光纖色散ppt課件-文庫吧資料
2025-05-18 07:07本頁面
【正文】 so travel faster than others. Since every pulse consists of a range of wavelengths it will spread out to some degree during its travel. Waveguide Dispersion ? The shape (profile) of the fibre has a very significant effect on the group velocity. This is because the amount that the fields overlap between core and cladding depends strongly on the wavelength. The longer the wavelength the further the the electromagic wave extends into the cladding. ? since a greater proportion of the wave at shorter wavelengths is confined within the core, the shorter wavelengths “see” a higher RI than do longer wavelengths. Therefore shorter wavelengths tend to travel more slowly than longer ones. Polarisation Mode Dispersion (PMD) ? There is usually a very slight difference in RI for each polarization. It can be a source of dispersion, usually less than ps/nm/km. ? The effect is to cause a circular or elliptical（ 橢圓的） polarization to form as the signal travels along the fiber. ? Dispersion resulting from the birefringent （ 雙折射的） properties of fiber is called “Polarization Mode Dispersion” (PMD). Calculating Dispersion ? In a typical singlemode fibre using a laser with a spectral width of 6 nm over a distance of 10 km : Dispersion = 17ps/nm/km 6 nm 10 km = 1020 ps ? At 1 Gbps a pulse is 1 ns long. So the system would not work. (20% is a good guideline for the acceptable limit.) But it would probably work quite well at a data rate of 155 Mbps (a pulse length of ns). ? A narrow spectral width laser might produce only one line with a linewidth of 300 MHz. Modulating it at 1 Gbps will add 2 GHz. 2,300 MHz is just less than .02 nm (at 1500 nm). So now: ? Dispersion = 17ps/nm/km .02 nm 10 km = ps ? In th
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