7.8. LONG-HAUL LIGHTWAVE SYSTEMS 305
power was kept below 3 mW to reduce the effects of fiber nonlinearity [95]. In an-
other study, the amplifier spacing was found to play an important role; transmission
over 9000 km was feasible by keeping the amplifiers 40 km apart [98]. The choice of
the operating wavelength with respect to the zero-dispersion wavelength was also crit-
ical. In the anomalous-dispersion region (β 2 <0), the periodic variation of the signal
power along the fiber link can lead to the generation of additional sidebands through the
phenomenon ofmodulation instability[110]. This instability can be avoided if the dis-
persion parameter is relatively large [D>10 ps/(km-nm)]. This is the case for standard
fibers near 1.55μm. It should be remarked that the maximum transmission distance
depends critically on many factors, such as the FWM efficiency, the input power, and
the amplifier spacing, and may decrease to below 3000 km, depending on the operating
parameters [96].
The use of OPC for long-haul lightwave systems requires periodic use of optical
amplifiers and phase conjugators. These two optical elements can be combined into one
by usingparametric amplifiers, which not only generate the phase-conjugated signal
through the FWM process but also amplify it. The analysis of such a long-haul system
shows that 20- to 30-ps input pulses can travel over thousands of kilometers despite a
high GVD; the total transmission distance can exceed 15,000 km for dispersion-shifted
fibers withβ 2 =−2ps^2 /km near 1.55μm [111]. The phase-conjugation technique is
not used in practice as parametric amplifiers are not yet available commercially. The
next section focuses on the techniques commonly used for dispersion management in
long-haul systems.
7.8 Long-Haul Lightwave Systems.....................
This chapter has so far focused on lightwave systems in which dispersion management
helps to extend the transmission distance from a value of∼10 km to a few hundred
kilometers. The important question is how dispersion management can be used for
long-haul systems for which transmission distance is several thousand kilometers. If
the optical signal is regenerated electronically every 100–200 km, all techniques dis-
cussed in this chapter should work well since the nonlinear effects do not accumulate
over long lengths. In contrast, if the signal is maintained in the optical domain over the
entire link by using periodic amplification, the nonlinear effects such as SPM, cross-
phase modulation (XPM), and FWM [106] would limit the system ultimately. Indeed,
the impact of nonlinear effects on the performance of dispersion-managed systems has
been a subject of intense study [112]–[137]. In this section we focus on long-haul
lightwave systems in which the loss and dispersion-management schemes are used si-
multaneously.
7.8.1 Periodic Dispersion Maps
In the absence of the nonlinear effects, total GVD accumulated over thousands of kilo-
meters can be compensated at the receiver end without degrading the system perfor-
mance. The reason is that each optical pulse recovers its original position within the
bit slot for a linear system (except for the amplifier-induced timing jitter) even if it was