A D V E N T U R E S in C Y B E R S O U N DThis Busy Ray : The Story of Communication by Light Beam
Note : This file now replaced by the Telstra 'Learn It' series file...This Busy Ray: The Story of Communication by Light Beam - The history of experimentation with light and its uses as a signalling device in telecommunications, plus an introduction to optical fibre and its many applications http://www.telstra.com.au/classroom/sec_2_1.htm
Section 1 : Light conversations - Bonfires to Photophones Light has a long history of being used to convey long distance messages. Three millennia ago, the Greek victory in the Trojan War was telegraphed by lighting hilltop bonfires one after the other. But there were limitations. While a bonfire conveyed a message far faster than any messenger could run, it could only carry one piece of information - an agreed 'Yes' signal that the war was over. The Heliograph system used a mirror which flashed the sun's rays in one direction, sending a coded message. In the early 1800s, the usual code was semaphore (a system which could also operate using flags). Both bonfires and heliographs shared other limitations. They were slow, unusable in bad weather and labour-intensive. Multiple observers, all within sight of each other, had to take down the message and repeat it along the chain. With a few exceptions (heliographs are still occasionally used), light-communications began to be phased out in the 1840s, with the invention of the electric telegraph. The electric telegraph carried signals through metal wires regardless of the weather, and could have its repeater stations hundreds of kilometres apart. Then, in 1876, came the telephone - and, around 1900, the radio (a method which did not even need wires, and so was called the 'wireless'). But the idea of using light for communication was never entirely shelved. Selenium - The half-metal Jacob Berzelius, a Swedish chemist, discovered selenium (Greek: selene = moon) in 1818. Its properties place it halfway between a metal and a non-metal. Selenium has several forms, a red non-crystalline one (the most common), a black glassy one and a grey metallic crystalline one. It is the grey metallic form that is photosensitive. Just three years before Bell completed the telephone, a British firm the Telegraph Construction and Maintenance Company, was laying a cable under the sea from Java to Darwin. To simulate a long-distance cable, a senior electrician, Willoughby Smith, needed test material with high electrical resistance. He decided to try using selenium. Bars of selenium were kept in a box with a sliding lid. Smith and his team discovered that the metal's resistance changed according to the position of the lid. When the lid was closed the resistance was high, with the lid open and light exposed to the selenium the resistance was low. Bell's 'Photophone' In Britain in 1878, Alexander Graham Bell suggested that if his telephone were placed in circuit with the selenium, a shadow might be heard to fall. His reasoning, the telephone produced sound in response to a changing electrical current. A shadow also changed selenium's resistance - so that too, should be audible. Willoughby Smith tried it out - and a few days later announced that he had, indeed, heard a shadow fall. Back in Washington, Bell and his assistant Summer Tainter tried using light to produce and reproduce sound. Fifty devices later, they produced a fairly simple working model. Its basis was a mirror (made of a wafer of silvered mica), mounted on a tightly-stretched canvas diaphragm. A light beam, focused on the mirror, was bounced back to a waiting listening post. Then, when someone spoke directly into the diaphragm, two things happened. The diaphragm vibrated, copying the sound of the speaker's voice and the mirror flexed and deformed, changing the intensity of the reflected light beam. At the listening post (which had to be in line-of-sight) was a selenium cell, in circuit with a battery and a pair of Bell telephone receivers. As the light beam's intensity changed, so did the current passing through the selenium cell. The result, a clear, recognisable sound exactly like that vibrating the mirror. Yes, but...While tests over 200 metres were successful, Bell's invention had some crucial shortcomings. The transmitter (mirror) and receiver (listening post) had to be placed within line of sight, , it was mechanically unreliable and it was subject to optical diffusion fog, smoke or rain put it out of commission. It was almost another century before voice communication by light beam became practicable.
Section 2 : Light - What it is and its Speed of Travel Turn on a torch on a dark night, and a beam of light appears. Turn it off, and the light vanishes. During a thunderstorm, lightning and thunder happen at the same moment. But there's a delay before the thunder clap, while the flash seems to appear in an instant. Experiences like these suggest that light travels very fast - far faster than the speed of sound. But how fast? How can its speed be measured? The Timeless Medium Greek philosophers, around 300BC, said that light's speed was infinite. It was everywhere at once, moving instantly from its source to the objects it struck. There were good reasons for this argument. Although light has a definite speed, it was not until a few centuries ago that anyone came close to measuring it. Galileo's Lantern Method Galileo 16th century scientist and astronomer, tried to measure the speed of light by sending two men with lanterns to hilltops a few kilometres apart. At a set time, Man 1 uncovered a lantern. As soon as Man 2 saw it, he uncovered a second lantern. Man 1 then noted how long it was before he received an answering signal. To Galileo's disappointment, the answer was exactly the same however far apart the hills were, the delay was entirely due to human reaction time. Roemer's Astronomical Method About 100 years later a Danish astronomer, Olaus Roemer, came close to calculating the speed of light. He was observing the moons of the planet Jupiter, whose movements, by then, were accurately known. To his surprise he found that when the planet's position was furthest from Earth the moon's arrival time was late and when the planet's position was closest to Earth, the moon's arrival time was early Many observations later, Roemer realised that when Jupiter was further away, light took longer to reach Earth than when it was closer. By calculating the time of the delay, and dividing it into the Earth's distance from Jupiter, Roemer produced a figure for the speed of light - 227,000 kilometres per second. The accepted modern figure for the speed of light is 229,792 kilometres (or approximately 3x10 8 metres) per second What is Light? Long after light's speed was known, debate (which had begun before Roemer's time) continued to rage about whether light consisted of waves or particles. The wave theory held that light was a wave travelling through space the way ripples travel through water and the particle theory held that light was like small hard balls which zipped along and bounced off objects. The decision, after long debate, was that light can act both as particles and as waves. A useful way of thinking of light is as small wave packets or photons (Greek: photos = light). Photons sometimes behave like particles, sometimes like waves. For present purposes, light is most usefully thought of as waves. Light as a Wave Light waves - like ripples on a pond, or a waves in the ocean - are described by wavelength, the distance between two wave crests, amplitude, the distance between the top of wave crests and the bottom of troughs and speed, which varies with the medium in which the wave is travelling. A wave's frequency is a calculation based on how many crests pass a fixed point in each second. If the wave crests are far apart, fewer will pass by than if they are close together. White light - a Mixture of Colours White light, because it is a mixture of all the visible colours of light, does not have a specific wavelength. When a rainbow appears, or when light passes through a prism, the spectrum of visible wavelengths is revealed. (*Latin spectrum = something seen; see section 3 ). In the rainbow, light with the highest visible wavelength is red, while light with the lowest visible wavelength is violet. Just beyond the visible spectrum are the ultraviolet (Latin: ultra = beyond) and infrared (Latin: infra = below) wavelengths. Light as Electromagnetic Radiation Light is a form of electromagnetic radiation - so called because it is made up of linked electric and magnetic fields moving through space. It is one small part of an electromagnetic spectrum which ranges from cosmic rays at one extreme to electric power on the others.
Section 3 : The Electromagnetic Spectrum The electromagnetic spectrum is arranged on a logarithmic scale. Each step represents ten times the previous step. Wavelength (in metres) is related by the formula W = c/F where W is the wavelength, c is the speed of light (3 X 10 8 metres per second) and F is the frequency in cycles per second.
Section 4 : About Lasers The word laser is an acronym - a word made up from the initials of other words. It stands for Light Amplification by Stimulated Emission of Radiation. A property known as coherence makes laser light different from every other kind of light. It is coherent while all other light (whether from sun, an electric torch or a glow-worm) is incoherent. Incoherent Light With any form of white light, close examination of the light waves would show that they are of varying frequencies and lthough the light appears to be white, it is an almost-random mixture of colours. Light waves travel in every direction. The light needs a shade or a reflector to focus it at all - and even then, it quickly fades and dissipates. Furthermore they possess a unique phase. Even if two waves have the same frequency, their crests are unlikely to be in step. When two waves of the same frequency meet, waves exactly IN step (crest-meets-crest) reinforce each other. Waves exactly OUT of step (crest-meets-trough) cancel each other out. Coherent Light While a light bulb produces incoherent light, a laser produces an intense, narrow beam of coherent light. Light waves emitted from a laser all have the same frequency, all travel in the same direction and are all in phase. With a laser, light - travelling at 299,792 kilometres per second - becomes a source of high-energy radiation. A beam can be focused to strike a single human cell, supply 40 x 10 13 watts of power to an atom and achieve the same heat as the sun's surface - 100 million degrees. Laser light does not occur in nature, and (since white light is a mixture of many frequencies of colour) it can't be white. if it were, it wouldn't be the laser light. A laser beamed at the moon (a trip of about 400,000 kilometres), lit up a patch only 10 metres wide. In 1969, when the Apollo astronauts placed mirrors there, laser beams were bounced back to Earth, where powerful telescopes picked them up. Atomic Matter According to atomic theory, all matter is made up of atoms. Each atom consists of a small, comparatively heavy nucleus, surrounded by a large arrangement of electrons, which orbit the nucleus the way the earth orbits the Sun. For present purposes, atoms are most usefully thought of as complete, indivisible units. Producing a Laser Beam The key to understanding how a laser is produced is the Stimulated Emission part of the laser acronym. When energy - heat, for example - is pumped into any kind of material, its atoms become excited. Momentarily, they enter a kind of high-level orbit. Then, while dropping back to their ground state, they emit radiation. This radiation is - typically - incoherent. Creating a Laser Beam If, during its brief excitement phase, the atom is hit by radiation of a certain frequency, it emits radiation of the same frequency and phase as the radiation that stuck it. The incoming radiation is reinforced, and the newly-released radiation strikes more of the excited atoms, which are then - in turn - stimulated. As the effect continues to snowball, most of the atoms in the material emit light of the same frequency, in phase. The result is a laser beam. Sources of Lasers Lasers were first demonstrated at the Hughes Aircraft Company in 1960 by a researcher called Theodore Maiman. His experiment used a ruby crystal rod. Two mirrors were placed at the ends of the rod, between which the emitted light bounced back and forth.
Section 5 : Fibre Optic Communication - Light Comes the Full Circle Radio waves, microwaves and light waves are all - as Section 3 shows - forms of electromagnetic radiation. In wavelength, these waves differ enormously. Some examples - radio waves kilometres - metres, microwaves metres - millimetres and light waves millimetres - nanometres (nanometre is a thousand-millionth of a metre) It had long been established that the shorter the wavelength, the more information a wave could carry. Light, then, should be able to carry massive amounts of information. But how was the wave to be transmitted and controlled? Lasers as a Light Source To test Maiman's laser as a possible answer, researchers -in the early 60s - began firing laser beams between towers several kilometres apart. Although partly successful, the method experienced the same difficulties as Bell's Photophone. In particular, fog or rain blocked communication. The answer seemed to be to guide the beam through pipes or cables. For that, the choice was optical fibres - hairs made of pure silica glass, with a core only 10-50 microns (millionths of a metre) in diameter. Inventor's Bonanza As far back as the 1930s a method for manufacturing optical fibres had been patented... "in case", commented the inventor, "someone ever finds a use for it". Keeping Light Inside the Fibre The first problem with optical fibres was to keep the transmitted light from passing through them. The eventual answer was to enclose the core in a glass jacket (or cladding) with a diameter of 125 microns. The glass for the core was made with a slightly different refractory index from its cladding. That meant that as light travelled along the core, most rays struck the core-to-cladding interface and were bounced back inside the core. Called Total Internal Reflection, this effect occurs even if the fibre is bent - as long as the bend is fairly shallow. Finding a Laser Light Source At first, lasers were large, expensive and - often - unreliable. Then came light-emitting diodes (LEDs) and semi-conductor lasers. Both sources suit fibre-optic communication. For example, they emit light in the frequencies least absorbed by silica - the basic ingredient of optical fibres. From the beginning, most fibre-optic communication has used the light of infrared (or near-infra red) frequencies. Several signals, usually with slightly different infra red frequencies, can now be piped down the same fibre at once. But semi-conductor lasers are now almost the universal choice, except with multi-mode fibres (explained later in this section) as they emit light which can be piped down narrow fibres without causing spectrum smear (dispersion), they can be turned off and on in less than 1 nanosecond and can transmit data at rates of more than 1 Gbit/s. A 1960s Pipe Dream Long after light could be piped down a glass fibre, one problem remained, how could it be transmitted long distance? Calculations showed that the emitted light needed to keep at least 1% of its intensity after travelling for a kilometre, otherwise, too many repeater stations would be needed for it to become a useable, everyday communications link. One early answer was to fill a hollow fibre-optic cable with liquid. The attraction, after travelling for a kilometre through a liquid-filled cable, laser light kept 10% of its original intensity. The idea, put forward by the CSIRO in 1971, was developed by Telecom's forerunner, the Postmaster-General's Department. But liquid-filled fibres proved difficult to make and handle - mainly because the liquid leaked. Analogue and Digital Another problem, how could light communicate the sound of the human voice? The answer was to transmit signals as digital pulses - achieved by turning the light on and off millions of times a second. At the far end of the link, these pulses are picked up by a photo detector and converted back into the smoothly-varying analogue signals that represent the human voice. Multi-Mode Fibres Multi-mode fibres have a core of 50 microns (compared with the tiny 10 microns of single-mode fibres) and so could be manufactured more easily and reliably. In Australia, AWA began making multi-mode fibres (of the graded-index type) for Telecom in 1978. With LEDs as the light source, they achieved moderate transmission rates, and needed repeaters only about every 10 kilometres. Telecom Australia still installs some multi-mode fibres - for example, for wiring new buildings, and for short-distance computer links. How Multi-Mode Fibres Work With multi-mode fibres, the material at the centre of the core has a higher refractive index than the material further away from it. Light travels faster in material with a lower refractive index, so that rays diverging from the centre (and with further to travel) also travel faster. If the refractive indexes are properly chosen, all light rays will arrive at the other end at the same time. But since 1985, in Australia and throughout the world, most fibre-optic communications links have used single-mode fibres.
Section 6 : Experiments - The First Steps Within five years of silica glass fibres becoming a practical proposition, Telecom installed a 1.5 km loop near its laboratories in Clayton, Victoria. The need for new work practices became immediately evident. To splice an optical fibre, the two ends of the cable must be aligned with absolute precision, then fused by a burst of electricity. Discussions were held with unions and staff associations, and from those discussions emerged - in 1982 - the first Australian field trials (see Table below). Testing the First Fibre-Optic Systems
Trial 1While the first trial loop used single-mode fibre, manufacturing such fine glass hairs (explained in Section 5) was at first a problem. That led to the use of multi-mode fibres for the remaining trials. The fibres - of the graded-index type - were made by different manufacturers, to allow on-site tests and comparisons. The LED and laser transmission equipment was supplied by NEC (Australia). These experiments now seem modest. But they were the basis of a decade of spectacular changes in the telecommunications network - changes which are discussed in Section 7.
Section 7 : The Growth of the 'Glass Network' In the early 1980s, the Sydney-Melbourne link - Australia's busiest - faced increasing demand. Much of it was because of computers, and the need for digital communications. (*Each conversation needs two fibres) Although the link had a coaxial cable, and had been given six new microwave bearers in 1984, it was again being pushed to the limits of its capacity. Fibre and its Rivals The main options (with copper pairs as a benchmark), lined up as follows...
Copper 'Pair'Against optical fibres was its cost. But its main contenders - coaxial cable and microwave radio - also had drawbacks. Coaxial cable (a 12 Mhz system) was cumbersome - and therefore expensive - to install, (It also had a much lower bit-rate than fibre-optics) and microwave radio would need three extra radio bands. That would mean a new route, since all bands on the existing route were full. Those factors tipped the choice in favour of optical fibre - specifically, a single mode, 30-fibre cable operating at a rate of 565 Mbit/s. These optical fibres link smoothly with the other broadband carriers in the network - coaxial cable, microwave radio and the solar-powered Digital Radio Concentrator System (DRCS) satellite. They also link with the copper network, a major Australian asset which continues to work well. The Repeater Question In Europe, transmitter spacings of 15 km are typical. But Australian geography makes repeaters a key issue in costing a fibre-optic system. For example, the Perth/Brisbane link, with its spur links to Adelaide and Sydney, is one of the longest in the world. With all fibre-optic transmissions at a small wavelength (1300 nanometres), average repeater spacings are now as follows... Major Trunk Routes - Spacings
565 Mbit/s - 55 kmMinor Trunk Routes - Spacings
34 Mbit/s - 75 kmThe exact layout for a fibre-optic cable is decided with the help of computer models, designed to minimise repeater spans while maintaining communications. By Leaps and Bounds In today's systems, each optical fibre can carry 7,680 one-way voice circuits - or (in data terms) 100 average-sized novels each second. About to be installed on the main intercapital routes systems operating at 2.5 Gbit/s, which will multiply those figures by five. Cabling Australia Installed in 1987, the Melbourne-Sydney optical fibre link ran alongside the old coaxial cable. Almost 1000 km long, it had one immediate advantage, repeaters were needed in only every third hut along the route. Now, the Melbourne-Sydney route has three fibre-optic links. All network expansions are done with optical fibre, and there are more than 1.5 million kilometres of optical fibre in the network.
Section 8 : Fibre-Optics for Businesses and Homes For customers, the attraction of fibre-optics is that they can carry broadband services - high-speed data, video, image and voice. So, with links established between exchanges and on trunk routes, optical fibres began to be installed in the Customer Access Network, beginning with high communications traffic areas, businesses whose services (such as PABXs or computers) required digital links of 2 Mbit/s or more and places susceptible to electromagnetic interference (optical fibres have no metal elements). In 1992, a multimedia system called Lasercast was installed in some 4 and 5-star hotels in Sydney. Based on optical fibres, the system was interactive, allowing guests to view and choose tourist information. Fibre to the Home For home users, broadband services offer a bundle of possibilities, including access to video libraries and CDs, Pay-TV, shopping from home, working from home (the home office) and studying from home. But costs meant that optical fibres could not be supplied to homes until close to the year 2000. To bridge the gap, Telecom researchers invented PON, the Passive Optical network PON : Copper meets Fibre-Optic How it Works Copper pairs link a cluster of about 20 homes. A single optical fibre runs from the local exchange to a spot at the kerb and an optical coupler allows transmission between the copper and optical circuits. PONs are cheap to install, and - because they have no electronic components-need no maintenance in the field. PON gave rise to laserlink, a program launched 1992 with the aim of giving most network users access to optical fibres by 1994/95. Ordinary - or B-ISDN? Will fibre-optics replace copper for ordinary telephone calls? The answer to that question is more complex than it at first seems. In the past, ordinary telephone calls were sharply divided from services such as videophones, which required special links. Now, that dividing line is becoming blurred, fibre-optics are likely to become part of what is being called the Broadband Integrated Services Digital network (B-ISDN). With B-ISDN, communication can become multimedia and multipoint. One conversation may include several people or groups and - when required - will be able to switch in video, graphics, or computer data. Two-way voice conversations - basic to the design of the present network - may no longer be the norm. Instead, they will be just one item on an extensive telecommunications menu. Technological Futures Researchers at the Telecom research Laboratories are investigating enhancing a fibre's capacity - for example, by sending signals to different time slots or wavelengths, fast optical switches ("switch" is the new term for exchange) and in-line optical amplifiers, the probable replacement for today's electronic repeaters.
Edited by Dr Russell Naughton, January 2000
Note : This file now replaced by the Telstra 'Learn It' series file...This Busy Ray: The Story of Communication by Light Beam - The history of experimentation with light and its uses as a signalling device in telecommunications, plus an introduction to optical fibre and its many applications http://www.telstra.com.au/classroom/sec_2_1.htm
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