446 CHAPTER 9. SOLITON SYSTEMS
In the case of terrestrial systems operating over standard fibers and dispersion-
managed using DCFs,|β 2 |exceeds 20 ps^2 /km in both sections of the dispersion map.
Consider, as an example, the typical situation in which the map consists of 60–70
km of standard fiber whose dispersion is compensated with 10–15 km of DCFs. The
parameterTmapexceeds 25 ps for such maps. At the same time, the map strength is
large enough that the pulse width oscillates over a wide range. These features make it
difficult to realize a bit rate of even 40 Gb/s although such a map permitted by 1997
transmission of four 20-Gb/s channels over 2000 km [149]. Numerical simulations
show the possibility of transmitting 40-Gb/s DM solitons over 2000 km of standard
fiber if the average GVD of the dispersion map is kept relatively low [150]. In a 1999
experiment, 40-Gb/s DM solitons were indeed transmitted over standard fibers but the
distance was limited to only 1160 km [151]. Interaction between solitons was the most
limiting factor in this experiment. By 2000, the use of highly nonlinear fibers together
with synchronous in-line modulation permitted transmission of 40-Gb/s solitons over
transoceanic distance [152].
To design high-speed soliton systems operating at 40 Gb/s or more, theTmappa-
rameter should be reduced to below 10 ps. If only a single-channel is transmitted,
one can use fibers with values of local GVD below 1 ps^2 /km. However, in the case
of WDM systems the GVD parameter should be relatively large (|β 2 |>4ps^2 /km) in
both the normal and anomalous-GVD fiber sections for suppressing the nonlinear ef-
fects such as cross-phase modulation (XPM) and four-wave mixing (FWM). It follows
from Eq. (9.4.11) that the map period should become smaller and smaller as the bit
rate increases. For example, when|β 2 |=5ps^2 /km in the two sections, the map period
should be less than 15 km for realizingTmap<6 ps. Such a map is suitable at bit rates
of 40 Gb/s with only 25-ps bit slot. As an example, Fig. 9.19 shows the evolution of a
DM soliton with the map used earlier for Figs. 9.15 and 9.16; the pulse parameters cor-
respond to those of Fig. 9.16(b). The system design employs 8 map periods per 80-km
amplifier spacing. Pulses at each amplifier maintain their shape even after 10 Mm even
though pulse width and chirp oscillate within each map period as seen in Fig. 9.16(b).
The map used for Fig. 9.19 is suitable for 40-Gb/s systems but would not work at a
bit rate of 100 Gb/s. A 100-Gb/s system has a bit period of only 10 ps and would require
a dispersion map withTmap<4 ps. Such systems require the use ofdenseorshort-
perioddispersion management, a scheme in which the map period is a small fraction
of the amplifier spacing [153]–[158]. Numerical simulation show the possibility of
transmission at bit rates as high as 320 Gb/s [157] if the map period is reduced to
below 1 km. Such fiber links may be hard to construct although a short-period DM
cable with the map period of 9 km has been made [159]. In this cable, the dispersion
map consisted of two 4.5-km fiber sections with GVD values of 17 and−15 ps/(km-
nm), resulting in an average dispersion of 1 ps/(km-nm). It was used to demonstrate
640-Gb/s WDM transmission (32 channels at 20 Gb/s) over 280 km. The parameter
Tmapis about 6 ps for this dispersion map and can be reduced further by decreasing the
local and the average GVD. In principle, such a fiber cable should be able to operate at
40 Gb/s per channel.
If a lightwave system is designed to support a single channel only, the dispersion
map can be made using low-GVD fibers. For example, whenβ 2 =± 0 .5ps^2 /km and
section lengths are nearly equal to 40 km,Tmap≈ 3 .2 ps. Such a soliton system can op-