OPTICAL FIBRES

Optical fibres are used to carry signals in the form of pulses of light over distance from up to 50km. they do this by total internal reflection. that's why optical fibres can guide light for such long distance because the walls of the fibre don't absorb any light at all as long as the angle of incidences is greater than the critical angle.

Two conditions are necessary for the total internal reflection :

• The angle of incidence must be greater than the critical angle (i>C)
• The Refractive index of the first medium is greater than the Refractive index of the second medium (N1>N2).

A fibre optic cable is made from a glass core that carries the light surrounded by glass cladding of lower  refractive index , which reflects escaping light back into the core. Without the cladding the light would pass between the fibres as they are all made of glass. This would not have a a lower refractive index boundary  for total internal reflection to occur at and therefore would allow light transfers. they are about 120 micrometers in diameters

Fibres optic cable can carry signal with much less energy loss than normal cable wires loose than normal cable wires looses signal energy as heat due to resistance.

they have the same bandwidth and less pace is required in cable duct.

they are immune to electromagnetic interference from radio signals.

they can be routed safely through exposing and flammable atmosphere

optical fibres are more expensive per meter than copper and is manufactured to high standard

they cannot be joined together as easily as copper cable installed by specialist.

In single mode fibres the light can only take one route down the fibre , which means there is less pulse broadening. They are used for High-data rate, long distance systems. Because the core is so small, single mode cables need high precision connectors and highly directional light sources. Single mode fibres have a very small acceptance angle. They are usually used with high power lasers. Single mode system will usually be more expensive than multi mode system.

In multi mode fibres the light can take many different routes through the fibre. The lengths of these paths will not be the same, so all the light Will not arrive at the detector at the same time, resulting in pulse broadening. the longer the fibre is, the greater difference there will be in the lengths of the light paths, so the more the pulse will spread. As a result, there is a trade off between length of fibre and the data ranges that can be transmitted- the longer the fibre, the lower the data rates have to be.

Graded index fibres overcome this problem by having a refractive index which changes gradually at the boundary between the core and the cladding.

Multi mode graded fibres have higher bandwidth and can transmit over longer distances than multi mode step-index fibres. Multi mode step-index fibres are rarely used in modern telecommunications.