Optical Networks - _2_6 Solitons_28

Optical Networks - _2_6 Solitons_28 - 2.6 Solitons 99 ber....

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2.6 Solitons 99 fiber. Its effects in positive chromatic dispersion fiber can be minimized by using lower power levels. (In the next section, we will see that due to the same interaction between SPM and chromatic dispersion that causes modulation instability, a family of narrow, high-power pulses with specific shapes, called solitions, can propagate without pulse broadening.) WDM systems cannot operate around the zero-dispersion wavelength of the fiber due to the severity of four-wave mixing. For positive chromatic dispersion fiber, the dispersion zero lies below the 1.55 μ m band, and not in the L-band. Hence, systems using positive chromatic dispersion fiber can be upgraded to use the L-band (see Figure 2.2). This upgradability is an important feature for terrestrial systems. Thus, positive chromatic dispersion fiber is preferred for terrestrial systems, and the power levels are controlled so that modulation instability is not significant. For undersea links, however, the use of higher power levels is very important due to the very long link lengths. These links are not capable of being upgraded anyway—since they are buried on the ocean floor—so the use of the L-band in these fibers at a later date is not possible. Hence negative chromatic dispersion fiber is used for undersea links. Since negative chromatic dispersion fiber is used for undersea links, the chromatic dispersion can be compensated using standard single-mode fiber (SMF), which has positive chromatic dispersion. That is, alternating lengths of negative chromatic dispersion fiber and (positive chromatic dispersion) SMF can be used to keep the total chromatic dispersion low. This is preferable to using dispersion compensating fibers since they are more susceptible to nonlinear effects because of their lower effective areas. Note that all the fibers we have considered have positive chromatic dispersion slope; that is, the chromatic dispersion increases with increasing wavelength. This is mainly because the material dispersion slope of silica is positive and usually dom- inates the negative chromatic dispersion slope of waveguide dispersion (see Fig- ure 2.12). Negative chromatic dispersion slope fiber is useful in chromatic dispersion slope compensation, a topic that we discuss in Section 5.7.3. While it is possible
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Optical Networks - _2_6 Solitons_28 - 2.6 Solitons 99 ber....

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