342Transmission System Engineeringcurrent to be increased as the laser ages, inducing a small wavelength shift. Withtypical channel spacings of 100 GHz or thereabouts, this is not a problem, but withtighter channel spacings, it may be desirable to operate the laser in constant currentmode and tolerate the penalty (if any) due to the reduced output power.5.10Design of Soliton SystemsAlthough much of our discussion in this chapter applies to the design of solitonsystems as well, there are a few special considerations in the design of these systems,which we now brieﬂy discuss.We discussed the fundamentals of soliton propagation in Section 2.6. Solitonpulses balance the effects of chromatic dispersion and the nonlinear refractive indexof the fiber, to preserve their shapes during propagation. In order for this balance tooccur, the soliton pulses must have not only a specific shape but also a specific energy.
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