448 CHAPTER 9. SOLITON SYSTEMS
-200 -150 -100 -50 0 50 100 150 20080006000400020000Distance (km)-200 -150 -100 -50 0 50 100 150 20080006000400020000Time (ps)Distance (km)Figure 9.20: Propagation of a 32-bit soliton stream over 8000 km when the input pulse parame-
ters correspond to those used in Fig. 9.16. Pulse energy equals 0.1 pJ in the top panel but has its
optimum for the bottom panel.
be around 1.6 [165]. This value corresponds to a pulse energy in the vicinity of the
minimum seen in Fig. 9.15. As an example, Fig. 9.20 shows propagation of a pulse train
consisting of a 32-bit pattern over 8000 km forE 0 = 0 .1 pJ (top panel) and its optimum
value (bottom panel). The periodic evolution of the chirp and pulse width in these two
cases is shown in Fig. 9.16. When the pulse energy is larger than the optimum value
(higher map strength), solitons begin to collide after 3000 km. In contrast, solitons can
propagate more than 8000 km before colliding when the pulse parameters are suitably
optimized.
A new multiplexing technique, called intrachannel polarization multiplexing, can
be used to reduce interaction among solitons. This technique is different from the
conventional polarization-division multiplexing in which two neighboring channels at
different wavelengths are made orthogonally polarized. In the case of intrachannel
polarization multiplexing, the bits of a single-wavelength channel are interleaved in
such a way that any two neighboring bits are orthogonally polarized. The technique was
used for solitons as early as 1992 and has been studied extensively since then [171]–
[179].
Figure 9.21 shows the basic idea behind polarization multiplexing schematically.
At first glance, such a scheme should not work unless polarization-maintaining fibers