9.7. WDM SOLITON SYSTEMS 467
Figure 9.29: Maximum transmission distance as a function of local dispersion for a WDM
soliton system with 40-Gb/s channels spaced apart by 100 GHz. The curves with labels “In-
teractions” and “XPM” show limitations due to intrachannel and interchannel pulse interac-
tions, respectively. The PIM curves correspond to the case of polarization multiplexing. (After
Ref. [248]);©c2001 OSA; reprinted with permission.)
be achieved for all channels because of dispersion-slope compensation realized using
reverse-dispersion fibers. In a 2001 experiment, a system capacity of 2.56 Tb/s was
realized (32 channels at 80 Gb/s) by interleaving two orthogonally polarized 40-Gb/s
WDM pulse trains but the transmission distance was limited to 120 km [262]. The
use of polarization multiplexing in combination with the carrier-suppressed RZ format
permitted a spectral efficiency of 0.8 (b/s)/Hz in this experiment.
Many experiments have focused on soliton systems for transoceanic applications.
The total bit rate is lower for such systems because of long distances over which soli-
tons must travel. Transmission of eight channels at 10-Gb/s over transoceanic distances
was realized as early as 1996 [205]. Eight 20-Gb/s channels were transmitted in a
1998 experiment but the distance was limited to 4000 km [254]. By 2000, the 160-
Gb/s capacity was attained by transmitting eight 20-Gb/s channels over 10,000 km
using optical filters and synchronous modulators inside a 250-km recirculating fiber
loop [258]. It was necessary to use a polarization scrambler and a phase modulator
at the input end. The 160-Gb/s capacity was also realized using two 80-Gb/s chan-
nels. In another experiment, up to 27 WDM channels were transmitted over 9000 km
using a hybrid amplification scheme in which distributed Raman amplification (with
backward pumping) compensated for nearly 70% of losses incurred over the 56-km
map period [259]. In general, the use of distributed Raman amplification improves the
system performance considerably as it reduces the XPM-induced interactions among
solitons [248]. These experiments show that the use of DM solitons has the potential of
realizing transoceanic lightwave systems capable of operating with a capacity of 1 Tb/s
or more.