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1、8.2 LED(Light Emitting Diode)Index of refraction折射率折射率 binary 二元的二元的Graded-index fiber渐变型光纤渐变型光纤 ternary 三元的三元的 Step index fiber突变型光纤突变型光纤 quaternary四元的四元的Multimode fibers多模光纤多模光纤Single-mode fibers单模光纤单模光纤 nitrogen 氮氮Spontaneous emission自发辐射自发辐射Stimulated emission受激辐射受激辐射Infrared 红外红外 phonon 声子声子Lig
2、ht-emitting diode(LED)发光二极管发光二极管 东南大学考研固态电子器件-chaUnder forward biased, the injected carriers will diffuse and recombine. During recombination process, the energy will be released. Principle of Action东南大学考研固态电子器件-chaFor a material with a direct band gap, the energy will be light emission.h Efficient
3、radiative recombinationGaAsGenerators of lightInjection electroluminescence effect东南大学考研固态电子器件-chaFor a material with an indirect band gap, such as Si or Ge, recombination releases the heat to the lattice. The momentum will change when the recombination occurs. A phonon and a photon will be released
4、. Inefficient radiative recombination东南大学考研固态电子器件-chaLED:A LED is an incoherent light source, in that photons are emitted randomly from the junction in all directions and not in phase with each other.东南大学考研固态电子器件-cha8.2.1 Light-Emitting Material and radiating wavelengthsDifferent material have diffe
5、rent band gap, therefore, different emission wavelength.Binary compounds: from GaN(3.4eV) to InSb(0.18eV) , corresponding to from blue(or ultraviolet) to infrared; GaAs:Eg=1.43eV(infrared) ? GaP: Eg=2.26eV(green) ? GaN（3.4ev,blue)东南大学考研固态电子器件-chaTernary compounds: GaAs1-xPx,The most common alloy com
6、-position usedin LED display is x0.4. red light东南大学考研固态电子器件-chaQuaternary compounds: InAlGaP etal. for yellow and green wavelength region.MaterialEnergy gap(eV)Wavelengths(nm)GaAs1.424876InP1.35924InGaAs0.75-1.241664-1006AlGaAs1.42-1.92879-650InGaAsP0.75-1.351664-924东南大学考研固态电子器件-chalGaAs:Eg=1.43eV(i
7、nfrared) ? GaP: Eg=2.26eV(green) ? GaN（3.4ev,blue)Return东南大学考研固态电子器件-chaSome kinds of LED: (1)EELED (Edge Emitting LED, 边发射发光二极边发射发光二极管管);东南大学考研固态电子器件-chal(2)SELED (Surface Emitting LED，面面发射发光二极管发射发光二极管)Heterojunction 东南大学考研固态电子器件-chaOutput Characteristics(1)P-I CharacteristicsP: output power of lig
8、ht emissionI: forward biased current东南大学考研固态电子器件-cha(2)Wavelength and Spectrum widthRadiated wavelength often referred to as a peak wavelength, is determined by band gap, Eg; Spectrum width, is measured as full width at half maximum, FWHM. from several nm to several hundred nmLight emitted from a LE
9、D is not monochromatic light!东南大学考研固态电子器件-chaLight sourceLED , laserfiberdetector8.2.2 Fiber Optic CommunicationsPhotodiode东南大学考研固态电子器件-chaOptical fiber material（石英）（石英）An outer layer of very pure fused silicaA core of germanium doped glassIndex of refractionn1n2Single-mode fibers 单模光纤单模光纤(2a10m)Mul
10、ti-mode fibers 多模光纤（多模光纤（2a=50-125m）东南大学考研固态电子器件-chaStep-index突变型光纤突变型光纤(阶跃折射阶跃折射率光纤）率光纤）Graded-index渐变渐变（折射率）（折射率）型光纤型光纤东南大学考研固态电子器件-chaIndex profile（折射率分布）（折射率分布） 阶跃折射率阶跃折射率分布分布(Step Index fiber渐变折射率渐变折射率分布分布GI fiberWhere， 1东南大学考研固态电子器件-cha(cable)光光缆缆东南大学考研固态电子器件-chaFabrication(光纤的制造光纤的制造)东南大学考研固态
11、电子器件-cha硫化（固化）硫化（固化）东南大学考研固态电子器件-cha(1/km) is attenuation coefficient, and the attenuation is not the same for all wavelengths.Attenuation or loss(衰减或损耗）衰减或损耗）东南大学考研固态电子器件-cha1310 1310 nm : 0.35 - 0.5 dB/Kmnm : 0.35 - 0.5 dB/Km1550 nm : 0.2 -0.3dB/Km1550 nm : 0.2 -0.3dB/Km850 nm : 2.3 -3.4 dB/Km850
12、nm : 2.3 -3.4 dB/Km光纤熔接点损耗：光纤熔接点损耗：0.2dB/0.2dB/点点光纤熔接点光纤熔接点 1 1点点/2km/2km东南大学考研固态电子器件-chaDispersion Dispersion is another important consideration in choice of operating wavelength. Dispersion can cause signal disorder. ps/nmkm东南大学考研固态电子器件-cha(1)Chromatic Dispersion（材料色散）（材料色散）: wavelength dependence
13、 of the refractive index (2)Intermodal dispersion（模间色散）（模间色散）: different modes propagate with different path lengths (it can be reduced by grading the index of the core). (3) Waveguide dispersion（波导色散）（波导色散）: small part of signal power propagate in cladding, different group velocities between the po
14、wer propagating in core and in cladding At =1.27m, chromatic dispersion is zero东南大学考研固态电子器件-chaPhotonic Crystal Fiber(光子晶体光纤光子晶体光纤)By making a suitable geometry of the periodic dielectricmedium, a phonic crystal can be used as an optical fiber.东南大学考研固态电子器件-cha东南大学考研固态电子器件-cha8.3 LasersLaser: Light A
15、mplification by Stimulated Emission of RadiationSome advantages:Highly directionalMonochromaticCoherent东南大学考研固态电子器件-cha8.3.1 spontaneous emission(自发辐射自发辐射) and stimulated emission(受激辐射受激辐射)东南大学考研固态电子器件-chaSpontaneous emission Stimulated absorptionThis radiation will be monochromatic since each photo
16、n will have an energy of preciselyand will be coherent, because all the photons released will be in phase.Stimulated Emission东南大学考研固态电子器件-cha For classical particles, at Equilibrium ( 12)Energy density: the total energy in the radiation field per unit volume and per unit requency.东南大学考研固态电子器件-cha A2
17、1: spontaneous emission probability;W21 ： stimulated emission probability;W12： stimulated absorption probabilityA21,B21 and B12 are called Einstein coefficients东南大学考研固态电子器件-cha8.3.2 optical resonant cavity (Fabry-Perot)法布里法布里-珀罗共振腔珀罗共振腔)The way to enhance the stimulated emission is to have a very la
18、rge photon field energy density ( 12) : an optical resonant cavity in which the photon density can build up to a large value through multiple internal reflections at certain frequencies( 12 ). 东南大学考研固态电子器件-chaHow to build up? There must be the medium in the cavity to provide the gain.东南大学考研固态电子器件-ch
19、a8.3.3 population inversion(分布反转分布反转)If stimulated emission is to dominate over absorption of photons from the radiation field, we must have a way of maintaining more electrons in the upper level than in the lower level, that is n2n1.How to maintain n2n1?Pump!东南大学考研固态电子器件-chaGain MediumOptical Resonant CavityPump东南大学考研固态电子器件-cha
