The Basics about Fibre Optics

Fibre optic cable consists of a glass / plastic core through which light is guided. A material covering called ‘Cladding’ surrounds the core and this has a refractive index (approx 1% less than the core) which confines the light to the core, so achieving the Total Internal Reflection needed to allow the fibre to transmit light rays.

Even though Fibre has massive benefits over copper in terms of bandwidth. As with everything there are limits linked to achievable bandwidth and cable lengths, yet the effective use of repeaters can over-come such problems. Transmission of light by fibre optics is not 100% efficient due to 2 main reasons, leaking of light from cladding and absorption of light into cladding. Both of these problems lead to signal reduction / attenuation.
Light can travel through an optical fibre in a stable number of ways or modes. Light entering a fibre at a specific angle will travel through the fibre following a particular path, this will be done with a specific number reflections off the cladding. If, however, the angle of light entering the fibre changes then the light will follow a different path with fewer or even more reflections.
An increase in core diameter leads to an increase in modes and therefore equals an increase in the number of ways the light can travel through the fibre. This type of fibre is called multi-mode, whereas a reduction in core diameter equals a decrease in the in the number of ways the light can travel through the fibre. This type of fibre is called single-mode. Therefore bandwidth of a single-mode fibre is much higher than that of a multi-mode fibre.

Multi-Mode Fibre

Multi-mode fibre has the following characteristics:
As mentioned Multi mode fibre has a lower bandwidth than single mode, which obviously has an relative effect on cost, making the multimode a cheaper option. This fibre work very efficiently in transmitting a light source to a receiver to work with low-cost light emitting diodes or LED’s as well as this when working with Multi mode fibre high precision connections are not required because the large core diameter allows wide-tolerance on mechanics.

Single-Mode Fibre

The characteristics of single-mode fibre are:
The Bandwidth of single mode fibre is much higher allowing with much lower attenuation and greater unrepeated transmission over long distances, however, this comes at a cost as very high precision equipment and connectors are required.