Communications project

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Communications project

By Ruben Thumbadoo 9S

Contents

Communication System

Fibre-Optics

Signals

Storage

Transducers

History of Communication

Radio Waves

Modulation

Satellites

Communication System

In an Communication system there are many building blocks which create the whole system. There are:

Encoder

An  puts a signal into code before it is transmitted. This can increase security.

Modulator

Subtracts the  (the radio wave) from the incoming signal so that the information it is carrying can be used e.g. as a message or radio/TV programme.a

Decoder

Device that decodes a message once it is received.

Storage

Records the data to be used later.

Transmitter

A  takes a signal and sends it to a .
This may be a television
 sending the  carrying TV programmes to your television aerial. There is a  in your mobile phone that sends the signal to the nearest mobile phone mast (antenna).For light signals the  is a  that sends light along the .

Receiver

A  is a device that will receive the transmitted signal e.g. a radio or mobile phone.

Transducer

A  is a device that takes an  and changes it into a different type of signal. For example  is a  that changes sound into an electrical signal. A  is a  that takes an electrical signal (varying electrical current) and changes it into sound (due to the movement of the  cone).

Amplifier

Device that increases the  of a signal by adding energy to it e.g. making a sound louder

Fibre-Optics

You hear about fibre-optic cables whenever people talk about the , the  or the Internet. Fibre-optic lines are strands of optically pure glass as thin as a human hair that carry digital information over long distances. They are also used in medical imaging and mechanical engineering inspection.

In this article, we will show you how these tiny strands of glass transmit light and the fascinating way that these strands are made.

What are Fibre Optics?
Fibre optics (optical fibres) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit  signals over long distances.

If you look closely at a single optical fibre, you will see that it has the following parts:

Core - Thin glass centre of the fibre where the light travels

Cladding - Outer optical material surrounding the core that reflects the light back into the core

Buffer coating - Plastic coating that protects the fibre from damage and moisture

Hundreds or thousands of these optical fibres are arranged in bundles in optical cables. The cable’s outer covering, called a jacket, protects the bundles.

Optical fibres come in two types:

Single-mode fibres 

Multi-mode fibres 

See  for a good explanation.

Single-mode fibres have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) and transmit infrared  light (wavelength = 1,300 to 1,550 nanometers). Multi-mode fibres have larger cores (about 2.5 x 10-3 inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300 nm) from  (LEDs).

Some optical fibres can be made from plastic. These fibres have a large core (0.04 inches or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.

Let's look at how an optical fibre works.

How Does an Optical Fibre Transmit Light?
Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the beam straight down the hallway -- light travels in straight lines, so it is no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway is very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is exactly what happens in an optical fibre.

The light in an optic-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fibre, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibres show much less signal degradation -- less than 10 percent/km at 1,550 nm.

Optic-Optic Relay System
To understand how optical fibres are used in communications systems, let's look at an example from a World War II movie or documentary where two naval ships in a fleet need to communicate with each other while maintaining  silence or on stormy seas. One ship pulls up alongside the other. The captain of one ship sends a message to a sailor on deck. The sailor translates the message into Morse code (dots and dashes) and uses a signal light (floodlight with a venetian blind type shutter on it) to send the message to the other ship. A sailor on the deck of the other ship sees the Morse code message, decodes it into English and sends the message up to the captain.

Now, imagine doing this when the ships are on either side of the ocean separated by thousands of miles and you have an optic-optic communication system in place between the two ships. Optic-optic relay systems consist of the following:

Transmitter - Produces and encodes the light signals

Optical fibre - Conducts the light signals over a distance

Optical regenerator - May be necessary to boost the light signal (for long distances)

Optical receiver - Receives and decodes the light signals

Transmitter
The
transmitter is like the sailor on the deck of the sending ship. It receives and directs the optical device to turn the light "on" and "off" in the correct sequence, thereby generating a light signal.

The transmitter is physically close to the optical fibre and may even have a lens to focus the light into the fibre. Lasers have more power than LEDs, but vary more with changes in temperature and are more expensive. The most common wavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the ).

Optical Regenerator
As mentioned above, some
signal loss occurs when the light is transmitted through the fibre, especially over long distances (more than a half mile, or about 1 km) such as with undersea cables. Therefore, one or more optical regenerators are spliced along the cable to boost the degraded light signals.

An optical regenerator consists of optical fibres with a special coating (doping). The doped portion is "pumped" with a . When the degraded signal comes into the doped coating, the energy from the laser allows the doped molecules to become lasers themselves. The doped molecules then emit a new, stronger light signal with the same characteristics as the incoming weak light signal. Basically, the regenerator is a laser amplifier for the incoming signal. See  for more details.

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Optical Receiver
The
optical receiver is like the sailor on the deck of the receiving ship. It takes the incoming digital light signals, decodes them and sends the electrical signal to the other user's ,  or  (receiving ship's captain). The receiver uses a photocell or photodiode to detect the light.

Advantages of Fibre Optics
Why are optic-optic systems revolutionizing telecommunications? Compared to conventional metal wire (copper wire), optical fibres are:

Less expensive - Several miles of optical cable can be made cheaper than equivalent lengths of copper wire. This saves your provider (cable TV, Internet) and you money.

Thinner - Optical fibres can be ...

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