Optical Fiber

Optical Fiber

Physical Description An optical fiber is a thin (2 to125 mm ), flexible medium capable of guiding an optical ray.Various glasses and plastics can be used to make optical fibers. The lowest losses have been obtained using fibers of ultra pure fused silica. Ultrapure fiber is difficult to manufacture; higher loss multi-component glass fibers are more economical and still provide good performance. Plastic fiber is even less costly and can be used for short haul links, for which moderately high losses are acceptable.
An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket (Figure 4.2c). The core is the inner most section and consists of one or more very thin strands, or fibers, made of glass or plastic; the core has a diameter in the range of 8 to Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different
from those of the core and a diameter of The interface between the core
and cladding acts as a reflector to confine light that would otherwise escape the
core. The outermost layer, surrounding one or a bundle of cladded fibers, is the
jacket. The jacket is composed of plastic and other material layered to protect
against moisture, abrasion, crushing, and other environmental dangers.
Applications Optical fiber already enjoys considerable use in long
distance
telecommunications, and its use in military applications is growing. The continuing
improvements in performance and decline in prices, together with the inherent
advantages of optical fiber, have made it increasingly attractive for local area net
working. The following characteristics distinguish optical fiber from twisted pair or
coaxial cable:
• Greater capacity: The potential bandwidth, and hence data rate, of optical
fiber is immense; data rates of hundreds of Gbps over tens of kilometers have
been demonstrated. Compare this to the practical maximum of hundreds of
Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or up
to 100 Mbps to 10 Gbps over a few tens of meters for twisted pair.
• Smaller size and lighter weight: Optical fibers are considerably thinner than
coaxial cable or bundled twisted
pair cable—at least an order of magnitude
thinner for comparable information transmission capacity. For cramped con
duits in buildings and underground along public rights
of
way, the advantage
of small size is considerable. The corresponding reduction in weight reduces
structural support requirements.
• Lower attenuation: Attenuation is significantly lower for optical fiber than for
coaxial cable or twisted pair (Figure 4.3c) and is constant over a wide range.
• Electromagnetic isolation: Optical fiber systems are not affected by external
electromagnetic fields. Thus the system is not vulnerable to interference,
impulse noise, or crosstalk.By the same token, fibers do not radiate energy, so
there is little interference with other equipment and there is a high degree of
security from eavesdropping. In addition, fiber is inherently difficult to tap.
• Greater repeater spacing: Fewer repeaters mean lower cost and fewer sources
of error. The performance of optical fiber systems from this point of view has
been steadily improving. Repeater spacing in the tens of kilometers for optical
fiber is common, and repeater spacings of hundreds of kilometers have been
demonstrated. Coaxial and twisted
pair systems generally have repeaters
every few kilometers.