光纤通信技术,英文版,chapter.ppt

Chapter 3 Optical Fibers – Basics  Fiber Optic Communication Technology Contents How Optic fibers Conduct Light Attenuation Intermodal and Chromatic Dispersion Bit Rate and Bandwidth Reading a Data Sheet Optical Fiber structure 全反射，total internal reflection 形成全反射的条件？ Conditions of total internal reflection occurring? Figure: Photonic crystal fibers’ structures vary according to application: (a) highly nonlinear fiber; (b) endlessly single-mode fiber; (c) polarization maintaining fiber; (d) high NA fiber. Critical incident angle, and critical propagation angle 临界传播角αc=900-θc (临界入射角) To save light inside a strand of fiber, we need to have it strike the core-cladding boundary at the critical incident angle, θ1c, or above it, in order to provide total reflection of this light; to make light fall at or above that angle, we have to direct it so that it is at or below the critical propagation angle, αc , with respect to the centerline of the fiber , as we’ve already seen. Launching Light: Understanding Numerical Aperture Acceptance angle: (接受角) Principle: To save light inside a fiber all rays must propagate at critical angle or less. In order for us to maintain the light inside the fiber at this angle , we have to direct it from outside the fiber at θa or less. Exceeding acceptance angle 2θa is just beyond the requirement for having total reflection inside a fiber. Numerical aperture (数值孔径) NA=sinθa= Attenuation is the decrease in light power during light propagation along an optical fiber. Bending losses Scattering Absorption Macrobending Loss ( 宏弯损耗) Bending an optical fiber introduces a loss in light power, or attenuation. For example, one turn at a 32mm diameter mandrel causes a 0.5dB bending loss for one popular type of fiber. Manufacturers include the minimal bending radius in their data sheets 一般弯曲半径应超过光纤包层直径的150倍 Microbending loss (微弯损耗) Definition: Macrobending is loss caused by the curvature of the entire fiber axis. Microbending is loss ca