s where /?2 (XJ and Xs are respectively expressed in ps2/km and nm. For the so called standard single mode fibers (SSF) at 1.55 pm, fi2(Xs) and D(XS) equal respectively -20 ps2/km and 17 ps/(nm.km). An important parameter of the fiber is the zero dispersion wavelength Xq, where P2(X)) cancels. For standard singlemode fibers, A<> is around 1.3 pm and dispersion is then much smaller around 1.3 jim, ensuring that propagating signals suffer less distortion, which explains why the 1.3 pm window has been used. But the counterpart is a larger attenuation than in the 1.55 pm window. Dispersion Shifted Fibers (DSF) exhibit around 1.55 pm a dispersion much smaller than standard fibers, typically less than 3.5 ps/(nm.km) in absolute value.
An impulse propagating along a fiber will then be distorted and broadened, which results into mutual overlapping when successive impulses are transmitted, and then into intersymbol interference, a universal impairement encountered in all the digital communication systems. Chromatic dispersion will then limit the bitrate for a given distance, or the distance for a given bitrate. Computer calculations are needed to get distortion suffered by the signal. Nevertheless, in the particular case of a gaussian impulse, computation can be carried out analytically. The envelope widens and remains gaussian, while the signal suffers also a phase modulation. The complex envelopes of the input and output signal after propagation over a length L of fiber are respectively expressed as [Agrawal, 1989] :
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