Download Optical Fibre

Telecommunication
1.2 – Communication Using
Cables
Communication Using Cables
Electrical cables and optical fibre cables
used in some telecommunication systems.
transmitted (sent) by a transmitter and
received (detected) by a receiver.
Signals are
are
Electrical Cables
An electrical
cable contains many
metal wires side by side.
These
are covered in plastic so that electric
current cannot pass from one to
another.
An electrical signal can be transmitted (sent) along a
metal wire at a speed much greater than the speed of sound:
almost 3x108
ms-1 (300,000,000 ms-1).
A microphone changes
sound
energy to
electrical
energy.
Electrical signals pass along the metal wire of the microphone and can
be displayed as a pattern on the screen of an oscilloscope.
quiet sound
loud sound
Quiet sounds
Loud sounds
have a low
have a high
energy, so the energy, so the
pattern has a
low amplitude.
pattern has a
high
amplitude.
low frequency
sound
high frequency
sound
Low
frequency
High
frequency
sounds (e.g. a
groan) have a
low number of
sounds (e.g. a
scream) have a
high number of
every second so
the pattern has
a low number of
wavelengths.
every second so
the pattern has
a high number
of
wavelengths.
vibrations
vibrations
Optical Fibres
An optical fibre is a long, flexible length
of pure glass about 0.1 mm in diameter (smaller
than human hair).
An optical fibre cable contains many optical
fibres side by side. No light escapes from the
outside of the optical fibre, so they are not
covered in plastic.
A light
signal can be transmitted (sent) along an optical
fibre at a very high speed of 2x108 ms-1 (200,000,000 ms-1).
This is not as fast as the speed of light through air.
How does the speed of the light
signal in an optical fibre
compare with the speed of the electrical signal in a metal
wire?
Answer
The speed of a light signal in an optical fibre is slower than the speed
of an electrical signal in a metal wire.
Reflection of Light
Light travels in straight
lines called rays.
When a light ray hits a
surface like a plane mirror, the
light ray is reflected off
the surface.
A normal line is a dashed line
drawn at 90° to a surface
where a light rays hits the
surface.
plane mirror
i
r
normal
line
During reflection, the angle
of incidence (i) is always
equal to the angle of
reflection (r).
i = r
Total Internal Reflection
An optical fibre has a dense solid glass core surrounded by a less dense
solid glass coating. Unlike a mirror there is not a silvered surface in
the optical fibre.
When a light ray hits the boundary between the core and coating at an
angle
ray is
greater than thei critical angle
reflected iback into the fibre.
This is known as
total internal reflection.
all of the light
Electrical Wires v Optical Fibres
Optical
Fibre
Notes
Smaller Size

more fibres in 1 cable
Least Weight

easier to lay
Transmits More
Signals

less cables required
Clearest & Highest
Quality Signal

less interference
Less Signal Loss
per km

fewer repeaters
Property
Fastest Signal
Speed
Electrical
Cable

Tutorial 1 – Distance, Time and Speed of Light Calculations
Involving Optical Fibres
1. It takes light 250 μs to travel 50 km along an optical fibre.
Calculate the speed of light in the fibre.
2. A pulse of laser light takes 0.3 ms to travel along an optical fibre.
How long is the fibre?
3. Calculate the time it takes for light to travel along a 100 km length
of optical fibre.
4. Calculate the speed of light in a 75 km optical fibre assuming it
takes 375 μs to travel through the fibre.
5. If light takes 2 ms to travel along an optical fibre system, what
distance does the light travel?
6. How long does it take a pulse of laser light to travel the entire
length of a 250 km optical fibre telecommunications system.
Electrical Cable Telecom Systems
The Morse Code Telegraph
The Morse Code Telegraph was the first form of
coded messages as
electrical signals through metal wires.
telecommunication system to send
By pressing a tap key at one end of the metal wires, a pulse of
electrical current was sent through the wires to a buzzer at the
other end.
The Code
Made up of short (dots) and long
(dashes) presses.
Letter
A
B
C
Symbol
Battery/Power Supply
Provides
electrical
energy.
Tap Key
This is a transmitter which
sends out coded messages
Long Metal Wires
These carry the coded
message as pulse of
electrical energy from
the tap key ( transmitter)
to the buzzer (receiver)
Buzzer
This is a receiver which
picks up the coded message.
The Telephone
The telephone is an example of long
range communication
(telecommunication) between a transmitter and receiver.
electrical signals are
transmitted (sent) along metal wires at a speed of almost
3x108 ms-1.
During telephone communication, changing
The electrical
signal sent along
the wires keeps changing because the
loudness and frequency of our
voice changes during a conversation.
Long Metal Wires
Carry the changing electrical signals
as an electric current.
The signals are changing as our voice
changes ( loudness and frequency ).
Mouthpiece
It sends out electrical
signals through the metal
telephone wires.
Energy change in the
microphone is:
sound to electrical .
Battery/Power Supply
Provides electrical energy.
Earpiece is the telephones
receiver.
It contains a loudspeaker.
It picks up electrical signals
from the metal telephone
wires.
Energy change in the
loudspeaker is:
Mouthpiece is the
telephones transmitter .
It contains a microphone .
Earpiece
electrical to sound .
Repeater Unit
(amplifier)
The electrical signals
lose energy as they
travel along the metal
wire.
These signals have their
energy increased every
4km or so to be strong
enough to be detected
by the receiver
Optical Fibre Telecom System
An optical fibre telecommunication system sends coded signals as
pulses of laser light through optical fibres.
Electrical to Light
Converter
Changes electrical signals
to coded light signals.
Optical Fibre
Metal Wire
Metal wire carrying the
changing electrical signal
from transmitter.
Coded light signals carried
through optical fibre by
total internal reflection.
Repeater Unit (amplifier)
As the coded light signal travels
along the optical fibre, it loses
energy.
The signal needs to have its energy
increased every 100km so the
signal is strong enough to be
detected by the receiver.
Light to Electrical
Converter
Changes coded light
signals to electrical
energy.
Metal Wire
Metal wire carrying the
changing electrical
signal to receiver.