Technical data

48 Theoretical and general applications www.westermo.com
Material
The material used for the core and cladding differ on different types of fibre. The most
common material used is glass. The glass used for these is extremely pure, silicon diox-
ide (silica). Other types of cable are PCS (Plastic-Clad Silica) with a core of glass and an
outer cladding of plastic, or a plastic fibre cable with both the core and outer cladding
of plastic.
Glass cable gives the best performance, but is more complicated to terminate. Plastic
fibre on the other hand is easier to terminate, but offers the worst performance.
Attenuation in multimode fibre
Different thickness’ of core material form different types of fibre cable. There are two
main types that you should be aware of, these are multimode and singlemode fibre.
Multimode
A multimode fibre has a dimension that affords space for several modes in a core.
Multimode cables are available in two categories, these are graded index and step
index. In a step index fibre, as modes reflect through the cable, some have to travel
further than others and in doing so the light pulse will spread. This is one disadvantage
which means the fibre has a lower bandwidth. The solution to this problem is graded
index. In these cables the refractive index reduces gradually from the core’s centre
towards the cladding. This means that a light beam travelling mainly in the centre of the
cable travels more slowly than those further out. The overall effect keeps the pulse
together.
The most common dimension
of multimode cable is a 62.5
µm core and 125 µm outer
cladding (the cable is then des-
ignated 62.5/125).
The most common singlemode
cable dimension is 9 µm core
and 125 µm cladding (9/125).
Transmitted
light pulse
Light paths in multimode graded index fibre Received
light pulse
B A C K