 You can send light down any piece of glass tube.
However there is likely to be leakage of light through
imperfections in the surface
 The simplest optical fibres used for communications
are made of two layers of glass or plastic.
 They have a core with a diameter of 50 µm - 200µm
which is covered in a cladding which is usually 25µm
thick. The cladding is usually covered with a
protective sheath.
 The image above shows a step-index
fibre.
 They are called step index because
there is a sudden change in (or step) in
the refractive index between the core
and the cladding
 The refractive index of the cladding is
lower than that of the core.
 Total internal reflection keeps the light
rays inside the core.
 The image above shows a step-index
fibre.
 They are called step index because
there is a sudden change in (or step) in
the refractive index between the core
and the cladding
 The refractive index of the cladding is
lower than that of the core.
 Total internal reflection keeps the light
rays inside the core.
 Step index fibres have the disadvantage
that light rays travel different distances
along the fibre, depending on the angle
that the light enters.
 As a result the parts of a signal transmitted
at the same moment arrive spread out over
time (see Fig 1A)
 A sharp square wave signal becomes
degraded – this is called multimode or
multipath distortion.
Figure 2
a) a square
wave signal as it
enters a fibre
b) a degraded
signal as it
emerges from a
step index fibre
Figure 1
 A graded index fibre helps reduce
multipath distortion by gradually
reducing the refractive index from
the core to the cladding (Fig 1B)
 This gradual change makes the
rays of light follow a curved path
 All the rays of light arrive at the
same time and this helps to
reduce the distortion