光纤的色散与非线性效应幻灯片.ppt

光纤的色散与非线性效应
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随着脉冲在光纤中传输，脉冲的宽度被展宽
劣化的程度随数据速率的平方增大
决定了电中继器之间的距离
光纤的色散
模间色散(Mode Dispersion)
色度色散(Cromatic Dispersion)
偏振色散(Polarization Mode Dispersion)
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色散对传输的限制
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Material (Chromatic) Dispersion
This is caused by the fact that the refractive index of the glass we are using varies (slightly) with the wavelength. Some wavelengths therefore have higher group velocities and so travel faster than others. Since every pulse consists of a range of wavelengths it will spread out to some degree during its travel.
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Group Velocity Dispersion” (GVD)
Normal Dispersion Regime :the long wavelengths travel faster than the short ones! Thus after travelling on a fibre wavelengths at the red end of the pulse spectrum will arrive first. This is called a positive chirp!
Anomalous Dispersion Regime: the short wavelengths (blue end of the spectrum) travel faster than the long wavelengths (red end). After travel on a fibre the shorter wavelengths will arrive first. This is considered a negative chirp.
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Polarisation Mode Dispersion (PMD)
There is usually a very slight difference in RI for each polarisation. It can be a source of dispersion, usually less than .5 ps/nm/km.
The effect is to cause a circular or elliptical polarisation to form as the signal travels along the fibre.
Dispersion resulting from the birefringent properties of fibre is called “Polarisation Mode Dispersion” (PMD).
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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 electromagnetic 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 wavelength