Fiber-Optic Communication Technology

Fiber-Optic Communication Technology
Chapter 3 Optical Transmitters

Chapter 3. Optical Transmitters
Introduction Basic concepts Semiconductor lasers (Laser Diode) Laser Characteristics Light-Emitting Diodes (LED) Transmitter Design 2018/11/15 OE, HUST

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 2018/11/15 OE, HUST

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 Group index 群折射率 External cavity 外腔 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 2018/11/15 OE, HUST

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 2018/11/15 OE, HUST

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 2018/11/15 OE, HUST

(b) External Modulation
Biased current Modulation current (≥10Gb/s) (≤2.5Gb/s) (a) Direct Modulation (b) External Modulation 2018/11/15 OE, HUST

3.1.2 Requirements for Optical Source
MQW DFB LD 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, phase noise 6. direct modulation ! ? 7. high efficiency& low threshold: MQW-LD, Ith ~ 10mA 2018/11/15 OE, HUST

Introduction Basic concepts Semiconductor lasers (Laser Diodes) Laser Characteristics Light-Emitting Diodes (LED) Transmitter Design 2018/11/15 OE, HUST

3.2 Basic Concepts 3.2.1 Three fundamental transition processes
1. Spontaneous Emission → LED 2. Stimulated Emission → LD, SOA 3. Stimulated Absorption → PIN / APD Light Emission 2018/11/15 OE, HUST

3.2.2 Emission and Absorption Rates
: spectral density of the electromagnetic energy In thermal equilibrium, according to Boltzmann Statistics : kB: Boltzmann Constant T: Absolute Temperature According to Planck’s formula: 2018/11/15 OE, HUST

? , thermal sources visible or near-infrared region, room temperature
thermal equilibrium laser operation ? Operation condition for laser: N2>N1, Rstim>Rabs (population inversion) External pumping source is needed: injection current, pumping light etc. 2018/11/15 OE, HUST

Energy bands in semiconductor
conduction band & valence band 原子是由原子核和绕原子核旋转的电子组成。最里层的电子距原子核最近，受原子核束缚最强，能量最低（包括电子的动能和势能）。越外层的电子受原子束缚越弱，能量越高；电子只能处于特定的能级之上；能级图用一系列高低不同的水平横线来表示电子所能取的确定能量；原子中的电子通过和外界交换能量的方式发生能级的跃迁——热跃迁和光跃迁。 2018/11/15 OE, HUST

实际物体是由大量原子构成的，每一原子的电子特别是外层电子除受本身原子的势场作用外，还受到相邻原子的作用。
半导体材料中原子在共价键的作用下形成紧密相间、周期排列的晶格结构。电子能级受晶格作用发生分裂而形成能带； Si 由于原子的外层电子与原子核之间库仑作用力的大小与其距原子核的距离成反比。各原子相应的一些外层电子运动轨道将发生不同程度的交叠，相邻原子最外壳层交叠最多，内壳层交叠较少。原子组成晶体后，由于电子壳层的交叠，电子不再完全局限在某一个原子上，可以由一个原子转移到相邻的原子上去，因而，电子将可以在整个晶体中运动。这种运动称为电子的共有化运动。在共价晶体中，每个原子最外层的电子和邻近原子形成共价键，整个晶体就是通过共价键把原子联系起来的。作共有化运动的电子受到周期性排列着的原子的共同作用，它们的势能具有晶格的周期性 2018/11/15 OE, HUST

价带：由共价键束缚的价电子所占据的能带为价带；导带：由自由电子占据的能带为导带，导带位于价带之上；
禁带：导带和价带之间被宽度为Eg的带隙分开，称为禁带；绝缘体：Eg ～ 7eV，电子不容易跃迁到导带；半导体： Eg～1eV，电子容易跃迁到导带；导体： Eg～0eV，没有带隙。 2018/11/15 OE, HUST

Energy bands in semiconductor
recombination between electrons and holes 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 2018/11/15 OE, HUST

: joint density of states, which describe 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 2018/11/15 OE, HUST

population-inversion condition:
in thermal equilibrium: pumping energy into semiconductor by injecting current To get laser output, 2018/11/15 OE, HUST

3.2.3 p-n junctions 1. Type of semiconductor Intrinsic semiconductor:
undoped, Fermi level is 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 2018/11/15 OE, HUST

p-type semiconductor & n-type semiconductor
Intrinsic p-type forward biased p-type semiconductor & n-type semiconductor 2018/11/15 OE, HUST

(a) in thermal equilibrium
(b) under forward biased 2. p-n junctions under forward biased: built-in electric field is reduced diffusion of electrons and holes across the junction electrons and holes are present simultaneously in depletion region generate light through spontaneous emission or stimulated emission in thermal equilibrium: the Fermi level must be continuous across the p–n junction achieved through diffusion of electrons and holes across the junction. 2018/11/15 OE, HUST

4. Homojunction & heterojunction
Homojunction: equal bandgaps the same semiconductor material wide region for electron-hole recombination difficult to realize high carrier densities Heterojunction: different bandgaps 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. 2018/11/15 OE, HUST

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higher density of carriers →higher index → waveguide (1D)
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 carriers & optical field 0.85µm: cladding/active: GaAlAs/GaAs 1.31µm, 1.55µm: cladding/active: InP/InGaAsP 2018/11/15 OE, HUST

3.2.4 Nonradiative Recombination
1. electron-hole recombination Trap of defects Surface recombination Auger Nonradiative recombination 2018/11/15 OE, HUST

2. internal quantum efficiency
Rrr : radiative recombination rate Rnr : nonradiative recombination rate Rtot : total recombination rate τ : recombination time Nonradiative recombination, especially Auger recombination (temperature dependent) is harmful to devices! positive feedback 2018/11/15 OE, HUST

B : spontaneous radiation
k1 k2 (1) direct-bandgap (GaAs, InP) (2) indirect-bandgap (Si, Ge) 3. carrier lifetime A : defects & traps B : spontaneous radiation C : Auger 2018/11/15 OE, HUST

3.2.5 Semiconductor Materials
Quality of the heterojunction interface depends on the lattice constant of the two materials. (matching !) ternary compound 2018/11/15 OE, HUST

0.85µm: GaAlAs/GaAs (cladding/active)
quaternary compound 0.85µm: GaAlAs/GaAs (cladding/active) 1.31µm, 1.55µm: InP/InGaAsP (cladding/active) 2018/11/15 OE, HUST

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3.3 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) 2018/11/15 OE, HUST

Components of Semiconductor Lasers
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Injection current Resonant cavity Gain medium Model of laser z=0 z=L
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3.3.1 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 ? 2018/11/15 OE, HUST

Blue or red shifting of peak wavelength when injected current increases ?
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. 2018/11/15 OE, HUST

3.3.2 Feedback and Laser Threshold
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Threshold 2018/11/15 OE, HUST

oscillating frequency
Amplitude condition Phase condition threshold gain oscillating frequency spacing of oscillating frequency MLM 2018/11/15 OE, HUST

3.3.3 LD Structures Broad-area LD
X Y Broad-area LD distribution in near field light-confinement mechanism in the direction perpendicular to the junction plane introduced by double heterostructure 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. 2018/11/15 OE, HUST

the light generated spreads over the entire width of the laser.
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. How about spatial mode in waveguide and distribution in far field ? 2018/11/15 OE, HUST

Gain-guided semiconductor lasers
Stripe lasers Gain-guided semiconductor lasers Y X 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. 2018/11/15 OE, HUST

the spot size is still not stable as the laser power is increased.
solve the light-confinement problem by limiting current injection over a narrow stripe. the spot size is still not stable as the laser power is increased. Injection current induced index variety! 2018/11/15 OE, HUST

Index-guided semiconductor lasers
Y X Figure 3.14: Cross section of two index-guided semiconductor lasers: (a) ridge-waveguide structure for weak index guiding; (b) etched-mesa buried heterostructure for strong index guiding. 2018/11/15 OE, HUST

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） 2018/11/15 OE, HUST

Double heterostructure
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! 2018/11/15 OE, HUST

3.3.4 Control of Longitudinal Modes
Loss SLM MLM Side Mode Suppression Ratio (SMSR): or 2018/11/15 OE, HUST

Distributed Feedback (DFB) 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，某一入射波长几乎被全反射，光栅起到了对波长选择性反射的作用。光栅周期满足： 2018/11/15 OE, HUST

Coupled-cavity laser Figure 3.18: Coupled-cavity laser structures (a) external-cavity laser; (b) cleaved-coupled cavity laser; (c) multisection DBR laser. 2018/11/15 OE, HUST

External cavity laser λc 增益介质反射镜准直透镜透镜光纤增透膜滤光片高反膜 2018/11/15
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Sampled Grating DBR Laser
The Sampled Grating (SG) DBR laser uses the Vernier effect between two mirrors with comb-shaped reflectivity spectrum. Just like the Vernier calliper the effect uses the fact that if the two comb-shaped spectra have a different period, a small relative displacement between the two gives a large shift in the frequency where they overlap. A simultaneous tuning of both gratings tunes over different cavity modes just like in the DBR laser. Finally a phase section makes the tuning quasi continuous. The SG DBR laser can achieve high side mode suppression ratio and a large tuning range. Due to the mirrors after the gain section it is easily integrated with for example SOAs and electro absorber modulators. Because of free carrier absorption in the DBRs the power variation can be substantial when tuning. Without an additional SOA the output power is low. DBR: distributed Bragg reflector 2018/11/15 OE, HUST

Cleaved-coupled cavity laser
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VCSEL 2018/11/15 OE, HUST

思考题 1. 现有半导体激光器的F-P谐振腔，长度为400m，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％，另一端没有镀膜。现有半导体激光器工作在1550nm附近，要求谐振腔谐振的阈值增益系数小于75cm－1，请问：如何选择半导体材料和组分？谐振腔内部损耗系数应满足什么条件？ 2018/11/15 OE, HUST

3.4 Laser Characteristics
3.4.1 CW Characteristics For a SLM laser, the rate equations: P, N: number of photons & carriers Net rate of stimulated emission—optical gain: g: peak gain of material : gain cross section, or differential gain. Photon lifetime: 2018/11/15 OE, HUST

Threshold of current & carrier
CW operation: For I > Ith (R1=R2) 2018/11/15 OE, HUST

P-I curves Threshold of P-I curves Spontaneous emission
Stimulated emission I0: constant T0: characteristic temperature GaAs: T0=120K, InGaAsP: T0=50 ~ 70K Bending of P-I curves Rnr: mainly depending on Auger recombination in InGaAsP LDs Solution: built-in thermoelectric cooler is used to deal with temperature sensitivities of InGaAsP LDs 2018/11/15 OE, HUST

Efficiencies Internal quantum efficiency:
Slope efficiency: Differential quantum efficiency: External quantum efficiency: wall-plug efficiency: 2018/11/15 OE, HUST

small-signal modulation:
Frequency response 2018/11/15 OE, HUST

Modulation bandwidth the efficiency is reduced when the modulation frequency exceeds ΩR by a large amount. Figure 3.21: Modulation response of a laser as a function of modulation frequency at several bias levels. 2018/11/15 OE, HUST

3.4.3 Large-Signal Modulation
Frequency chirp leading edge: mode frequency shifts toward the blue side trailing edge: mode frequency shifts toward the red side : amplitude-phase coupling parameter, ex. bulk material: 4~8; MQW: ~3 External modulation for high speed transmission! 2018/11/15 OE, HUST

Electro-optical Delay & Relaxation Oscillation
请参见江剑平编著的《半导体激光器》 Pre-biased to reduce delay time! 2018/11/15 OE, HUST

Pattern effect TB I P “11” 当电光延迟时间与电调制速率对应的的码元持续时间相近时，会使“0”码后的第一个“1”码脉冲宽度变窄，幅度变小，严重时使单个“1”码丢失，这种现象即“码型效应”。连“0”数越多，调制速率越高，该效应越明显。用适当的“过调制”补偿，可以消除码型效应。 2018/11/15 OE, HUST

Operated far from kink zone!
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。 P I O P I Operated far from kink zone! 2018/11/15 OE, HUST

Simulation 1-Direct modulation
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Simulation 2-External modulation
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Ib & Im LD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。 2018/11/15 OE, HUST

3.5 Light-Emitting Diodes (LEDs)
3.5.1 Power-Current Characteristics a forward-biased p-n junction → spontaneous emission → LED 2018/11/15 OE, HUST

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Power-conversion efficiency (wall-plug efficiency)
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(2) bending of P-I curve:
Responsivity: (1) responsivity remains constant when I is small (2) bending of P-I curve: (3) no threshold 2018/11/15 OE, HUST

3.5.2 LED Spectrum an approximate expression:
LEDs are suitable for LAN with low bit rate & short distance ! 2018/11/15 OE, HUST

超宽带光源白光LED Figure 3.7: (b) spectrum of the emitted light for a typical 1.3-μm LED. 2018/11/15 OE, HUST

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3.5.3 Modulation Response Rate equation: Sinusoidal modulation:
: injection carrier : carrier of recombination (nonradiative & spontaneous emission) Sinusoidal modulation: Ib : bias current Im : modulation current ωm : modulation frequency 2018/11/15 OE, HUST

Since modulated power is related to linearly
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(a) surface-emitting LED
3.5.4 LED Structures (a) (b) (a) surface-emitting LED (b) edge-emitting LED 2018/11/15 OE, HUST

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；
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。 2018/11/15 OE, HUST

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；
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。 2018/11/15 OE, HUST

A、LD的激射波长一定是自发辐射的峰值波长； B、条形激光器中也存在双异质结结构；
C、双异质结中对载流子的限制作用是因为存在内建折射率波导； D、通过选择合适的组分x和y，基于In1-xGaxAsyP1-y的半导体光源可设计工作于0.85m处； E、LD有谐振腔，而LED没有； F、LD的P-I曲线有阈值，而LED的P-I曲线没有阈值； G、LD和SOA中最初的光子均来源于自发辐射； H、激光器的小信号调制带宽会随着偏置电流的增加而增大； I、偏置电流选择合理可适当减小张驰振荡和电光延时效应的影响； J、单纵模LD用作光源时，色散容限大。 2. 以下关于半导体材料和发光机理论述错误的是： 2018/11/15 OE, HUST

3.6 Transmitter Design 3.6.1 Basic concept Analog & Digital Modulation
(a) LED analog modulation (b) LED digital modulation (c) LD digital modulation for LD, biased near threshold! 2018/11/15 OE, HUST

Digital Logic Electrical Level
TTL: 0 ~ 0.8V ~ 5.0V (-5V) ECL: V V (+5V) PECL: V V Extinction Ratio P P1 P0 t 2018/11/15 OE, HUST

Source-fiber coupling
Packaging source fiber coating lensed fiber Rf die submount PD heat sink TEC cooler fiber metal shell 2018/11/15 OE, HUST

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Butterfly packaged LD 2018/11/15 OE, HUST

LiNbO3 modulator in Mach-Zehnder configuration
External Modulator LiNbO3 modulator in Mach-Zehnder configuration V 2018/11/15 OE, HUST

EA V=0 V(t) T T1 T2 λ 2018/11/15 OE, HUST

3.6.2 Driving circuit Digital modulation circuit with APC for LD
2018/11/15 OE, HUST

射极耦合电路三极管T1和T2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； LD
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、T2和T3的导通电压分别有－2.5mV/C的负温度特性，利用D1、D2对T2、T3的温度特性进行补偿，使温度变化时驱动电流保持恒定。 LD 2018/11/15 OE, HUST

APC电路 2018/11/15 OE, HUST

ATC电路 2018/11/15 OE, HUST

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、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。 2018/11/15 OE, HUST

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