Fiber Optic Cable

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]


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.


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.

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
  • ADSS: All-Dielectric Self-Supporting

Jacket material

The jacket material is application specific. The material determines the mechanical robustness, aging due to UV radiation, oil resistance, etc. Nowadays PVC is being replaced by halogen free alternatives, mainly driven by more stringent regulations.

Material Halogen-free UV Resistance Remark
LSFH Polymer Yes Good[5] Good for indoor use
Polyvinyl chloride (PVC) No Good[6] Being replaced by LSFH Polymer
Polyethylene (PE) Yes Poor[7][8][9] Good for outdoor applications
Polyurethane (PUR) Yes ? Highly flexible cables
Polybutylene terephthalate (PBT) Yes Fair?[10] Good for indoor use
Polyamide (PA) Yes Good[11]-Poor[12] Indoor and outdoor use

Color coding

Patch cords

The buffer or jacket on patchcords is often color-coded to indicate the type of fiber used. The strain relief "boot" that protects the fiber from bending at a connector is color-coded to indicate the type of connection. Connectors with a plastic shell (such as SC connectors) typically use a color-coded shell. Standard color codings for jackets and boots (or connector shells) are shown below:

Buffer/jacket color Meaning
Yellow single-mode optical fiber
Orange multi-mode optical fiber
Aqua 10 gig laser-optimized 50/125 micrometer multi-mode optical fiber
Grey outdated color code for multi-mode optical fiber
Blue Sometimes used to designate polarization-maintaining optical fiber
Connector Boot Meaning Comment
Blue Physical Contact (PC), 0° mostly used for single mode fibers; some manufacturers use this for polarization-maintaining optical fiber.
Green Angle Polished (APC), 8°
Black Physical Contact (PC), 0°
Grey, Beige Physical Contact (PC), 0° multimode fiber connectors
White Physical Contact (PC), 0°
Red High optical power. Sometimes used to connect external pump lasers or Raman pumps.

Remark: It is also possible that a small part of a connector is additionally colour-coded, e.g. the leaver of an E-2000 connector or a frame of an adapter. This additional colour coding indicates the correct port for a patchcord, if many patchcords are installed at one point.

Multi-fiber cables

Individual fibers in a multi-fiber cable are often distinguished from one another by color-coded jackets or buffers on each fiber. The identification scheme used by Corning Cable Systems is based on EIA/TIA-598, "Optical Fiber Cable Color Coding." EIA/TIA-598 defines identification schemes for fibers, buffered fibers, fiber units, and groups of fiber units within outside plant and premises optical fiber cables. This standard allows for fiber units to be identified by means of a printed legend. This method can be used for identification of fiber ribbons and fiber subunits. The legend will contain a corresponding printed numerical position number and/or color for use in identification.[13]

EIA598-A Fiber Color Chart[13]
Position Jacket color Position Jacket color
1 37 px
blue
13 37 px
blue/black
2 37 px
orange
14 37 px
orange/black
3 37 px
green
15 37 px
green/black
4 37 px
brown
16 37 px
brown/black
5 37 px
slate
17 37 px
slate/black
6 37 px
white
18 37 px
white/black
7 37 px
red
19 37 px
red/black
8 37 px
black
20 37 px
black/yellow
9 37 px
yellow
21 37 px
yellow/black
10 37 px
violet
22 37 px
violet/black
11 37 px
rose
23 37 px
rose/black
12 37 px
aqua
24 37 px
aqua/black
Color coding of Premises Fiber Cable[13]
Fiber Type / Class Diameter (µm) Jacket Color
Multimode 1a 50/125 Orange
Multimode 1a 62.5/125 Slate
Multimode 1a 85/125 Blue
Multimode 1a 100/140 Green
Singlemode IVa All Yellow
Singlemode IVb All Red

Propagation speed and delay

Optical cables transfer data at the speed of light in glass (slower than vacuum). This is typically around 180,000 to 200,000 km/s, resulting in 5.0 to 5.5 microseconds of latency per km. Thus the round-trip delay time for 1000 km is around 11 ms.[14]

Losses

Typical modern multimode graded-index fibers have 3 dB/km of attenuation loss at 850 nm and 1 dB/km at 1300 nm. 9/125 singlemode loses 0.4/0.25 dB/km at 1310/1550 nm. POF (plastic optical fiber) loses much more: 1 dB/m at 650 nm. Plastic optical fiber is large core (about 1mm) fiber suitable only for short, low speed networks such as within cars.[15]

Each connection made adds about 0.6 dB of average loss, and each joint (splice) adds about 0.1 dB.[16] Depending on the transmitter power and the sensitivity of the receiver, if the total loss is too large the link will not function reliably.

Invisible IR light is used in commercial glass fiber communications because it has lower attenuation in such materials than visible light. However, the glass fibers will transmit visible light somewhat, which is convenient for simple testing of the fibers without requiring expensive equipment. Splices can be inspected visually, and adjusted for minimal light leakage at the joint, which maximizes light transmission between the ends of the fibers being joined.

The charts at Optical power loss (attenuation) in fiber illustrate the relationship of visible light to the IR frequencies used, and show the absorption water bands between 850, 1300 and 1550 nm.

Safety

Because the infrared light used in communications can not be seen, there is a potential laser safety hazard to technicians. In some cases the power levels are high enough to damage eyes, particularly when lenses or microscopes are used to inspect fibers which are inadvertently emitting invisible IR. Inspection microscopes with optical safety filters are available to guard against this.

Small glass fragments can also be a problem if they get under someone's skin, so care is needed to ensure that fragments produced when cleaving fiber are properly collected and disposed of appropriately.

See also

Notes and references

External links

  • Fiber Optic Association The FOA Reference Guide To Fiber Optics
  • Accurately Testing Fiber Optic Cables
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