Optical Technologies
Fiber Optics
The History of Light Theory's
Fiber optics is a concept that amazes many people. To fully
understand the concept, the composition of light needs to be examined. Light has
been characterized by six major theories over the past 3,000 years. The six
theories are known as:
1. The tactile theory
2. The emission theory
3. The corpuscular theory
4. The wave theory
5. The electromagnetic theory
6. The quantum theory
The tactile theory was based on the ability to touch. The
theory stated that the eye sent out invisible probes to "feel"
objects. The emissions theory, however, was the opposite of the tactile theory.
The emissions theory stated that bright objects sent out beams or particles
that would ricochet off objects and enter the eye. The emission theory became
generally accepted over the tactile theory by the eleventh century.
The next two theories that manifested were the corpuscular
theory of Sir Isaac Newton and the wave theory of Christian Huygens. These
theories contradicted each other. Newton corpuscular theory stated that light
traveled in straight lines. He described light as a system of corpuscles or
particles that were emitted in all directions from a source in straight lines.
Newton's laws of physics were able to explain light reflecting off objects.
Huygens wave theory disagreed with Newton. Huygens argued
that if light were made of particles, then two crossed beams would cancel each
other. He compared this point to two streams of water interrupting each other.
Light, however, did not exhibit this phenomenon. Huygens proposed that light
was a wave. He described light and its actions completely based on a wave
theory. His description, on the other hand, presented Newton's original
objection that if light were truly a wave, light would bend around corners.
Modern research, however, has since shown that light can bend around corners,
but its wavelength is so short that it is difficult to detect under ordinary circumstances.
The wave theory held through the 1800s and the particle theory was relatively
ignored until about 1900.
At the end of the nineteenth century, James Clerk Maxwell
combined electricity, magnetism and light into one theory. He called his theory
the electromagnetic theory. According to Maxwell, light was an electromagnetic
wave. Because light was an electromagnetic wave, light carried the same
properties as the other electromagnetic waves. Maxwell was able to predict the
speed of light by using electrical and magnetic constants. His calculations
proved to be very close to the accepted value. Maxwell's theory, however, was
unable to explain the photoelectric effect.
In 1887, Heinrich Hertz discovered that electrons could be
emitted from metal if a certain light was allowed to shine on it. In 1900, Max
Planck described another theory about light. He suggested that light was
transmitted and absorbed in small bundles of energy. He called the bundles of
energy "quanta." The amount of energy in the quanta, he stated, was
proportional to the frequency of the light. Albert Einstein agreed with
Planck's theory and stated that light not only was transmitted and absorbed as
quanta, but also traveled as quanta. Einstein termed the units of quanta as
photons. In 1905, Einstein was able to explain the photoelectric effect using
the quantum theory. The quantum theory combines the two major theories of
light, which are the particle theory and the wave theory. This is necessary
because light does not always behave as a particle and light does not always
behave as a wave. Light is a form of energy. The unit of light energy is a
photon, which is a bundle of energy (quanta). Photons exist as a photon only
when they are in motion. The speed of light is 3 x 108 meters per second in a
vacuum.
One of the most important properties of light relating to
fiber optics is reflection. Newton's laws govern how light is reflected.
Newton's laws states "the angle of incidence will equal the angle of
reflection." Another very important property light exhibit is that of
refraction. Refraction results from light traveling through different media.
Light will speed up, or slow down depending on the medium it travels in. For
example, light travels faster in air than in glass. Due to light having to slow
down, light will bend towards the normal, which is the point at which the light
would intersect the medium at 90°, as it passes through the glass. The exact
opposite would occur for a light wave traveling through a slower medium to a
faster one.
A third property of light that is important to fiber optics
is recognizing the critical angle. The critical angle is the minimum angle at
which total internal reflection takes place. This recognized by Snell's Law.
Snell's Law gives n1· sin(q 1) = n2· sin(q 2) where n is the index of
refraction of that material and theta is the angle of the ray from the normal.
The critical angle then occurs when q 2 = 90°. The critical angle is the crux
of fiber optic technology.
Fiber optic technology is a result of light technology
evolving throughout the past few centuries. Light communication has been around
for a long time. Since early history, fire was used as a signaling device. As
society advanced, more sophisticated means of signaling came about. Major
advancements in fiber optic technology have taken place relatively recently,
although light experimentation began centuries ago.
Back in 1621, Willebrord Snell formulated his law on the
behavior of light. He realized light would refract as it entered one medium
from another. Alexander Graham Bell, in 1860, demonstrated voice transmission
using mirrors that were vibrated by the sound waves of the voice. The light
reflected off the mirrors was modulated by the sound. He was able to focus the
modulations onto a selenium plate, which caused resistance changes. These
changes occurred as the intensity of the light changed. From these changes, a
device similar to a speaker was activated. In 1870, John Tyndall experimented
with light in a stream of water. He concluded that light could propagate
through a medium. When in a medium, he realized that light could follow a
curved path as well as a straight line. In 1897, John William Strutt presented
some basic laws regarding light propagation. At the turn of the twentieth
century, Max Planck developed the quantum theory. He developed Planck's
constant, which relates electron and photon energy.
In 1905, Albert Einstein, using the quantum theory, was able
to explain the photoelectric effect. In 1930, Willis Lamb, Jr. started some of
the earliest experiments with guiding light in a glass fiber.
The Beginning of Fiber
In 1951, some American researchers were able to demonstrate
the transmission of an image through a bundle of glass fibers. With experiments
with light propagation in glass fibers well under way, Narinder Singh Kapany in
1953 developed fibers with cladding. The cladded fibers greatly improved
transmission characteristics. The cladding significantly reduced the amount of
dispersion of the light. The laser came about in 1960 being developed by
Theodore Maiman. Two years later, Maiman invented the semiconductor laser. The
laser was very important to advancing fiber optic technology because now it was
possible to have a coherent light source. A coherent light source consists of a
light beam of a single wavelength. In 1966, a collaboration effort between
Charles Kao and Charles Hockman proposed the use of fiber optics for
long-distance communication.
In 1970, Corning Glass Company produced the first set of
low-loss fibers. This began to revolutionize the fiber optic industry.
AT&T, in 1980, began the first major fiber optic communication link between
Boston, Massachusetts, and Richmond, Virginia. The next year, Corning Glass
Company modified their low loss fibers and came up with single-mode fibers with
high bandwidth and low loss capabilities to increase data transmission rates.
Starting in the mid-eighties, the major communication companies began
installing long-distance fiber optic communication links using single-mode
fibers.
In 1988, the first transatlantic fiber optic cable was
installed. Starting during the end of the eighties and working into the
nineties, cable providers began implementing widespread usage of fiber optic
networks. During the past few years cable companies began replacing their
coaxial cable lines with fiber optic cable.
Fiber Advantages
Fiber optic systems have many advantages over their
conventional coaxial and radio transmission counterparts. Fiber optic cable can
transmit at a higher capacity than conventional means. Current technology
allows fiber that can carry data at more than a gigabit-per-second. Laboratory
testing is being done on fibers allowing an increase of 10 times that amount.
The increased amount of data that can be transmitted means better service for
telecommunications, high-speed data communications, video, High Definition
Television, and other communication needs. This technology also offers new
services that require large bandwidths a low cost means of transmission. Fiber
optic technology offers higher reliability over other methods. Fiber optic
systems have very low bit error ratios on the magnitude of less than 1 x 10-11.
With fiber optics, there is no electromagnetic interference because the transmissions
all occur through light waves.
The transmission is also unaffected by rain, temperature and
humidity. Fiber optic cable is usually buried three to four feet underground.
This provides substantial protection from the elements. Fiber technology
survived hurricane Hugo in 1989. Telephone service was not completely disrupted
because the cables were buried underground and survived substantial flooding.
Fiber optic systems are more cost effective than
conventional means. Commercially available fiber optic systems can transmit
data for miles without requiring a repeater. A copper wire, however, requires
repeater spacing about every mile. On the horizon, repeater spacing is expected
to increase which will reduce the overall cost of the line and increase
reception quality.
Fiber optic systems also offer higher security. Fiber optic
transmissions neither induce nor emit any external energy. Because of its
security, fiber optic communication is virtually untappable if it is monitored
carefully. A signal loss can be detected almost immediately provided the system
is monitored. The military and banking applications are unlimited.
Fiber optics also provides a great size and weight advantage
over conventional means. For example, a shipboard advanced radar system
requires about 750 feet of coaxial cable, with a weight of seven tons and a
diameter of eighteen inches. These cables can be replaced by an optical fiber
weighing forty pounds and measuring one inch in diameter. This reduces the
weight of the ship above the water line.
Fiber optic systems also possess an unlimited growth
potential. The capacity of a fiber optic system is not limited by the fiber,
but rather by the equipment used to terminate the fiber. This enables the
system to expand as technology advances.
Fiber optic systems also possess a greater financial
benefit. The initial and maintenance costs of traditional copper systems are
higher than fiber systems. Copper is becoming a scarcer natural resource, and its
reserves are expected to be depleted within sixty years, whereas the primary
ingredient of fiber optics is silica, which is readily available. Fiber optic
systems can also transmit more data and at a faster rate with smaller wires and
fewer repeaters than a copper system. Fiber, because it is not metal, is
relatively unaffected by its environment. The fiber will not corrode in the
atmosphere. Using fiber optic cable also prevents any type of wear from an
electrical connection. The system is electrically separated from both ends. A
beam of light is the only bridge between the sender and the receiver. With its
superior quality and cost advantages, fiber optic systems will only be
implemented into more applications.
Fiber Optic System
A fiber optic system consists of five parts. The three main
parts of a system are transmit circuitry and light source, light detector and
receive circuitry, and fiber.
1. Transmit Circuitry and Light Source
The transmitter circuitry's main job is to convert the
electronic signal to modulate a light source. The transmitter circuitry codes
the information it receives and gives it to a light source for transmission.
There are generally two types of light sources. The light
source could either be a semiconductor diode laser or a light emitting diode
(LED). The application at hand will determine which type of light source to
use. LED's are relatively inexpensive transmitters with low output power. This
type of system is more appropriate for applications requiring only moderate data
rates (200 million bits per second), within a relatively short communication
distance. A semiconductor laser system is usually more expensive. This system
has a higher output power, which enables it to transmit high-speed data (1-2
billion bits per second), over a considerable distance. These two parts, the
transmitter circuitry, and the light source are usually packaged together and
commonly referred to as the transmitter. The transmitter provides an essential
part of the system because without it, the modulated light signal would not
occur.
2. Light detector and receiver circuitry
At the other end of the system are two other very important
components. Although they are similar to the components at the beginning of the
system, they have their own special characteristics. The light transmitted by
the light source needs to be received by something that can interpret the
signal. The equipment that can interpret the light signal is a photoelectric
light detector. This detector takes the modulated light signal and converts it
into an electronic form that the receiver circuitry can utilize. An example of
a simple detector is a solar cell. A solar cell can detect light and transform
it into an electric voltage. This type of detector is not used in sensitive applications
because it is slow and generally insensitive. The faster and more sensitive
detectors are electrically reverse-biased. This type of detector can be used to
handle faster applications like voice and video communication.
The other part of the system is referred to as the receiver
circuitry. The receiver circuitry takes the information from the photoelectric
detector and converts it back into the information that was inputted into the
system. This circuitry can also amplify the signal received. Often, a receiver
is normally coupled with an amplifier. The detector and the circuitry are also
usually packaged together to form the unit called the receiver.
3. Fiber
Tying together the two ends of the system, transmitters and
receivers, are the fibers. A fiber is made up of generally three parts, core,
cladding, and buffer coating.
The light transmitted is guided down the fiber by reflecting
off the outside of the core. The core's index of refraction is slightly higher
than that of the surrounding cladding to insure internal refraction. The core
is surrounded by optical material called the cladding. The cladding causes the
light to remain in the core. The core and the cladding are usually made of
ultra-pure glass, although some fibers are plastic. The materials need to be
ultra-pure because impurities in the material can lead to a reduction of power
output. Impurities can add to absorption and scattering, which would reduce the
effectiveness of the fiber. The buffer coating covers the core and the cladding.
The buffer coating is generally made of plastic, which protects the fiber from
moisture and other damage.
There are two types of optical fibers in common use,
multimode and singlemode fibers. Multimode fibers have a larger core, about
62.5 microns. This type of fiber is used with LED sources at wavelengths of
850-1300 nm, and is commonly used for local area networks (LANs). The
singlemode fiber has a much smaller core, about 9 microns. This type of fiber
is used with laser sources at wavelengths of 1300-1550 nm, and is usually used
for telephone and cable television applications. Due to the higher wavelength,
this cable is able to handle high data transmission speeds.
Handle With Care
Optical fibers are fragile and care must be taken when they
are installed. For this reason, optical fibers are mated with a cabling
structure to increase the mechanical characteristics of the fiber. The cables
need to protect the fibers from tensile, torsional, and bending stresses. These
stresses can occur at any time during the installation process or during normal
use. If a fiber is stretched too far, it can break. The glass fibers are also
very susceptible to twisting. Twisting a fiber could easily compromise its
integrity. The bending of fibers must also be watched. If a fiber is bent too
far, the optical characteristics of the fiber can be changed. Understanding the
causes of stress on the fiber enables the engineering of cables that can handle
the application. Depending on the application, the cable could consist of a
Kevlar coating or a simple thickened rubberized material.
The worst problems come from contractors with backhoes and
other earth-moving equipment, who dig up buried cables, applying sharp forces
and snapping the cables as well as from falling objects on aerial cables.
Although cable designs cannot prevent out of the ordinary mishaps from
occurring, with proper design considerations a cable strong enough for the
application intended can easily be manufactured.
Fiber optic cable is an ideal medium for transmitting
information. Fiber optics can provide faster data transmission at better
quality and at a lower cost that their conventional counterparts. They are also
considerably smaller and lighter than traditional means enabling usage in
locations which otherwise could not accept traditional wiring. Fiber optics is
playing an increasing role in the medical profession. Doctors are now able to
look inside a person's body through a small incision, whereas a few years ago
this procedure required a large operation.
Future of Fiber
Fiber technology is playing an increasing role in cable television systems. It will soon be possible for interactive voice and video communication using fiber optics. The speeds of data transmission required for such applications can only be achieved with fiber. The computer applications of fiber optic networks have only first begun to be utilized. Fiber linked networks will enable faster communication which will make downloading outdated. The Internet will be able to provide instant two-way communication. Fiber optics is being used in almost every industry from the military to banking to the automated industries. The potential applications of fiber optics are limited only by the imagination of the designer.
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