Optical fiber cable
From Wikipedia, the free encyclopedia
A
TOSLINK optical fiber cable with a clear jacket. These cables are used mainly for digital audio connections between devices.
An
optical fiber cable is a
cable containing one or more
optical fibers.
The optical fiber elements are typically individually coated with
plastic layers and contained in a protective tube suitable for the
environment where the cable will be deployed.
Design
Optical fiber consists of a
core and a
cladding layer, selected for
total internal reflection due to the difference in the
refractive index between the two. In practical fibers, the cladding is usually coated with a layer of
acrylate polymer or
polyimide. This coating protects the fiber from damage but does not contribute to its
optical waveguide properties. Individual coated fibers (or fibers formed into ribbons or bundles) then have a tough
resin buffer
layer and/or core tube(s) extruded around them to form the cable core.
Several layers of protective sheathing, depending on the application,
are added to form the cable. Rigid fiber assemblies sometimes put
light-absorbing ("dark") glass between the fibers, to prevent light that
leaks out of one fiber from entering another. This reduces
cross-talk between the fibers, or reduces
flare in fiber bundle imaging applications.
[1]
Left:
LC/PC connectors
Right: SC/PC connectors
All four connectors have white caps covering the
ferrules.
For indoor applications, the jacketed fiber is generally enclosed, with a bundle of flexible fibrous
polymer strength members like
aramid (e.g.
Twaron or
Kevlar), in a lightweight plastic cover to form a simple cable. Each end of the cable may be
terminated with a specialized
optical fiber connector to allow it to be easily connected and disconnected from transmitting and receiving equipment.
Fibre-optic cable in a Telstra pit
An optical fiber breakout cable
For use in more strenuous environments, a much more robust cable construction is required. In
loose-tube construction the fiber is laid
helically
into semi-rigid tubes, allowing the cable to stretch without stretching
the fiber itself. This protects the fiber from tension during laying
and due to temperature changes. Loose-tube fiber may be "dry block" or
gel-filled. Dry block offers less protection to the fibers than
gel-filled, but costs considerably less. Instead of a loose tube, the
fiber may be embedded in a heavy polymer jacket, commonly called "tight
buffer" construction. Tight buffer cables are offered for a variety of
applications, but the two most common are "
Breakout" and "
Distribution".
Breakout cables normally contain a ripcord, two non-conductive
dielectric strengthening members (normally a glass rod epoxy), an aramid
yarn, and 3 mm buffer tubing with an additional layer of Kevlar
surrounding each fiber. The ripcord is a parallel cord of strong yarn
that is situated under the jacket(s) of the cable for jacket removal.
[2]
Distribution cables have an overall Kevlar wrapping, a ripcord, and a
900 micrometer buffer coating surrounding each fiber. These
fiber units are commonly bundled with additional steel strength members, again with a helical twist to allow for stretching.
A critical concern in outdoor cabling is to protect the fiber from
contamination by water. This is accomplished by use of solid barriers
such as copper tubes, and water-repellent jelly or water-absorbing
powder surrounding the fiber.
Finally, the cable may be armored to protect it from environmental
hazards, such as construction work or gnawing animals. Undersea cables
are more heavily armored in their near-shore portions to protect them
from boat anchors, fishing gear, and even
sharks, which may be attracted to the electrical power that is carried to power amplifiers or repeaters in the cable.
Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches,
dual use as power lines, installation in conduit, lashing to aerial telephone poles,
submarine installation, and insertion in paved streets.
Capacity and market
Modern fiber cables can contain up to a thousand fibers in a single
cable, with potential bandwidth in the terabytes per second. In some
cases, only a small fraction of the fibers in a cable may be actually
"lit". Companies can lease or sell the unused fiber to other providers
who are looking for service in or through an area. Companies may
"overbuild" their networks for the specific purpose of having a large
network of
dark fiber for sale, reducing the overall need for trenching and municipal permitting.
[citation needed]
Reliability and quality
Optical fibers are inherently very strong, but the strength is
drastically reduced by unavoidable microscopic surface flaws inherent in
the manufacturing process. The initial fiber strength, as well as its
change with time, must be considered relative to the stress imposed on
the fiber during handling, cabling, and installation for a given set of
environmental conditions. There are three basic scenarios that can lead
to strength degradation and failure by inducing flaw growth: dynamic
fatigue, static fatigues, and zero-stress aging.
Telcordia GR-20,
Generic Requirements for Optical Fiber and Optical Fiber Cable, contains reliability and quality criteria to protect optical fiber in all operating conditions.
[3]
The criteria concentrate on conditions in an outside plant (OSP)
environment. For the indoor plant, similar criteria are in Telcordia
GR-409,
Generic Requirements for Indoor Fiber Optic Cable.
[4]
Cable types
- OFC: Optical fiber, conductive
- OFN: Optical fiber, nonconductive
- OFCG: Optical fiber, conductive, general use
- OFNG: Optical fiber, nonconductive, general use
- OFCP: Optical fiber, conductive, plenum
- OFNP: Optical fiber, nonconductive, plenum
- OFCR: Optical fiber, conductive, riser
- OFNR: Optical fiber, nonconductive, riser
- OPGW: Optical fiber composite overhead ground wire
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