"Fiber Optic Cables"
"Fiber Optic Cables"
Here you can read all about Fiber Optic Cable from what type -to- installation.
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Evolution of Fiber |
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Fiber Types |
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Protecting Fibers |
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Some Common Cables |
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Optical Connectors |
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Installing Connectors |
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Optical Splices |
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Cutting Cables & Cleaving Fibers |
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Installing Optical Cables |
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Tools of Trade |
The theory of using light as a transmission medium has been around for quite sometime.
Back in the 1880's Alexander Graham Bell demostrated that light
could carry voice through the air with the use of wires. Bell's Photo Phone reproduced
voices by detecting vibrations in the amount of sunlight reaching the reciever. His theory
was correct. However, it was not very practical, as any objects that got in the way of the
light beam caused a disruption at the reciever.
During the 1930's several patents surfaced that used tubing as a
waveguide for light. The tubing was big and bulky, and thus impractical for use undergound
or in buildings.
Interest in optical fiber technology began to grom significantly in the
1950's, as a patent utlizing a two-layer glass waveguide surfaced. The principal behind
the two-layer waveguide was to confine the light signal withing the inner layer (core) by
the use of a second layer (cladding) that would reflect the light back into the core, much
like the way light is contained within water.
This development became the foundation for optical fiber transmission
as we know it today. What was needed at this point was alight source capable of traveling
the length of the waveguide.
Un the early 1960's, a laser was first used as a light source, with
tremendous results. The high cost of optical lasers, however, still prevented the
practical use of optical fiber technology for communications.
In the late 1960's, it was discovered that the high loss of light in
optical fiber was do to the impurities of the glass, not its intrinsinic properties.
In the early m1970's, engineers at Corning Glass Works refined the
manufacturing process of optical fiber construction, thereby allowing for the use of
lower-cost light sources, such as LEDs.
In the 1980's, optical fiber technology began to find its place as the
backbone of long distance telephone networks throughout North America.
Presently, with the advances in digital technology and the further
development of standards, optical fiber technolgy has become an intergal part of the
networks of today, as they have become foundation for tomorrow.
Fiber types
There are two primary types of optical fibers in widespread use
today, multimode and single-mode.
Simply put, multi-mode fibers are those with multiple
pathways through which light travels. Within the core of these cables are several hundred
layers of glass, each with a lower index of refraction as you move outward from the
center. Since light travels faster in the glass with lower indexes of refraction, the
light waves refracted to the outside of the fiber are speeded up to match those traveling
in the center.
If that explanation is too complicated, don't worry. The most important
thing to know about multi-mode fibers is that they are fine for distances of up to about a
kilometer and slower bandwidths. That makes them suitable for data networks within
buildings or between nearby buildings.
Single-mode fibers, on the other hand, are considered
the more appropriate choice when running cable longer than a kilometer, when putting
through a signal of more than 2 or 3 Gb/s, or when the system must be
"future-proof."
The reason single-mode fibers have such a great capacity lies in their
design. The core of a single-mode fiber cable is extremely small-usually between 8 and 9
microns (compared with 62.5 microns in the typical multi-mode fiber cable)-and only one
light wave at a time can be transmitted down the core. Because of quantum mechanical
effects, the light traveling in the very narrow core stays together in packets, rather
than bouncing around the core of the fiber.
While this enables single-mode fibers to handle far more signal over
far greater distances than their multi-mode cousins, single-mode fibers are considered to
be a bit more difficult to terminate than multi-mode fibers. Plus, single-mode fibers
require lasers as their light sources, which are more expensive than the LEDs that are
typically used with multi-mode fibers.
You might think that optical fibers are quite fragile-they're made
of glass, after all. But optical fibers are a lot stronger than fine glassware, and, once
they're enclosed in protective cables, they can be even tougher than copper communication
cables.
To achieve this strength, all optical fibers are designed in three
concentric layers. At the center is the core, the central piece of glass that carries the
light. Surrounding the core is the cladding, a second layer of glass, which keeps the
light from escaping the core. And around both of these lies the buffer, an outer layer of
plastic, which provides the real protection and strength.
There are two basic types of buffers in use today:
tight buffers and loose tubes.
Tight buffers consist of a thin layer of plastic
extruded over the individual fiber, much like the insulation placed on copper wires
(except a lot smaller). Tight-buffer configurations are generally used with indoor cables,
though they can also be found on some outdoor cables.
Loose tubes, meanwhile, are used in most outdoor
cables. In a loose-tube setup, one or more fibers (usually up to 12) are installed inside
of a plastic tube that resembles a drinking straw, and the assembly is then bound into the
cable. Frequently these tubes are filled with a water-block gel (usually and descriptively
called "icky-pick" on the job site), which prevents water from entering the
cable.
The loose-tube assembly provides particularly good protection for the
fibers, but such cables are messy to terminate and, because the fibers have a tendency to
fall out of the vertical cables, should never be used as risers.
For added strength during pulling, most fiber cables also contain a
strength member. Except for very short, easy runs (like 20 feet, between two outlets in
the same room), every time you pull a fiber cable, you must attach the pull string of the
fish tape to the strength member. If you do not, you stand a good chance of
ruining the cable.
With larger cables, strength members may be made of a steel rod, steel
wires, or fiberglass/epoxy rods. Most strength members, however, consist of a layer of
Kevlar yarn in the cable. (Warning: Use special scissors to cut the Kevlar. This is the
stuff they make bullet-proof vests out of, and cutting it with your lineman's pliers will
ruin them quickly.)
Beyond these basic configurations, fiber optic cables can take many
forms. Here's a look at some of the most common:
Simplex and zip cord cables consist
of one or two fibers that have been tight-buffered and Kevlar-reinforced and jacketed.
Such cables are used mostly for patch-cord and backplane applications.
Tightpack cables, also known as distribution cables, are
made up of up to several tight-buffered fibers bundled under the same jacket with Kevlar
reinforcement. These cables are small in size, but because their fibers are not
individually reinforced, they need to be terminated inside a patch panel or junction box.
They are generally used for short, dry conduit runs as well as riser and plenum
applications.
Breakout cables are made of several simplex units,
cabled together to produce a strong, rugged design suitable for conduit runs and riser and
plenum applications. Because each fiber is individually reinforced, this type of cable
allows for a strong termination to connectors and can be brought directly to a computer
back plane. Breakout cables are larger and more expensive than tightpack cables.
Loose-tube cables are considered ideal for outside
plant trunking applications. Depending on the actual construction, they can be used in
conduits, strung overhead, or buried directly into the ground.
Composite cables are those that contain a number of
copper conductors (jacketed and sheathed depending on the application) in the same cable
assembly as optical fibers. Be careful of the terminology there. Prior to 1993 NEC, such
cables were known as hybrid cables, but today that term typically refers to
cables that contain only optical fibers - but both multi-mode and single-mode fibers.
It used to be that you had to be an engineer to terminate optical fibers. Now, it's almost easy enough for kids to do. Thanks for this ease goes to optical connectors, which are used as terminating fixtures for non-fixed joints. As such, they are designed to be plugged in and disconnected several times. Optical connectors are available in a wide variety of styles and types. Choosing which one to use is usually decided by the type of connector already installed in the equipment to be connected. Nevertheless, adapters are generally available in either sleeve connectors or patch cords to allow coupling of different types of connectors. Connectors of the same style but from different manufacturers should be compatible with one another. For example, AT&T's ST connector can be used interchangeably with AMP's ST connector. Some popular connectors for various applications are listed below.
| Popular Connector Styles | |
| DATA
Communications Applications (Mostly Multi-mode) ST - most commonly used |
Telecommunications
Applications (mostly single-mode) FC/PC -
widely used |
All of the common types of connectors are fairly simple to install,
although you can expect a 10-percent loss until installers have a few days worth of
experience. After that, figure on losses of 2 to 5 percent, depending on the cleanliness
of the area in which the connections are made.
Before the installation of connectors onto a fiber cable, a breakout
kit may have to be installed. This procedure will not be necessary on breakout cables
having 2-mm buffered fibers, but will be required on 250-, 500-, and 900-micron
tight-buffer cables. The breakout kit consists of a buffer tubing (usually 2 mm) over a
900-micron inner tubing. The bare fibers are inserted into these buffer tubes to provide
handling protection and strength when mounted onto connectors.
Installing a fiber connector onto a pigtail or unbuffered fiber can be
done in several ways. The three most common are epoxy glue with oven-cure, then polish;
Hot Melt pre-glue, then polish; and cleave and crimp, no polish.
The epoxy-glue method is the oldest and is still
widely used today. This process involves filling the connector with a mixed two-part
epoxy, then insetting the prepared and cleaned fiber into the connector. After curing the
epoxy in an oven for the specified period of time (usually 5 to 20 minutes) the fiber is
scribed and cleaved nearly flush with the end of the connector. Finally, it's polished
with a succession of finer and finer lapping papers (typically ranging from 3-micron
grit down to 0.3- micron grit).
With the Hot Melt method (a trademark of 3M Co.), the
connector come preloaded with glue and must be place into an oven to soften the glue.
Clean, prepared fiber is then inserted into the connector, then left to cool. After
cooling, fiber is scribed and polished in the same process as used in the epoxy method.
Cleave and crimp connectors do not require a polish
procedure since these connectors already have a polished ferrule tip. Thus, installation
simply involves inserting a properly cleaved fiber to butt against the connector's
internal fiber "stub." The fiber connector is then crimped to hold the fiber in
place. Each mounting method has advantages and disadvantages, varying from ease of
installation to cost per connector to performance qualities.
In contrast to optical connectors, optical splices are used for fixed (more-or-less
permanent) joints.
Generally speaking, splices offer a lower return loss (light reflected
back from a connector), lower attenuation (light loss), and greater physical strength than
connectors. Also, because only one splice is required where there might be two connectors,
splices are usually less expensive and less labor-intensive than connectors. In addition,
they offer a better hermetic seal than connectors, constitute a smaller joint for
inclusion into splice closures, and allow either individual or mass splicing.
The two basic types of splices are fusion and mechanical.
Fusion splices are made by using an electric are to ionize
the space between prepared fibers to eliminate air and to heat the fibers to 2,000º F.
The heated fibers take a semi-liquid form and meld together, producing a single fiber
(rather than two joined fibers) when done properly. The splice is then covered with a
plastic sleeve or other protective device.
The completed splice is placed in an appropriate splicing tray, which is then fit into a
splice organizer and, in turn, into a splice closure.
One downside to fusion splicing is that it generally must be performed
in a dust- and contaminant-free environment, such as a splicing van or trailer. In
addition, fusion splicing is prohibited in manholes because the gases frequently found in
such locations could create a deadly explosion when exposed to the electric arc generated
during the splicing process.
Mechanical splicing is considered a quick and easy
method for repairing joints, though it's also used in new construction, especially on
small projects.
In this method, cleaned and prepared fibers are glued, crimped, or
faced together. The splice is then covered with some type of index matching gel or liquid
and placed in a V-groove or tube-type device to align the fibers. Finally, the groove or
tube is either crimped or snapped to hold the fibers in place. As with fusion splicing the
completed splice is placed in an appropriate splicing tray, then splice organizer, then
splice closure.
Unlike fusion splicing, mechanical splicing does not require a
controlled environment other than a reasonable level of dust control. Mechanical splices
are not without drawbacks, however. For instance, the index matching gel or liquid is
subject to contamination and aging, as is any adhesive glue used.
These factors, along with the following others, usually determine which
splicing method to use.
Type
of fiber: Most single-mode fiber is fusion-spliced. Multi-mode fiber, with its
complicated core structure, does not always fusion-splice easily, so mechanical splices
are generally used.
Attenuation,
including return loss: New fusion splicers provide incredibly low loss when used
properly, while a mechanical splice may allow as much light loss as a properly terminated
connector.
Physical
durability: The "welding" process used in the fusion splice results in
higher strength and greater durability than a mechanical splice. In addition, with fusion
splices, the mechanical tensile strength of the fiber remains near that of the original
fiber.
Ease
of installation: Today's fusion splicers are at least partially automated if not
fully automated in a one-button process. Mechanical splicing devices vary, but are usually
easier to use than connector kits.
Cost
per splice: While mechanical splices typically cost only $7 to $12 a piece, a fusion
splicer can run anywhere from $3,000 to $30,000. Once this machine is purchased, however,
the per splice cost is negligible.
Cutting Cables
& Cleaving Fibers
An important part of installing fiber optic cables is cutting those cables-and the in
individual fibers within them. And that means cutting very different processes to use:
cutting and cleaving.
Cutting, in these terms, applies to the cable. To cut the cable, first
carefully remove the cable jacket with a cable slitter. Next, gather the Kevlar into a
"rope," then, using a Kevlar cutter, snip the rope, leaving enough to terminate
into any required stress relief lugs. This will expose the loose buffer tubes or tight
buffered fibers.
At this point, cut the inner central strength member with the
appropriate tool (usually regular sidecutters) and secure it in its lug. (Depending on
manufacturer -specs, the Kevlar and the central strength member may share the same lug.)
If there are loose buffer tubes in the cable, carefully remove them
with a buffer tube cutter. Tight buffers can be stripped off using a jacket stripper to
reveal the coated fibers. A coating stripper sized for the job (the coating can be from
250 to 900 microns thick) then brings the fibers down to bare glass. Finally, the bare
fiber is cleaned using just one firm wipe of an alcohol-saturated pad.
Cleaving-scoring and cutting- is the process used on the individual
fibers. Remember, optical fibers are glass, and simply cutting them with a a pair of side
cutters would ruin the surface. In fact, cleaving fibers is much like cutting heavier
glass.
Start by using the sharp blade of a scribe or cleaver (a cleaver is
generally preferable) to put a surface defect on the fiber. Next pull carefully to allow a
crack to propagate across the fiber. In most cases, fiber ends should be close as possible
to perpendicular to the fiber axis . Some systems, however, require fibers to be cleaved
at a angle - usually about 8 degrees off of perpendicular - which is the best accomplished
with special angled cleavers.
Before fitting the fibers with splices or connectors, check them (and
polished connector ferrules) for possible defects using a microscope with a stage modified
to hold the fiber or connector in the field of view. Cleaved fiber are usually viewed from
the side, to see breakover and lip. Connectors are typically viewed end-on or at a small
angle to find polishing defects such as scratches.
Installing Optical
Cable
The general process of installing optic cable is much like that of pulling electrical
cables: Cables are pulled into raceways using lubricant and a pull line.
Of course, just as there's more than that to pulling electrical cables,
there's more to installing fiber. Here's a look at some techniques unique to fiber and
some tips for getting the job done right:
Connect the strength member of the cable to the pull line. Unless cables are designed for
such use, basket grips and similar methods of attachment should not be used.
Use a swivel while pulling to prevent the cable from being twisted by rigid attachment to
a fish tape. To keep pulling tension within careful limits, do not use power pullers with
fiber, unless you use a tension meter as well. As a general rule, the tension
should not exceed 300 pounds with indoor cables and 600 pounds with outdoor cables. But
always check the specifications for the cable to be sure.
Use lots of large pull boxes and divide pulls in half or even thirds at corners or pull
boxes. A pulling operation must be stopped if pulling tensions reach the cable's limit.
The discontinued pull will have to be aborted, the cable pulled back out and replaced on
the reel, and the pull started again using shorter runs, better lubrication, or both. The
cable may be reused if the pull or extraction has not damaged it.
Identify exact terminating locations in advance, keeping in mind that the cable must be
pulled all the way to where it will be terminated, not just into the room. This obvious
though often overlooked step can lead to costly mistakes if forgotten.
Carefully measure the cable's path before the conduit pull to avoid the need for splices.
Not only do splices cost time and money, but they also cause loss in both signal quality
and strength. In fact, most designers specify point-to-point or device-to-device runs to
avoid needless signal loss from splices.
The preferred way to measure is to use a pull tape. This product,
sometimes called mule tape, is a flat ribbon, consecutively numbered in feet, which is
usually made of polyester or aramid yarn and may be coated with plastic for waterproofing.
It comes in different lengths and pull strengths.
Pull the pull tape into the conduit, or along the cable path, for an
initial measurement, and add to that extra length for splices, terminations, and future
access coils, if specified. The amount of additional cable needed for splicing depends on
site conditions, splicing method used, and long-range plans for cable usage. For
termination length, figure on about 2 to 3 meters of additional cable beyond where the
terminal will be mounted. And add about 10 to 12 meters per span for any access or repair
coils. A good rule of thumb to cover all these additional cable lengths is to allow I
percent extra for outside plant (outdoor) cable runs and 5 to 7 percent extra for inside
cable runs.
Choose crews based on the length of the pull, total degrees of bend, tail loading (force
required to pull cable off feed reel), use of lubricant, and use of power putting
equipment. In most cases, two- or three-person crews are ideal. That way, one person pays
the cable off the reel and into the duct, while one or two others pull at the other end.
No job can be completed without the right tools, and installing fiber optic cables is no exception. To get the work done, you will need, at a minimum:
Cleaning fluid and rags (or an approved cable cleaner) Buffer tube cutter Reagent-grade isopropyl alcohol in a non spill bottle or on presoaked pads Canned air Tape (masking tape or "scotch" invisible) Coating Stripper Cleaver or Scribe Microscope or Cleaver checker Splicer Connector Supplies
These tools can be purchased indivisually or in a prepackaged kit, and can cost from as little as $750.00 or as much as $5000.00 depending on the quality and quantity of tools purchased.
- This article was written by Paul Rosenberg.
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