Optical Networks - _5_11 Design of Dispersion-Managed Soliton Systems_66

Optical Networks - _5_11 Design of Dispersion-Managed Soliton Systems_66

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Unformatted text preview: 5.11 Design of Dispersion-Managed Soliton Systems 343 Here, the distance and time are measured in terms of the chromatic dispersion length of the fiber and the pulse width, respectively. The pulse U(, + )e i( t + 2 / 2 (5.29) is also a soliton for any frequency shift , and thus solitons can alter their frequency without affecting their shape and energy. Because of the chromatic dispersion of the fiber, however, changes in pulse fre- quencies are converted into changes in the pulse arrival times, that is, timing jitter. This jitter is called Gordon-Haus jitter, in honor of its discoverers, and is a significant problem for soliton communication systems. A potential solution to this timing jitter problem is the addition of a bandpass filter whose center frequency is close to that of the launched soliton pulse. In the presence of these filters, the solitons change their center frequencies to match the passband of the filters. For this reason, these filters are called guiding filters. This has the effect of keeping the soliton pulse frequencies stable, and hence minimizing the timing jitter. This phenomenon is similar to the solitons reshaping themselves when their shape is perturbed by the added ASE. The problem with the above solution is that the ASE noise accumulates within the passband of the chain of filters. As a result, the transmission length of the system, before the timing jitter becomes unacceptable, is only moderately improved compared to a system that does not use these filters. The solution to this problem is to change the center frequencies of the filters progressively along the link length. For example, if the filters are used every 20 km, each filter can be designed to have a center frequency that is 0.2 GHz higher than the previous one. Over a distance of 1000 km, this corresponds to a change of 10 GHz. The soliton pulses track the center frequencies of the filters, but the accumulation of ASE noise is lessened. This technique of using sliding-frequency guiding filters significantly minimizes timing jitter and makes transoceanic soliton transmission practical....
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Optical Networks - _5_11 Design of Dispersion-Managed Soliton Systems_66

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