Page 260 - Optical Communications Essentials

Performance Impairments

250 Chapter Fifteen

than 0.306 of a bit period. If B is the bit rate and ∆λ is the spectral width of the
light source, then for a link of length L this limitation can be expressed as

D CD LB∆λ 0.306 (15.1)
As an example, if D CD 2ps/(km nm), B 2.5Gbps, and ∆λ 0.1nm, then the
maximum allowed link length is L 612km.
An important factor to remember when you are designing a WDM system is
that in order to mitigate some of the nonlinear effects described in Sec. 15.5, the
dispersion must be a positive value (or a negative value) across the entire spec-
tral band of the WDM system. As described in Sec. 4.8, this was the motivation
for designing the G.655 and G.655b optical fibers.

15.3. Dispersion Compensation
A large base of G.653 dispersion-shifted fiber has been installed throughout the
world for use in single-wavelength transmission systems. As described in Sec.
15.5, a nonlinear effect called four-wave mixing (FWM) can be a significant prob-
lem for these links when one attempts to upgrade them with high-speed dense
WDM technology in which the channel spacings are less than 100GHz and the
bit rates are in excess of 2.5Gbps. One approach to reducing the effect of FWM
is to use dispersion compensation techniques that negate the accumulated dis-
persion of the transmission fiber. Two possible methods are the insertion of a dis-
persion-compensating fiber into the link or the use of a chirped Bragg grating.

15.3.1. Dispersion-compensating fiber
A dispersion-compensating fiber (DCF) has a dispersion characteristic that is
opposite that of the transmission fiber. Dispersion compensation is achieved by
inserting a loop of DCF into the transmission path. The total dispersion in the
DCF loop needs to be equal and opposite to the accumulated dispersion in the
transmission fiber. If the transmission fiber has a low positive dispersion [say,
2.3ps/(nm km)], then the DCF will have a large negative dispersion [say,
90ps/(nm km)]. With this technique, the total accumulated dispersion is zero
after some distance, but the absolute dispersion per length is nonzero at all
points along the fiber. The nonzero absolute dispersion value causes a phase
mismatch between wavelength channels, thereby destroying the possibility of
effective FWM production.
Figure 15.2 shows that the DCF can be inserted at either the beginning or the
end of an installed fiber span between two optical amplifiers. A third option is
to have a DCF at both ends. In precompensation schemes the DCF is located
right after the optical amplifier and thus just before the transmission fiber. Con-
versely, in postcompensation schemes the DCF is placed right after the trans-
mission fiber and just before the optical amplifier. Figure 15.2 also shows plots
of the accumulated dispersion and the power level as functions of distance along
the fiber. These plots are called dispersion maps and power maps, respectively.

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