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1. INTRODUCTION
In recent years it has become apparent that fiber-optics
are steadily replacing copper wire as an appropriate
means of communication signal transmission. They span
the long distances between local phone systems as well
as providing the backbone for many network systems.
Other system users include cable television services,
university campuses, office buildings, industrial plants,
and electric utility companies. This is an overview
of fiber-optic technology, telecommunication applications,
fiber-optic advantages and disadvantages, and fiber-optic
economics.
2. FIBER-OPTIC TECHNOLOGY
A fiber-optic system is similar to the copper wire
system that fiber-optics is replacing. The difference
is that fiber-optics use light pulses to transmit information
down fiber lines instead of using electronic pulses
to transmit information down copper lines. Looking at
the components in a fiber-optic chain will give a better
understanding of how the system works in conjunction
with wire based systems.
At one end of the system is a transmitter. This is
the place of origin for information coming on to fiber-optic
lines. The transmitter accepts coded electronic pulse
information coming from copper wire. It then processes
and translates that information into equivalently coded
light pulses. A light-emitting diode (LED) or an injection-laser
diode (ILD) can be used for generating the light pulses.
Using a lens, the light pulses are funneled into the
fiber-optic medium where they transmit themselves down
the line.
Light pulses move easily down the fiber-optic line
because of a principle known as total internal reflection.
"This principle of total internal reflection states
that when the angle of incidence exceeds a critical
value, light cannot get out of the glass; instead, the
light bounces back in. When this principle is applied
to the construction of the fiber-optic strand, it is
possible to transmit information down fiber lines in
the form of light pulses.
There are generally five elements that make up the
construction of a fiber-optic strand, or cable: the
optic core, optic cladding, a buffer material, a strength
material and the outer jacket (Fig. 1). The optic core
is the light carrying element at the center of the optical
fiber. It is commonly made from a combination of silica
and germania. Surrounding the core is the optic cladding
made of pure silica . It is this combination that makes
the principle of total internal reflection possible.
The difference in materials used in the making of the
core and the cladding creates an extremely reflective
surface at the point in which they interface. Light
pulses entering the fiber core reflect off the core/cladding
interface and thus remain within the core as they move
down the line.
Fig. 1. Cut away of a fiber-optic cable.
Surrounding the cladding is a buffer material used
to help shield the core and cladding from damage. A
strength material surrounds the buffer, preventing stretch
problems when the fiber cable is being pulled. The outer
jacket is added to protect against abrasion, solvents,
and other contaminants.
Once the light pulses reach their destination they
are channeled into the optical receiver. "The basic
purpose of an optical receiver is to detect the received
light incident on it and to convert it to an electrical
signal containing the information impressed on the light
at the transmitting end. The electronic information
is then ready for input into electronic based communication
devices, such as a computer, telephone, or TV.
3. FIBER-OPTIC APPLICATIONS
The use of fiber-optics was generally not available
until 1970 when Corning Glass Works was able to produce
a fiber with a loss of 20 dB/km. It was recognized that
optical fiber would be feasible for telecommunication
transmission only if glass could be developed so pure
that attenuation would be 20dB/km or less. That is,
1% of the light would remain after traveling 1 km. Today's
optical fiber attenuation ranges from 0.5dB/km to 1000dB/km
depending on the optical fiber used. Attenuation limits
are based on intended application.
The applications of optical fiber communications have
increased at a rapid rate, since the first commercial
installation of a fiber-optic system in 1977. Telephone
companies began early on, replacing their old copper
wire systems with optical fiber lines. Today's telephone
companies use optical fiber throughout their system
as the backbone architecture and as the long-distance
connection between city phone systems.
Cable television companies have also began integrating
fiber-optics into their cable systems. The trunk lines
that connect central offices have generally been replaced
with optical fiber. Some providers have begun experimenting
with fiber to the curb using a fiber/coaxial hybrid.
Such a hybrid allows for the integration of fiber and
coaxial at a neighborhood location. This location, called
a node, would provide the optical receiver that converts
the light impulses back to electronic signals. The signals
could then be fed to individual homes via coaxial cable.
Local Area Networks (LAN) is a collective group of
computers, or computer systems, connected to each other
allowing for shared program software or data bases.
Colleges, universities, office buildings, and industrial
plants, just to name a few, all make use of optical
fiber within their LAN systems.
Power companies are an emerging group that have begun
to utilize fiber-optics in their communication systems.
Most power utilities already have fiber-optic communication
systems in use for monitoring their power grid systems.
4. FIBER-OPTIC ADVANTAGES AND DISADVANTAGES
There are several advantages that have been established
with the development and implementation of fiber-optic
cable systems. Compared to copper, optical fiber is
relatively small in size and light in weight. This characteristic
has made it desirable as intra-floor conduits and wiring
duct space has become increasing plugged with expanded
copper cable installation .
Optical fiber is also desirable because of it's electromagnetic
immunity. Since fiber-optics use light to transmit a
signal, it is not subject to electromagnetic interference,
radio frequency interference, or voltage surges. This
may be an important consideration when laying cables
near electronic hardware such as computers or industrial
equipment. As well, since it does not use electrical
impulses, it does not produce electric sparks which
can be an obvious fire hazard.
Advances in optical fiber technology has lead to decreases
in signal loss, or attenuation. As an electric pulse
or a light pulse travels down it's respective cable
line, it will eventually lose signal energy due to imperfections
in the transmission medium. To keep the signal going,
it must be boosted every so often along the medium line.
A signal regenerator is used to boost the electronic
pulse in a copper cable. An optical repeater is used
to boost the light pulse in a fiber-optic cable. The
advantage of optical fiber is that it performs better
with respect to attenuation. Fiber-optic cable needs
fewer boosting devices, along the same length of line,
than copper cable.
A characteristic feature of optical fiber that has
yet to be fully realized is it's potentially wide bandwidth.
Bandwidth refers to the amount of information that a
fiber can carry. The greater the bandwidth, the greater
the carrying capacity of the optical fiber. It is said
that currently, the fastest fibre circuits used in trunk
connections between cities and countries carry information
at up to 2.5 gigabits per second, enough to carry 40,000
telephone conversations or 250 television channels.
Experts predict larger bandwidths than this as light
frequency separation becomes available. Private communication
systems are already using much higher bandwidths.
A disadvantage of the fiber-optic system is it's incompatibility
with the electronic hardware systems that make up today's
world. This inability to interconnect easily requires
that current communication hardware systems be somewhat
retrofitted to the fiber-optic networks. Much of the
speed that is gained through optical fiber transmission
can be inhibited at the conversion points of a fiber-optic
chain. When a portion of the chain experiences heavy
use, information becomes jammed in a bottleneck at the
points where conversion to, or from, electronic signals
is taking place. Bottlenecks like this should become
less frequent as microprocessors become more efficient
and fiber-optics reach closer to a direct electronic
hardware interface.
5. FIBER-OPTIC ECONOMICS
One of the initial economic factors to consider when
converting to fiber-optics is the cost of replacing
wire systems with fiber. Increased demand for optical
fiber has brought the prices down within competitive
range of copper. Cable sales are expected to increase.
However, since transmitters, converters, optical repeaters,
and a variety of connecting hardware will be needed,
the initial cost of changing over to fiber can be expensive.
Increased demand, advances in the technology, and competition
has brought the prices down somewhat.
Short term and long term gains should be considered
when updating a communications system. In the short
term it is often less expensive to continue using copper
cabling for covering expanded communication needs. By
simply adding more wire to an existing system, expanded
needs can be covered. This avoids the expense of adding
the transmitters and receivers needed for integrating
optical fiber. Long term needs, however, may require
more expansion in the future.
In the long term it may be more cost effective to invest
in conversion to fiber-optics. This cost effectiveness
is due to the relative ease of upgrading fiber optics
to higher speeds and performance. It has already been
seen in the industry as communication providers are
wiring customers with optical fiber bandwidth that exceed
consumer bandwidth needs. This is in anticipation of
future bandwidth needs. It is generally accepted that
customers will need increased bandwidth as the information
highway grows. Replacing copper with fiber today would
avoid continued investment in a soon to be outdated
copper system.
Recent changes in the laws regulating the telecommunications
industry have helped to promote and spur the use of
fiber optics. The passage of the Telecommunications
Act of 1996 has helped this effort by allowing television
and telephone companies to enter each others markets.
Fiber optics will play a pivotal role in this race since
the bandwidth needed for providing an all-in-one service
with television, telephone, interactive multimedia,
and internet access is not available in much of the
wiring of America.
6. SUMMARY
Based on industry activity, it is evident that fiber-optics
have become the industry standard for terrestrial transmission
of telecommunication information. The choice is not
whether to convert to optical fiber, but rather when
to convert to optical fiber. The bandwidth needs of
the Information Superhighway require a medium, like
optical fiber, that can deliver large amounts of information
at a fast speed. It will be difficult for copper cable
to provide for future bandwidth needs. Satellite and
other broadcast media will undoubtedly play a role alongside
fiber optics in the new world telecommunications order.
Considering all the services that the telecommunications
industries are announcing to be just around the corner,
and a modern society that seems to be expecting them,
it is evident that fiber optics will continue to be
a major player in the delivery of these services.
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Fiber Optic Basics