Fibre Cables for High Speed Transmission Over Long Distances

An optical fibre is a glass or plastic fibre designed to guide light along its length by confining as much light as possible in a propagating form. In fibre cables with large core diameter, the confinement is based on total internal reflection. When this principle is applied to the construction of the fibre optic strand, it is possible to transmit information down fibre lines in the form of light pulses. Fibre cables are widely used in fibre-optic communication, which permits transmission over longer distances and at higher data rates than other forms of wired and wireless communications.

Fibre cables became practical for use in communications in the late 1970s, since then, several technical advances have been made to improve the attenuation and dispersion properties of optical fibres (i.e., allowing signals to travel farther and carry more information), and lower the cost of fibre cables.

The size of the optical fibre cable is commonly referred to by the outer diameter of its core, cladding and coating. Example: 50/125/250 indicates a fibre with a core of 50 microns, cladding of 125 microns, and a coating of 250 microns. The coating is always removed when joining or connecting fibres.

Fibre cables can be identified by the type of paths that the light rays, or modes, travel within the fibre core. There are two basic types of fibres: multimode and single-mode.

Multimode fibre cores may be either step index or graded index. Step index multimode fibre derives its name from the sharp step like difference in the refractive index of the core and cladding.

In the more common graded index multimode fibre the light rays are also guided down the fibre in multiple pathways. But unlike step index fibre, a graded index core contains many layers of glass, each with a lower index of refraction as you go outward from the axis.

The result is that a graded index fibre equalizes the propagation times of the various modes so that data can be sent over a much longer distance and at higher rates before light pulses start to overlap and become less distinguishable at the receiver end.

Graded index fibres are commercially available with core diameters of 50, 62.5 and 100 microns. The single mode fibre allows only a single light ray or mode to be transmitted down the core. This virtually eliminates any distortion due to the light pulses overlapping. The core of the single mode fibre is extremely small, approximately five to ten microns.

The single mode has a higher capacity and capability than either of the two multimode types. For example, undersea telecommunications cables can convey 60,000 voice channels on a pair of single mode fibres.

Fibre cabling is the medium on which the broadband technology is based. Fibre cables have the ability to transmit multiple pieces of data simultaneously and to carry signals from different network carriers. It is simply amazing to see the many uses of Fibre optic cables.

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