Download optical fibres

So what is an optical fibre?
An optical fibre is a strand of material (commonly glass fibre)
through which light or infra–red radiation can travel.
The diagram below shows thin strands of plastic optical fibres on the end
of a torch.
Optical fibres carrying light from a torch.
As you can imagine, a long thin strand of glass the thickness of a hair is
not very strong. For this reason, optical fibres require cladding.
The cladding needs to be of lower refractive index than the strand of
glass to ensure total internal reflection within the glass fibre.
There are three main types of optical fibres used for telecommunications.
Multimode optical fibre
total internal reflection
of several light impulses
within the optic fibre
cladding with lower
refractive index
than optical fibre
optic fibre
light entering
cladded optical
fibre
Multimode optical fibre.
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Information systems
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Thousands of different digital transmissions may be sent along a
multimode optical fibre. The light impulses are often sent in the
infra–red band of the electromagnetic spectrum. The glass inside the
cladding has a consistent refractive index throughout the fibre.
Graded optical fibre
optic fibre has a graded
density causing light to travel
faster on the edges of the
fibre than the centre
cladding with lower
refractive index or
optical density
than optical fibre
graded optic fibre
Graded optical fibre.
Imagine running 100 metres, but instead of running in a straight line, you
zig–zagged across the track to the finish line. Would you have travelled
further than if you had run in a straight line? Of course you would have.
Light impulses bouncing off the edges of an optical fibre would also
travel much further than light that barely touched the edges. This causes
impulses to arrive at their destination at slightly different times.
Graded optical fibres have a higher refractive index material towards the
centre of the fibre. The refractive index gradually decreases towards the
outer edges of the fibre. This causes light inside the optical fibre to
appear to curve, although light is still travelling in a straight line.
The result of graded optical fibres is all the light transmissions,
regardless of how many times they are internally reflected, arrive at their
destination at the same time. This prevents time delays in transmission.
Quality of output information is better than for the multimode optic fibre.
Part 6: Fibre optics
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Small diameter core
small diameter core optical fibre (3–5 mm)
cladding
Small diameter core.
The glass strand inside this optical fibre is only 3–5 µm (3 –5 x 10–6 m)
in diameter and only transmits a single impulse at any one time.
Only one light wave can fit inside the inner core at one time. An impulse
of light travels through the high refractive index material centre.
Because there is no interference with other waves and the light contained
inside the core cannot spread out due to the size of the core, the quality of
the information transmitted is excellent and does not need to be boosted
for 500 km. Small diameter optical fibres are therefore often used in
long distance cables, however the precision lasers required to beam the
impulses through these fibres and the technology required to align
their tiny cores cause them to be more expensive than other optic
fibre varieties.
1
What is the difference between a multimode optic fibre and a graded
optic fibre?
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2
Why is an optic fibre strand encased in cladding?
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3
What problem does graded optical fibre attempt to overcome?
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4
Which type of optical fibre is more effective in long distance
telecommunications and why?
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You have identified a glass strand and its cladding however optical fibres
are surrounded by several more layers for strength and durability.
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6
Label the diagram below with the following:
•
the thinnest tube is the glass core and cladding combined;
together they form the optical fibre
•
the next layer is silicone
•
the next layer is the buffer jacket
•
the second last layer is the strength layer
•
the outside layer is polyurethane
Label the above diagram as a single optical fibre.
Check your answers.
Even with the protective layers, optical fibres are quite flexible.
Rarely is one optical fibre laid alone in telecommunications. Bundles of
optical fibres such as the one above are usually encased inside a cable in
telecommunications.
Turn to Exercise 6.1 at the back of this part to summarise the properties of
optical fibres.
Part 6: Fibre optics
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