384
10 Wavelength Division Multiplexing (WDM)
shows how WDM methodologies apply to several categories of communication
networks.
10.1 Concepts of WDM
The original optical fiber links that were deployed around 1980 consisted of simple
point-to-point connections. These links contained a single fiber with one light source
at the transmitting end and one photodetector at the receiving end. In these early
systems, signals from different light sources used separate and uniquely assigned
optical fibers. Because the spectral width of a typical laser source occupies only
a narrow slice of optical bandwidth, these simplex systems greatly underutilize the
large bandwidth capacity of a fiber. The first use of WDM was to upgrade the capacity
of installed point-to-point transmission links. This was achieved with wavelengths
that were separated from several tens up to 200 nm in order not to impose strict
wavelength-tolerance requirements on the different laser sources and the wavelengthseparating components at the receiving end.
With the advent of high-quality light sources with extremely narrow spectral
emission widths (less than 1 nm), many independent wavelength channels spaced
less than a nanometer apart could be placed simultaneously on the same fiber. Thus the
use of WDM allows a dramatic increase in the capacity of an optical fiber compared
to the original simple point-to-point link that carried only a single wavelength. For
example, if each wavelength supports an independent transmission rate of 10 Gb/s,
then each additional channel provides the fiber with significantly more capacity.
Another advantage of WDM is that the various optical channels can support different
transmission formats. Thus, by using separate wavelengths, differently formatted
signals at any data rate can be sent simultaneously and independently over the same
fiber without the need for a common signal structure.
10.1.1 WDM Operational Principles
A characteristic of WDM is that the discrete wavelengths form an orthogonal set
of carriers that can be separated, routed, and switched without interfering with each
other. This isolation between channels holds as long as the total optical power intensity is kept sufficiently low to prevent nonlinear effects such as stimulated Brillouin
scattering and four-wave mixing processes from degrading the link performance (see
Chap. 12).
The implementation of sophisticated WDM networks requires a variety of passive
and active devices to combine, distribute, isolate, and amplify optical power at
different wavelengths. Passive devices require no external control for their operation,
so they are somewhat limited in their application flexibility. These components are
mainly used to split and combine or tap off optical signals. The wavelength-dependent
10 Wavelength Division Multiplexing (WDM)
shows how WDM methodologies apply to several categories of communication
networks.
10.1 Concepts of WDM
The original optical fiber links that were deployed around 1980 consisted of simple
point-to-point connections. These links contained a single fiber with one light source
at the transmitting end and one photodetector at the receiving end. In these early
systems, signals from different light sources used separate and uniquely assigned
optical fibers. Because the spectral width of a typical laser source occupies only
a narrow slice of optical bandwidth, these simplex systems greatly underutilize the
large bandwidth capacity of a fiber. The first use of WDM was to upgrade the capacity
of installed point-to-point transmission links. This was achieved with wavelengths
that were separated from several tens up to 200 nm in order not to impose strict
wavelength-tolerance requirements on the different laser sources and the wavelengthseparating components at the receiving end.
With the advent of high-quality light sources with extremely narrow spectral
emission widths (less than 1 nm), many independent wavelength channels spaced
less than a nanometer apart could be placed simultaneously on the same fiber. Thus the
use of WDM allows a dramatic increase in the capacity of an optical fiber compared
to the original simple point-to-point link that carried only a single wavelength. For
example, if each wavelength supports an independent transmission rate of 10 Gb/s,
then each additional channel provides the fiber with significantly more capacity.
Another advantage of WDM is that the various optical channels can support different
transmission formats. Thus, by using separate wavelengths, differently formatted
signals at any data rate can be sent simultaneously and independently over the same
fiber without the need for a common signal structure.
10.1.1 WDM Operational Principles
A characteristic of WDM is that the discrete wavelengths form an orthogonal set
of carriers that can be separated, routed, and switched without interfering with each
other. This isolation between channels holds as long as the total optical power intensity is kept sufficiently low to prevent nonlinear effects such as stimulated Brillouin
scattering and four-wave mixing processes from degrading the link performance (see
Chap. 12).
The implementation of sophisticated WDM networks requires a variety of passive
and active devices to combine, distribute, isolate, and amplify optical power at
different wavelengths. Passive devices require no external control for their operation,
so they are somewhat limited in their application flexibility. These components are
mainly used to split and combine or tap off optical signals. The wavelength-dependent