6.5. SYSTEM APPLICATIONS 271
Figure 6.24:Qfactor as a function of the average input power for a 9000-km fiber link: (a)
5-Gb/s system; (b) improvement realized with 150-GHz optical filters; (c) 6-Gb/s operation
near the zero-dispersion wavelength; (d) 10-Gb/s system with dispersion management. (After
Ref. [134];©c1996 IEEE; reprinted with permission.)
behavior can be understood by noting that as the input power increases, the system
performance improves initially because of a better SNR but becomes worse at high
input powers as the nonlinear effects (SPM) begins to dominate.
The role of dispersion can be minimized either by operating close to the zero-
dispersion wavelength of the fiber or by using a dispersion management technique in
which the fiber GVD alternates its sign in such a way that the average dispersion is
close to zero (see Chapter 7). In both cases, the GVD parameterβ 2 fluctuates because
of unintentional variations in the zero-dispersion wavelength of various fiber segments.
The curve (c) in Fig. 6.24 is drawn for a 6-Gb/s system for the case of a Gaussian
distribution ofβ 2 with a standard deviation of 0.3 ps^2 /km. The filter bandwidth is
taken to be 60 GHz [134]. The curve (d) shows the dispersion-managed case for a 10-
Gb/s system with a filter bandwidth of 50 GHz. All other parameters remain the same.
Clearly, system performance can improve considerably with dispersion management,
although the input pump power needs to be optimized in each case.
6.5.5 WDM-Related Impairments
The advantages of EDFAs for WDM systems were demonstrated as early as 1990 [149]–
[154]. In a 1993 experiment, four channels were transmitted over 1500 km using 22
cascaded amplifiers [150]. By 1996, 55 channels, spaced apart by 0.8 nm and each
operating at 20 Gb/s, were transmitted over 150 km by using two in-line amplifiers,
resulting in a total bit rate of 1.1 Tb/s and theBLproduct of 165 (Tb/s)-km [151].
For submarine applications, one needs to transmit a large number of channels over a
distance of more than 5000 km. Such systems employ a large number of cascaded
amplifiers and are affected most severely by the amplifier noise. Already in 1996,
transmission at 100 Gb/s (20 channels at 5 Gb/s) over a distance of 9100 km was pos-
sible using the polarization-scrambling and forward-error correction techniques [152].
By 2001, transmission at 2.4 Tb/s (120 channels at 20 Gb/s) over 6200 km has been
realized within the C band using EDFAs every 50 km [154]. The adjacent channels