From surgical instruments and precision guides in construction to barcode scanners and compact disc readers, lasers are integral to many aspects of modern life and work. But perhaps the farthest-flung contribution of the 20th century's combination of optics and electronics has been in telecommunications. With the advent of highly transparent fiber-optic cable in the 1970s, very high-frequency laser signals now carry phenomenal loads of telephone conversations and data across the country and around the world.
| 1917 |
Theory of stimulated emission Albert Einstein proposes the theory of stimulated emission—that is, if an atom in a high-energy state is stimulated by a photon of the right wavelength, another photon of the same wavelength and direction of travel will be created. Stimulated emission will form the basis for research into harnessing photons to amplify the energy of light. |
| 1954 |
"Maser" developed Charles Townes, James Gordon, and Herbert Zeiger at Columbia University develop a "maser" (for microwave amplification by stimulated emission of radiation), in which excited molecules of ammonia gas amplify and generate radio waves. The work caps 3 years of effort since Townes's idea in 1951 to take advantage of high-frequency molecular oscillation to generate short-wavelength radio waves. |
| 1958 |
Concept of a laser introduced Townes and physicist Arthur Schawlow publish a paper showing that masers could be made to operate in optical and infrared regions. The paper explains the concept of a laser (light amplification by stimulated emission of radiation)—that light reflected back and forth in an energized medium generates amplified light. |
| 1960 |
Operable laser invented Theodore Maiman, a physicist and electrical engineer at Hughes Research Laboratories, invents an operable laser using a synthetic pink ruby crystal as the medium. Encased in a "flash tube" and book ended by mirrors, the laser successfully produces a pulse of light. Prior to Maiman’s working model, Columbia University doctoral student Gordon Gould also designs a laser, but his patent application is initially denied. Gould finally wins patent recognition nearly 30 years later. |
| 1960 |
Continuously operating helium-neon gas laser invented Bell Laboratories researcher and former Townes student Ali Javan and his colleagues William Bennett, Jr., and Donald Herriott invent a continuously operating helium-neon gas laser. The continuous beam of laser light is extracted by placing parallel mirrors on both ends of an apparatus delivering an electrical current through the helium and neon gases. On December 13, Javan experiments by holding the first telephone conversation ever delivered by a laser beam. |
| 1961 |
Glass fiber demonstration Industry researchers Elias Snitzer and Will Hicks demonstrate a laser beam directed through a thin glass fiber. The fiber’s core is small enough that the light follows a single path, but most scientists still consider fibers unsuitable for communications because of the high loss of light across long distances. |
| 1961 |
First medical use of the ruby laser In the first medical use of the ruby laser, Charles Campbell of the Institute of Ophthalmology at Columbia- Presbyterian Medical Center and Charles Koester of the American Optical Corporation use a prototype ruby laser photocoagulator to destroy a human patient’s retinal tumor. |
| 1962 |
Gallium arsenide laser developed Three groups—at General Electric, IBM, and MIT’s Lincoln Laboratory—simultaneously develop a gallium arsenide laser that converts electrical energy directly into infrared light and that much later is used in CD and DVD players as well as computer laser printers. |
| 1963 |
Heterostructures Physicist Herbert Kroemer proposes the idea of heterostructures, combinations of more than one semiconductor built in layers that reduce energy requirements for lasers and help them work more efficiently. These heterostructures will later be used in cell phones and other electronic devices. |
| 1966 |
Landmark paper on optical fiber Charles Kao and George Hockham of Standard Telecommunications Laboratories in England publish a landmark paper demonstrating that optical fiber can transmit laser signals with much reduced loss if the glass strands are pure enough. Researchers immediately focus on ways to purify glass. |
| 1970 |
Optical fibers that meet purity standards Corning Glass Works scientists Donald Keck, Peter Schultz, and Robert Maurer report the creation of optical fibers that meet the standards set by Kao and Hockham. The purest glass ever made, it is composed of fused silica from the vapor phase and exhibits light loss of less than 20 decibels per kilometer (1 percent of the light remains after traveling 1 kilometer). By 1972 the team creates glass with a loss of 4 decibels per kilometer. Also in 1970, Morton Panish and Izuo Hayashi of Bell Laboratories, along with a group at the Ioffe Physical Institute in Leningrad, demonstrate a semiconductor laser that operates continuously at room temperature. Both breakthroughs will pave the way toward commercialization of fiber optics. |
| 1973 |
Chemical vapor deposition process John MacChesney and Paul O’Connor at Bell Laboratories develop a modified chemical vapor deposition process that heats chemical vapors and oxygen to form ultratransparent glass that can be mass-produced into low-loss optical fiber. The process still remains the standard for fiber-optic cable manufacturing. |
| 1975 |
First commercial semiconductor laser Engineers at Laser Diode Labs develop the first commercial semiconductor laser to operate continuously at room temperatures. The continuous-wave operation allows the transmission of telephone conversations. |
| 1977 |
Telephone companies fiber optic trials Telephone companies begin trials with fiber-optic links carrying live telephone traffic. GTE opens a line between Long Beach and Artesia, California, whose transmitter uses a light-emitting diode. Bell Labs establishes a similar link for the phone system of downtown Chicago, 1.5 miles of underground fiber that connects two switching stations. |
| 1980 |
Fiber-optic cable links major cities AT&T announces that it will install fiber-optic cable linking major cities between Boston and Washington, D.C. The cable is designed to carry three different wavelengths through graded-index fiber—technology that carries video signals later that year from the Olympic Games in Lake Placid, New York. Two years later MCI announces a similar project using single-mode fiber carrying 400 bits per second. |
| 1987 |
"Doped" fiber amplifiers David Payne at England’s University of Southampton introduces fiber amplifiers that are "doped" with the element erbium. These new optical amplifiers are able to boost light signals without first having to convert them into electrical signals and then back into light. |
| 1988 |
First transatlantic fiber-optic cable The first transatlantic fiber-optic cable is installed, using glass fibers so transparent that repeaters (to regenerate and recondition the signal) are needed only about 40 miles apart. The shark-proof TAT-8 is dedicated by science fiction writer Isaac Asimov, who praises "this maiden voyage across the sea on a beam of light." Linking North America and France, the 3,148-mile cable is capable of handling 40,000 telephone calls simultaneously using 1.3-micrometer wavelength lasers and single-mode fiber. The total cost of $361 million is less than $10,000 per circuit; the first transatlantic copper cable in 1956 costs $1 million per circuit to plan and install. |
| 1991 |
Optical Amplifiers Emmanuel Desurvire of Bell Laboratories, along with David Payne and P. J. Mears of the University of Southampton, demonstrate optical amplifiers that are built into the fiber-optic cable itself. The all-optic system can carry 100 times more information than cable with electronic amplifiers. |
| 1996 |
All-optic fiber cable that uses optical amplifiers is laid across the Pacific Ocean TPC-5, an all-optic fiber cable that is the first to use optical amplifiers, is laid in a loop across the Pacific Ocean. It is installed from San Luis Obispo, California, to Guam, Hawaii, and Miyazaki, Japan, and back to the Oregon coast and is capable of handling 320,000 simultaneous telephone calls. |
| 1997 |
Fiber Optic Link Around the Globe The Fiber Optic Link Around the Globe (FLAG) becomes the longest single-cable network in the world and provides infrastructure for the next generation of Internet applications. The 17,500-mile cable begins in England and runs through the Strait of Gibraltar to Palermo, Sicily, before crossing the Mediterranean to Egypt. It then goes overland to the FLAG operations center in Dubai, United Arab Emirates, before crossing the Indian Ocean, Bay of Bengal, and Andaman Sea; through Thailand; and across the South China Sea to Hong Kong and Japan. |
Its secret was light—a very special kind of radiance produced by devices called lasers and channeled along threads of ultrapure glass called optical fibers. Today, millions of miles of the hair-thin strands stretch across continents and beneath oceans, knitting the world together with digital streams of voice, video, and computer data, all encoded in laser light.
When the basic ideas behind lasers occurred to Columbia University physicist Charles Townes in 1951, he wasn't thinking about communications, much less the many other roles the devices would someday play in such fields as manufacturing, health care, consumer electronics, merchandising, and construction. He wasn't even thinking about light. Townes was an expert in spectroscopy—the study of matter's interactions with electromagnetic energy—and what he wanted was a way to generate extremely short-wavelength radio waves or long-wavelength infrared waves that could be used to probe the structure and behavior of molecules. No existing instrument was suitable for the job, but early one spring morning as he sat on a park bench wrestling with the problem, he suddenly recognized that molecules themselves might be enlisted as a source.
All atoms and molecules exist only at certain characteristic energy levels. When an atom or molecule shifts from one level to another, its electrons emit or absorb photons—packets of electromagnetic energy with a tell-tale wavelength (or frequency) that may range from very long radio waves to ultrashort gamma rays, depending on the size of the energy shift. Normally the leaps up and down the energy ladder don't yield a surplus of photons, but Townes saw possibilities in a distinctive type of emission described by Albert Einstein back in 1917.
If an atom or molecule in a high-energy state is "stimulated" by an impinging photon of exactly the right wavelength, Einstein noted, it will create an identical twin—a second photon that perfectly matches the triggering photon in wavelength, in the alignment of wave crests and troughs, and in the direction of travel. Normally, there are more molecules in lower-energy states than in higher ones, and the lower-energy molecules absorb photons, thus limiting the radiation intensity. Townes surmised that under the right conditions the situation might be reversed, allowing the twinning to create amplification on a grand scale. The trick would be to pump energy into a substance from the outside to create a general state of excitement, then keep the self-duplicating photons bouncing back and forth in a confined space to maximize their numbers.
Not until 1954 did he and fellow researchers at Columbia prove it could be done. Using an electric field to direct excited molecules of ammonia gas into a thumb-sized copper chamber, they managed to get a sustained output of the desired radio waves. The device was given the name maser, for microwave amplification by stimulated emission of radiation, and it proved valuable for spectroscopy, the strengthening of extremely faint radio signals, and a few other purposes. But Townes would soon create a far bigger stir, teaming up with his physicist brother-in-law Arthur Schawlow to show how stimulated emission might be achieved with photons at the much shorter wavelengths of light—hence the name laser, with the "m" giving way to "l." In a landmark paper published in 1958 they explained that light could be reflected back and forth in the energized medium by means of two parallel mirrors, one of them only partly reflective so that the built-up light energy could ultimately escape. Six years later Townes received a Nobel Prize for his work, sharing it with a pair of Soviet scientists, Aleksandr Prochorov and Nikolai Gennadievich Basov, who had independently covered some of the same ground.
The first functioning laser—a synthetic ruby crystal that emitted red light—was built in 1960 by Theodore Maiman, an electrical engineer and physicist at the Hughes Research Laboratories. That epochal event set off a kind of evolutionary explosion. Over the next few decades lasers would take forms as big as a house and as small as a grain of sand. Along with ruby, numerous other solids were put to work as a medium for laser excitation. Various gases proved viable too, as did certain dye-infused liquids and some of the electrically ambivalent materials known as semiconductors. Researchers also developed many ways to excite a laser medium into action, pumping in the necessary energy with flash lamps, other lasers, electricity, and even chemical reactions.
As for the laser light itself, it soon came in a broad range of wavelengths, from infrared to ultraviolet, with the output delivered as either pulses or continuous beams. All laser light has the same highly organized nature, however. In the language of science, it is practically monochromatic (of essentially the same wavelength), coherent (the crests and troughs of the waves perfectly in step, thus combining their energy), and highly directional. The result is an extremely narrow and powerful beam, far less inclined to spread and weaken than a beam of ordinary light, which is composed of a jumble of wavelengths out of step with one another.
Lasers have found applications almost beyond number. In manufacturing, infrared carbon dioxide lasers cut and heat-treat metal, trim computer chips, drill tiny holes in tough ceramics, silently slice through textiles, and pierce the openings in baby bottle nipples. In construction the narrow, straight beams of lasers guide the laying of pipelines, drilling of tunnels, grading of land, and alignment of buildings. In medicine, detached retinas are spot-welded back in place with an argon laser's green light, which passes harmlessly through the central part of the eye but is absorbed by the blood-rich tissue at the back. Medical lasers are also used to make surgical incisions while simultaneously cauterizing blood vessels to minimize bleeding, and they allow doctors to perform exquisitely precise surgery on the brain and inner ear.
Many everyday devices have lasers at their hearts. A CD or DVD player, for example, reads the digital contents of a rapidly spinning disc by bouncing laser light off minuscule irregularities stamped onto the disc's surface. Barcode scanners in supermarkets play a laser beam over a printed pattern of lines and spaces to extract price information and keep track of inventory.
Pulsed lasers are no less versatile than their continuous-beam brethren. They can function like optical radar, picking up reflections from objects as small as air molecules, enabling meteorologists to detect wind direction or measure air density. The reflections can also be timed to measure distances—in some cases, very great indeed. A high-powered pulsed laser, aimed at mirrors that astronauts placed on the lunar surface, was used to determine the distance from Earth to the Moon to within 2 inches. The pulses of some lasers are so brief—a few quadrillionths of a second—that they can visually freeze the lightning-fast movements of molecules in a chemical reaction. And superpowerful laser pulses may someday serve as the trigger for controlled fusion, the long-sought thermonuclear process that could provide humankind with almost boundless energy.
Whatever the future holds, the laser's status as a world-changing innovation has already been secured by its role in long-distance communications. But that didn't happen without some pioneering on another frontier—fiber optics. At the time lasers emerged, the ability of flexible strands of glass to act as a conduit for light was a familiar phenomenon, useful for remote viewing and a few other purposes. Such fibers were considered unsuitable for communications, however, because any data encoded in the light were quickly blurred by chaotic internal reflections as the waves traveled along the channel. Then in 1961 two American researchers, Will Hicks and Elias Snitzer, directed laser beams through a glass fiber made so thin—just a few microns—that the light waves would follow a single path rather than ricocheting from side to side and garbling a signal in the process.
This was a major advance, but practical communication with light was blocked by a more basic difficulty. As far as anyone knew, conventional glass simply couldn't be made transparent enough to carry light far. Typically, light traveling along a fiber lost about 99 percent of its energy by the time it had gone just 30 feet. Fortunately for the future of fiber optics, a young Shanghai-born electrical engineer named Charles Kao was convinced that glass could do much better.
Working at Standard Telecommunications Laboratories in England, Kao collected and analyzed samples from glassmakers and concluded that the energy loss was mainly due to impurities such as water and minerals, not the basic glass ingredient of silica itself. A paper he published with colleague George Hockham in 1966 predicted that optical fibers could be made pure enough to carry signals for miles. The challenges of manufacturing such stuff were formidable, but in 1970 a team at Corning Glass Works succeeded in creating a fiber hundreds of yards long that performed just as Kao and Hockham had foreseen. Continuing work at Corning and AT&T Bell Labs developed the manufacturing processes necessary to produce miles of high quality fiber.
At about the same time, researchers were working hard on developing a light source to partner with optical fibers. Their efforts were focused on semiconductor lasers, sand-grain-sized mites that could be coupled to the end of a thread of glass. Semiconducting materials are solid compounds that conduct electricity imperfectly. When a tiny sandwich of differing materials is electrically energized, laser action takes place in the junction region, and the polished ends of the materials act as mirrors to confine the light photons while they multiply prolifically.
In 1967 Morton Panish and Izuo Hayashi at Bell Labs spelled out the basic requirements for a semiconductor laser for fiber optic communications. It would have to generate a continuous beam rather than pulses. It would need to function at room temperature and operate for hundreds of thousands of hours without failure. Finally, the laser's output would have to be in the infrared range, optimal for transmission down a fiber of silica glass. In 1970 Panish and Hayashi, and Zhorez Alferov at the Ioffe Institute, demonstrated the double hetrostructure concept which led to such lasers. The same basic lasers later proved essential for the light source in CD and DVD players, bar code scanners and other devices.
By the mid-1970s all the necessary ingredients for fiber-optic communications were ready, and operational trials got under way. The first commercial service was launched in Chicago in 1977, with 1.5 miles of underground fiber connecting two switching stations of the Illinois Bell Telephone Company. Improvements in both lasers and fibers would keep coming after that, further widening light's already huge advantage over other methods of communication.
Any transmission medium's capacity to carry information is directly related to frequency—the number of wave cycles per second, or hertz. The higher the frequency, the more wave cycles per second, and the more information can be packed into the transmission stream. Light used for fiber-optic communications has a frequency millions of times higher than radio transmissions and 100 billion times higher than electric waves traveling along copper telephone wires. But that's just the beginning. Researchers have learned how to send multiple light streams along a fiber simultaneously, each carrying a huge cargo of information on a separate wavelength. In theory, more than a thousand distinct streams can ride along a single glass thread at the same time.
Toward the 20th century's end, one of the few lingering constraints was removed by a device that is both laser and fiber. For all the marvelous transparency of silica glass, light inevitably weakens as it travels along, requiring amplification from time to time. In the early years of fiber optics, the necessary regeneration was done by devices that converted the light signals into electricity, boosted them, and then changed them back into light again. This limited the speed of transmission because the electronic amplifier was slower than the fiber. But the 1990s saw the appearance of vastly superior amplifiers that are lasers themselves. These optical amplifiers consist of short stretches of fiber, doped with the element erbium and optically energized by an auxiliary "pump" laser. The erbium-doped amplifiers revive the fading photons every 50 miles or so without the need for electrical conversion. The amplification can occur for a relatively broad range of wavelengths, allowing roughly 40 different wavelengths to be amplified simultaneously.
For the most part the devices that switch messages from one fiber to another (as from one router to another on the Internet) still must convert a message from light to electricity and back again. Yet even as researchers and engineers actively pursue the development of all-optical switches, this last bottleneck scarcely hampers the flow of information carried on today's fiber-optic systems. Flashing incessantly between cities, countries, and continents, the prodigious torrent strains the gossamer web not at all.
Charles H. Townes
Professor, Department of Physics
University of California, Berkeley
The laser invention happened because I wanted very much to be able to make an oscillator at frequencies as high as the infrared in order to extend the field of microwave spectroscopy in which I was working. I had tried several ideas, but none worked very well. At the time I was also chairman of a committee for the navy that was examining ways to obtain very short-wave oscillators. In 1951, on the morning before the last meeting of this committee in Washington, I woke up early worrying over our lack of success. I got dressed and stepped outside to Franklin Park, where I sat on a bench admiring the azaleas and mulling over our problem.
Why couldn't we think of something that would work at high frequencies? I went through the possibilities, including, of course, molecules, which oscillate at high frequencies. Although I had considered molecules before, I had dismissed them because of certain laws of thermodynamics. But suddenly I recognized, "Hey, molecules don't have to obey such a law if they are not in equilibrium." And I immediately took a piece of paper out of my pocket and wrote equations to see if selection of excited molecules by molecular beam methods could produce enough molecules to provide a feedback oscillator. Wow! It looked possible.
I went back to my hotel and told Art Schawlow about the idea, since he was staying at the same place. Back at Columbia University, I wrote the idea carefully in my notebook and had Schawlow witness it in preparation for the possibility of a patent. (In my previous career at Bell Labs, I had produced several patents in the field of radar and hence was familiar with patent requirements.) Soon my students and I began building the first maser—not yet producing light but demonstrating the principles. Its extension to waves as short as light came a few years later, after much excitement over the maser and as a result of my continued collaboration with Schawlow, then at Bell Labs. An essential element in this discovery, I believe, was my experience in both engineering and physics: I knew both quantum mechanics and the workings and importance of feedback oscillators.
When Schawlow and I first distributed our paper on how to make a laser, a number of friends teased me with the comment, "That's an invention looking for an application. What can it do?" To me, communications with a potentially large bandwidth and beam directionality seemed an obvious application. But the use of fibers did not occur to me, and that's what really changed communications, especially with the development of low-loss materials.
Lasers combine optics and electronics. And so in addition to their revolutionary role in communications, lasers by now have found a wealth of applications—in medicine, manufacturing, measurements and control, computing, possibly nuclear power, and much new science. Thirteen Nobel prizes have been awarded for work utilizing lasers or masers as scientific tools.
Both lasers and fiber optics are fields that can be expected to grow and develop further, including in ways still not foreseen. Consider that all of the separate principles and ideas involved in the invention of masers and lasers were known and understood by someone in the scientific or technical community at least as early as the mid-1930s. Yet it took 25 more years for these ideas to be put together to make a laser. What might we be missing or overlooking now?