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Fiber Cables

Definition: cables containing one or several optical fibers

German: Faserkabel

Category: fiber optics and waveguides

How to cite the article; suggest additional literature

An optical fiber cable (or fiber-optic cable) is a more or less flexible cable which contains one or several (sometimes even many hundreds of) optical fibers. Due to protective layers (made of polymers and sometimes also metals) which prevent excessive bending as well as damage due to externally applied stress, fiber cables can be much more robust than bare fibers. Also, fiber cables protect glass fibers against moisture.

A variety of constructions are used for fiber cables:

photograph of a fiber-coupled diode laser
Figure 1: Photograph of fiber-coupled diode lasers from Coherent, having different kinds of fiber-optic cables. The image was kindly provided by Coherent.

For indoor cables, fire safety is an important aspect, whereas for outdoor cables, moisture resistance and temperature tolerance are factors of high interest. Special aerial/self-supporting cables, which can be exposed to wind and sunlight in free air (hanging on poles), must have a particularly high pulling strength and overall robustness, using some steel or aramid yarn. Outdoor cables could easily tolerate indoor conditions, but are often not allowed for indoor use (or only with certain restrictions) due to issues with fire protection.

The detailed construction for a fiber cable can be relatively sophisticated, optimized for an appropriate trade-off between strength, diameter, weight, fire resistance, cost and other possibly relevant properties. For example, the mechanical details can have an influence on polarization mode dispersion.

The terminations (ends) of fiber cables are often equipped with fiber-optic connectors, which allow relatively simple plugging, as is possible with electrical cables. However, fiber-optic connections are usually more sensitive than electrical connections, and more sophisticated procedures and equipment are needed to prepare or clean them.

Various types of fiber-optic patch cables (patch cords) are available, which are terminated with standardized fiber connectors.

Long-distance cables are made by connecting multiple cables, each one being a few kilometers long. Stable connections can be made with splicing, in particular with fusion splicing.

NEC Classification of Optical Fiber Cables

There exist many variations of fiber cables, to which some classifications are applied. A common classification according to the National Electrical Code (NEC) in the United States is as follows:

  • OFC cables are conductive (C) fiber cables, i.e., cables containing metals in strengthening structures (which are not used for electrical connections). In contrast, OFN cables are non-conductive and therefore fully insulating.
  • Riser cables, marked with an additional “R” (e.g., OFCR or OFNR), are used for connections between different levels of a building (vertical shafts). They are optimized such that they do not support the spread of fire between different floors.
  • Plenum cables, marked with an additional “P” (e.g., OFCP or OFNP), are used in plenum or air-handling spaces – for example, for cables running along ceilings or ventilation ducks. When exposed to fire, they should not produce too much smoke and should be flame-retardant.
  • General purpose cables are marked with a “G”, e.g. OFCG or OFNG.

The above mentioned codes do not specify optical properties of the contained fibers; these can be single-mode or multimode fibers, for example, of any type.

Applications of Fiber Cables

Optical fiber cables are often used in optical fiber communications. Large and strong cables are used for onshore and submarine data transmission, often bridging distances of thousands of kilometers (using fiber amplifiers). Smaller fiber patch cables can be used e.g. to connect the components of fiber-optic data links within buildings. The key advantages of fiber-optic cables over electrical data cables are the enormous transmission bandwidth and the low losses (particularly in the 1.5-μm wavelength region), but one may also profit from the immunity against electromagnetic interference and from the possibility to have fully insulating cables.

Nowadays, many fiber cables for data transmission have been laid down which are not or not yet used. This is partly because burying a fiber is much cheaper when a channel in the ground is already opened e.g. for laying electrical cables. One may prefer the risk of never used fiber cables over the other risk that additional ground work will be required later on.

There are also many “dark fibers” within fiber cables, i.e., fibers which are not (yet) used. They may be held in reserve for future use.

In laboratory and industrial setups, fiber cables are often convenient for transporting light from a source to an application – for example, from a high-power fiber laser to a welding robot in a car factory, or from an optical sampling head to a measurement instrument. For short-distance transmission of low optical powers, fiber patch cables are often used.

Bibliography

[1]R. Paschotta, tutorial on "Passive Fiber Optics"

(Suggest additional literature!)

See also: fiber patch cables, fibers, fiber optics, fiber connectors, optical fiber communications
and other articles in the category fiber optics and waveguides

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