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Important Terms
Optical Fiber: An optical fiber (or fibre) is a glass or plastic fiber
that carries light along its length. Fiber optics is the overlap of
applied science and engineering concerned with the design and
application of optical fibers. Optical fibers are widely used in fiberoptic
communications, which permits transmission over longer
distances and at higher bandwidths (data rates) than other forms of
communications.
Refraction: Refraction is the change in direction of a wave due to a
change in its speed. This is most commonly observed when a wave
passes from one medium to another. Refraction of light is the most
commonly observed phenomenon, but any type of wave can refract
when it interacts with a medium, for example when sound waves
pass from one medium into another or when water waves move into
water of a different depth
Reflection: Reflection is the change in direction of a wavefront at
an interface between two different media so that the wavefront
returns into the medium from which it originated. Common
examples include the reflection of light, sound and water waves.
Internal Reflection
Scattering: Scattering is a general physical process where some
forms of radiation, such as light, sound, or moving particles, are
forced to deviate from a straight trajectory by one or more localized
non-uniformities in the medium through which they pass. In
conventional use, this also includes deviation of reflected radiation
from the angle predicted by the law of reflection.
Attenuation: is the gradual loss in intensity of any kind of flux
through a medium. For instance, sunlight is attenuated by dark
glasses, and X-rays are attenuated by lead.
Optical Fiber Cable (OFC)
An optical fiber (or fibre) is a glass or plastic fiber that carries light along
its length. Fiber optics is the overlap of applied science and engineering
concerned with the design and application of optical fibers. Optical fibers
are widely used in fiber-optic communications, which permits
transmission over longer distances and at higher bandwidths (data rates)
than other forms of communications. Fibers are used instead of metal
wires because signals travel along them with less loss, and they are also
immune to electromagnetic interference. Fibers are also used for
illumination, and are wrapped in bundles so they can be used to carry
images, thus allowing viewing in tight spaces. Specially designed fibers
are used for a variety of other applications, including sensors and fiber
lasers.
Light is kept in the core of the optical fiber by total internal reflection.
This causes the fiber to act as a waveguide. Fibers which support many
propagation paths or transverse modes are called multi-mode fibers
(MMF), while those which can only support a single mode are called
single-mode fibers (SMF). Multi-mode fibers generally have a larger core
diameter, and are used for short-distance communication links and for
applications where high power must be transmitted. Single-mode fibers
are used for most communication links longer than 550 meters (1,800 ft).
Joining lengths of optical fiber is more complex than joining electrical wire
or cable. The ends of the fibers must be carefully cleaved, and then
spliced together either mechanically or by fusing them together with an
electric arc. Special connectors are used to make removable connections.
Applications
Optical fiber communication
Optical fiber can be used as a medium for telecommunication and
networking because it is flexible and can be bundled as cables. It is
especially advantageous for long-distance communications, because light
propagates through the fiber with little attenuation compared to electrical
cables. This allows long distances to be spanned with few repeaters.
Additionally, the per-channel light signals propagating in the fiber can be
modulated at rates as high as 111 gigabits per second, although 10 or 40
Gb/s is typical in deployed systems. Each fiber can carry many
independent channels, each using a different wavelength of light
(wavelength-division multiplexing (WDM)). The net data rate (data rate
without overhead bytes) per fiber is the per-channel data rate reduced by
the FEC overhead, multiplied by the number of channels (usually up to
eighty in commercial dense WDM systems as of 2008). The current
laboratory fiber optic data rate record, held by Bell Labs in Villarceaux,
France, is multiplexing 155 channels, each carrying 100 Gbps over a 7000
km fiber.
For short distance applications, such as creating a network within an
office building, fiber-optic cabling can be used to save space in cable
ducts. This is because a single fiber can often carry much more data than
many electrical cables, such as Cat-5 Ethernet cabling. Fiber is also
immune to electrical interference; there is no cross-talk between signals
in different cables and no pickup of environmental noise. Non-armored
fiber cables do not conduct electricity, which makes fiber a good solution
for protecting communications equipment located in high voltage
environments such as power generation facilities, or metal communication
structures prone to lightning strikes. They can also be used in
environments where explosive fumes are present, without danger of
ignition. Wiretapping is more difficult compared to electrical connections,
and there are concentric dual core fibers that are said to be tap-proof.
Although fibers can be made out of transparent plastic, glass, or a
combination of the two, the fibers used in long-distance
telecommunications applications are always glass, because of the lower
optical attenuation. Both multi-mode and single-mode fibers are used in
communications, with multi-mode fiber used mostly for short distances,
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up to 550 m (600 yards), and single-mode fiber used for longer distance
links. Because of the tighter tolerances required to couple light into and
between single-mode fibers (core diameter about 10 micrometers),
single-mode transmitters, receivers, amplifiers and other components are
generally more expensive than multi-mode components.
Fiber optic sensors
Fibers have many uses in remote sensing. In some applications, the
sensor is itself an optical fiber. In other cases, fiber is used to connect a
non-fiberoptic sensor to a measurement system. Depending on the
application, fiber may be used because of its small size, or the fact that
no electrical power is needed at the remote location, or because many
sensors can be multiplexed along the length of a fiber by using different
wavelengths of light for each sensor, or by sensing the time delay as light
passes along the fiber through each sensor. Time delay can be
determined using a device such as an optical time-domain reflectometer.
Optical fibers can be used as sensors to measure strain, temperature,
pressure and other quantities by modifying a fiber so that the quantity to
be measured modulates the intensity, phase, polarization, wavelength or
transit time of light in the fiber. Sensors that vary the intensity of light
are the simplest, since only a simple source and detector are required. A
particularly useful feature of such fiber optic sensors is that they can, if
required, provide distributed sensing over distances of up to one meter.
Extrinsic fiber optic sensors use an optical fiber cable, normally a multimode
one, to transmit modulated light from either a non-fiber optical
sensor, or an electronic sensor connected to an optical transmitter. A
major benefit of extrinsic sensors is their ability to reach places which are
otherwise inaccessible. An example is the measurement of temperature
inside aircraft jet engines by using a fiber to transmit radiation into a
radiation pyrometer located outside the engine. Extrinsic sensors can also
be used in the same way to measure the internal temperature of electrical
transformers, where the extreme electromagnetic fields present make
other measurement techniques impossible. Extrinsic sensors are used to
measure vibration, rotation, displacement, velocity, acceleration, torque,
and twisting.
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Other uses of optical fibers
Fibers are widely used in illumination applications. They are used as light
guides in medical and other applications where bright light needs to be
shone on a target without a clear line-of-sight path. In some buildings,
optical fibers are used to route sunlight from the roof to other parts of the
building (see non-imaging optics). Optical fiber illumination is also used
for decorative applications, including signs, art, and artificial Christmas
trees. Swarovski boutiques use optical fibers to illuminate their crystal
showcases from many different angles while only employing one light
source. Optical fiber is an intrinsic part of the light-transmitting concrete
building product, LiTraCon.
Optical fiber is also used in imaging optics. A coherent bundle of fibers is
used, sometimes along with lenses, for a long, thin imaging device called
an endoscope, which is used to view objects through a small hole. Medical
endoscopes are used for minimally invasive exploratory or surgical
procedures (endoscopy). Industrial endoscopes (see fiberscope or
borescope) are used for inspecting anything hard to reach, such as jet
engine interiors.
In spectroscopy, optical fiber bundles are used to transmit light from a
spectrometer to a substance which cannot be placed inside the
spectrometer itself, in order to analyze its composition. A spectrometer
analyzes substances by bouncing light off of and through them. By using
fibers, a spectrometer can be used to study objects that are too large to
fit inside, or gasses, or reactions which occur in pressure vessels.
An optical fiber doped with certain rare earth elements such as erbium
can be used as the gain medium of a laser or optical amplifier. Rare-earth
doped optical fibers can be used to provide signal amplification by splicing
a short section of doped fiber into a regular (undoped) optical fiber line.
The doped fiber is optically pumped with a second laser wavelength that
is coupled into the line in addition to the signal wave. Both wavelengths of
light are transmitted through the doped fiber, which transfers energy
from the second pump wavelength to the signal wave. The process that
causes the amplification is stimulated emission.