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Fibre optics is a branch of
physics based on the transmission of light through transparent fibres of glass
or plastic. These optical fibres can
carry light over distances ranging from a few centimetres to more than 160
kilometres. Such fibres work
individually or in bundles. Some
individual fibres measure less than 0.004 millimetres in diameter. The principle on which this
transmission of light depends is that of total internal reflection: Light travelling
inside the fibre center, or core, strikes the outside surface at an
angle of incidence greater than the critical angle, so that all the light is
reflected toward the inside of the fibre without loss. Thus light can be
transmitted over long distances by being reflected inward thousands of times.
In order to avoid losses through the scattering of light by impurities on the
surface of the fibre, the optical fibre core is clad with a glass layer of much
lower refractive index; the reflections occur at the interface of the glass fibre
and the cladding. Optical fibres have a highly
transparent core of glass or plastic surrounded by a covering called a
cladding. Light from a laser, a light
bulb, or some other source enters one end of the optical fibre. As the light travels through the core, it is
typically kept inside it by the cladding.
The cladding bends or reflects inward, light rays that strike its inside
surface. At the other end of the fibre,
the light is received by a detector, such as a photosensitive device or the
human eye. Kinds of optical fibres. There are two basic kinds of
optical fibres--single-mode fibres and multi-mode fibres. Single-mode fibres are used for
long-distance transmissions. They have
extremely small cores, and they accept light only along the axis of the
fibres. As a result, single-mode fibres
require the use of special lasers as a light source, and they need to be
precisely connected to the laser, to other fibres in the system, and to the
detector. Multi-mode fibres have cores
larger than those of single-mode fibres, and they accept light from a variety
of angles. Multi-mode fibres can use
more types of light sources and cheaper connectors than can single-mode fibres,
but they cannot be used over long distances.
Uses of optical fibres. Optical fibres have a number of
uses. In fibre-optic communication
systems, special lasers transmit coded messages by flashing on and off at
extremely high speeds. The messages
travel through optical fibres to interpreting devices that decode the messages,
converting them back into the original signal.
Fibre-optic communication systems
have a number of features that make them superior to systems that use
traditional copper cables. They have a
much larger information-carrying capacity and are not subject to electrical
interference. Signals sent over
long-distance fibre-optic cables need less amplification than do signals sent
over copper cables of equal length.
Many communication companies have installed large networks of
fibre-optic cables. Underwater
fibre-optic cables carry signals across the Atlantic and Pacific oceans. In February 1996 Fujitsu Ltd.,
Nippon Telephone and Telegraph Corporation, and a team of researchers from
AT&T succeeded in transmitting information through an optical fibre at a
rate of 1 trillion bits per second—the equivalent of transmitting 300 years of
newspapers in a single second. This was accomplished by simultaneously sending
different wavelengths of light, each carrying separate information, through the
optical fibre. If it can be integrated into a network, this new technology will
make it easy, inexpensive, and incredibly fast to send information, such as
video and memory-sensitive three-dimensional images. One growing application of
optical fibres is in communication. Because the information-carrying capacity of a signal
increases with frequency, the use of laser light offers many advantages. Fibre-optic laser systems are being used in communications networks. Many
long-haul fibre communications networks for both transcontinental connections
and, through undersea cables, international connections are in operation. One
advantage of optical fibre systems is the long distances that can be maintained
before signal repeaters are needed to regenerate signals. These are currently
separated by about 100 km (about 62 mi), compared to about 1.5 km (about 1 mi)
for electrical systems. Newly developed optical fiber amplifiers can extend
this distance even farther. Telephone-transmission method
uses fibre-optic cable, which is made of bundles of optical fibres, long
strands of specially made glass encased in a protective coating. Optical fibres transmit energy in the form of light pulses. The technology is similar to that
of the coaxial cable, except that the optical fibres can handle tens of
thousands of conversations simultaneously. Optical fibres are well suited for medical use. They can be made in extremely thin, flexible strands for insertion into the blood vessels, lungs, and other hollow parts of the body. Optical fibres are used in a number of instruments that enable doctors to view internal body parts without having to perform surgery. Surgical lasers and devices for measuring temperature or pressure also use optical fibres. Arthroscopy is the technique of using an arthroscope to examine a joint of the body. An arthroscope is a straight, tube like instrument with a series of lenses and optical-fibre bundles. It comes in sizes from 2 to 5 millimetres in diameter. It can be inserted into a joint through a small incision. A light transmitted by the optical fibres to the tip of the arthroscope illuminates the joint. Using an arthroscope, a doctor can thoroughly examine a patient's joint and perform certain surgical operations. Doctors use arthroscopy mainly on shoulder, elbow, hip, and knee joints. The problem most commonly treated by arthroscopy is torn cartilage in the knee. The doctor diagnoses this problem by looking into the knee joint through the arthroscope. Then the cartilage is removed with other instruments through a second incision. The main advantage of arthroscopic surgery is that the operation can be performed through a small incision at the joint. As a result, a patient can sometimes have the surgery and leave the hospital the same day. Also, the patient experiences a minimum amount of discomfort, and healing time is much shorter than for other methods of surgery.
The simplest application of
optical fibers is the transmission of light to locations otherwise hard to
reach, for example, the bore of a dentist's drill. Also, bundles of several
thousand very thin fibers assembled precisely side by side and optically
polished at their ends, can be used to transmit images. Each point of the image
projected on one face of the bundle is reproduced at the other end of the
bundle, reconstituting the image, which can be observed through a magnifier.
Image transmission by optical fibres is widely used in medical instruments for
viewing inside the human body and for laser surgery, in facsimile systems, in
phototypesetting, in computer graphics, and in many other applications. Optical fibres are also being used in a wide variety of sensing devices, ranging from thermometers to gyroscopes. The potential of their applications in this field is nearly unlimited, because the light sent through them is sensitive to many environmental changes, including pressure, sound waves, and strain, as well as heat and motion. The fibres can be especially useful where electrical effects could make ordinary wiring useless, less accurate, or even hazardous. Fibres have also been developed to carry high-power laser beams for cutting and drilling.
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