Chapter 9
Soliton Systems
The wordsolitonwas coined in 1965 to describe the particle-like properties of pulses
propagating in a nonlinear medium [1]. The pulse envelope for solitons not only prop-
agates undistorted but also survives collisions just as particles do. The existence of
solitons in optical fibers and their use for optical communications were suggested in
1973 [2], and by 1980 solitons had been observed experimentally [3]. The potential
of solitons for long-haul communication was first demonstrated in 1988 in an experi-
ment in which fiber losses were compensated using the technique of Raman amplifica-
tion [4]. Since then, a rapid progress during the 1990s has converted optical solitons
into a practical candidate for modern lightwave systems [5]–[9]. In this chapter we fo-
cus on soliton communication systems with emphasis on the physics and design of such
systems. The basic concepts behind fiber solitons are introduced in Section 9.1, where
we also discuss the properties of such solitons. Section 9.2 shows how fiber solitons
can be used for optical communications and how the design of such lightwave systems
differs from that of conventional systems. The loss-managed and dispersion-managed
solitons are considered in Sections 9.3 and 9.4, respectively. The effects of amplifier
noise on such solitons are discussed in Section 9.5 with emphasis on the timing-jitter
issue. Section 9.6 focuses on the design of high-capacity single-channel systems. The
use of solitons for WDM lightwave systems is discussed in Section 9.7.
9.1 Fiber Solitons
The existence of solitons in optical fibers is the result of a balance between the group-
velocity dispersion (GVD) and self-phase modulation (SPM), both of which, as dis-
cussed in Sections 2.4 and 5.3, limit the performance of fiber-optic communication
systems when acting independently on optical pulses propagating inside fibers. One
can develop an intuitive understanding of how such a balance is possible by following
the analysis of Section 2.4. As shown there, the GVD broadens optical pulses during
their propagation inside an optical fiber except when the pulse is initially chirped in the
right way (see Fig. 2.12). More specifically, a chirped pulse can be compressed during
the early stage of propagation whenever the GVD parameterβ 2 and the chirp parameter