《5single mode fiber1》.ppt

Contents Review: (1)conception and calculation of modes (V parameter) (2)what is modal dispersion and how to calculate the pulse spreading (3)how intermodal dispersion restricts bit rate and how to calculate the maximum bit rate (4) basic structure of GI fiber and mechanism (5) basic structure of Single mode fiber (6)what is chromatic dispersion The refractive index depends on wavelength; that is n=n(λ). (7) how bit rate and bandwidth defined and the relationship between dispersion and bit rate. BR1/(4 ? t) Specialized Words: intermodal (Modal) Dispersion chromatic Dispersion pulse-widening （spreading) GI fiber (graded-index) SI fiber (step-index) propagating transmission medium short- and intermediate-distance networks zero-order mode (fundamental mode) zero-dispersion wavelength manufactures line codes interval coefficient formula BR BW SM MM Chapter 5: Singlemode Fibers Key points of this class: （1） HOW A SINGLE MODE FIBER WORKS (2) Gaussian Beam (Gaussian model) (3) conception about MFD( mode field diameter) (4) what is cutoff wavelength? (5) attenuation (6)chromatic dispersion, especially material dispersion and waveguide dispersion Preface: why do we introduce singlemode fiber? Because we want to solve the problems of intermodal dispersion caused by multimode fibers. Firstly, it is difficult to fabricate the core with the diameter less than 10 μm, but with the development of the fabrication technology, this is not the problem any more. Now, singlemode fiber is taking a dominant role in the fiber-optic communication fields, and by estimates, 88% of all installed fibers are of the singlemode type. 5.1 HOW DOES A SINGLE MODE FIBER WORK? 5.1.1 reviewing : How to make a fiber that only support only 1 mode? Step 1: calculating the V parameter Step 2: using the appropriate formulas to get the number of modes: For a step-index fiber: N=V2/2 For a graded-index fiber: N=V2/4 A real single mode condition is