Chapter 8
Multichannel Systems
In principle, the capacity of optical communication systems can exceed 10 Tb/s be-
cause of a large frequency associated with the optical carrier. In practice, however, the
bit rate was limited to 10 Gb/s or less until 1995 because of the limitations imposed by
the dispersive and nonlinear effects and by the speed of electronic components. Since
then, transmission of multiple optical channels over the same fiber has provided a sim-
ple way for extending the system capacity to beyond 1 Tb/s. Channel multiplexing
can be done in the time or the frequency domain through time-division multiplexing
(TDM) and frequency-division multiplexing (FDM), respectively. The TDM and FDM
techniques can also be used in the electrical domain (see Section 1.2.2). To make the
distinction explicit, it is common to refer to the two optical-domain techniques asop-
ticalTDM (OTDM) andwavelength-division multiplexing(WDM), respectively. The
development of such multichannel systems attracted considerable attention during the
1990s. In fact, WDM lightwave systems were available commercially by 1996.
This chapter is organized as follows. Sections 8.1–8.3 are devoted to WDM light-
wave systems by considering in different sections the architectural aspects of such sys-
tems, the optical components needed for their implementation, and the performance
issues such as interchannel crosstalk. In Section 8.4 we focus on the basic concepts
behind OTDM systems and issues related to their practical implementation. Subcarrier
multiplexing, a scheme in which FDM is implemented in the microwave domain, is
discussed in Section 8.5. The technique of code-division multiplexing is the focus of
Section 8.6.
8.1 WDM Lightwave Systems
WDM corresponds to the scheme in which multiple optical carriers at different wave-
lengths are modulated by using independent electrical bit streams (which may them-
selves use TDM and FDM techniques in the electrical domain) and are then transmitted
over the same fiber. The optical signal at the receiver is demultiplexed into separate
channels by using an optical technique. WDM has the potential for exploiting the large
bandwidth offered by optical fibers. For example, hundreds of 10-Gb/s channels can