Garth Naar - The Construction of the Fibre Optic Cables

Fibre optic technology relies on the fact that it is possible to send a light beam along a thin fibre suitably constructed. A fibre optic cable consists of a glass or silica core. The core of the optical fibre is surrounded by a similar material, i.e. glass or silica, called the cladding, that has a refractive index that is slightly lower than that of the core. Garth Naar says it is found that even when the cladding has a slightly higher refractive index, the light passing down the core undergoes total internal reflection, and it is thereby contained within the core of the optical fibre.

The Outside the cladding there is placed a plastic jacket. This is used to provide protection to the optical fibre itself. In addition to this, optical fibres are usually grouped together in bundles and these are protected by an overall outer sheath. This not only provides further protection but also serves to keep the optical fibres together.

Optical fibre types

There is a variety of different types of fibre optic cable that can be used, and there are a number of ways in which types may be differentiated. There are two major categories:

Step index fibre optic cabling

Graded index fibre optic cabling

The step index cable refers to cable in which there is a step change in the refractive index between the core and the cladding. This type is the more commonly used. The other type, as indicated by the name, changes more gradually over the diameter of the fibre. Using this type of cable, the light is refracted towards the centre of the cable.

Optical fibres or optical fibers can also be split into single mode fibre, and multimode fibre. Mention of both single mode fiber and multi-mode fiber is often seen in the literature.

Single mode fiber     This form of optical fibre is the type that is virtually exclusively used these days. It is found that if the diameter of the optical fibre is reduced to a few wavelengths of light, then the light can only propagate in a straight line and does not bounce from side to side of the fibre. As the light can only travel in this single mode, this type of cable is called a single mode fibre. Typically single mode fibre core are around eight to ten microns in diameter, much smaller than a hair.

Single mode fiber does not suffer from multi-modal dispersion and this means that it has a much wider bandwidth. Garth Naar says the main limitation to the bandwidth is what is termed chromatic dispersion where different colours, i.e. Wavelengths propagate at different speeds. Chromatic dispersion of the optical fibre cable occurs within the centre of the fibre itself. It is found that it is negative for short wavelengths and changes to become positive at longer wavelengths. As a result there is a wavelength for single mode fiber where the dispersions is zero. This generally occurs at a wavelength of around 1310 nm and this is the reason why this wavelength is widely used.

The disadvantage of single mode fibre is that it requires high tolerance to be manufactured and this increases its cost. Against this the fact that it offers superior performance, especially for long runs means that much development of single mode fiber has been undertaken to reduce the costs.

Multimode fiber     This form of fibre has a greater diameter than single mode fibre, being typically around 50 microns in diameter, and this makes them easier to manufacture than the single mode fibres.

Multimode optical fiber has a number of advantages. As it has a wider diameter than single mode fibre it can capture light from the light source and pass it to the receiver with a high level of efficiency. As a result it can be used with low cost light emitting diodes. In addition to this the greater diameter means that high precision connectors are not required. However this form of optical fibre cabling suffers from a higher level of loss than single mode fibre and in view of this its use is more costly than might be expected at first sight. It also suffers from multi-mode modal dispersion and this severely limits the usable bandwidth. As a result it has not been widely used since the mid 1980s. Single mode fiber cable is the preferred type.

Attenuation within an optical fibre

Although fibre optic cables offer a far superior performance to that which can be achieved with other forms of cable, they nevertheless suffer from some levels of attenuation. This is caused by several effects:

Loss associated with the impurities     There will always be some level of impurity in the core of the optical fibre. This will cause some absorption of the light within the fibre. One major impurity is water that remains in the fibre.

Loss associated with the cladding     When light reflects off the interface between the cladding and the core, the light will actually travel into the core a small distance before being reflected back. This process causes a small but significant level of loss and is one of the main contributors to the overall attenuation of a signal along an fibre optic cable.

Loss associated with the wavelength     It is found that the level of signal attenuation in the optical fibre depends the wavelength used. The level increases at certain wavelengths as a result of certain impurities.

Despite the fact that attenuation is an issue, it is nevertheless possible to transmit data along single mode fibres for considerable distances. Lines carrying data rates up to 50 Gbps are able to cover distances of 100 km without the need for amplification.