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1、1,2023/3/23,The Sch.Of Information Engineering,WHUT,Chapter2 Optical Fiber,2,2023/3/23,The Sch.Of Information Engineering,WHUT,OUTLINE,The nature of lightThe fabrication of optical fibersThe structure of an optical fiberThe propagation principle of light along a fiberThe transmission character of fi
2、ber,3,2023/3/23,The Sch.Of Information Engineering,WHUT,2.1 The nature of light,2.1.1 the basic concept,2.1.2 Light as an electromagnetic wave,2.1.3 Polarization,2.1.4 Basic optical law and definitions,4,2023/3/23,The Sch.Of Information Engineering,WHUT,2.1.1 the basic concept,1.Raygeometrical or ra
3、y opticalIt is a good description when it goes through an object that is many times larger than the wavelength.In classical physics,light consists of“ray”that could be reflected and refracted through mirrors and prisms etc.The rays show the direction of energy flow in the light beam.In optical fiber
4、 communication system,It always be used to discuss light propagation in multimode fibers.Problem:it is cannot explain many phenomena we make use of in optical fiber communication.,5,2023/3/23,The Sch.Of Information Engineering,WHUT,2.Electromagentic waves,In 1815,Fresnel gived the correct explanatio
5、n of diffraction.In 1864,Maxwell theorized that light waves must be electromagnetic in nature.The observation of polarization effects indicated that light wave are transverse.It is no different from a radio wave except that the wavelength is much shorter.,6,2023/3/23,The Sch.Of Information Engineeri
6、ng,WHUT,3.Photons,Light energy is always emitted or absorbed in discrete units called quanta of photons.In all experiments used to show the existence of photons,the photon energy is found to depend only on the frequency.The relationship between the energy and the frequency of a photon is given by,Wh
7、ere is Plancks constant.,7,2023/3/23,The Sch.Of Information Engineering,WHUT,2.1.2 Light as an electromagnetic wave,The electomagnetic spectrum,8,2023/3/23,The Sch.Of Information Engineering,WHUT,An electromagnetic wave consist of two field:an electric field an magnetic fieldthe two fields(electric
8、and magnetic)are oriented at precisely 90to one another,9,2023/3/23,The Sch.Of Information Engineering,WHUT,10,2023/3/23,The Sch.Of Information Engineering,WHUT,The curved line is intended to represent field strength.The field might start at a maximum in one direction,decay to a zero and then build
9、up in the other direction until it reaches amaximum in that other direction.The field strength changessinusoidally.,11,2023/3/23,The Sch.Of Information Engineering,WHUT,2.1.3 Polarization,1.Linear polarizationThe electric and magnetic field distributions in a train of plane electromagnetic waves at
10、a given instant in time are shown as following:The plane wave example given above has its electric field vector always pointing in the direction.Such a wave is linearly polarized.,12,2023/3/23,The Sch.Of Information Engineering,WHUT,Another linearly polarized wave which is independent of the first w
11、ave and orthogonal to it.,Where is the relative phase difference between the waves.,the resultant wave is then simply as following:,If is zero or an integer multiple of,the waves are in phase.,13,2023/3/23,The Sch.Of Information Engineering,WHUT,Just as any two orthogonal plane waves can be combined
12、 into a linearly polarized wave,an arbitrary linearly polarized wave can be resolved into two independent orthogonal plane waves that are in phase.,14,2023/3/23,The Sch.Of Information Engineering,WHUT,2.Ellipitical and circular polarization,For general values of the wave is elliptically polarized.Th
13、e general equation of an ellipse is,The axis of the ellipse makes an angle relative to the x axisas following,15,2023/3/23,The Sch.Of Information Engineering,WHUT,16,2023/3/23,The Sch.Of Information Engineering,WHUT,When,We have circularly polarized light.,Choose the positive sign for,the wave is ri
14、ght circulary polarized,Choose the negative sign for,the wave is left circulary polarized,17,2023/3/23,The Sch.Of Information Engineering,WHUT,2.1.4 Basic optical law and definitions,1.Refractive index,In free space a light wave travels at a speed,Typical values of n are 1.00 for air,1.33 for water,
15、1.50 for glass,And 2.42 for diamond.,18,2023/3/23,The Sch.Of Information Engineering,WHUT,2.Snell law,(1)According to the law of reflection,the angle at which the incident ray strikes the interface is exactly equal to the angle that the reflected ray makes with the same interface.(2)The incident ray
16、,the normal to the interface,and the reflected ray all line in the same plane,which is perpendicular to the interface plane between the two materials.This is call the plane incidence.,19,2023/3/23,The Sch.Of Information Engineering,WHUT,(3)External reflection:light traveling in a certain medium is r
17、eflected off an optically denser material(one with a higher refractive index).Internal reflection:the reflection of light off of less optically dense material(such as light traveling in glass being reflected at a glass-air interface).,20,2023/3/23,The Sch.Of Information Engineering,WHUT,2.2 The fabr
18、ication of optical fibers,2.2.1 Fiber Materials,2.2.2 fiber fabrication,21,2023/3/23,The Sch.Of Information Engineering,WHUT,2.2.1 Fiber Materials,1.The requirements of fiber materialsIt must be possible to make long,thin flexible fibers from the material.The material must be transparent at a partic
19、ular optical wavelength in order for the fiber to guide light efficiently.Physically compatible materials that have slightly different refractive indices for the core and cladding must be available.Materials that satisfy these requirements are glasses and plastics.,22,2023/3/23,The Sch.Of Informatio
20、n Engineering,WHUT,2.Glass fibersHigh-loss glass fibers with large cores used for short-transmission distancesVery transparent(low-loss)fibers employed in long-haul applicationsThe largest category of optically transparent glasses from which optical fibers are made consists of the oxide glasses.Of t
21、hese,the most common is silica(SiO2),which has a refractive index of 1.458 at 850nm.,23,2023/3/23,The Sch.Of Information Engineering,WHUT,To produce two similar materials that have slightly different indices of refraction for the core and cladding,either fluorine or various oxides(referred to as dop
22、ants),such as B2O3,GeO2,or P2O5,are added to the silica.The addition of GeO2 or P2O5 increases the refractive index,whereas doping the silica with fluorine or B2O3 decreases it,24,2023/3/23,The Sch.Of Information Engineering,WHUT,3.Halide glass fibers4.Active glass fibersIncorporating rare-earth ele
23、ments(atomic numbers 57-71)into a normally passive glass gibes the resulting material new optical and magnetic properties.These new properties allow the material to perform amplification,attenuation,and phase retardation on the light passing through it.Two commonly used materials for fiber lasers ar
24、e erbium and neodymiun.,25,2023/3/23,The Sch.Of Information Engineering,WHUT,5.Chalgenide glass fibersChalgenide glass is one candidate for these uses because of its high optical nonlinearity and its long interaction length.6.Plastic optical fibersHigh-bandwidth graded-index polymer(plastic)optical
25、fibers(POF)for use in a customer premises.Although they exhibit considerably greater optical signal attenuations than glass fibers,they are tough and durable,the core diameters of plastic fibers are 10-20 time larger,which allows a relaxation of connector tolerances without sacrificing optical coupl
26、ing efficiencies.,26,2023/3/23,The Sch.Of Information Engineering,WHUT,2.2.2 fiber fabrication,1.direct-melt methodsthis methods follows traditional glass-making procedures in that optical fibers are made directly from the molten state of purified components of silicate glasses.,27,2023/3/23,The Sch
27、.Of Information Engineering,WHUT,2.vapor-phase oxidation processHighly pure vapors of metal halides(eg.SiCl4 and GeCl4)react with oxygen to form a white powder of SiO2 particles.The particles are then collected on the surface of a bulk glass and are sintered(transformed to homogeneous glass mass by
28、heating without melting)by one of a variety techniques to form a clear glass rod or tube that is called a preform.It is typically around 10-25 mm in diameter and 60-120 cm long.The preform is precision-fed into a circular heater called the drawing furnace.Then the preform end is softened to the poin
29、t where it can drawn into a very thin filament,which becomes the optical fiber.An elastic coating is applied to the fiber in order to protect the bare glass fiber from external coontaminants,28,2023/3/23,The Sch.Of Information Engineering,WHUT,29,2023/3/23,The Sch.Of Information Engineering,WHUT,2.3
30、 The structure of an optical fiber,An optical fiber is a dielectric waveguide that operates at optical frequencies.This fiber waveguide is normally cylindrical in form.It confines electromagnetic energy in the form of light to within its surfaces and guides the light in a direction parallel to its a
31、xis.The transmission properties of an optical waveguide are dictated by its structural characteristics,which have a major effect in determining how an optical signal is affected as it propagates along the fiber.,30,2023/3/23,The Sch.Of Information Engineering,WHUT,2.3.1 fiber structure,1.core-claddi
32、ng structureThe most widely accepted structure is the single solid dielectric cylinder of radius a and index of refraction n1This cylinder is known as the core of the fiber.The core is surrounded by a solid dielectric cladding which has a refractive index n2 that is less than n1.,31,2023/3/23,The Sc
33、h.Of Information Engineering,WHUT,32,2023/3/23,The Sch.Of Information Engineering,WHUT,2.The purpose of using core-cladding structure:the cladding reduces scattering loss that results from dielectric discontinuities at the core surface.adds mechanical strength to the fiberprotects the core from abso
34、rbing surface contaminants with which it could come in contact.,33,2023/3/23,The Sch.Of Information Engineering,WHUT,2.3.2 fiber type,1.According the mode which propagate in the fiberSingle mode fiber(SM fiber)2a=212m a single mode optical source:LDMultimode fiber(MM fiber)2a=5100m optical source:LE
35、D the larger core radii of multimode fibers make it easier to launch optical power into the fiber and facilitate the connecting together of similar fibers,34,2023/3/23,The Sch.Of Information Engineering,WHUT,35,2023/3/23,The Sch.Of Information Engineering,WHUT,2.According to the refractive index of
36、the core,(1)Step-index fiber(SIOF),(2)Graded-index fiber(GIOF),The parameter is called the core-cladding index difference or simply the index difference.It is nominally 0.01.Typical values range from 1 to 3 percent for MM fibers and form 0.2 to 1.0 percent for SM fiber.,36,2023/3/23,The Sch.Of Infor
37、mation Engineering,WHUT,a is the core radius,the parameter determines the index profile:A step-index profile is approached in the limit of large A parabolic-index fiber corresponds to,37,2023/3/23,The Sch.Of Information Engineering,WHUT,2.4 The propagation principle of light along a fiber,Two method
38、 to study the propagation characteristics of light:Mode theoryGeometrical optics of ray-tracing approach,38,2023/3/23,The Sch.Of Information Engineering,WHUT,2.4.1 ray optics representation,1.The classification of rayMeridional ray they are confined to the meridian planes of the fiber,which are the
39、planes that contain the axis of symmetry of the fiber.meridional ray can be divided into two general classes:bound rays;unbound raysSkew ray they are not confined to a single plane,but instead tend to follow a helical-type path along the fiber.,39,2023/3/23,The Sch.Of Information Engineering,WHUT,40
40、,2023/3/23,The Sch.Of Information Engineering,WHUT,2.The analysis of meridional ray in SIOF,41,2023/3/23,The Sch.Of Information Engineering,WHUT,Refraction at the air-glass interface:Total internal reflection at the core-cladding interface ifDefine the numerical aperture(NA)of a step-index fiber for
41、 meridional ray,42,2023/3/23,The Sch.Of Information Engineering,WHUT,Multimode fibers suffer from modal dispersion.Shortest path length(along the fiber axis).Longest path length for the ray close to the critical angle.Pulse broadening:Modal dispersion:Limitation on the bit rateSingle-mode fibers ess
42、ential for high performace,43,2023/3/23,The Sch.Of Information Engineering,WHUT,3.The analysis of meridional ray in GIOF,Why intermodal or multipath dispersion is reduce for graded-index fibers.The trajectory of a paraxial ray is obtained by solvingFor All rays arrive simultaneously at periodic inte
43、rvalsLimitation on the bit rate:,44,2023/3/23,The Sch.Of Information Engineering,WHUT,2.4.2 mode theory for circular waveguides,45,2023/3/23,The Sch.Of Information Engineering,WHUT,Maxwell equation,46,2023/3/23,The Sch.Of Information Engineering,WHUT,1.The basic concept of mode,Solving Maxwells equa
44、tions subject to the cylindrical boundary conditions at the interface between the core and the cladding of the fiber.The basic equation of waving theory,Here is laplacian functor,47,2023/3/23,The Sch.Of Information Engineering,WHUT,Lengthways propagation factor:,Guide mode,radiation mode or cladding
45、 mode,Leaky mode,48,2023/3/23,The Sch.Of Information Engineering,WHUT,2.The mode in optical fiber,TE mode or TM mode correspond to meridional ray HE mode or EH mode correspond to skew ray TE mode:TM mode:HE or EH mode:,49,2023/3/23,The Sch.Of Information Engineering,WHUT,Weak guiding fiber and linea
46、rly polarized(LP)mode,and,if,is the main mode and no skew ray,The meaning of l and m in symbol,50,2023/3/23,The Sch.Of Information Engineering,WHUT,The V number is connect to the cutoff condition,When it realize that a single mode propagate in fiber,The V number can also be used to express the numbe
47、r of modesM in a multimode fiber when V is large.For this case,an estimate Of the total number of modes supported in a fiber is,51,2023/3/23,The Sch.Of Information Engineering,WHUT,2.5 the transmission character of fiber,What happen to signal fiber losses:limits the transmission distance chromatic d
48、ispersion:limits the bit rate through the pulse broadening nonlinear effect:distort the signal and limit the system performace,52,2023/3/23,The Sch.Of Information Engineering,WHUT,Signal degradation in optical fibersWhat are the loss or signal attenuation mechanism in a fiberWhy and to what degree d
49、o optical signals get distorted as they propagate long a fiber,53,2023/3/23,The Sch.Of Information Engineering,WHUT,2.5.1 attenuation,Attenuation of a light signal as it propagates along a fiber is an important consideration in the design of an optical communication system,since it plays a major rol
50、e in determining the maximum transmission distance between a transmitter an a receiver or an in-line amplifier.,54,2023/3/23,The Sch.Of Information Engineering,WHUT,1.Attenuation units,As light travels along a fiber,its power decreases exponentially with distance.If P(0)is the optical power in a fib
