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1、第第6讲讲 光纤通信光源光纤通信光源半导体激光二极管半导体激光二极管 LDContents （内容）（内容） Laser Principles (激光器原理激光器原理) Laser Diodes (半导体激光二极管半导体激光二极管LD) LD Operating Characteristics (LD运转特性运转特性)Z. Q. Luo6.33 LaserLight Amplification by StimulatedEmission of RadiationSection 6.3LASER PRINCIPLES （激光原理） 激光三要素l 增益介质（受激辐射光增益）l 谐振腔 （光反馈、选
2、模/波长）l 激励源 （提供能量）Z. Q. Luo6.34= filled state= formerly filled stateLower Level (低能级)Higher Level (高能级)original photon (原始光子)new photon (新光子) Optical Stimulated Emission 光受激辐射光受激辐射 A laser needs this condition, referred to as population inversion. （光受激辐射的前提条件是：粒子数反转粒子数反转）LASER PRINCIPLES(激光原理)Z. Q. L
3、uo6.35Laser threshold (激光阈值激光阈值) Because of the need for amplification beyond any attenuation that occurs, the laser will not lase until the applied power is beyond a threshold value. （只有当泵浦强度超过某个值后，受激辐射放大才能超过腔损耗，产生激光。即激光器存在一个泵浦阈值）LASER PRINCIPLES(激光原理)Z. Q. Luo6.36Longitudinal modes: （激光器纵模）1500 MH
4、zffffGain （增益介质增益带宽）Cavity Resonances (谐振腔)Output Spectrum (输出激光谱) Shown are the three longitudinal modes of the laser. (3个纵模激射)LASER PRINCIPLES(激光原理)Z. Q. Luo6.37The single-transverse-mode output has a Gaussian beam intensity pattern. （单横模激光是高斯光斑分布）Transverse modes: （激光器横模）LASER PRINCIPLES(激光原理)Z.
5、Q. Luo6.48Section 6.4 Laser Diode （激光二极管）（激光二极管） Heterojunction Metal Stripe Contact Laser Diode（异质结金属电极的激光二极管）（异质结金属电极的激光二极管）10 m0.1 - 0.3 m300 mJunction RegionnpZ. Q. Luo6.19An LD is a pn junction semiconductor. It emits light in the infrared or visible spectrum when forward biased. LD是是PN结结半半导导体体
6、器器件件，通通过过加加正正向向偏偏置置电电压压，它可在可见至近红外波段发光它可在可见至近红外波段发光pnLDiV+ -LD 工作原理工作原理Z. Q. Luo6.410To produce oscillation we need: （为了产生激光需要如下条件）（为了产生激光需要如下条件）Amplification （光放大）（光放大） Feedback （光反馈（光反馈/谐振腔）谐振腔） Pumping （电激励）（电激励）LASER DIODE (激光二极管激光二极管)Z. Q. Luo6.111LD 工作原理工作原理光放大光放大pn Energy Level Diagram for V =
7、 0 (zero bias) （LD PN结的能带图）结的能带图）WgConduction Band （导带）（导带）Valence Band （价带）（价带） - - - - - - -+ + + + + +Hole Energy （电子空穴复合）（电子空穴复合）电子能量电子能量Wg = bandgap energy （带隙能量）（带隙能量）Its value depends on the semiconductor used. （带隙能量大小依赖于半导体材料）（带隙能量大小依赖于半导体材料） Z. Q. Luo6.112PN Energy Level Diagram for V 0 (fo
8、rward bias)(LED PN结加正向偏振电压时结加正向偏振电压时)WgConduction BandValence Band- - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + Hole Energy（电子空穴复合）（电子空穴复合）hf = WgElectron Energy 释放能量释放能量光子光子LD 工作原理工作原理光放大光放大Z. Q. Luo6.113The released energy takes the form of a photon.电子空穴复合释放光子能量及波长为：电子空穴复合释
9、放光子能量及波长为：(6.1)Nowl Wg is in joules and is in meters （这里，（这里， Wg 单位为焦耳单位为焦耳J）LD 工作原理工作原理光放大光放大Z. Q. Luo6.114 Suppose Wg is given in eV. Compute the output wavelength.若若Wg 用电子伏特用电子伏特ev为单位，则上式可重写为：为单位，则上式可重写为：(6.2)with Wg in eV and in m. LD 工作原理工作原理光放大光放大Z. Q. Luo6.115Material ( m)Wg (eV)GaAs0.91.4AlG
10、aAs0.8 - 0.91.4 - 1.55InGaAs1.0 - 1.30.95 - 1.24InGaAsP0.9 - 1.70.73 - 1.35GaInP0.64 - 0.681.82 - 1.94 Common Characteristics （常见半导体（常见半导体Wg及发光波长）及发光波长）LD 工作原理工作原理光放大光放大Z. Q. Luo6.416 Forward current injects holes and electrons into the junction. (前向偏置电流注入前向偏置电流注入PN结（提供能量），即注入电结（提供能量），即注入电荷入荷入PN结结)
11、Photons in the junction stimulate electron-hole recombination, with emission of added photons. (电子空穴对在电子空穴对在PN结中复合，产生光子，受激辐射后放大结中复合，产生光子，受激辐射后放大) This process yields gain. If the gain exceeds the losses, oscillation occurs. (当受激辐射增益大于腔损耗，当受激辐射增益大于腔损耗，则产生激光则产生激光)LASER DIODE (激光二极管激光二极管)Z. Q. Luo6.417
12、Feedback （光反馈）（光反馈） Cleave the front and back faces of the semiconductor. This provides mirror-like reflections. (切割半导体切割半导体PN结结端面，利用菲涅耳反射形成光反馈端面，利用菲涅耳反射形成光反馈)Example: AlGaAs, n = 3.6LASER DIODE (激光二极管激光二极管)Z. Q. Luo6.418 The cavity also affects the output spectrum. （谐振腔也影响激光输出光谱）（谐振腔也影响激光输出光谱） Exam
13、ple: 谐振腔两相邻谐振波长的间隔为：谐振腔两相邻谐振波长的间隔为：(3.26)(3.25)LASER DIODE (激光二极管激光二极管)Z. Q. Luo6.419Assume: （假设如下参数）（假设如下参数） 0 = 0.82 m, L = 300 m, n = 3.6 = 2 nm (激光增益带宽激光增益带宽)ThenThe number of longitudinal modes is approximately 包含纵模数为：包含纵模数为：LASER DIODE (激光二极管激光二极管)Z. Q. Luo6.420819820 821Gain0.311 nm (nm) (nm)
14、Cavity Resonances cFor the laser diode, we have: LD的典型光谱图的典型光谱图 (nm)cOutput SpectrumLASER DIODE (激光二极管激光二极管)Z. Q. Luo21 From the previous slide we see that six longitudinal modes exist. 上面给出的是多纵模光谱（上面给出的是多纵模光谱（6个）个） A longitudinal single-mode laser has just one longitudinal mode. The spectrum looks
15、like: 典型单纵模光谱典型单纵模光谱 (nm)820 0.1 nmLASER DIODE (激光二极管激光二极管)Z. Q. Luo6.522Spectral Width （LD激光线宽）激光线宽） Multimode (longitudinal) laser diodes: is typically 1 to 5 nm （多纵模（多纵模LD典型线宽典型线宽15nm） Single-mode (longitudinal) laser diodes: is typically 0.01 to 0.2 nm （单纵模（单纵模LD典型线宽典型线宽0.01-0.2 nm）LASER DIODE O
16、PERATING CHARACTERISTICS （LD运转特性）运转特性）Z. Q. Luo6.523Section 6.5 Laser Diode Operating CharacteristicsLD运转特性运转特性P (mW)50I (mA)ITH = threshold currentExample: ITH = 35 mADiode Voltages 1.2 - 2 VoltsActualIdealITH100Only when the gain exceeds the losses, oscillation occurs.只有当增益大于损耗才有激光只有当增益大于损耗才有激光Z.
17、Q. Luo6.524Optical PowerOptical Power （光信号光信号）11 1100Input Current or Signal （电数字信号电数字信号）ttDigital Modulation （数字信号调制（数字信号调制LD）isIdcITHLASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Z. Q. Luo6.525iisIdcBias The dc bias, set just at (or near) threshold, makes the device turn on quicker for binary
18、 ones. 数字信号数字信号调制时，尽量让调制时，尽量让dc偏置在偏置在LD激光阈值附近激光阈值附近Digital Modulation （数字信号调制（数字信号调制LD）SignalLASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Z. Q. Luo6.526Optical PowerOptical Power (光信号光信号)Input Current (Signal)tAnalog Modulation （模拟信号调制（模拟信号调制LD）tPdcIdciPspIspITHLASER DIODE OPERATING CHARACTERI
19、STICS （LD运转特性）运转特性）Z. Q. Luo6.527The dc bias is set to operate in the middle of the linear region of the diodes characteristic curve.模拟信号调制时，直流偏置电流尽量放在线性工作区的中部模拟信号调制时，直流偏置电流尽量放在线性工作区的中部isIdcBias SignaliLASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Analog Modulation （模拟信号调制（模拟信号调制LD）Z. Q. Luo6.5
20、28Temperature Dependence （温度对（温度对LD的的PI曲线影响）曲线影响） P (mW)50i (mA)30C80C70100 温度越高，LD阈值越高 温度越高，输出功率越低（相同电流下）LASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Z. Q. Luo6.529 The laser diode wavelength changes with temperature because the diode materials index of refraction is temperature dependant. Th
21、us, the cavity resonant-frequency changes with temperature. 因温度变化导致半导体材料折射率因温度变化导致半导体材料折射率n(T)变化，故温度也改变变化，故温度也改变LD激激射波长射波长 Recall the cavity resonance frequencies: 谐振腔振荡频率谐振腔振荡频率(3.24)LASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Z. Q. Luo6.530Example: Wavelength Shift = 0.1 nm/oCT1 = 27 CGainC
22、avity ResonanceOutputCavity ResonanceOutputT2 = 30 CLASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Z. Q. Luo6.531Laser Diode Bandwidth（LD调制带宽调制带宽/传输速率极限）传输速率极限） Typical laser diode rise time: 0.1 1 ns （LD典型上升沿时间约典型上升沿时间约0.1 1 ns ） Bandwidth （极限带宽）（极限带宽） f3-dB = 0.5/tr Example: Let tr = 0.1 ns! L
23、ight from the laser diode can be modulated at rates over 40 GHz (40 Gb/s).LASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Z. Q. Luo6.532 | Half-Power Beamwidths | = 10 Parallel （发散角）（发散角） = 35 Perpendicular Radiation Pattern (LD横模光斑横模光斑)7 m0.2 mLASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Z
24、. Q. Luo6.533Laser Diode With Integral Fiber Pigtail (尾纤封装尾纤封装LD)DiodeTerminal (正电极正电极)Threaded (热层热层)ContactLDGroovedBlockCap (顶帽顶帽)Ground (接地接地)FiberPigtailLASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Z. Q. Luo6.534Grooved BlockLDGroundPhotodetectorDiode TerminalInsulatedStandoffSiliconSubstrateFiberLASER DIODE OPERATING CHARACTERISTICS （LD运转特性）运转特性）Z. Q. Luo6.535Fourteen-Pin Dual In-Line Laser Package （14针封装针封装LD）14 Electrical Pin ConnectionsMounting BracketFiber2024/7/27Thanks!
