光纤通信技术

光纤通信技术 第一章Telecommunication 电信Shannon-Hartley limit(农香公式：运送和传递信息的能力），SNR-Signal Noise Ratio（信噪比），BW-bandwidth（带宽）。Diffraction 衍射Interference 干涉the determinant factors of BW：frequence of carrier waverule of thumb：BW is about 10% of carrive wave frequency（经验规则）Employing higher frequency led to the birth of： radio television radar microwave communications. 10GHz optical communications Ultra VIolet（UV） 400-780nm Infrared（IR） 800nm-2.55umIn optical system: two transmission medium are used -an atmosphere channel -a guided-wave channel. (Optical fibers) Information is superimposed onto carrier wave,which is a high frequency Sinusoidally Varying electromagnetic wave. Amount of information is directly related the carrier frequency,the development is towards employing progressively higher frequencies. Apart from the transmission capacity,since 1990s,people have seen demand on communication-network for services.Time-division multiplexing(TMD)Frequency-division multiplexing(FDM)Video TV 1.5-6Mb/sFor TMD,the N independent information streams each running at a data rate.It is inconvenient that same hierarchies using different bit-rate levels are employed in different areas.SONET:Synchronous Optical Network the fundamental building block is 51.84Mb/sSDH:Synchronous Digital Hierarchy the fundamental building block is 155.52Mb/sATM:Asynchronous transfer mode Can transmit both narrowed and broadband communication service.ISDN:Integrated Services Digital NetworkHow does the information transmit in an optical communication system? Audio-Audio to electrical Converter-Light Source-optical fiber-Light Detector-Demodulator-Electrical toAudio Converter-Analog Audio.The four key components for optical communications. -optical fibers -light sources -photodetectors -optical amplifiersthe transmission capacity(传输的能量) has experienced a 10-fold increase every four years.The first generation links-850nm GaAs-based optical sources Si photodetectors multi-mode fibersThe third generation links-1550nm InGaAs-based optical source and photodetectors Single mode fibersThe fourth generation:optical amplifier GaAlAs-based optical Amplifier at 850nm Erbium-Doped Fiber Amplifier at 1550nm Praseodymium-doped Fiber Amplifier at 1310nmThe optical Soliton(光孤子) Communication Systems: A Soliton is a non dispersive pulse that make use of nonlinear dispersion properties in a fiber to cancel out chromatic dispersion effect(色度色散效应).The fifth generation links-WDM and DWDMWDM:wavelength-Division multiplexingDWDM:dense wavelength-Division multiplexingWDM and DWDM offers further boost in fiber transmission capacity. Before 1990,WDM is less used because that implementing high-speed electronic devices was easier than DWDM. In the 1990s,WDM had being used more and more because electric device neared their modulation limit. Mid-1990s,Combination of EDFAS & WDM was used to boost fiber communication capacity.Elements of an optical fiber transmission link(附图) 1.Transmitter(发射机) light source,drive circuit 2.Regenerator(再生) optical receiver and transmitter,electronics,optical amplifier 3.Receiver(接收机) photodetector,amplifier,signal restorer 4.Cable protecting the fiber contained inside 5.Additional components in the optical fiber communication system optical amplifiers,connectors,splices,couplers1.Elements of optical fiber communication2.Definition of Soliton 第二章Optical fiber,Structure,wave-guiding and FabricationWhat is optical fiber? 光纤的定义 An optical fiber is a dielectric wave-guide that confines electromagnetic energy in the form of light to within its surface and guides the light in a direction parallel to its axis.What is the structure of an optical fiber?(附图)Fiber types.(附图). 单模，多摸Advantage of multi-mode fiber: 1.the large core radii make it easier to launch optical power into the fiber. LED can be used as light source.Disadvantage of multimode fiber: Intermodal dispersion,which causes the pulse to spread out.Theory: 1.Modes:The electromagnetic light field that is guided along an optical fiber can be represented by a superposition of bound mode. 2.Rays theory:Geometrical optics or ray-tracing approach This method provides a good approximation when ratio(半径) of the fiber radius to the wavelength is large.multimode fiber:折射率差1-3%single mode fiber:折射率差0.2-1%Optical propagation: two types of rays: -meridional rays 子午光线 -skew rays 偏斜光线 What are meridional rays? They are confined to the meridian planes of the fiber .Here meridian planes contains the axis of symmetry of the fiber. What are skew rays? Those rays that zigzag down the fiber without necessarily crossing the fiber axis are called skew rays. When viewed looking down the fiber(its projection in a plane normal to the fiber axis),it traces out a polygon around the fiber axis.Therefore a skew ray has a helical path around the fiber.Guided modes in a step index fiber with 折射率差1 are called weakly building fibers.TE:transverse electric fieldTM:transverse magnetic field Guided modes are also called linearly polaried(LP,线偏振) and have transverse electric and magnetic characteristics. A guided LP mode along the fiber step-index fiber can be represented by the propagation of an electric field distribution E along Z. two integers l and m.E01-fundamental mode of step-index fiber. the field is maximum at the center of the core. The electric field can penetrate somewhat into the cladding due to the accompanying evanescent wave. The extent of penetration depends on the V-number of the fiber and hence on the wavelength.