8.1. WDM LIGHTWAVE SYSTEMS 333
Table 8.1 High-capacity WDM transmission experimentsChannels Bit Rate Capacity Distance NBLProduct
N B(Gb/s) NB(Tb/s) L(km) [(Pb/s)-km]
120 20 2.40 6200 14.88
132 20 2.64 120 0.317
160 20 3.20 1500 4.80
82 40 3.28 300 0.984
256 40 10.24 100 1.024
273 40 10.92 117 1.278This should be contrasted with the third-generation commercial lightwave systems,
which transmitted a single channel over 80 km or so at a bit rate of up to 2.5 Gb/s,
resulting inBLvalues of at most 0.2 (Tb/s)-km. Clearly, the use of WDM has the po-
tential of improving the performance of modern lightwave systems by a factor of more
than 100,000.
In practice, many factors limit the use of the entire low-loss window. As seen in
Chapter 6, most optical amplifiers have a finite bandwidth. The number of channels is
often limited by the bandwidth over which amplifiers can provide nearly uniform gain.
The bandwidth of erbium-doped fiber amplifiers is limited to 40 nm even with the use
of gain-flattening techniques (see Section 6.4). The use of Raman amplification has
extended the bandwidth to near 100 nm. Among other factors that limit the number of
channels are (i) stability and tunability of distributed feedback (DFB) semiconductor
lasers, (ii) signal degradation during transmission because of various nonlinear effects,
and (iii) interchannel crosstalk during demultiplexing. High-capacity WDM fiber links
require many high-performance components, such as transmitters integrating multiple
DFB lasers, channel multiplexers and demultiplexers with add-drop capability, and
large-bandwidth constant-gain amplifiers.
Experimental results on WDM systems can be divided into two groups based on
whether the transmission distance is∼100 km or exceeds 1000 km. Since the 1985
experiment in which ten 2-Gb/s channels were transmitted over 68 km [3], both the
number of channels and the bit rate of individual channels have increased considerably.
A capacity of 340 Gb/s was demonstrated in 1995 by transmitting 17 channels, each
operating at 20 Gb/s, over 150 km [15]. This was followed within a year by several
experiments that realized a capacity of 1 Tb/s. By 2001, the capacity of WDM systems
exceeded 10 Tb/s in several laboratory experiments. In one experiment, 273 channels,
spaced 0.4-nm apart and each operating at 40 Gb/s, were transmitted over 117 km
using three in-line amplifiers, resulting in a total bit rate of 11 Tb/s and aBLproduct of
1300 (Tb/s)-km [16]. Table 8.1 lists several WDM transmission experiments in which
the system capacity exceeded 2 Tb/s.
The second group of WDM experiments is concerned with transmission distance
of more than 5000 km for submarine applications. In a 1996 experiment, 100-Gb/s
transmission (20 channels at 5 Gb/s) over 9100 km was realized using the polarization-
scrambling and forward-error-correction techniques [17]. The number of channels was