Fibre Optic Cable

• What is Fibre Optic Cable
• Advantages of Fibre Optic Cable
• Data Transmission over Fibre Optic
• Single Mode & Multi Mode Fibre
• Wave Division Multiplexing

Fibre optic cable consists of one or more optical fibres.  Each individual fibre is made from a very thin strand of glass which carry data signals using pulses of laser light.

A single optical fibre is about the thickness of a human hair (typically .025mm - 0.5mm) and consists of 3 layers:
 
  1). The core made from pure glass which carries data in the form of
       light signals.
  2). The cladding which is a layer of even purer glass with a lower
       refractive index that keeps light trapped in the core.
  3). The buffer - Acrylate plastic coating that protects the glass
       cladding against scratches which could cause light leakage.

It may then be covered with a plastic jacket and/or bundled with other fibres to make up a larger cable.  Various types of fibre optic cables can carry numerous amounts of optical fibres bundled together depending upon bandwidth requirements.

Construction of the cable may also vary, for example some cables may have steel armour coating, whilst others may use kevlar yarns - or even a combination of both - depending upon where it will be used and the flexibility of the cable requirements.

Transmitting data over fibre optic cable requires 2 main electronic components

• Transmitter - Converts electrical signals into light rays.
  Contains an optical source such as Light Emitting Diodes (LEDs) or Laser Diodes.
• Receiver - Contains an optical detector. Converts the pulses of light back to electrical signals.

A combination unit containing both transmitter and receiver is called a transceiver.

The transmitter converts electrical data into pulses of light which are beamed down an optical fibre. 

The ray of light is now trapped in the fibre core - light travels in a straight line - but if the light ray hits the wall of the core (eg if the cable has a bend) then the higher refractive index of the cladding reflects the light ray to keep it within the core. 

At the other end of the fibre, an optical detector converts the light pulses back into electrical data.

Depending on the fibre type and wave length of the light ray it can travel hundreds of km without any reduction in power of the light signal.  Attenuation is measured in dB/km. Repeaters and amplifiers can be used to extend reach even further.

Multi-mode cable can carry multiple light rays at the same time which means high bandwidth and high speeds.  Multi mode fibre has a larger core and light waves are dispersed into numerous paths called modes.  Each mode has a slightly different reflection angle.

Unfortunately using multiple modes causes modal dispersion which spreads the optical pulse and weakens the data signal at the receiving end.  For this reason Multi Mode fibre is seldom used over distances of more than 500m.

BTw uses a combination of the following cables in their network : Remote Single Mode Fibre, Local Single Mode Fibre, Multi-mode Fibre 62.5 μm @ 850nm.  Aqua jacketed cable is usually OM3 or OM4 (50/125) which are capable of 1Gbps @ 550/1000 metres respectively.