464 CHAPTER 9. SOLITON SYSTEMS
Figure 9.27: Fraction of soliton energy in an FWM sideband during a single collision when the
exponential GVD profile is approximated by a staircase with two, three, and four steps. The
case of constant-dispersion fibers is shown for comparison. (After Ref. [222]);©c1996 OSA;
reprinted with permission.)
of the integermbecause a peak occurs wheneverLA= 2 πmLD/Ω^2 ch. Numerical simu-
lations consider 20-ps solitons in two channels, spaced 75 GHz apart. Clearly, FWM
can be nearly suppressed, with as few as three fiber sections, for an amplifier spacing
below 60 km. An experiment in 1996 achieved transmission of seven 10-Gb/s channels
over 9400 km using only four fiber segments in a recirculating fiber loop [224]. In a
1998 experiment, eight 20-Gb/s channels were transmitted over 10,000 km by using the
same four-segment approach in combination with optical filters and modulators [225].
Periodic Dispersion Maps
Similar to the single-channel soliton systems discussed in Section 9.4.2, periodic dis-
persion maps consisting of two fiber segments with opposite GVD benefit the WDM
soliton systems enormously. Issues such as interchannel collisions, timing jitter, and
optimum dispersion maps were studied extensively during the 1990s [226]–[250]. The
use of design optimization techniques has resulted in WDM soliton systems capable of
operating at bit rates close to 1 Tb/s [251]–[262].
An important issue for WDM systems making use of DM solitons is how a dis-
persion map consisting of opposite-GVD fibers affects interchannel collisions and the
timing jitter. It is easy to see that the nature of soliton collisions is changed drastically
in such systems. Consider solitons in two different channels. A shorter-wavelength
soliton travels faster in the anomalous-GVD section but slower in the normal-GVD
section. Moreover, because of high local GVD, the speed difference is large. Also,
the pulse width changes in each map period and can become quite large in some re-
gions. The net result is that two colliding solitons move in a zigzag fashion and pass
through each other many times before they separate from each other because of the