. Optical Transmitters Introduction Basic concepts Light-Emitting Diodes (LED) Semiconductor lasers (Laser Diodes) Laser Characteristics Transmitter Design
  • 65.2021/9/26 OE of HUST 65 3.5 Laser Characteristics 3.5.1 CW Characteristics For a single-mode laser, the rate equations: P, N: number of photons & carriers Net rate of stimulated emission—optical gain: Photon lifetime: CW operation
  • 66.2021/9/26 OE of HUST 66 threshold current Spontaneous emission Stimulated emission P-I Curves
  • 67.2021/9/26 OE of HUST 67 Ith (T) T0: characteristic temperature GaAs: T0=120K, InGaAsP: T0=50 ~ 70K Bending of P-I curves Rnr: mainly depending on Auger recombination in InGaAsP LD Solution: built-in thermoelectric cooler used to deal with temperature sensitivities of InGaAsP Lasers
  • 68.2021/9/26 OE of HUST 68 Threshold of current & carrier For I>Ith threshold
  • 69.2021/9/26 OE of HUST 69 Efficiencies Internal quantum efficiency: Slope efficiency: Differential quantum efficiency: External quantum efficiency: wall-plug efficiency:
  • 70.2021/9/26 OE of HUST 70 3.5.2 Small-Signal Modulation :amplitude-phase coupling parameter, ex. bulk material: 4~8; MQW: ~3 Frequency response small-signal modulation: Frequency chirp Modulation:
  • 71.2021/9/26 OE of HUST 71
  • 72.2021/9/26 OE of HUST 72 Figure 3.21: Measured (solid curves) and fitted (dashed curves) modulation response of a 1.55-μm DFB laser as a function of modulation frequency at several bias levels.
  • 73.2021/9/26 OE of HUST 73 Frequency chirp 3.5.3 Large-Signal Modulation leading edge: mode frequency shifts toward the blue side trailing edge: mode frequency shifts toward the red side External modulation for high speed transmission!
  • 74.2021/9/26 OE of HUST 74 Electro-optical Delay & Relaxation Oscillation Pre-biased to reduce delay time!
  • 75.2021/9/26 OE of HUST 75 Self-pulsation 不同于张弛振荡，没有阻尼，脉动频率范围为0.2~4GHz 容易发生在阈值附近和P-I特性的扭曲区 造成自脉动的机理涉及量子噪声效应、有源区的缺陷及温度感应的变化等因素 抑制这种现象主要靠控制材料的质量，尽量减少有源区的缺陷。 Operated far from kink zone! O P I
  • 76.2021/9/26 OE of HUST 76 Pattern effect TB I P biased above threshold! “11” “11”
  • 77.2021/9/26 OE of HUST 77 Chapter 3. Optical Transmitters Introduction Basic concepts Light-Emitting Diodes (LED) Semiconductor lasers (Laser Diodes) Laser Characteristics Transmitter Design
  • 78.2021/9/26 OE of HUST 78 3.6.1 Basic concept Analog & Digital Modulation 3.6 Transmitter Design (a) LED analog modulation (b) LED digital modulation (c) LD digital modulation for LD, biased above threshold!
  • 79.2021/9/26 OE of HUST 79 Digital Logic Electrical Level 0 1 TTL: 0 ~ 0.8V 2.0 ~ 5.0V (-5V) ECL: -1.75 V -0.85 V (+5V) PECL: +3.25 V +4.15 V Extinction Ratio P P1 P0 0 t
  • 80.2021/9/26 OE of HUST 80 Source-fiber coupling packaging source fiber Rf coating lensed fiber die submount PD heat sink TEC cooler fiber metal shell
  • 81.2021/9/26 OE of HUST 81
  • 82.2021/9/26 OE of HUST 82 Butterfly packaged LD
  • 83.2021/9/26 OE of HUST 83 External Modulator LiNbO3 modulator in Mach-Zehnder configuration V
  • 84.2021/9/26 OE of HUST 84 EA 0 V=0 V(t) T T1 T2 λ
  • 85.2021/9/26 OE of HUST 85 3.6.2 Driving circuit Digital modulation circuit with APC for LD
  • 86.2021/9/26 OE of HUST 86 射极耦合电路 三极管VT1和VT2是轮流截止和导通的，避免了载流子恢复时间的影响，因而可工作于更高的速率 射极耦合电路为恒流源，总电源电流可以保持不变，所以电源电流噪声小 D1和D2是温度补偿二极管，由于D1、D2、VT2和VT3的导通电压分别有－2.5mV/C的负温度特性，利用D1、D2对VT2、VT3的温度特性进行补偿，使温度变化时驱动电流保持恒定。 LD
  • 87.2021/9/26 OE of HUST 87
  • 88.2021/9/26 OE of HUST 88 Ib & Im LD的驱动电路中偏置电流和调制电流大小的选取至关重要，偏置电流的选择合适与否直接影响激光器的高速调制输出特性。 加大直流偏置，使其接近阈值，可以减小电光延迟时间，也可使张驰振荡得到一定程度的抑制。 当激光器偏置在阈值附近时，较小的调制电流就能得到足够高的输出光脉冲，调制效率较高，而且由于偏置电流与最大电流相差不大，可以大大减小码型效应和结发热效应的不良影响。 过大的直流偏置电流会使消光比恶化，影响接收机灵敏度。 激光器恰好偏置在阈值时，散粒噪声会增强，直接影响信号的信噪比。 因此偏置电流的选取应兼顾上述各种影响，根据所用光源的特性与具体系统的要求适当选取。
  • 89.2021/9/26 OE of HUST 89 ATC circuit
  • 90.2021/9/26 OE of HUST 90 Review 光纤通信对光源的要求，光谱线宽和阈值电流 半导体发光的物理基础：三种跃迁过程，费米能级，粒子数反转，正向偏置PN结，双异质结结构对半导体发光器件的性能改善，非辐射复合及其危害，如何决定半导体材料的组分 LED的特性，为什么LED适合用在短距离、低速、模拟通信中？ LD的工作条件，阈值条件，纵模条件 LD的典型结构，增益导引和折射率导引条形激光器，同质结、异质结、条形激光器、多量子阱结构如何实现阈值电流的降低和输出功率的提高？ 如何实现单纵模？DBR、DFB、外腔、C3腔、VCSEL的基本原理。 LD的工作特性：P-I特性, 大信号调制的瞬态效应 光发射机驱动电路，LED和LD驱动电路的不同，PI曲线表示调制过程， 带光反馈的LD数字驱动电路
  • 91.2021/9/26 OE of HUST 91 Optical transmitter：光发射机 LED: 发光二极管 LD：激光二极管 Spontaneous emission：自发辐射 Stimulated emission：受激发射 Stimulated absorption：受激吸收 Boltzman statistics：玻尔兹曼统计分布 Thermal equilibrium：热平衡 Spectral density：光谱密度 Population inversion：粒子数反转 Fermi-Dirac distribution：费米狄拉克分布 Conduction band：导带 Valence band：价带 Forward-biased ：正向偏置 Junction：结 Fermi level：费米能级 Bandgap：带隙 Heavy doping：重掺杂 Homojunction：同质结 Heterojunction：异质结 Double heterostructure：双异质结 Electron-hole recombination：电子空穴复合 Cladding layer：包层 Auger recombination：俄歇复合 Kinetic energy：动能 Nonradiative recombination：非辐射复合 Surface recombination：表面复合 Internal quantum efficiency：内量子效率 Direct bandgap：直接带隙 Indirect bandgap：非直接带隙 Carrier lifetime：载流子寿命 Lattice constant：晶格常数 Ternary and quaternary compound：三元系和四元系化合物 Substrate:衬底 LPE：液相外延 VPE：汽相外延 MBE：分子束外延 MOCVD：改进的化学汽相沉积 MQW: 多量子阱
  • 92.2021/9/26 OE of HUST 92 Electron-hole pairs 电子空穴对 External quantum efficiency 外量子效率 Fresnel transmissivity 菲涅耳透射率 Lambertian source 朗伯光源 Power-conversion efficiency 功率转换效率 Wall-plug efficiency 电光转换效率 Responsivity 响应度 Rate equation 速率方程 Surface-emitting 表面发射 Beam divergence 光束发散 Edge-emitting 边发射 Resonant cavity 谐振腔 Gain coefficient 增益系数 Differential gain 微分增益 Laser threshold 激光阈值 Threshold current 阈值电流 Longitudinal model 纵模 Group index 群折射率 Broad area 宽面 Stripe geometry 条形 Diffusion 扩散 Index-guided 折射率导引 Ridge waveguide laser 脊波导激光器 Buried heterostructure 掩埋异质结 Lateral 侧向 Transverse 横向 SLM: Single Longitudinal mode单纵模 MSR: Mode suppression ratio 模式抑制比 DFB: Distributed Feedback 分布式反馈 Bragg diffraction 布拉格衍射 Bragg condition 布拉格条件 DBR: distributed Bragg reflector 分布式布拉格反射器 Phase-shifted DFB laser 相移DFB激光器 Gain coupled 增益耦合 Coupled cavity 耦合腔
  • 93.2021/9/26 OE of HUST 93 External cavity 外腔 Superstructure grating 超结构光栅 VCSEL: vertical cavity surface-emitting lasers 垂直腔表面发射激光器 Photon lifetime 光子寿命 Spontaneous-emission factor 自发辐射因子 Characteristics temperature 特征温度 Slope efficiency 斜率效率 Differential quantum efficiency 微分量子效率 Linewidth enhancement factor 线宽加强因子
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