Technical data

www.westermo.com Theoretical and general applications 47
Fibre Optic Communications
The greatest advantage of fibre cable is that it is completely insensitive to electrical
and magnetic disturbances. It is therefore ideal for harsh industrial environments. It
provides reliable transmission and has a very high data transfer capacity. Fibre
cable can be used on specific sensitive sections of networks and be combined
using a modem with, for example, 4-wire cable in a system. The investment to
install a fibre network is still slightly higher than copper wire, but it offers many benefits,
however the market is growing and prices are dropping.
The Westermo range of fibre products converts electrical signals to light, which are
then transferred to the cable via a fibre optic transmitter with a light emitting diode or
laser. It is possible to communicate over longer distances when using a laser and at
higher speeds. However, laser diodes are more expensive components and for this
reason light emitting diodes are in more widespread use. The receiver houses a photo-
diode, which converts the light pulses back into electrical signals.
Fibre cable
In principle, a fibre cable is made up of two types of glass with different refractive
indices. The central part is known as the core and the surrounding part the cladding.
When a light pulse enters the fibre, the pulse is reflected through the cable as the
boundary between the two lay-
ers act like a mirror provided
that the angle of incidence of
the light entering the fibre is not
too great.
The core and cladding of the
fibre cable are enclosed by an
outer sheath, whose sole task is
to protect the fibre from exter-
nal influences.
Selection of a cable depends on functions such as
… The material
… Singlemode or multimode
… Step or graded index
… Wave length of the transmitter
Buffer
Aramide fibres
Outer coating
Glass fibre
Cladding refractive index n
1
Buffer (jacket)
Core refractive index n
2
Light rays
Acceptance
angle
B A C K