《光通信技术》课件：Lecture2new169.pptVIP

Fiber-Optic Communications TechnologyLecture 3X. WuDepartment of Optical Engineering ZJU TopicsLight traveling in fiberRectangular waveguide: an analogyCircular waveguideCut-off wavelengthGroup velocityDispersionDepartment of Optical Engineering ZJU Interesting Observation: when multimode fiber meets single mode fiberReduce end separation to observe output modesDepartment of Optical Engineering ZJUEnd separationMultimodeSingle mode Department of Optical EngineeringLight Propagation in WaveguidesMaxwell’s equations Rectangular waveguideDetermined by boundary conditionApply to z-component First find Ez and Hz, and you have all the rest componentsDepartment of Optical Engineering ZJUNo TEM: Ez=Hz=0, then Ex=Ey=Hx=Hy=0TE: Ez=0, E-field lines ? axis of waveguideTM: Hz=0, B-field lines ? axis of waveguide TE and TM modesTE: TM: Department of Optical Engineeringh2 h2 = TE and TM FieldFor TM modes, l=0 and m=0 are not possible, TM11 is the lowest TM mode. Department of Optical EngineeringCutoff frequency and wavelength?=?+j?h2= ?2＋k2， k＝?/vIf ?0, waveguide attenuation occurs, not support propagationCutoff frequencyCutoff occurs Evanescent: no propagationPropagation: allow traveling Department of Optical EngineeringCutoff frequency and wavelengthCutoff frequencyPhase constant With ?’= ?√(??)Three wavelengths: Unbounded wavelength ?=v/f: free space, no boundaryGuided wavelength ?g=?/[1-(f/fc)2]1/2 : Allow to travel !Cutoff wavelength ?c=v/fc : attenuation/evanescent Mode specific??c ? a /m or b/n How field looks between parallel platesTM modesm=1m=2m=3Department of Optical Engineering What about the field in circular waveguidesNo need to prove e??z term (why?)Again you only need to find Ez and HzLaplacian equationDepartment of Optical Engineering TE and TM in circular waveguideTE: E?(a,?,z)=0TM: Ez(a,?,z)=0Department o