Communicating with light – fibre optics

Key text

The lights used to guide ships at sea are another way of using light as a signal. Each light gives out flashes of different timing and length so that sailors can tell which light they are seeing many kilometres away – just by timing the light and looking it up in a guide book.

But there are some major problems with using light this way to send messages. For one thing, even very powerful lights such as those used at sea (which can have millions of candlepower) can only be seen from a relatively short distance. This is partly because the Earth is curved, and partly because as light radiates out from a source it gradually becomes weaker because of the effects of dust and smoke in the air and because of the inverse square law of radiation.

Pushing light through a pipe

A cleverer way of using light to send messages is to push it through a pipe – much in the same way as water is moved around through pipes. This is where 'light pipes' – better know as optical fibres – come in. Optical fibres are long strands of transparent material which let the light pass through the middle. Of course, the light tries to get out (left to itself, light will always travel in a dead straight line) but the outer walls of the optical fibres act like a continual tube of mirror. So the light travels along the fibre bouncing off the mirror-like outer casing until it arrives at the other end of the fibre.

These optical fibres – which are thinner than a human hair – work when bent around corners, laid underground or even laid on the ocean floor. And because the light is contained within the walls of the fibre and can't disperse or radiate away, it takes very little light energy to send a signal over a long distance. In theory, if you had a single optical fibre that ran right across Australia, you could use a torch to flash a message to a person watching the other end! Also, as light travels at about 300,000 kilometres per second, you could use your torch to flash a signal right around the world in next to no time.

Of course, actually doing it is much more complicated than that, but that is the principle on which it works.

Three questions to consider

Three questions had to be answered when optical fibres were considered for use in telecommunications.

• How does light behave when it is sent through a long fibre?
• What sort of physical equipment do you need to make it work?
• How are you going to use the flashes of light to carry a message?

The 1997 Australia Prize winners worked in these areas (Box 1: The Australia Prize).

• How light behaves

  When light passes down an optical fibre, it continues to travel in straight lines – until it hits the mirror-like side of the fibre and bounces off. It then travels in a straight line again until it bounces off another part of the fibre wall. Because of the angle of reflection, the light cannot go back on itself so it must always go in the same direction, bouncing its way along until it reaches the end. Various things affect the way the light is transmitted – including the quality of the original light source, the exact composition of the transmitting fibre, and the material used for the walls of the fibre. Each of these qualities must be understood in order to predict what the light will do under differing circumstances.

• What sort of equipment do you need?

  To send a flash or 'pulse' of light along an optical fibre you need something to generate the light in the first place. You could use a hand-held torch but this would be very slow and inefficient. The optical fibre communications network uses lasers to generate a suitable light source. Lasers can produce very tightly focused pulses of light – and they can do it many times a second. The pulses are then picked up at the other end of the fibre by a light-sensitive cell which can convert the pulses of light into pulses of electricity. These pulses of electricity are then fed into a computer and decoded to reveal the message.

• Using the flashes of light to carry a message

  Simply flashing messages down an optical fibre on the 'one flash for yes, two flashes for no' principle would take a very long time. So, complex digital codes have been worked out to take advantage of the very high speed and volume of data that can be sent through an optical fibre (Box 2: Digital communication and Box 3: Binary numbers). Using a standard commercial system, it is possible to send the entire contents of the 32 volumes of the Encyclopaedia Britannica through an optical connection in less than one second! In fact, using a combination of codes, many messages can be sent along an optical fibre at the same time.

Optical fibres are now used in many telecommunications systems, so the next time you pick up a phone to speak – or use a computer modem to send a message – you may well be using an optical fibre system to do it.

Related Nova topic:

Wireless but not clueless

Box 1. The Australia Prize

Professor Allan Snyder

Early in his career, Professor Snyder studied how photoreceptors in the retina of the human eye transmit light images to the brain. At the time he had no interest in telecommunications, but in the course of his retina work, he was struck by the similar light transmission properties of photoreceptors and optical fibres.

This led him to study how light travels down an optical fibre. There was already a theory describing this, but because it was complex it was very difficult for engineers to apply it to optical fibres. Professor Snyder worked to simplify the theory.

Professor Snyder's work became the basis of the theory on light transmission in an optical fibre, which in turn made optical fibre technology possible. From this start, telecommunications was revolutionised, with millions of kilometres of fibre optic cable being laid around the world.

Professor Rodney Tucker

Professor Rodney Tucker specialises in creating devices which help control the transmission of light through optical fibres.

Professor Tucker's research in the early 1980s led to a ten-fold increase in the carrying capacity of the fibre optic network. His research resulted in a new class of devices which are now used internationally.

Professor Tucker sees the day when telecommunications will allow people to be in two places at once (well almost). Their 'real' self will be almost indistinguishable from their 'virtual' self somewhere else. Professor Tucker calls this 'telepresence'.

Telepresence will come from a merging of virtual reality, high definition/three dimensional television and telecommunications. It will allow a person to sit in their home or office using goggles or perhaps a helmet and be able to attend a meeting at a distant place.

What they'll see and hear will be like actually being there. Other people at the meeting will share the same virtual experience. Professor Tucker says that using telepresence, sports fans will be able to attend major events from their lounge chair. It'll be as if they were in the stadium for big games, no matter where they're held.

Dr Gottfried Ungerboeck

The old telephone system was designed to transmit the sound of people talking, but it didn't work well when used to communicate between computers. Computer information (data) could be sent on phone lines only if the rate of transmission wasn't too fast (a maximum of 9600 bits per second). When engineers tried to make it go faster, the distortion which you can hear (but ignore) on most phone lines wrecked the messages sent by computers. To get around this problem, Dr Ungerboeck used his mathematical talents to invent an information coding system called trellis-coded modulation. Dr Ungerboeck's system made it possible to transmit data reliably over ordinary telephone cables at far higher speeds than was ever dreamed of.

Using his revolutionary coding system, Dr Ungerboeck was able to represent data in a different way so that the distortion on the telephone line was much less of a problem.

When Dr Ungerboeck's trellis-coded modulation system was used, modem speed went up from 9600 to 14,400. Most modems now transmit at 28,000 bits per second and beyond. Trellis-coded modulation helped remove the log jam in world communications.

These three winners have helped make telecommunications what it is today – without them the information super-highway might be just a slow and bumpy bush track!

Related sites

• Allan Snyder – Optical Physicist/Visual Scientist (Tall Poppy Campaign, Australian Institute of Political Science)

• Australian Photonics CRC

Box 2. Digital communication

Like much technical jargon, the word 'digital' or 'digit' has been completely changed from its original meaning – which was simply a 'finger'. By holding up three fingers or digits you can show somebody you want three of something, so it also came to mean 'number'. The word got borrowed when clocks with numbers (instead of hands) appeared and got called 'digital clocks'. The word was further stretched when somebody realised the easiest way for computers to talk to each other was in numbers rather than words. So information passing from computer to computer became known as a 'digital transmission' or 'digital signal' – and the word 'digital' came to mean communicating information by numbers.

But why do computers use numbers? Computers may be fast but they are basically pretty dumb. We humans have ten fingers (or digits) and can use them to count up to ten. And we are smart enough to have different names for each of the numbers between one and ten. Also, while we have a ten-based numbering system, computers have trouble coping with something so complicated. Computers only have two 'fingers' (or digits) to count on, so to them it is either one finger or the other. Computers use only the numbers '0' and '1' or 'on' and 'off'. They make up numbers larger than one by using a whole string of zeroes and ones (if you have ever studied a binary or two-based numbering system you will know about this). To give you an example, if you write out the date in ten-based numbers you would get this:

17 November 1998.

But a date in binary computer-talk might look something like this:
101010010010010111100101010100100101010001010101.

When a computer wants to send a message (eg, a word-processing document), it first converts the message to a large number of zeroes and ones and then sends the message down a cable to another computer. The receiving computer reads all the ones and zeroes and reforms them into something we can understand.

Put simply, computers use digital codes to move information about – and like many things to do with the internal workings of computers it can make pretty dull reading until the computer translates it back into 'human talk' for us. The process is similar in almost any electronic machine: fax, phone, mobile phone, tape recorder, CD player, modem – the list goes on. All you have to know is that when you hear people talk about 'digital', they are probably talking about a kind of electronic language that is now used to send almost all messages through the telecommunications system.

Box 3. Binary numbers

What is the difference between these two number systems?

In 'decimal' arithmetic, which is based on the number 10, the positions of the digits in a number, reading from the right, mean 'units', 'tens', 'hundreds', 'thousands', and so on. (The value of each position goes up by a factor of 10.) The decimal system uses 10 numerals (0,1,2,3,4,5,6,7,8,9) and the number 453 means 3 units, 5 tens and 4 hundreds.

In the binary representation of a number, the position of the digits, reading from the right mean 'units', 'twos', 'fours', 'eights', 'sixteens', and so on. (The value of each position goes up by a factor of 2.) The binary number system uses two numerals (0,1) and 1101 means 1 unit, no twos, 1 four and 1 eight, or 1 + 4 + 8, which equals 13. (A binary digit (a 1 or a 0) is called a bit.)

Here are some other examples:

Decimal Number Binary representation

2 10

3 11

5 101

6 110

15 1111

56 111000

100 1100100

Binary numbers and computers

The binary number system is ideal for use in computer programs because the two digits can be represented by the two states of an electronic circuit (off = 0 and on = 1).

Although computers are based on the binary number system, we don't have to use binary numbers when using a computer or calculator. Instead, we enter decimal numbers which computer programs translate into binary representations before manipulating them.

Representing letters with a binary code

Binary codes can represent the letters of the alphabet, numerals, common symbols, and commands such as 'space' or 'enter'. These are not represented by single bits, but rather by eight-bit assemblages called bytes.

Using groupings of eight means that it is possible to generate 2⁸ or 256 different combinations, each of which can be assigned a different meaning. A byte can represent an individual letter, numeral, symbol or command in a text document, in which case every character in the document uses a byte. (The computer memory required to store the 8 bits is also called a byte.)

There are 47 keys on an ordinary computer keyboard, and each can be used with or without the 'Shift' key, making 94 basic symbols to be encoded for numbers, small letters, capitals, and common punctuation. (The 'Space' bar and the 'Return' key add two more to this.) The ASCII code for computers uses a 7-bit code which can represent 128 symbols – more than enough to encode all 'ordinary' keyboard symbols.

If grouping of eight are used (256 possible combinations), there are a large number of 'spare' codes for other symbols which are used in an extended ASCII system. These symbols are encoded using either the 'Control' or the 'Alternate' keys together with one of the 47 ordinary keys (making an extra 94 symbols). Finally there are codes for the 12 'Function' keys and for the other special keys (eg, arrow keys) at the right of the keyboard. Once the keystrokes are encoded in binary form, the computer can recognise the difference between numbers and other symbols and can process them accordingly.

Digital communications

Computing is not the only area of technology that uses the binary number system. 'Digital' representations based on binary numbers are used for CDs, mobile phones as well as for fibre optic and satellite communications. In these examples, the system detects the difference between a 1 and a 0 in the signal, just as a computer does. We now live in a digital world, and the most important digits are 1 and 0!

Related sites

• On binary numbers (Telstra Learning Centre, Australia)

• How bytes and bits work (How Stuff Works, USA)

Activities

• Helix (CSIRO, Australia)
  • Fibre optics in the kitchen – demonstrates the properties of optical fibres.

• Newton's Apple (USA)
  • Telecommunications – an activity on optical fibres and an introduction to telecommunications.
  • Satellite technology – an activity on sending and receiving digital images.

• UniServe Science (Australia)
  • An introduction to optics and communication – a series of 16 lesson plans relevant to optics and communication. Also provides an introduction to photonics, and teacher and student notes.
  • Explaining the principles of photonics and optical communications – a series of eight experiments in photonics and communication.

Activity 1. Optical fibres as 'light pipes'

Materials (for the class)

• solid glass rod about 50 to 60 centimetres long, with a bend in the middle
  
• 2 clamps and stands
  
• sheet of cardboard (approximately 50 × 50 centimetres)
  
• torch
  
• piece of white paper (A4 or smaller)

Procedure

1. Put the glass rod into the clamp on the stand.

2. Make a hole in the centre of the sheet of cardboard and slide it over the end of the rod. The cardboard will act as a shield against light that isn't focused down the rod.

3. Focus the torch on one end of the glass rod, and clamp it into position. (The glass of the torch can be touching the glass rod.)

4. Hold a piece of white paper a short distance from the other end of the rod.

5. Observe the beam of light on the paper.

Questions

1. Given that light always travels in a straight line, where would you expect the light from the torch to land?

2. Explain why the light follows the bend in the glass rod.

3. There is only a weak beam of light transmitted through the glass rod. Why?

Teachers notes

Preparation

Obtain a rod of glass of about 3 to 5 millimetres in diameter. Put a 30-40^o bend in it by heating the middle of the rod with a Bunsen burner, until the glass softens.

You can use a small torch with a diameter similar to the diameter of the glass rod.

If you use a larger torch, you may need to use black tape or paper around the space between the light and the rod to reduce the amount of light that is not focused on the glass rod.

A small halogen lamp could be used instead of a torch, but this will get too hot to allow you to wrap the intervening space with tape or paper.

The results are easiest to see if the room is very dark, so use a darkroom if one is available.

Try the following variations to focus more light down the rod:

• Remove the glass of the torch and put the rod as close as possible to the bulb.
  
• Use a lens to focus the light onto the end of the rod.

This experiment could be done in small groups, if you have enough equipment.

Answers to questions

1. Normally the beam from the torch would land on a spot directly in front of the torch.

2. When light travelling through the glass rod meets the air-glass interface at a small enough grazing angle, the light is reflected back into the rod and none escapes.

3. Much of the light from the torch has been absorbed by the glass rod. All glass absorbs light. For example, when you look through a window pane, only about one half of the external light is visible through the pane. If the pane of glass was half a metre thick, much more light would be absorbed.

With new glass that has been developed for optical fibres, light can travel more than 10 kilometres before 90 per cent of it is absorbed. This is a big improvement over ordinary glass. When light travels through ordinary glass 90 per cent of the light has been absorbed after only about 20 metres.

Students may notice that the light travelling down the rod is coloured. Glass does not absorb all the wavelengths of light equally (eg, Pyrex glass absorbs blue and red wavelengths, transmitting yellow-coloured light; ordinary soda glass transmits green light best). Infrared light is used to send messages down optical fibres because glass absorbs least in the infrared part of the spectrum.

Even with ideal conditions, a fibre optics network requires an amplifier every 10 or 20 kilometres of optical fibre to boost the light signal, but this distance is being increased as glass technology improves.

Activity 2. Illustrating the inverse square law of radiation

In this activity you will determine how illumination varies with the distance from the light source.

Materials (for the class)

• electric light with a 60-watt bulb
  
• camera with a built-in light meter
  
• sheet of greaseproof paper (approximately 10 × 10 centimetres)
  
• sticky tape

Materials (for each person)

• a sheet of log-log graph paper

Procedure

1. Prepare a table with headings as follows:

   Distance between camera and light source

   Exposure time

2. Attach the piece of greaseproof paper so that it covers the lens of the camera. (Do not put any tape on the camera lens.)

3. Darken the room.

4. Place the light source at one end of the darkened room.

5. Get as far away as possible from the light (5-10 metres).

6. Set the camera shutter on the slowest speed (1 second).

7. Point the camera at the light. Adjust the lens aperture setting so that the pointer on the exposure meter comes to a clearly defined mark (the 'correct' exposure) on its scale.

8. Measure the distance between the light source and the camera.

9. Record on your table the distance and the exposure time.

10. Move the camera closer to the light, by about one-third of the distance, and measure that distance. (For example, if the initial distance was 9 metres, move the camera to 6 metres.)

11. Point the camera at the light. Leave the lens aperture setting unchanged and change the shutter speed until the pointer returns to the same position that it was on in step 7.

12. Record on your table the distance and the exposure time.

13. Repeat steps 10, 11, and 12 until you get to the shortest exposure time allowed by the camera (1/1000 second). (The subsequent distances for the example given in step 10 would be 4 metres, 2.7 metres, 1.8 metres, 1.2 metres, 0.8 metres, 0.5 metres, and 0.33 metres.)

14. Plot your exposure setting against distance on log-log graph paper.

15. Draw a line of best fit and calculate the slope of the line.

Questions

1. What is the slope of your line?

2. Illumination is proportional to the reciprocal of the exposure setting. What does the slope of your line tell you about the relationship between distance and illumination?

Teachers notes.

1. The graphed line should have a slope of +2.

2. Since illumination is proportional to the reciprocal of the exposure setting, students should see an inverse square relation.

The equation to describe the behaviour of light is the inverse square law of radiation where

E = I/D2

E is the illumination
I is the luminous intensity of the light source
D is the distance between the source and the point or object.

Because the energy from a point source spreads out equally in all directions, the illumination it produces diminishes as the inverse square of the distance.

Activity 3. Morse code

1. Computers use only two numbers in their communication system, 1 and 0. This is called a binary or base-two numbering system.
   • Working with a partner, try to devise binary code (1 and 0, or on and off) to send a simple message.

   • Write down why you had difficulty in sending a binary code.

2. Morse code is a system of dots, dashes and spaces to represent the letters of the alphabet. It is used in telegraphy and signalling.

   A

   Alpha

   .-

   B

   Bravo

   -

   C

   Charlie

   -.-.

   D

   Delta

   -..

   E

   Echo

   .

   F

   Foxtrot

   ..-.

   G

   Golf

   - -.

   H

   Hotel

   ....

   I

   India

   ..

   J

   Juliet

   . – - -

   K

   Kilo

   - . -

   L

   Lima

   .-..

   M

   Mike

   - -

   N

   November

   -.

   O

   Oscar

   - – -

   P

   Papa

   .- -.

   Q

   Quebec

   - -.-

   R

   Romeo

   .-.

   S

   Sierra

   ...

   T

   Tango

   -

   U

   Uniform

   ..-

   V

   Victor

   ...-

   W

   Whisky

   .- -

   X

   X-ray

   -..-

   Y

   Yankee

   -.- -

   Z

   Zulu

   - -..

   • Use Morse code to send a simple message to your partner.

   • Write down why Morse code is easier for people to use than a simple binary system of 1 and 0.

Teachers notes

1. It is difficult and laborious for us to communicate using a binary code. We can't easily identify a sequence of consecutive zeros or consecutive ones because we don't have an accurate enough inbuilt sense of time. It is possible to devise a binary code to eliminate this problem but it is generally not very efficient. For example, we could always send a 0 after each 1, and have a large number of zeroes (eg, five) between symbols. Then we could identify each letter of the alphabet by a specified number of 'dots' (each a 1, 0 sequence).

2. Morse code is easier than the simple binary system because it uses two active symbols, a dot and a dash. Also the simple dot-dash of 'A' is much simpler than its binary equivalent.

You may want to point out that because of the ability of computers to handle simple codes at very high speed, they are able to communicate in a very different way from humans.

Further reading

Useful sites

A good introduction to fibre optics, with helpful illustrations. Some explanations (eg. 'How are optical fibers made?') are technical.
http://www.howstuffworks.com/fiber-optic.htm

The busy ray: The story of communication by light beam (Telstra Learning Centre, Australia)

Provides the history of experimentation with light and its uses as a signalling device in telecommunications. Also includes an introduction to optical fibre and its applications.
http://www.telstra.com.au/abouttelstra/learning/light_beam/beam.cfm

A brief history of fiber optic technology (Fiberoptics.info)

Includes the history of the technology as well as current and future applications.
http://www.fiber-optics.info/fiber-history.htm

A fiber optic chronology (Jeff Hecht, USA)

A timeline compiled by science and technology writer Jeff Hecht. This chronology is an early version of the one that appears in his book 'City of Light: The Story of Fiber Optics', published by Oxford University Press as part of the Sloan Technology Series.
http://www.sff.net/people/Jeff.Hecht/chron.html

Optic fibre FAQs (m2m, University of Southampton, UK)

Click on a question to find out more about optical fibres.
http://www.m2m.ecs.soton.ac.uk/default.asp?id=48

Modern communication: The laser and fibre-optic revolution (Beyond Discovery, National Academy of Sciences, USA)

Describes how basic research into laser and optical fibres has led to important practical applications. (A PDF file of the complete article is available.)
http://www.beyonddiscovery.org/content/view.article.asp?a=438

Australian Broadcasting Corporation (transcripts)

• Developments in fibre optics (The Science Show, 28 May 2005)
  Researchers at Adelaide University are developing a new kind of optical fibre with holes which will be able to carry more information.
  http://www.abc.net.au/rn/science/ss/stories/s1375428.htm

• The telecommunications industry – part 1 (The Media Report, 24 April 1997)
  Explains how telecommunications is changing from a public utility to a corporate giant.
  http://www.abc.net.au/rn/talks/8.30/mediarpt/mstories/mr970424.htm

• The telecommunications industry – part 2 (The Media Report, 1 May 1997)
  Explains how telecommunications is changing from a public utility to a corporate giant.
  http://www.abc.net.au/rn/talks/8.30/mediarpt/mstories/mr970501.htm

Glossary

ASCII uses a 7-bit code which produces 128 different combinations to represent different symbols. The decimal numbers 0 to 47 code for symbols and computer commands. Decimal numbers 48 to 57 code for the numerals (0-9), decimal numbers 65 to 90 code for capital letters (A-Z), and decimal numbers 97 to 122 code for lower case letters (a-z). For example, in a computer using the ASCII code, 'A' is represented by the decimal number 65. The computer 'reads' this as the binary number 01000001 and encodes the letter A.

Most computers use an 8-bit code (extended ASCII) which produces 256 different combinations to represent symbols. In addition to the regular character set represented by ASCII (in the decimal range from 0 to 127), extended ASCII has an additional 128 codes that can be used to represent additional symbols (eg, non-English characters or graphical symbols).

For more information see ASCII – What is it and why should I care? (Tela Communications, USA).

binary code. A digital coding system that uses a sequence of only two types of symbols (eg, 0 and 1) to represent data. The two symbols are called bits (an abreviation for binary digits). For more information see How bits and bytes work (How Stuff Works, USA).

bit. Binary digit. The smallest unit of information in a digital system. A bit can be 0 or 1. For more information see How bits and bytes work (How Stuff Works, USA).

digital codes (digital system of codes). Information that is represented as a series of discrete digits (numbers).

distortion. An undesired change in the shape of an electrical wave or signal. Distortion results in the loss of clarity in reception or reproduction, or even the loss of information in a digital system.

fibre optics (fibre optic communications). The transmission of information by the passage of light through flexible, glass fibres. Electrical impulses are converted into light which is then transmitted through the optical fibre. The light is then re-converted into electrical impulses at its destination.

inverse square law of radiation. States that illumination at a point varies inversely as the square of the distance from the light source.

If you halve the distance from a radiation source (such as a light, a fire, or a radio transmitter) you multiply by four the intensity of the radiation. This means that if you hold your hand 1 metre away from a light, then move your hand halfway towards the light (so it is just 50 centimetres away), there will be not double but four times as much light reaching your hand. If you halve the distance again to just 25 centimetres there will be sixteen times as much light reaching you hand compared to when it was 1 metre away. The same thing happens with radiated heat – which is why if you sit with your feet pointing at a camp fire, your feet can feel very hot while your chest is cool. Or, why a heat source strong enough to make metal glow red, may feel only pleasantly warm a short distance away.

laser. Light amplification by stimulated emission of radiation. A device that produces a high-intensity, directional, monochromatic beam of light.

light-sensitive cell. A device having a photoelectric property such as the ability to generate a current or change its electrical resistance when exposed to light.

modem. Modulator/demodulator. A device connected between a computer and a telephone line. It consists of a modulator that converts digital computer signals into audio signals for transmission over the telephone line and a corresponding demodulator to convert the incoming audio signals into digital form.

optical fibre. A glass thread that acts as a guide for lightwaves. Fibres used in telecommunications usually have a cladding of glass of a lower refractive index. In a communication system, several fibres are made up into a cable.

photoreceptor. A light-sensitive cell.

retina. The light-sensitive cell layers of the inner lining of the back of the eye.

telecommunications. The communication of information over a distance by means of radio waves, optical signals or along a transmission line.

trellis-coded modulation. A coding system used for high speed, reliable data transmission over telephone lines.

virtual reality. An artificial environment created by computers, in which people can immerse themselves and feel that this artificial reality really does exist. For more information see Virtual reality (Whatis.com, USA).