9.6. HIGH-SPEED SOLITON SYSTEMS 449
llelFigure 9.21: (a) Polarization multiplexing scheme and (b) the improvement realized with its use
for a 40-Gb/s soliton system. (After Ref. [161];©c1999 IEEE; reprinted with permission.)
are used since the polarization state of light changes randomly because of birefrin-
gence fluctuations resulting in polarization-mode dispersion (PMD). It turns out that
even though the polarization states of the bit train does change in an unpredictable
manner, the orthogonal nature of any two neighboring bits is nearly preserved. Be-
cause of this orthogonality, the interaction among solitons is much weaker compared
with the copolarized-solitons case. Figure 9.21 shows how the reduced interaction low-
ers the bit-error rate (BER) and increases the transmission distance of a 40-Gb/s soliton
system.
The use of polarization multiplexing helps to increase the bit rate as solitons can
be packed more tightly because of reduced interaction among them [172]. Its imple-
mentation is not difficult in practice when the OTDM technique is used. It requires
the generation of two bit streams using orthogonally polarized optical carriers and then
interleave them using an optical delay line (see Section 8.4). A polarizing beam split-
ter can be used in combination with a polarization controller to demultiplex the two
orthogonally polarized channels at the receiver end.
An important factor limiting the performance of polarization-multiplexed soliton
systems is the PMD induced by random changes in the fiber birefringence [177]. In
fact, PMD seriously limits the use of this technique for nonsoliton systems through
pulse depolarization (different parts of the pulse have different polarizations). The sit-
uation is different for solitons which are known to be much more robust to the PMD
effects [180]. The natural tendency of a soliton to preserve its integrity under vari-
ous perturbations also holds for perturbations affecting its state of polarization. Unlike
linear pulses, the state of polarization remains constant across the entire soliton (no
depolarization across the pulse), and the effect of PMD is to induce a small change in
the state of polarization of the entire soliton (a manifestation of its particle-like nature).
Such resistance of solitons to PMD, however, breaks down for large amounts of PMD.
The breakdown occurs forDp> 0. 3 D^1 /^2 [181], whereDpis the PMD parameter intro-
duced in Section 2.3.5 and expressed in ps/
√
km andDis the dispersion parameter in
units of ps/(nm-km). Since typicallyDp< 0 .1 ps/
√
km for high-quality optical fibers,
Dmust exceed 0.06 ps/(nm-km). In the case of DM solitons,Dcorresponds to the
average GVD of the link.