9.4. DISPERSION-MANAGED SOLITONS 427
Figure 9.14: Recirculating-loop configuration used in a 1991 experiment for transmitting soli-
tons over 12,000 km. (After Ref. [74];©c1991 IEE; reprinted with permission.)
become quite common for both the soliton and nonsoliton systems. Its revival was
possible because of the technological advances in the fields of semiconductor and fiber
lasers, both of which can provide power levels in excess of 500 mW. The use of
dispersion management also helps in reducing the timing jitter. We turn to dispersion-
managed solitons next.
9.4 Dispersion-Managed Solitons
As discussed in Chapter 7, dispersion management is employed commonly for mod-
ern wavelength-division multiplexed (WDM) systems. It turns out that soliton sys-
tems benefit considerably if the GVD parameterβ 2 varies along the link length. This
section is devoted to such dispersion-managed solitons. We first consider dispersion-
decreasing fibers and then focus on dispersion maps that consist of multiple sections of
constant-dispersion fibers.
9.4.1 Dispersion-Decreasing Fibers
An interesting scheme proposed in 1987 relaxes completely the restrictionLALD
imposed normally on loss-managed solitons, by decreasing the GVD along the fiber
length [75]. Such fibers are calleddispersion-decreasingfibers (DDFs) and are de-
signed such that the decreasing GVD counteracts the reduced SPM experienced by
solitons weakened from fiber losses.
Since dispersion management is used in combination with loss management, soli-
ton evolution in a DDF is governed by Eq. (9.3.6) except that the second-derivative
term has a new parameterdthat is a function ofξbecause of GVD variations along
the fiber length. The modified NLS equation takes the form
i∂v
∂ξ+
1
2
d(ξ)∂^2 v
∂τ^2+p(ξ)|v|^2 v= 0 , (9.4.1)