8.1. WDM LIGHTWAVE SYSTEMS 331
Figure 8.1: Low-loss transmission windows of silica fibers in the wavelength regions near 1.3
and 1.55μm. The inset shows the WDM technique schematically.
be transmitted over the same fiber when channel spacing is reduced to below 100 GHz.
Figure 8.1 shows the low-loss transmission windows of optical fibers centered near 1.3
and 1.55μm. If the OH peak can be eliminated using “dry” fibers, the total capacity of
a WDM system can ultimately exceed 30 Tb/s.
The concept of WDM has been pursued since the first commercial lightwave sys-
tem became available in 1980. In its simplest form, WDM was used to transmit two
channels in different transmission windows of an optical fiber. For example, an ex-
isting 1.3-μm lightwave system can be upgraded in capacity by adding another chan-
nel near 1.55μm, resulting in a channel spacing of 250 nm. Considerable attention
was directed during the 1980s toward reducing the channel spacing, and multichannel
systems with a channel spacing of less than 0.1 nm had been demonstrated by 1990
[1]–[4]. However, it was during the decade of the 1990s that WDM systems were de-
veloped most aggressively [5]–[12]. Commercial WDM systems first appeared around
1995, and their total capacity exceeded 1.6 Tb/s by the year 2000. Several laboratory
experiments demonstrated in 2001 a system capacity of more than 10 Tb/s although
the transmission distance was limited to below 200 km. Clearly, the advent of WDM
has led to a virtual revolution in designing lightwave systems. This section focuses on
WDM systems by classifying them into three categories introduced in Section 5.1.
8.1.1 High-Capacity Point-to-Point Links
For long-haul fiber links forming the backbone or the core of a telecommunication
network, the role of WDM is simply to increase the total bit rate [14]. Figure 8.2 shows
schematically such a point-to-point, high-capacity, WDM link. The output of several
transmitters, each operating at its own carrier frequency (or wavelength), is multiplexed
together. The multiplexed signal is launched into the optical fiber for transmission to
the other end, where a demultiplexer sends each channel to its own receiver. WhenN