Optical technology: Lighting up our lives
“Behold, that which has removed the extreme in the pervading darkness, Light became the throne of light, light coupled with light"
—BASAVANNA, twelfth century, Bhakti poet, Karnataka
“A splendid light has dawned on me about the absorption and emission of radiation.”
—ALBERT EINSTEIN, in a letter to Michael Angelo Besso, in November 1916
“Roti, kapda, makaan, bijlee aur bandwidth (food, clothing, housing, electricity and bandwidth) will be the slogan for the masses.”
—DEWANG MEHTA, an IT evangelist
It is common knowledge that microchips are a key ingredient of modern IT. But optical technology, consisting of lasers and fibre optics, is not given its due. This technology already affects our lives in many ways; it is a vital part of several IT appliances and a key element in the modern communication infrastructure.
Let us look at lasers first. In popular perception, lasers are still identified with their destructive potential—the apocalyptic ‘third eye’ of Shiva. It was no coincidence that the most powerful neodymium glass laser built for developing nuclear weapon technology at the Lawrence Livermore Laboratory in the US (back in 1978) was named Shiva.
Villains used lasers in the 1960s to cut the vaults of Fort Knox in the James Bond movie, Goldfinger. In 1977 Luke Skywalker and Darth Vader had their deadly duels with laser swords in the first episode of Star Wars. As if to prove that life imitates art, Ronald Reagan poured millions of dollars in the 1980s, in a ‘ray gun’, a la comic-strip super-hero stories, with the aim of building the capability to shoot down Soviet nuclear missiles and satellites.
THE BENIGN THIRD EYE
In our real, daily lives, lasers have crept in without much fanfare:
• All sorts of consumer appliances, including audio and video CD players and DVD players use lasers.
• Multimedia PCs are equipped with a CD-ROM drive which uses a laser device to read or write.
• Light emitting diodes (LED)—country cousins of semiconductor lasers—light up digital displays and help connect office computers into local area networks.
• LED-powered pointers have become popular in their use with audiovisual presentations.
• Who can forget the laser printer that has revolutionised publishing and brought desk top publishing to small towns in India?
• Laser range finders and auto-focus in ordinary cameras have made ‘expert photographers’ of us all.
• The ubiquitous TV remote control is a product of infrared light emitting diodes.
• The bar code reader, used by millions of sales clerks and storekeepers, and in banks and post offices, is one of the earliest applications of lasers.
• Almost all overseas telephone calls and a large number of domestic calls whiz through glass fibres at the speed of light, thanks to laser-powered communications.
• The Internet backbone, carrying terabits (tera = 1012, a million million) of data, uses laser-driven optical networks.
C.K.N. Patel won the prestigious National Medal of Science in the US in 1996 for his invention of the carbon dioxide laser, the first laser with high-power applications, way back in 1964 at Bell Labs. He says, “Modern automobiles have thousands of welds, which are made by robots wielding lasers. Laser welds make the automobile safer and lighter by almost a quintal. Even fabrics in textile mills and garment factories are now cut with lasers.”
Narinder Singh Kapany, the inventor of fibre optics, was also the first to introduce lasers for eye surgery. He did this in the 1960s along with doctors at Stanford University. Today’s eye surgeons are armed with excimer laser scalpels that can make incisions less than a micron (thousandth of a millimetre) wide, on the delicate tissues of the cornea and retina.
So what are lasers, really? They produce light that has only one wavelength and a high directionality and is coherent. What do these things mean, and why are they important? Any source of light, man-made or natural, gives out radiation that is a mixture of wavelengths—be it a kerosene lantern, a wax candle, an electric bulb or the sun. Different wavelengths of light correspond to different colours.
When atoms fly around in a gas or vibrate in a solid in random directions, the light (photons) emitted by them does not have any preferred direction; the photons fly off in a wide angle. We try to overcome the lack of direction by using a reflector that can narrow the beam, as from torchlight, to get a strong directional beam. However, the best of searchlights used, say, outside a circus tent or during an air raid, get diffused at a distance of a couple of miles.
The intensity of a spreading source at a distance of a metre is a hundred times weaker than that at ten centimetres and a hundred million times weaker at a distance of one kilometre. Since there are physical limits to increasing the strength of the source, not to mention the prohibitive cost, we need a highly directional beam. Directionality becomes imperative for long-distance communications over thousands of kilometres..
Why do we need a single wavelength? When we are looking for artificial lighting, we don’t. We use fluorescent lamps (tube lights), brightly coloured neon signs in advertisements or street lamps filled with mercury or sodium. All of them produce a wide spectrum of light. But a single wavelength source literally provides a vehicle for communications. This is not very different from the commuter trains we use en masse for efficient and high-speed transportation. Understandably, telecom engineers call them ‘carrier waves’. In the case of radio or TV transmission, we use electronic circuits that oscillate at a fixed frequency. Audio or video signals are superimposed on these carrier channels, which then get a commuter ride to the consumer’s receiving set.
With electromagnetic communications, the higher the frequency of the carrier wave, the greater the amount of information that can be sent piggyback on it. Since the frequency of light is million times greater than that of microwaves, why not use it as a vehicle to carry our communications? It was this question that led to optical communications, where lasers provide the sources of carrier waves, electronics enables your telephone call or Internet data to ride piggyback on it, and thinner-than hair glass fibres transport the signal underground and under oceans.
We have to find ways to discipline an unruly crowd of excited atoms and persuade them to emit their photons in some order so that we obtain monochromatic, directional and coherent radiation. Lasers are able to do just that.
Why coherence? When we say a person is coherent in his expression, we mean that the different parts of his communication, oral or written, are connected logically, and hence make sense. Randomness, on the other hand, cannot communicate anything. It produces gibberish.
If we wish to use radiation for communications, we cannot do without coherence. In radio and other communications this was not a problem since the oscillator produced coherent radiation. But making billions of atoms radiate in phase necessarily requires building a new kind of source. That is precisely what lasers are.
Like many other ideas in modern physics, lasers germinated from a paper by Albert Einstein in 1917. He postulated that matter could absorb energy in discrete quanta if the size of the quantum is equal to the difference between a lower energy level and a higher energy level. The excited atoms, he noted, can come down to the lower energy state by emitting a photon of light spontaneously.
On purely theoretical considerations, Einstein made a creative leap by contending that the presence of radiation creates an alternative way of de-excitation, called stimulated emission. In the presence of a photon of the right frequency, an excited atom is induced to emit a photon of the exact same characteristics. Such a phenomenon had not yet been seen in nature.
Stimulated emission is like the herd effect. For example, a student may be in two minds about skipping a boring lecture, but if he bumps into a couple of friends who are also cutting classes, then he is more likely to join the gang.
A most pleasant outcome of this herd behaviour is that the emitted photon has the same wavelength, direction and phase as the incident photon. Now these two photons can gather another one if they encounter an excited atom. We can thus have a whole bunch of photons with the same wavelength, direction, and phase. There is one problem, though; de-excited atoms may absorb the emitted photon, and hence there may not be enough coherent photons coming out of the system.
What if the coherent photons are made to hang around excited atoms long enough without exiting the system in a hurry? That will lead to the same photon stimulating more excited atoms. But how do you make photons hang around? You cannot slow them down. Unlike material particles like electrons, which can be slowed down or brought to rest, photons will always zip around with the same velocity (of light of course!)—300,000 km per second.
THE BARBERSHOP SOLUTION
Remember the barber’s shop, with mirrors on opposite walls showing you a large number of reflections? Theoretically, you could have an infinite number of reflections, as if light had been trapped between parallel facing mirrors. Similarly, if we place two highly polished mirrors at the two ends of our atomic oscillator, coherent photons will reflected back and forth, and we will get a sustainable laser action despite the usual absorptive processes.
At the atomic level, of course, we need to go further than the barber’s shop. We need to adjust the mirrors minutely so that we can achieve resonance, i.e., when the incident and reflected photons match one another in phase, and standing waves are formed. Lo and behold, we have created a light amplification by stimulated emission of radiation (laser).
In the midst of disorderly behaviour we can see order being created by a laser. Physics discovered that the universe decays spontaneously into greater and greater disorder. If you are a stickler, ‘the entropy—measure of disorder—of an isolated system can only increase’. This is the second law of thermodynamics. So are we violating this law? Are we finally breaking out of thermodynamic tyranny?
It should be noted, however, that the universe becomes interesting due to the creation of order. Evolution of life and its continuous reproduction is one of the greatest acts of creating order. However, rigorous analysis shows that even when order is created in one part of the universe, on the whole, disorder increases. Lasers are humanity’s invention of an order-creating system.
Charles Townes, a consultant at Bell Labs, first created microwave amplification through stimulated emission in 1953. He called the apparatus a maser. Later work by Townes and Arthur Schawlow at Bell Labs, and Nikolay Basov and Aleksandr Prokharov in the Soviet Union led to the further development of laser physics. Townes, Basov and Prokharov were awarded the Nobel Prize for their work in 1964. Meanwhile, in 1960, Theodore Maiman, working at the Hughes Research Laboratory, had produced the first such instrument for visible light—hence the first laser—using a ruby crystal.
Since then many lasing systems have been created. At Bell Labs C K N Patel did outstanding work in gas lasers and developed the carbon dioxide laser in 1964. This was the first high power continuous laser and since then it has been perfected for high power applications in manufacturing.
THE SEMICONDUCTOR REVOLUTION IN LASERS
What made lasers become hi-tech mass products was the invention of semiconductor lasers in 1962 by researchers at General Electric, IBM, and the MIT Lincoln Laboratory. These researchers found that diode devices based on the semiconductor gallium arsenide convert electrical energy into light. They were highly efficient in their amplification, miniature in size and eventually inexpensive. These characteristics led to their immediate application in communications, data storage and other fields.
Today, the performance of semiconductor lasers has been greatly enhanced by using sandwiches of different semiconductor materials. Such ‘hetero-junction’ lasers can operate even at room temperature, whereas the older semiconductor lasers needed cooling by liquid nitrogen (to around -77 0C). Herbert Kroemer and Zhores Alferov were awarded the Nobel Prize in physics in 2000 for their pioneering work in hetero-structures in semiconductors. Today, various alloys of gallium, arsenic, indium, phosphorus and aluminium are used to obtain the best LEDs and lasers.
One of the hottest areas in semiconductor lasers is quantum well lasers, or cascade lasers. This area came into prominence with the development of techniques of growing semiconductors layer by layer using molecular beam epitaxy. Researchers use this technique to work like atomic bricklayers. They build a laser by placing a layer of a semiconductor with a particular structure and then placing another on top with a little bit of cementing material in between. By accurately controlling the thickness of these layers and their composition, researchers can adjust the band gaps in different areas. This technique is known as ‘band gap engineering’.
If the sandwich is thin enough, it acts as a quantum well for electrons. The electrons confined in this way lead to quantum systems called quantum wells (also known as particle in a box). The gap in the energy levels in such quantum wells can be controlled minutely and used for constructing a laser. Further, by constructing a massive club sandwich, as it were, we can have several quantum wells next to each other. The electron can make a stimulated emission of a photon by jumping to a lower level in the neighbouring well and then the next one and so on. This leads to a cascade effect like a marble dropping down a staircase. The system ends up emitting several photons of different wavelengths, corresponding to the quantum energy staircase. Frederico Capasso and his team built the first such quantum cascade laser at Bell Labs in 1994.
Once a device can be made from semiconductors, it becomes possible to miniaturise them while raising performance levels and reducing their price. That’s the pathway to mass production and use. This has happened in the case of lasers too.
We can leave the physics of lasers at this point and see how lasers are used in appliances of daily use:
A bar-code reader uses a tiny helium-neon laser to scan the code. A detector built into the reader detects reflected light and the white-and black bars are then converted to a digital code that identifies the object.
A laser printer uses static electricity; that’s what makes your polyester shirt or acrylic sweater crackle sometimes. The drum assembly inside the laser printer is made of material that conducts when exposed to light. Initially, the rotating drum is given a positive charge. A tiny movable mirror reflects a laser beam on to the drum surface, thereby rendering certain points on the drum electrically neutral. A chip controls the movement of the mirror. The laser ‘draws’ the letters and images to be printed as an electrostatic image.
After the image is set, the drum is coated with positively charged toner (a fine, black powder). Since it has a positive charge, the toner clings to the discharged areas of the drum, but not to the positively charged ‘background’. The drum, with this powder pattern, rolls over a moving sheet of paper that has already been given a negative charge stronger than the negative charge of the image. The paper attracts the toner powder. Since it is moving at the same speed as the drum, the paper picks up the image exactly. To keep the paper from clinging to the drum, it is electrically discharged after picking up the toner. Finally, the printer passes the paper through a pair of heated rollers. As the paper passes through these rollers, the toner powder melts, fusing with the paper, which is why pages are always warm when they emerge from a laser printer.
Compact discs are modern avatars of the old vinyl long-playing records. Sound would be imprinted on the LPs by a needle as pits and bumps. When the needle in the turntable head went over the track, it moved in consonance with these indentations. The resultant vibrations were amplified mechanically to reproduce the sound we heard as music. Modern-day CDs and DVDs are digital versions of the old Edison’s phonograph. Sound or data is digitised and encoded in tiny black or white spots corresponding to ones and zeros. These spots are then embedded in tiny bumps that are 0.5 microns wide, 0.83 microns long and 0.125 micron high. The bumps are laid out in a spiral track much as in the vinyl record. A laser operating at a 0.780-micron wavelength lights up these spots and the reflected signal is then read by a detector as a series of ones and zeroes, which are translated into sound.
In the case of DVDs, or digital versatile discs, the laser operates at an even smaller wavelength, and is able to read much smaller bumps. This allows us to increase the density of these bumps in the track on a DVD with more advanced compression and coding techniques. This means we can store much more information on a DVD than we can on a CD. A DVD can store several GB of information compared with the 800 MB of data a CD can store.
A CD is made from a substratum of polycarbonate imprinted with microscopic pits and coated with aluminium, which is then protected by a thin layer of acrylic. The incredibly small dimensions of the bumps make the spiral track on a CD almost five kilometres long! On DVDs, the track is almost twelve kilometres long.
To read something this small you need an incredibly precise discreading mechanism. The laser reader in the CD or DVD player, which has to find and read the data stored as bumps, is an exceptionally precise device.
The fundamental job of the player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminium layer, and hits an opto-electronic device that detects changes in light. The bumps reflect light differently than the rest of the aluminium layer, and the opto-electronic detector senses the change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes. These are then processed as audio or video signals.
With the turntables of yesterday’s audio technology, the vibrating needles would suffer wear and tear. Lasers neither wear themselves out nor scratch the CDs, and they are a thousand times smaller than the thinnest needle. That is the secret of high-quality reproduction and the high quantity of content that can be compressed into an optical disc.
C.K.N. Patel recalls how, in the 1960s, the US defence department was the organisation that evinced the greatest interest in his carbon dioxide laser. “The launch of the Sputnik by the Soviet Union created virtual panic,” he says. “That was excellent, since any R&D project which
the military thought remotely applicable to defence got generously funded.” ‘Peacenik’ Patel, who is passionate about nuclear disarmament, is happy to see that the apocalyptic ‘Third Eye’ has found peaceful applications in manufacturing and IT. Patel refuses to retire and is busy,
in southern California, trying to find more applications of lasers for health and pollution problems.
To get into the extremely important application of lasers in communications, we need to look at fibre optics more closely.
DUG UP ROADS
Outside telecom circles, fibre optics is not very popular among city dwellers in India. Because, in the past couple of years, hundreds of towns and cities in India have been dug up on an precedented scale. The common refrain is: “They are laying fibre-optic cable”. Fibre optics has created an obstacle course for pedestrians and drivers while providing grist to the mills of cartoonists like R.K. Laxman. Being an optimist, I tell my neighbours, “Soon we will have a bandwidth infrastructure fit for the twenty-first century.” What is bandwidth? It is an indication of the amount of information you can receive per second, where ‘information’ can mean words, numbers, pictures, sounds or films.
Bandwidth has nothing to do with the diameter of the cable that brings information into our homes. In fact, the thinnest fibres made of glass— thinner than human hair—can bring a large amount of information into our homes and offices at a reasonable cost. And that is why fibre optics is playing a major role in the IT revolution.
It is only poetic justice that words like fibre optics are becoming popular in India. Very few Indians know that an Indian, Narinder Singh Kapany, a pioneer in the field, coined them in 1960. We will come to his story later on, but before that let us look at what fibre optics is.
It all started with queries like: Can we channel light through a curved path, even though we know that light travels in a straight line? Why is that important? Well, suppose you want to examine an internal organ of the human body for diagnostic or surgical purposes. You would need a flexible pipe carrying light. Similarly, if you want to communicate by using light signals, you cannot send light through the air for long distances; you need a flexible cable carrying light over such distances.
The periscopes we made as class projects when we were in school, using cardboard tubes and pieces of mirror, are actually devices to bend light. Bending light at right angles as in a periscope was simple. Bending light along a smooth curve is not so easy. But it can be done, and that is what is done in optic fibre cables.
For centuries people have built canals or viaducts to direct water for irrigation or domestic use. These channels achieve maximum effect if the walls or embankments do not leak. Similarly, if we have a pipe whose insides are coated with a reflecting material, then photons or waves can be directed along easily without getting absorbed by the wall material. A light wave gets reflected millions of times inside such a pipe (the number depending on the length and diameter of the pipe and the narrowness of the light beam). This creates the biggest problem for pipes carrying light. Even if we can get coatings with 99.99 per cent reflectivity, the tiny leakage’ of 0.01 per cent on each reflection can result in a near-zero signal after 10,000 reflections.
Here a phenomenon called total internal reflection comes to the rescue. If we send a light beam from water into air, it behaves peculiarly as we increase the angle between the incident ray and the perpendicular. We reach a point when any increase in the angle of incidence results in the light not leaving the water and, instead, getting reflected back entirely. This phenomenon is called total internal reflection. Any surface, however finely polished, absorbs some light, and hence repeated reflections weaken a beam. But total internal reflection is a hundred per cent, which means that if we make a piece of glass as non-absorbent as possible, and if we use total internal reflection, we can carry a beam of light over long distances inside a strand of glass. This is the principle used in fibre optics.
The idea is not new. In the 1840s, Swiss physicist Daniel Collandon and French physicist Jacques Babinet showed that light could be guided along jets of water. British physicist John Tyndall popularised the idea further through his public demonstrations in 1854, guiding light in a jet of water flowing from a tank. Since then this method has been commonly used in water fountains. If we keep sources of light that change their colour periodically at the fountainhead, it appears as if differently coloured water is springing out of the fountain.
Later many scientists conceived of bent quartz rods carrying light, and even patented some of these inventions. But it took a long time for these ideas to be converted into commercially viable products. One of the main hurdles was the considerable absorption of light inside glass rods.
Narinder Singh Kapany recounted to the author, “When I was a high school student at Dehradun in the beautiful foothills of the Himalayas, it occurred to me that light need not travel in a straight line, that it could be bent. I carried the idea to college. Actually it was not an idea but the statement of a problem. When I worked in the ordnance factory in Dehradun after my graduation, I tried using right-angled prisms to bend light. However, when I went to London to study at the Imperial College and started working on my thesis, my advisor, Dr Hopkins, suggested that I try glass cylinders instead of prisms. So I thought of a bundle of thin glass fibres, which could be bent easily. Initially my primary interest was to use them in medical instruments for looking inside the human body. The broad potential of optic fibres did not dawn on me till 1955. It was then that I coined the term fibre optics.”
Kapany and others were trying to use a glass fibre as a light pipe or, technically speaking, a ‘dielectric wave guide’. But drawing a fibre of optical quality, free from impurities, was not an easy job.
Kapany went to the Pilkington Glass Company, which manufactured glass fibre for non-optical purposes. For the company, the optical quality of the glass was not important. “I took some optical glass and requested them to draw fibre from that,” says Kapany. “I also told them that I was going to use it to transmit light. They were perplexed, but humoured me.” A few months later Pilkington sent spools of fibre made of green glass, which is used to make beer bottles. “They had ignored the optical glass I had given them. I spent months making bundles of fibre from what they had supplied and trying to transmit light through them, but no light came out. That was because it was not optical glass. So I had to cut the bundle to short lengths and then use a bright carbon arc source.”
Kapany was confronted with another problem. A naked glass fibre did not guide the light well. Due to surface defects, more light was leaking out than he had expected. To transmit a large image he would have needed a bundle of fibres containing several hundred strands; but contact between adjacent fibres led to loss of image resolution. Several people then suggested the idea of cladding the fibre. Cladding, when made of glass of a lower refractive index than the core, reduced leakages and also prevented damage to the core. Finally, Kapany was successful; he and Hopkins published the results in 1954 in the British journal Nature.
Kapany then migrated to the US and worked further in fibre optics while teaching at Rochester and the Illinois Institute of Technology. In 1960, with the invention of lasers, a new chapter opened in applied physics. From 1955 to 1965 Kapany was the lead author of dozens of technical and popular papers on the subject. His writings spread the gospel of fibre optics, casting him as a pioneer in the field. His popular article on fibre optics in the Scientific American in 1960 finally established the new term (fibre optics); the article constitutes a reference point for the subject even today. In November 1999, Fortune magazine published profiles of seven people who have greatly influenced life in the twentieth century but are unsung heroes. Kapany was one of them.
BELL WAS THERE, TOO
If we go back into the history of modern communications involving electrical impulses, we find that Alexander Graham Bell patented an optical telephone system in 1880. He called this a ‘photophone’. Bell converted speech into electrical impulses, which he converted into light flashes. A photosensitive receiver converted the signals back into electrical impulses, which were then converted into speech. But the atmosphere does not transmit light as reliably as wires do; there is heavy atmospheric absorption, which can get worse with fog, rain and other impediments. As there were no strong and directional light sources like lasers at that time, optical communications went into hibernation. Bell’s earlier invention, the telephone, proved far more practical. If Bell yearned to send signals through the air, far ahead of his time, we cannot blame him; after all, it’s such a pain digging and laying cables.
In the 1950s, as telephone networks spread, telecommunications engineers sought more transmission bandwidth. Light, as a carrying medium, promised the maximum bandwidth. Naturally, optic fibres attracted attention. But the loss of intensity of the signal was as high as a decibel per metre. This was fine for looking inside the body, but communications operated over much longer distances and could not tolerate losses of more than ten to twenty decibels per kilometre.
Now what do decibels have to do with it? Why is signal loss per kilometre measured in decibels? The human ear is sensitive to sound on a logarithmic scale; that is why the decibel scale came into being in audio engineering, in the first place. If a signal gets reduced to half its strength over one kilometre because of absorption, after two kilometres it will become a fourth of its original strength. That is why communication engineers use the decibel scale to describe signal attenuation in cables.
In the early 1969s signal loss in glass fibre was one decibel per metre, which meant that after traversing ten metres of the fibre the signal was reduced to a tenth of its original strength. After twenty metres the signal was a mere hundredth its original strength. As you can imagine, after traversing a kilometre no perceptible signal was left.
A small team at the Standard Telecommunications Laboratories in the UK was not put off by this drawback. This group was headed by Antoni Karbowiak, and later by a young Shanghai-born engineer, Charles Kao. Kao studied the problem carefully and worked out a proposal for long-distance communications through glass fibres. He presented a paper at a London meeting of the Institution of Electrical Engineers in 1966, pointing out that the optic fibre of those days had an information-carrying capacity of one GHz, or an equivalent of 200 TV channels, or more than 200,000 telephone channels. Although the best available low-loss material then showed a loss of about 1,000 decibels/kilometre (dB/km), he claimed that materials with losses of just 10-20 dB/km would eventually be developed.
With Kao almost evangelistically promoting the prospects of fibre communications, and the British Post Office (the forerunner to BT) showing interest in developing such a network, laboratories around the world tried to make low-loss fibre. It took four years to reach Kao’s goal of 20 dB/km. At the Corning Glass Works (now Corning Inc.), Robert Maurer, Donald Keck and Peter Schultz used fused silica to achieve the feat. The Corning breakthrough opened the door to fibre-optic communications. In the same year, Bell Labs and a team at the Ioffe Physical Institute in Leningrad (now St Petersburg) made the first semiconductor lasers, able to emit a continuous wave at room temperature. Over the next several years, fibre losses dropped dramatically, aided by improved fabrication methods and by the shift to longer wavelengths where fibres have inherently lower attenuation. Today’s fibres are so transparent that if the Pacific Ocean, which is several kilometres deep, were to be made of this glass we could see the ocean bed!
Note one point here. The absorption of light in glass depends not only on the chemical composition of the glass but also on the wavelength of light that is transmitted through it. It has been found that there are three windows with very low attenuation: one is around 900 nanometres, the next at 1,300 nm and the last one at 1,550 nm. Once engineers could develop lasers with those wavelengths, they were in business. This happened in the 1970s and 1980s, thanks to Herbert Kroemer’s hetero-structures and many hard-working experimentalists.
REPEATERS AND ‘CHINESE WHISPERS’
All telephone systems need repeater stations at every few kilometres to receive the signal, amplify it and re-send it. Fibre optic systems need stations every few kilometres to receive a weak light signal, convert it into electronic signal, amplify it, use it to modulate a laser beam again, and re-send it. This process is exposed to risk of noise and errors creeping into the signal; the system needs to get rid of the noise and re-send a fresh signal. It is like a marathon run, where the organisers place tables with refreshing drinks all along the route so that the tired and dehydrated runners can refresh themselves. This means a certain delay, but the refreshment is absolutely essential.
Submarine cables must have as few points as possible where the system can break down because, once the cable is laid several kilometres under the sea, it becomes virtually impossible to physically inspect faults and repair them.
The development, in the 1980s, of fibre amplifiers, or fibres that act as amplifiers, has greatly facilitated the laying of submarine optic fibre cables. This magic is achieved through an innovation called the erbium doped fibre amplifier. Sections of fibre carefully doped with the right amount of erbium—a rare earth element—act as laser amplifiers.
While fibre amplifiers reduce the requirement of repeater stations, they cannot eliminate the need for them. That is because repeater stations not only amplify the signal, they also clean up the noise (whereas fibre amplifiers amplify the signal, noise and all). In fact, they add a little bit of their own noise. This is like the popular party game called Chinese whispers. If there is no correction in between, the message gets transmitted across a distance, but in a highly distorted fashion.
Can we get rid of these repeater stations altogether and send a signal which does not need much amplification or error correction over thousands of kilometres? That’s a dream for every submarine cable company, though perhaps not a very distant one.
The phenomenon being used in various laboratories around the world to create such a super-long-distance runner is called a ‘soliton’ or a solitary wave. A Dutch gentleman first observed solitary waves nearly 300 years ago while riding along the famous canals of the Netherlands. He found that as boats created waves in canals, some waves were able to travel enormously long distances without dissipating themselves. They were named solitary waves, for obvious reasons. Scientists are now working on creating solitons of light that can travel thousands of kilometres inside optical fibres without getting dissipated.
As and when they achieve it, they will bring new efficiencies to fibre optic communications. Today, any signal is a set of waves differing in wavelength by very small amounts. Since the speeds of different wavelengths of light differ inside glass fibres, over a large distance the narrow packet tends to loosen up, with some portion of information appearing earlier and some later. This is called ‘dispersion’, something similar to the appearance of a colourful spectrum when light passes through a glass prism or a drop of rain. Solitons seem to be unaffected by dispersion. Long-distance cable companies are eagerly awaiting the conversion of these cutting-edge technologies from laboratory curiosities to commercial propositions.
Coming down to earth, we find that even though fibre optic cable prices have crashed in recent years, the cost of terminal equipment remains high. That is why it is not yet feasible to lay fibre optic cable to every home and office. For the time being, we have to remain content with such cables being terminated at hubs supporting large clusters of users, and other technologies being used to connect up the ‘last mile’ between the fibre optic network and our homes and offices.
1. “Zur Quantentheorie der strahlung” (Towards a quantum theory of radiation)—Albert Einstein, Physikalische Zeitschrift, Volume 18 (1917), pp 121-128 translated as “Quantum Theory of Radiation and Atomic Processes,” in Henry A. Boorse and Lloyd Motz (eds.) The World of the Atom, Volume II, Basic Books, 1966, pp 884-901
2. Charles Townes—Nobel Lecture, 1964 (www.nobel.se/physics/laureates/ 1964/townes-lecture.html)
3. N.G. Basov—Nobel Lecture, 1964 (http://www.nobel.se/physics/laureates/1964/basov-lecture.html)
4. Lasers Theory and Applications—K. Thyagarajan, A.K. Ghatak, Macmillan India Ltd, 2001.
5. Chaos, Fractals and Self-Organisation: New perspectives on complexity in nature—Arvind Kumar, National Book Trust, India, 1996
6. Semiconductor devices Basic principle—Jasprit Singh, John Wiley, 2001.
7. Herbert Kroemer—Nobel Lecture, 2000 (www.nobel.se/physics/laureates/2000/kroemer-lecture.html ).
8. Zhores Alferov—Nobel Lecture, 2000 (www.nobel.se/physics/laureates/2000/alferov-lecture.html ).
9. Diminishing Dimensions—Elizabeth Corcoran and Glenn Zorpette,Scientific American, Jan 22, 1998.
10. Fiber Optics—Jeff Hecht, Oxford University Press, New York, 1999.
11. Fiber Optics—N.S. Kapany, Scientific American, November 1960.
12. Fibre Optics—G.K. Bhide, National Book Trust, India, 2000.
13. “Beyond valuations”—Shivanand Kanavi, Business India, Sep 17-30, 2001.