9.1.2 Bandwidth and Capacity of WDM Systems

WDM has the potential for exploiting the large bandwidth offered by optical fibers. Figure 9.2 shows the loss spectrum of a typical silica fiber and two low-loss transmission windows of optical fibers centered near 1.3 and 1.55 µm. Each of these spectral widows extends over more than 10 THz. If the so-called OH peak, resulting from residual water vapors trapped inside the core during manufacturing of silica fibers, can be eliminated using "dry" fibers, the entire spectral region extending from 1.25 to 1.65 µm can be exploited through WDM.

The ultimate capacity of WDM systems can be estimated by assuming that the 300-nm wavelength range extending from 1,300 to 1,600 nm is employed for transmission using dry fibers. The minimum channel spacing can be as small as 50 GHz (or 0.4 nm) for 40-Gb/s channels. Since 750 channels can be accommodated over the 300-nm bandwidth, the resulting capacity can be as large as 30 Tb/s. If we assume that such a WDM signal can be transmitted over 1,000 km using optical amplifiers with dispersion management, the NBL product can exceed 30,000 (Tb/s)-km with the use of WDM technology. This 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 10 Gb/s, resulting in NBL values of at most 0.8 (Tb/s)-km.

Figure 9.2: Typical loss spectrum of silica fibers and low-loss transmission windows (shaded regions) near 1.3 and 1.55 µm. The inset shows the basic idea behind WDM schematically.

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. The use of Raman amplification can solve the amplifier bandwidth problem to some extent. Among other factors that limit the number of channels are (1) wavelength stability and tunability of distributed feedback (DFB) lasers, (2) signal degradation during transmission because of various nonlinear effects, and (3) 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 1,000 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 [11]. 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 capacity of 11 Tb/s and a NBL product of 1.28 (Pb/s)-km [12]. Table 9.1 lists several WDM experiments in which the NBL product exceeded 1 Pb/s. In this table, OFC and ECOC stand, respectively, for the Optical Fiber Communication Conference and European Conference on Optical Communication, the two conferences where most record-breaking results are often presented.

Table 9.1: High-capacity WDM transmission experiments

Another group of WDM experiments is concerned with transmission distances of more than 5,000 km for submarine applications. In a 1996 experiment, 100-Gb/s transmission (20 channels at 5 Gb/s) over 9,100 km was realized using the polarization-scrambling and forward-error-correction techniques [13]. The number of channels was later increased to 32, resulting in a 160-Gb/s transmission over 9,300 km [9]. In a 2003 experiment, a 3.73-Tb/s WDM signal (373 channels, each operating at 10 Gb/s) was transmitted over 11,000 km, resulting in a NBL product of almost 41 (Pb/s)-km (see Table 9.1). This should be compared with the first fiber-optic cable laid across the Atlantic Ocean (TAT-8); it operated at 0.27 Gb/s with NBL ≈ 1.5 (Tb/s)-km. The use of WDM improved by 2003 the capacity of undersea systems by a factor of 27,000.

On the commercial side, WDM systems with a capacity of 40 Gb/s (16 channels at 2.5 Gb/s or 4 channels at 10 Gb/s) were available in 1996. The 16-channel system covered a wavelength range of about 12 nm in the 1.55-µm region with a channel spacing of 0.8 nm. WDM fiber links operating at 160 Gb/s (16 channels at 10 Gb/s) appeared in 1998. By 2001, WDM systems with a capacity of 1.6 Tb/s (realized by multiplexing 160 channels, each operating at 10 Gb/s) were available. This should be contrasted with the 10-Gb/s capacity of the third-generation systems available before the advent of the WDM technique. The use of WDM had improved by 2001 the capacity of commercial terrestrial systems by a factor of more than 6,000. Although systems with even higher capacities have been designed, they were not deployed commercially by 2004 because of the downturn in the telecommunication industry that began in 2001.