25-08-2017, 09:32 PM
Optical Fiber and its Applications
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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.
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.
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.