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Optical fibre transmission involves changing electrical signals into pulses of light, using an opto-electronic transmitter, and sending the pulses down the core of an optical fibre. Because the core and its surrounding cladding glass have different compositions, the light is trapped within the core. It has nowhere to go but down the length of the fibre. At the opposite end, a receiver changes the pulses back to electrical signals.

When it comes to network installation, optical fibre cable offers many benefits. First, its small size and lightweight actually make it easy to install. Despite some lingering misconceptions, optical fibre cable is quite strong. Its pull strength is 200 lbs. for a two-fibre cable. And the bend radius for two-fibre cable and four-pair UTP copper cable is the same.

Besides, current cable designs provide ample protection for fibres, so there's little need to worry about damage. Recently, for example, a crane was driven into an installation site and stopped on top of a Siecor optical fibre cable and the crane executed a 900 turn while on top of the cable, before cable installers could stop the crane. The fibre cable was slightly deformed but all the fibres remained operational.

Perhaps most important, optical fibre is reliable enough to be virtually worry-free. Because it is dielectric, fibre eliminates most of the concern over factors affecting link performance. It is immune to crosstalk, electromagnetic interference (EMI), radio frequency interference (RFI), impedance mismatches, transmission frequency variability, and ground loops (all pitfalls of copper-based systems).

Fibre immunity to EMI means installers need have few concerns about where they run optical fibre cable. No need to worry about coming too close to electric motors or fluorescent lights, for example. This helps make for smoother, simpler jobs, with easier installation, fewer testing problems, and no callbacks.

Only four performance factors should concern installers. The four are bandwidth; environmental effects, such as temperature dependence; continuity (unbroken transmission of a signal from one point to another); and attenuation (acceptable signal loss over distance).

Installers need concern themselves only with continuity and attenuation. Using quality optical fibre, cable, and connectors helps to minimize these concerns, or even prevent them altogether. Besides, testing for continuity and attenuation is very simple.

The key to fibres worry free attributes is a high quality fibre manufacturing process. This will ensure consistency in optical and mechanical performance; purity, which provides strength and low attenuation; and control of the physical attributes of the fibre. So, those who choose cable need to be aware that, just as all cable is different, the fibres inside are not all created equal.

One process for manufacturing fibre involves using ultrapure, vapor-deposited chemicals. Some companies use computer systems to continuously measure each fibre dimensions along its entire length, while others measure at either end of a spool.

Whatever method is used, the important result is predictable consistency in fibre profile and geometry. Fibre geometry, the physical characteristics of a fibre, is vital when connectorising or splicing, joining the core of one fibre with the core of another while losing as little light as possible. Consistent fibre geometry helps get splices right the first time during installation.

In a large core fibre such as multimode, rays of light may travel along the fibre at many different angles / following different paths. These different paths are called Modes, and so a fibre, which supports many of these modes, is known as Multimode fibre.

In singlemode fibre all the light follows the exact same path right along the axis of the fibre. This means that the light / laser cannot spread out, as they do in multimode fibre, and therefore this means that singlemode fibre has a very much higher bandwidth than multimode.

The three main parameters, which characterise the performance of optical fibres, are: -

The amount of light that can be launched into the fibre depends upon the area of the core of the fibre, as well as the output power of the light source and how well it is aligned to the fibre.

As the light travels along the fibre it fades away. This is due to absorption by the impurities in the fibre, scattering losses from minute variations in the silica structure, & bending losses. It is the attenuation or loss of the fibre that effects how far the signal may travel down before being boosted up.

As the individual pulses of light which make up a digital signal along the fibre they suffer some distortion. In multimode fibres this is due to the energy travelling in different modes, so some of the light energy in a pulse travels along a longer path than other parts of the pulse. This is called modal dispersion and is the limiting factor of the bandwidth of a multimode fibre.

The central part of the fibre where the light is carried is called the core. The core is usually made from pure silica.

The cladding surrounds the core and is usually made from pure silica, which will have a lower refractive index than the core. The core and the cladding cannot be physically separated, they appear as one solid filament.

The primary coating is an acrylate layer over the fibre, which is applied as the fibre, is manufactured. It protects the silica from scratches or other damage which would weaken the fibre, its other use is for identification, i.e. colour coding.

The key dimensions of fibre geometry are cladding outer diameter, core/cladding concentricity, and cladding non-circularity.

Outer diameter.

Tight tolerances in cladding outer diameter determine the precision with which each fibre fits into ferrule type connectors. If the fiber is too thick, it won't fit, and connectorising times are increased. If it is too thin, the cores won't align properly, and power losses are increased. In effect, tight tolerances on both the fibre and the connector optimize link loss performance.

Core/cladding concentricity.

Core/clad concentricity is a measure of how well the fibre core is centered in the cladding glass. Because the outer cladding is referenced when connectorising and splicing to align the two cores, tight centering tolerances translate to closer alignments and less power loss. Cladding non-circularity. The uniform ovality of the cladding glass is known as cladding noncircularity. Consistency in noncircularity along with cladding diameters ensures successful connectorisation of optical fibres.

Will be added This Month...........Honest Gov

Revised: Saturady, 3^rd June 1999