Fiber Cable
BRIEF OVERVIEW OF FIBER OPTIC CABLE AND ADVANTAGES
OVER COPPER:
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SPEED: Fiber
optic networks operate at high speeds - up into the
gigabits
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BANDWIDTH: large
carrying capacity
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DISTANCE: Signals
can be transmitted further without needing to be "refreshed" or
strengthened.
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RESISTANCE:
Greater resistance to electromagnetic noise such as radios, motors or
other nearby cables.
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MAINTENANCE:
Fiber optic cables costs much less to maintain.
In recent years it has become apparent that fiber-optics are
steadily replacing copper wire as an appropriate means of communication
signal transmission. They span the long distances between local phone
systems as well as providing the backbone for many network systems. Other
system users include cable television services, university campuses,
office buildings, industrial plants, and electric utility
companies.
A fiber-optic system is similar to the copper wire system that
fiber-optics is replacing. The difference is that fiber-optics use light
pulses to transmit information down fiber lines instead of using
electronic pulses to transmit information down copper lines. Looking at
the components in a fiber-optic chain will give a better understanding of
how the system works in conjunction with wire based systems.
At one end of the system is a transmitter. This is the place of
origin for information coming on to fiber-optic lines. The transmitter
accepts coded electronic pulse information coming from copper wire. It
then processes and translates that information into equivalently coded
light pulses. A light-emitting diode (LED) or an injection-laser diode
(ILD) can be used for generating the light pulses. Using a lens, the light
pulses are funneled into the fiber-optic medium where they transmit
themselves down the line.
Think of a fiber cable in terms of very long cardboard roll (from
the inside roll of paper towel) that is coated with a mirror. If you
shine a flashlight in one you can see light at the far end - even if bent
the roll around a corner.
Light pulses move easily down the fiber-optic line because of a
principle known as total internal reflection. "This principle of total
internal reflection states that when the angle of incidence exceeds a
critical value, light cannot get out of the glass; instead, the light
bounces back in. When this principle is applied to the construction of the
fiber-optic strand, it is possible to transmit information down fiber
lines in the form of light pulses.
There are three types of fiber optic cable commonly used: single
mode, multimode and plastic optical fiber (POF).
Transparent glass
or plastic fibers which allow light to be guided from one end to the other
with minimal loss.
Fiber optic cable functions as a "light guide," guiding the light
introduced at one end of the cable through to the other end. The light
source can either be a light-emitting diode (LED)) or a laser.
The light source is pulsed on and off, and a light-sensitive
receiver on the other end of the cable converts the pulses back into the
digital ones and zeros of the original signal.
Even laser light shining through a fiber optic cable is subject to
loss of strength, primarily through dispersion and scattering of the
light, within the cable itself. The faster the laser fluctuates, the
greater the risk of dispersion. Light strengtheners, called repeaters, may
be necessary to refresh the signal in certain applications.
While fiber optic cable itself has become cheaper over time - a
equivalent length of copper cable cost less per foot but not in capacity.
Fiber optic cable connectors and the equipment needed to install them are
still more expensive than their copper counterparts.
Single Mode cable
is a single stand of glass fiber with a diameter of 8.3 to 10 microns that
has one mode of transmission. Single Mode Fiber with a relatively
narrow diameter, through which only one mode will propagate typically 1310
or 1550nm. Carries higher bandwidth than multimode fiber, but requires a
light source with a narrow spectral width. Synonyms mono-mode optical
fiber, single-mode fiber, single-mode optical waveguide, uni-mode
fiber.
Single-mode
fiber gives you a higher transmission rate and up to 50 times more
distance than multimode, but it also costs more. Single-mode fiber has a
much smaller core than multimode. The small core and single light-wave
virtually eliminate any distortion that could result from overlapping
light pulses, providing the least signal attenuation and the highest
transmission speeds of any fiber cable type.
Single-mode optical fiber is an optical fiber in
which only the lowest order bound mode can propagate at the wavelength of
interest typically 1300 to 1320nm.
jump to single mode fiber page
Multimode cable is made of of glass
fibers, with a common diameters in the 50-to-100 micron range for the
light carry component (the most common size is 62.5). POF is a newer
plastic-based cable which promises performance similar to glass cable on
very short runs, but at a lower cost.
Multimode
fiber gives you high bandwidth at high speeds over medium distances. Light
waves are dispersed into numerous paths, or modes, as they travel through
the cable's core typically 850 or 1300nm. Typical multimode fiber core
diameters are 50, 62.5, and 100 micrometers. However, in long cable runs
(greater than 3000 feet [914.4 ml), multiple paths of light can cause
signal distortion at the receiving end, resulting in an unclear and
incomplete data transmission.
The use of fiber-optics was generally not available until 1970 when
Corning Glass Works was able to produce a fiber with a loss of 20 dB/km.
It was recognized that optical fiber would be feasible for
telecommunication transmission only if glass could be developed so pure
that attenuation would be 20dB/km or less. That is, 1% of the light would
remain after traveling 1 km. Today's optical fiber attenuation ranges from
0.5dB/km to 1000dB/km depending on the optical fiber used. Attenuation
limits are based on intended application.
The applications of optical fiber communications have increased at
a rapid rate, since the first commercial installation of a fiber-optic
system in 1977. Telephone companies began early on, replacing their old
copper wire systems with optical fiber lines. Today's telephone companies
use optical fiber throughout their system as the backbone architecture and
as the long-distance connection between city phone systems.
Cable television companies have also began integrating fiber-optics
into their cable systems. The trunk lines that connect central offices
have generally been replaced with optical fiber. Some providers have begun
experimenting with fiber to the curb using a fiber/coaxial hybrid. Such a
hybrid allows for the integration of fiber and coaxial at a neighborhood
location. This location, called a node, would provide the optical receiver
that converts the light impulses back to electronic signals. The signals
could then be fed to individual homes via coaxial cable.
Local Area Networks (LAN) is a collective group of computers, or
computer systems, connected to each other allowing for shared program
software or data bases. Colleges, universities, office buildings, and
industrial plants, just to name a few, all make use of optical fiber
within their LAN systems.
Power companies are an emerging group that have begun to utilize
fiber-optics in their communication systems. Most power utilities already
have fiber-optic communication systems in use for monitoring their power
grid systems.
Fiber
Some 10 billion digital bits can be transmitted per second along an
optical fiber link in a commercial network, enough to carry tens of
thousands of telephone calls. Hair-thin fibers consist of two concentric
layers of high-purity silica glass the core and the cladding, which are
enclosed by a protective sheath. Light rays modulated into digital pulses
with a laser or a light-emitting diode move along the core without
penetrating the cladding.
The light stays confined to the core because the cladding has a
lower refractive index—a measure of its ability to bend light. Refinements
in optical fibers, along with the development of new lasers and diodes,
may one day allow commercial fiber-optic networks to carry trillions of
bits of data per second.
Total internal refection confines light within optical fibers
(similar to looking down a mirror made in the shape of a long paper towel
tube). Because the cladding has a lower refractive index, light rays
reflect back into the core if they encounter the cladding at a shallow
angle (red lines). A ray that exceeds a certain "critical" angle escapes
from the fiber (yellow line).
STEP-INDEX MULTIMODE FIBER has a large core, up to 100 microns in
diameter. As a result, some of the light rays that make up the digital
pulse may travel a direct route, whereas others zigzag as they bounce off
the cladding. These alternative pathways cause the different groupings of
light rays, referred to as modes, to arrive separately at a receiving
point. The pulse, an aggregate of different modes, begins to spread out,
losing its well-defined shape. The need to leave spacing between pulses to
prevent overlapping limits bandwidth that is, the amount of information
that can be sent. Consequently, this type of fiber is best suited for
transmission over short distances, in an endoscope, for
instance.
GRADED-INDEX MULTIMODE FIBER contains a core in which the
refractive index diminishes gradually from the center axis out toward the
cladding. The higher refractive index at the center makes the light rays
moving down the axis advance more slowly than those near the cladding.
Also, rather than zigzagging off the cladding, light in the core curves
helically because of the graded index, reducing its travel distance. The
shortened path and the higher speed allow light at the periphery to arrive
at a receiver at about the same time as the slow but straight rays in the
core axis. The result: a digital pulse suffers less
dispersion.
SINGLE-MODE FIBER has a narrow core (eight microns or less), and
the index of refraction between the core and the cladding changes less
than it does for multimode fibers. Light thus travels parallel to the
axis, creating little pulse dispersion. Telephone and cable television
networks install millions of kilometers of this fiber every
year.
Basic Cable Design
Two Basic cable designs are loose-tube cable, used in the
majority of outside plant installations in North America and
tight-buffered cable, primarily used inside the buildings.
The modular design of loose-tube cables typically holds up
to 12 fibers per buffer tube with a maximum per cable fiber count of more
than 200 fibers. Loose-tube cables can be all dielectric or optionally
armored. The modular buffer tube design permits easy drop-off of groups of
fibers at intermediate points, without interfering with other protected
buffer tubes being routed to other locations. The loose-tube design also
helps in the identification and administration of fibers in the
system.
Single fiber tight-buffered cables are used as pigtails,
patch-cords and jumpers to terminate loose-tube cables directly into
opto-electronic transmitters, receivers and other active and passive
components
Multi-fiber tight buffered cables are also available and
used primarily for alternative routing and handling flexibility and ease
within buildings.
Loose-Tube Cable
In a loose-tube cable design color coded plastic buffer
tubes house and protect optical fibers. A gel filling compound impedes
water penetration. Excess fiber length (relative to buffer tube length)
insulates fibers from stress of installation and environmental
loading. Buffer tubes are stranded around a dielectric or steel central
member, which serves as an anti-buckling element. The cable core,
typically uses aramid yarn as the primary tensile strength member. The
outer polyethylene jacket is extruded over the core. If armoring is
required a corrugated steel tape is formed around the single jacketed
cable with an additional jacket extruded over the armor. Loose-tube cables
are typically used for outside plant installation in aerial, duct and
direct-buried applications.
Tight-Buffered Cable:
With tight-buffered cable designs, the buffering material is
in direct contact with the fiber. This design is suited for jumper cables,
which connect outside plant cables to terminal equipment and also for
linking various devices in premises network.
Multi-fiber tight-buffered cables often are used for
intra-building, risers, general building and plenum
applications.
The tight-buffered design provides a rugged cable structure
to protect individual fibers during handling, routing and connectorisation
yarn strength members keep the tensile load away from the fiber
As with the loose-tube cables, Optical specifications for
the tight-buffered cables also should include the maximum performance of
all fibers over the operating temperature range and life of the cable.
Averages should not be acceptable
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