9.6. HIGH-SPEED SOLITON SYSTEMS 447
(^6040200)
(^2040600)
2000
4000
6000
8000
0
0.2
0.4
0.6
0.8
1
Distance (km
)
Time (ps)
Normalized Power
Figure 9.19: Evolution of a DM soliton over 8000 km with dense dispersion management (8
map periods per amplifier spacing). The map and pulse parameters correspond to those of
Fig. 9.16(b).
erate at 40 Gb/s if the average GVD is kept low. In a 1998 experiment, 40-Gb/s solitons
were indeed transmitted over 8600 km using a 140-km-long fiber loop with an aver-
age dispersion of only− 0 .03 ps^2 /km [160]. Interaction between solitons was the most
limiting factor in this experiment. As discussed later in this section, it can be reduced
by alternating the polarization of neighboring bits. Indeed, the use of this technique
permitted by 1999 the transmission of 40-Gb/s solitons over more than 10 Mm with-
out employing any jitter-control technique [161]. The use of synchronous modulation
allowed transmission of even 80-Gb/s solitons over 10 Mm [162]. Much higher capac-
ities can be realized using the combination of WDM and OTDM techniques [163]. In
a 2000 experiment, a single OTDM channel at a bit rate of 1.28 Tb/s was transmitted
over 70 km using 380-fs optical pulses [164]. The transmission distance of such sys-
tems is limited by fiber dispersion; it was necessary to compensate for dispersion up to
fourth order.
9.6.2 Soliton Interaction
The interaction among neighboring pulses becomes a critical issue as the bit rate in-
creases. The reason is that the bit slot becomes so small (only 10 ps at 100 Gb/s)
that one is often forced to pack the solitons closely. The interaction between two DM
solitons can be studied numerically or by using a variational technique [165]–[170].
The qualitative features are similar to those discussed in Section 9.2.2 for the standard
solitons. In general, the parameterq 0 , related to the bit rate asB=( 2 q 0 T 0 )−^1 , should
exceed 4 for the system to work properly.
A new feature of the interaction among DM solitons is that the collision length
depends on details of the dispersion map. As a result, soliton systems can be optimized
by choosing the pulse and the map parameters appropriately. Numerical simulations
show that the system performance is optimum when the map strength is chosen to