The use of fibre optic cables has grown rapidly over recent years, particularly in the areas of telecoms and data networking. With the cost of the fibre transmitters and receivers dropping to acceptable levels, the benefits of using this extremely high bandwidth technology has also been seen to be of advantage in the CATV market for long runs where repeaters or boosters may otherwise be needed with coax.

But what actually is a fibre optic cable?
In reality, it is no more than an extremely thin glass rod covered with a cladding to protect it from the environment and to stop light escaping from it. This glass rod comes in 3 different sizes with diameters of 9 microns, 50 microns and 62.5 microns. The cladding over this brings the diameter of each up to 125 microns. The fibre can then be specified by the size you need. Either 9/125 micron, (also known as Single Mode fibre), or 50/125 micron or 62.5/125 micron (these being known as Multimode fibre).

Data is sent along a Single Mode fibre using a laser as the light source that transmits light pulses in a single beam along the centre of the fibre. Such fibres have a very low attenuation (approx 0.3dB/Km) and a very high bandwidth. As an example, they will transmit 10Gbps Ethernet over a distance of 3Km.

Multimode fibre uses an LED as the light source and this transmits a much wider scattering of light along multiple pathways in the fibre. This scattering of light in a Multimode fibre results in some light travelling along the centre of the fibre arriving at the receiver before light that is reflected internally along the fibre. This results in a poor quality of reception, and Multimode fibres are therefore “doped” to produce a fibre that is optically more dense at the centre, graduating out to the edges of the fibre. This effectively “slows down” the light travelling at the centre of the fibre so that it arrives at the receiver at the same time as light travelling along the outer part of the fibre.

All sizes of fibre can then be made up into either Loose Tube or Tight Buffer constructions. Loose Tube fibres are those that are laid inside a gel-filled tube. The gel acts as a water blocker and also as a protection for the fibre against crushing. These are commonly used outdoors. Tight Buffer fibre has a very tight extra cladding put around the fibre to protect it against impact. These types are commonly used indoors.

Terminations can either be done on site or by means of pre-terminated fibre. The latter is particularly useful as a cable can be delivered to site already terminated and fully tested, ready to install and run without the need of investing in termination equipment and training.

So, why do we use fibre?
1. It has a huge bandwidth and a low attenuation.
2. Unlike copper cables, fibre is immune to EMI and RFI.
3. Unlike copper cables, it is immune to lightning.
4. It is lightweight.
5. It offers secure data transmission.

Belden fibre optic cables are available in the UK from Solutions Group.