Page 32 - Optical Communications Essentials

Optical Communication Systems Overview

22 Chapter Two

copper wires and made them highly attractive for simple on/off keyed links.
Included in these advantages are the following:

■ Long transmission distance. Optical fibers have lower transmission losses
compared to copper wires. This means that data can be sent over longer dis-
tances, thereby reducing the number of intermediate repeaters needed for
these spans. This reduction in equipment and components decreases system
cost and complexity.
■ Large information capacity. Optical fibers have wider bandwidths than copper
wires, which means that more information can be sent over a single physical
line. This property results in a decrease in the number of physical lines needed
for sending a certain amount of information.
■ Small size and low weight. The low weight and the small dimensions of fibers
offer a distinct advantage over heavy, bulky wire cables in crowded under-
ground city ducts or in ceiling-mounted cable trays. This also is of importance
in aircraft, satellites, and ships where small, lightweight cables are advanta-
geous, and in tactical military applications where large amounts of cable must
be unreeled and retrieved rapidly.
■ Immunity to electrical interference. An especially important feature of optical
fibers relates to the fact that they consist of dielectric materials, which means
they do not conduct electricity. This makes optical fibers immune to the elec-
tromagnetic interference effects seen in copper wires, such as inductive pickup
from other adjacent signal-carrying wires or coupling of electrical noise into
the line from any type of nearby equipment.
■ Enhanced safety. Optical fibers do not have the problems of ground loops,
sparks, and potentially high voltages inherent in copper lines. However, pre-
cautions with respect to laser light emissions need to be observed to prevent
possible eye damage.
■ Increased signal security. An optical fiber offers a high degree of data security,
since the optical signal is well confined within the fiber and any signal emis-
sions are absorbed by an opaque coating around the fiber. This is in contrast
to copper wires where electric signals often can be tapped off easily. This makes
fibers attractive in applications where information security is important, such
as in financial, legal, government, and military systems.

Now let us look at a very brief history of optical communications and how this
led to present-day systems. For an interesting detailed account of the development
of optical communications from ancient to modern times, the reader is referred
to the book City of Light by Jeff Hecht.

2.2. Evolution of Optical Communications
A challenge in using an optical fiber for a communications channel is to have a
flexible, low-loss medium that transfers a light signal over long distances without

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