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1、8.2 LED(Light Emitting Diode),Index of refraction折射率 binary 二元的 Graded-index fiber渐变型光纤 ternary 三元的 Step index fiber突变型光纤 quaternary四元的 Multimode fibers多模光纤 Single-mode fibers单模光纤 nitrogen 氮 Spontaneous emission自发辐射 Stimulated emission受激辐射 Infrared 红外 phonon 声子 Light-emitting diode(LED)发光二极管,Under f
2、orward biased, the injected carriers will diffuse and recombine. During recombination process, the energy will be released.,Principle of Action,For a material with a direct band gap, the energy will be light emission.,h,Efficient radiative recombination,GaAs,Generators of light,Injection electrolumi
3、nescence effect,For 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.,Inefficient radiative recombination,LED: A LED is an incoherent light source, in
4、 that photons are emitted randomly from the junction in all directions and not in phase with each other.,8.2.1 Light-Emitting Material and radiating wavelengths,Different material have different band gap, therefore, different emission wavelength. Binary compounds: from GaN(3.4eV) to InSb(0.18eV) , c
5、orresponding to from blue(or ultraviolet) to infrared; GaAs:Eg=1.43eV(infrared) ? GaP: Eg=2.26eV(green) ? GaN（3.4ev,blue),Ternary compounds: GaAs1-xPx,The most common alloy com- position used in LED display is x0.4. red light,Quaternary compounds: InAlGaP etal. for yellow and green wavelength region
6、.,GaAs:Eg=1.43eV(infrared) ? GaP: Eg=2.26eV(green) ? GaN（3.4ev,blue),Return,Some kinds of LED: (1)EELED (Edge Emitting LED, 边发射发光二极管);,(2)SELED (Surface Emitting LED，面发射发光二极管),Heterojunction,Output Characteristics,(1)P-I Characteristics,P: output power of light emission I: forward biased current,(2)
7、Wavelength and Spectrum width Radiated 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 nm,Light emitted from a LED is not monochromatic light!,Light source LED , las
8、er,fiber,detector,8.2.2 Fiber Optic Communications,Photodiode,Optical fiber material（石英） An outer layer of very pure fused silica A core of germanium doped glass Index of refraction n1n2,Single-mode fibers 单模光纤(2a10m) Multi-mode fibers 多模光纤（2a=50-125m）,Step-index突变型光纤 (阶跃折射率光纤） Graded-index渐变（折射率）型光
9、纤,Index profile（折射率分布）,阶跃折射率分布(Step Index fiber,渐变折射率 分布GI fiber,Where，,， 1,(cable)光缆,Fabrication(光纤的制造),硫化（固化）,(1/km) is attenuation coefficient, and the attenuation is not the same for all wavelengths.,Attenuation or loss(衰减或损耗）,1310 nm : 0.35 - 0.5 dB/Km 1550 nm : 0.2 -0.3dB/Km 850 nm : 2.3 -3.4
10、dB/Km 光纤熔接点损耗：0.2dB/点 光纤熔接点 1点/2km,Dispersion Dispersion is another important consideration in choice of operating wavelength. Dispersion can cause signal disorder. ps/nmkm,(1)Chromatic Dispersion（材料色散）: wavelength dependence of the refractive index (2)Intermodal dispersion（模间色散）: different modes pr
11、opagate with different path lengths (it can be reduced by grading the index of the core). (3) Waveguide dispersion（波导色散）: small part of signal power propagate in cladding, different group velocities between the power propagating in core and in cladding,At =1.27m, chromatic dispersion is zero,Photoni
12、c Crystal Fiber(光子晶体光纤) By making a suitable geometry of the periodic dielectric medium, a phonic crystal can be used as an optical fiber.,8.3 Lasers,Laser: Light Amplification by Stimulated Emission of Radiation,Some advantages: Highly directional Monochromatic Coherent,8.3.1 spontaneous emission(自
13、发辐射) and stimulated emission(受激辐射),Spontaneous emission,Stimulated absorption,This radiation will be monochromatic since each photon will have an energy of precisely and will be coherent, because all the photons released will be in phase.,Stimulated Emission,For classical particles, at Equilibrium,(
14、12)Energy density: the total energy in the radiation field per unit volume and per unit requency.,A21: spontaneous emission probability; W21 ： stimulated emission probability; W12： stimulated absorption probability,A21,B21 and B12 are called Einstein coefficients,8.3.2 optical resonant cavity (Fabry
15、-Perot)法布里-珀罗共振腔),The way to enhance the stimulated emission is to have a very large 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 ).,How to build up?,There must
16、be the medium in the cavity to provide the gain.,8.3.3 population inversion(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!,Gain Medium Optical Resonant Cavity Pump,
