Download Lasers - Sackville School

M. Manser (2014)
Module P1:
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Lasers, Morse code and signalling
(multiplexing)
M. Manser
Sackville School
Describe how light is transmitted through optical fibres
Describe the differences between analogue and digital signals
M. Manser
Sackville School
Lasers produce a focussed, intense beam of light.
All of the light waves in a laser are:
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the same frequency (this is why a laser is one pure
colour)
in phase with each other
This means that all the waves have a constant phase
difference.
M. Manser
Sackville Schooll
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CD players
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Communication with optic fibre cables
M. Manser
Sackville Schooll
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Compact Discs are made from a 1.2 mm thick disc of very pure
polycarbonate plastic.
CD data is stored digitally as a series of tiny indentations. On the
side the laser reads from, they are called bumps.
The incredibly small dimensions of the bumps make the spiral
track on a CD extremely long. If you could lift the data track off
a CD and stretch it out into a straight line, it would be 0.5
microns wide and almost 5 km (3.5 miles ) long!
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To read something this small you
need an incredibly precise discreading mechanism – a laser.
The laser focuses in on the
track of bumps (digital info).
The light from the laser is
reflected differently from the
different bumps. This produces
a digital signal.
A sensor detects the reflected
light and interprets this into an
analogue signal which is
amplified for us to hear.
http://electronics.howstuffworks.com/cd5.htm
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Light flashes have been
used to send messages as
MORSE CODE for a long
time. Signals would be
relayed between stations to
cover large distances
Pulses of electricity can also
be used
Pulses of light are a digital
signal since the light is
either ON or OFF. It cannot
be anything in between.
The Morse codes used for
the letters of the alphabet.
M. Manser
Sackville Schooll
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There are only 2 distinct values or states.
Examples would include, an SOS signal sent
with a flash light, a bar code on a bought
item, the power button on a radio.
Values of the signal can be 0 or 1, ON or
OFF, or HIGH and LOW.
A digital
wave signal
M. Manser
Sackville School
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Can have many different values
Examples include, price tags on clothes, the
volume control knob on a radio, a dimmer
switch.
An analogue
wave signal
M. Manser
Sackville School
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Morse code can also be sent using electrical
signals along copper wires.
Over long distances, the signal weakens
More power is needed to send electrical
signals long distances.
Electrical signals can get “mixed” resulting in
“crosstalk”
Optic fibre cables can carry more information
due to MULTIPLEXING.
M. Manser
Sackville Schooll
A fibre optic is a very narrow length of glass or plastic.
The light shines in at one end of the fibre and does not
emerge until it reaches the other end.
The light (or infra-red signal) is totally internally
reflected a number of times inside the cable. It
cannot be seen until it leaves the fibre.
Lasers can send millions or billions of signals of pulses down
an optical fibre every second. So a fibre can carry millions
of calls at the same time.
 The message is divided up into a series of batches or
pulses.
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The laser flashes the first batch of your message, then a
batch from someone else’s, then a batch from a third
message.
M. Manser
Sackville Schooll
M. Manser
Sackville Schooll