Chapter 7
Dispersion Management
It should be clear from Chapter 6 that with the advent of optical amplifiers, fiber losses
are no longer a major limiting factor for optical communication systems. Indeed, mod-
ern lightwave systems are often limited by the dispersive and nonlinear effects rather
than fiber losses. In some sense, optical amplifiers solve the loss problem but, at the
same time, worsen the dispersion problem since, in contrast with electronic regener-
ators, an optical amplifier does not restore the amplified signal to its original state.
As a result, dispersion-induced degradation of the transmitted signal accumulates over
multiple amplifiers. For this reason, several dispersion-management schemes were de-
veloped during the 1990s to address the dispersion problem [1]. In this chapter we
review these techniques with emphasis on the underlying physics and the improve-
ment realized in practice. In Section 7.1 we explain why dispersion management is
needed. Sections 7.2 and 7.3 are devoted to the methods used at the transmitter or re-
ceiver for managing the dispersion. In Sections 7.4–7.6 we consider the use of several
high-dispersion optical elements along the fiber link. The technique of optical phase
conjugation, also known as midspan spectral inversion, is discussed in Section 7.7.
Section 7.8 is devoted to dispersion management in long-haul systems. Section 7.9
focuses on high-capacity systems by considering broadband, tunable, and higher-order
compensation techniques. Polarization-mode dispersion (PMD) compensation is also
discussed in this section.
7.1 Need for Dispersion Management
In Section 2.4 we have discussed the limitations imposed on the system performance
by dispersion-induced pulse broadening. As shown by the dashed line in Fig. 2.13, the
group-velocity dispersion (GVD) effects can be minimized using a narrow-linewidth
laser and operating close to the zero-dispersion wavelengthλZDof the fiber. How-
ever, it is not always practical to match the operating wavelengthλ withλZD.An
example is provided by the third-generation terrestrial systems operating near 1.55μm
and using optical transmitters containing a distributed feedback (DFB) laser. Such
systems generally use the existing fiber-cable network installed during the 1980s and