310 CHAPTER 7. DISPERSION MANAGEMENT
played an important role, these experiments are thought to be making use of soliton
properties (see Chapter 9). The main limitation stems from a large pulse broadening in
the standard-fiber section of the dispersion map, resulting in the nonlinear interaction
between the neighboring overlapping pulses. Such nonlinear effects have been studied
extensively [127]–[134] and are referred to as theintrachanneleffects to distinguish
them from theinterchannelnonlinear effects that occur when pulses in two neighboring
channels at different wavelengths overlap in the time domain (see Section 8.3).
The origin of intrachannel nonlinear effects can be seen from Eq. (7.8.4) by consid-
ering three neighboring pulses and writing the total field asB=B 1 +B 2 +B 3. Equation
(7.8.4) then reduces to the following set of three coupled NLS equations [132]:
i∂B 1
∂z−
β 2
2∂^2 B 1
∂t^2+γ ̄[(|B 1 |^2 + 2 |B 2 |^2 + 2 |B 3 |^2 )B 1 +B^22 B∗ 3 ]= 0 , (7.8.12)i∂B 2
∂z−
β 2
2∂^2 B 2
∂t^2+γ ̄[(|B 2 |^2 + 2 |B 1 |^2 + 2 |B 3 |^2 )B 2 + 2 B 1 B∗ 2 B 3 ]= 0 , (7.8.13)i∂B 3
∂z−
β 2
2∂^2 B 3
∂t^2+γ ̄[(|B 3 |^2 + 2 |B 1 |^2 + 2 |B 2 |^2 )B 3 +B^22 B∗ 1 ]= 0. (7.8.14)The first nonlinear term corresponds to SPM. The next two terms result from XPM
induced by the other two pulses. The last term is FWM-like. Although it is common to
refer to its effect as intrachannel FWM, it is somewhat of a misnomer because all three
pulses have the same wavelength. Nevertheless, this term can create new pulses in the
time domain, in analogy with FWM that creates new waves in the spectral domain.
Such pulses are referred to as ghost pulses [128]. The ghost pulses can impact the
system performance considerably if they fall within the 0-bit time slots [134].
The intrachannel XPM affects only the phase but the phase shift is time dependent.
The resulting frequency chirp leads to timing jitter through fiber dispersion [130]. The
impact of intrachannel XPM and FWM on the system performance depends on the
choice of the dispersion map among other things [127]. In general, the optimization of
a dispersion-managed systems depends on many design parameters such as the launch
power, amplifier spacing, and the location of DCFs [129]. In a 2000 experiment, a
40-Gb/s signal was transmitted over transoceanic distances, in spite of its use of stan-
dard fibers, using the in-line synchronous modulation method originally proposed for
solitons [137]. Pseudolinear transmission of a 320-Gb/s channel has also been demon-
strated over 200 km of fiber whose dispersion of 5.7 ps/(km-nm) was compensated
using DCFs [138].
7.9 High-Capacity Systems.........................
Modern WDM lightwave systems use a large number of channels to realize a system
capacity of more than 1 Tb/s. For such systems, the dispersion-management technique
should be compatible with the broad bandwidth occupied by the multichannel signal.
In this section we discuss the dispersion-management issues relevant for high-capacity
systems.