光纤通信课件2-optical-fiber概论.pdf

Electromagnetic Wave Propagation in Step-Index Fibers Two methods for analyzation:  Ray optics (Geometrical optics)：Omitting wavelength, focusing on rays trajectories that stand for the energy transmission ( Grasping NA)  Maxwell’s Equations (Will show in Chinese for most of the contents) 1. What is mode? Or how to understand mode? 2. Solving what? 1 BUPT-EE Ray optics (Geometrical optics) • V= C/ n n, Refractive index in medium n: 1.00 for air, 1.33 for water, 1.50 for glass, 2.42 for diamond • Snell’s Law n sin f = n sin f 1 1 2 2 Critical angle fc sin f = n /n c 2 1 Total reflection condition ： f fc 1 2 BUPT-EE Numerical aperture (Numerical aperture数值孔径) 0 2 2 NA= sin = n sin(90 - f)= n n 2  c max 1 1 －n2 = 1 = ( n －n ) / n Fractional index change at the core- 1 2 1 cladding interface 3 BUPT-EE The time delay ??? 4 BUPT-EE B=1/T (2.1.6) 一个常见的多模光纤系统的带宽-距离乘积约是500MHz ×km 5 BUPT-EE Wave propagation based on Maxwell’s Equations 可以证明：