2010 marks the 50th anniversary of the first practical laser. We look at the history and technology of laser development.
Once exotic, futuristic and more-or-less synonymous with the ray gun of science-fiction tales, the laser now most commonly serves mankind by decoding disks in DVD players and scanning vegetables at the local supermarket. There are now so many applications of the laser that most of us think nothing of them - until we use a laser pointer or read an advert for laser eye surgery. But it took time for the invention to be recognised as important as it later proved to be.
In 1917, Albert Einstein published a paper describing what happens when energy is introduced to an atom, either through collision or electrical stimulation. Using an atomic model with electrons orbiting a nucleus in a series of defined energy levels, he suggested that, if enough energy were added to an atom, some of its electrons would move temporarily to a higher level before returning, or decaying, to their former state. Crucially, he also said that, when the decay occurs, a photon would be emitted. Because the energy transitions within the atom are quantised, the wavelength of that radiation would be predictable.
As with other theories, Einstein's writings on stimulated emission were well in advance of any technical capability for proof, and the first empirical evidence in support of the theory did not appear until the late 1920s, when experiments on electrical discharges in neon were conducted. But it was during the aftermath of the Second World War - when a wealth of surplus radar equipment was distributed among the universities of Europe and America - that the real advances occurred.
According to Dennis Cheek, a historian with the Templeton Foundation in Pennsylvania, it was American physicist Joseph Weber who described - at a 1952 electron tube conference in Canada - how the population of electrons in the higher energy states could be maintained using 'optical pumping' with the aid of a source such as an arc lamp. However, says Cheek, he was unable to produce the population inversion necessary to maintain the effect in the laboratory.
Birth of the MASER
Thus it fell to Charles Townes and co-workers James Gordon and Herbert Zeiger, at Columbia University, to produce such a sustained inversion process, which they did using ammonia vapour. A beam of ammonia molecules was projected through an electrostatic focuser to filter out the molecules in the lower quantum state and pass those in the higher state to a microwave cavity, to produce the population inversion.
The microwave energy coupled into the resonant cavity, at a frequency of 24GHz, induced the molecules to make downward transitions, thus releasing energy to amplify the radio-frequency (RF) wave that could then be coupled out of the cavity. The microwave output had been amplified by extracting energy from the ammonia molecules. Although they didn't name it at the time, they had succeeded in developing the basis of the first MASER ('microwave amplification by stimulated emission of radiation').
In 1954, Townes 'formally announced the operation of an oscillator' that could sustain the inversion process, explains Cheek, but in the same year Aleksandr Prokhorov and Nicolay Basov from Moscow's Lebedev Institute published a paper concerning 'similar results using microwave spectroscopes'. As a result, all three shared the 1964 Nobel Prize for Physics.
Interviewed for E&T, Townes claims priority: 'The first official notice of the maser was the publication in 1954 by myself, my student James Gordon, and my post-doc Herbert Zeiger.' It appeared in the Vol 95, July issue of The Physical Review.
'The paper had the typically academic title of 'Molecular Microwave Oscillator and New Hyperfine Structure in Microwave Spectrum of NH3', Townes adds.
'My students and I devised the acronym MASER at lunch together', he admits. 'The name was first used in our second publication on a working system, in 1955.'
Asked whether he viewed the process as a discovery or an invention, Townes says 'I recognised the idea as both a discovery and' one of the most important inventions of the century'.
However, in common with many inventions, it was not universally recognised at the time.
Had development ceased with the maser, Einstein's theory of stimulated emission would not have had the impact on society that it has today. The trouble was, as ever, that building equipment to operate at higher frequencies offered a significant challenge because of the smaller physical dimensions that shorter wavelengths imply.
If some were sceptical that the principle worked at microwave frequencies, 'very few people thought it could be made to work at much shorter wavelengths', confirms Townes, adding that even he needed time to 'get my ideas together' on how to do it.
This time it was Townes, and his brother-in-law Arthur Schawlow, working at Bell Laboratories where Townes was also a consultant, who would lay the groundwork for others to build upon. By 1958, they had determined that a carefully crafted optical cavity with mirrors at either end would allow photons released by the pumping process to re-enter the medium to strike other electrons, which would in turn release further photons. The result would be an intense, highly focused beam of visible light with all the photons at the same wavelength and in phase with each other: a coherent monochromatic beam. The device was known as an optical maser.
Gordon Gould, a Columbia student at the time of the invention, is credited with the acronym that would later become a familiar term to engineers and science fiction readers alike: LASER when 'microwave' was replaced by 'light' in the original term.
The first Laser
Theodore Maiman, on 16 May 1960 at the Hughes Research Laboratory in California, succeeded in turning Townes' and Schawlow's ideas into reality. Using a synthetic ruby crystal as the lasing material and a helical flash lamp as the pumping-energy source, he produced a beam of red laser light at a wavelength of 694nm. This was the first confirmed demonstration of a laser.
Perhaps predictably, given that most people's knowledge of 'coherent monochromatic beams' was informed by pulp magazines and B-movies, American headline writers had a field day with the likes of 'LA Man Invents Death Ray'. Maiman himself seemed less certain of its future applications in an interview with the Vancouver Sun in 2000: 'It worked the first time,' Maiman said. 'I was exhilarated. I thought, 'Wow, it's working!', but to tell you the truth, I was a little numbed, I did not appreciate the gravity of what I had done.'
However, it didn't take Maiman long to acquire that appreciation. He left Hughes to form his own laser development company, Korad Corporation, and, in 1972, co-founded Laser Video to develop large-screen, laser-driven video displays.
Maiman's original laser was deceptively simple in construction. The optical cavity, known as a Fabry-Pérot cavity, was formed from a cylinder of synthetic chromium-doped ruby whose ends had been accurately polished to be flat within a fraction of a wavelength and parallel within a few seconds of arc. Each end was coated with evaporated silver, to allow multiple internal reflections as the beam increased in intensity, but with one end less reflective than the other to allow the amplified beam to escape.
Since the pumping function was produced by a xenon flash tube that required recharging after each flash, Maiman's laser delivered only millisecond pulse-lengths and could not be 'on' continuously. Nevertheless, such was the excitement generated that, by 1962, Bell Labs scientists Donald Nelson and Willard Boyle, who in 1969 would develop the charged-coupled device with George Smith, had constructed a continuously lasing ruby by replacing the flash lamp with an arc lamp.
The ruby laser found application as widespread as creating holographic artworks and in tattoo removal.
As is often the case with new fields of research, related developments were being hotly pursued in parallel with Maiman's. Ali Javan and colleagues at Bell Labs, which was fast becoming a Mecca for laser development, were building the first gas laser using a mixture of helium and neon. Javan explains: 'My original invention took place in the summer of 1958, when nothing was known of lasers, but my successful operation of the original helium-neon gas-discharge laser took place on 12 December 1960.' With a view to historical accuracy, he adds: 'The historic event occurred at 4:20pm'.
Javan is keen to point out that his invention bears no relation to 'that of Theodore Maiman's pulsed ruby laser, which took place in the summer of 1960'. Seemingly to further distance himself from the Maiman laser, Javan contrasts its production of a 'single high intensity, short duration laser pulse at a frequency which is uncorrelated from one pulse to the next' with his gas laser, which he confirms was 'the world's first laser capable of operating continuously, and at an unprecedented colour purity and accuracy'.
As Javan is proud to relate, it didn't take long for an application to be demonstrated, not only in the lab but in a press conference at New York's Park Plaza Hotel. In February 1961, using the very same laser that had made history the previous year, Bell Labs demonstrated how the beam could be modulated with a voice signal from a telephone, propagated across the function room and demodulated by an optical receiver connected to a second telephone. 'A two-way conversation could be held between the two telephones', says Javan. The scene was set for the optical fibres of the 1970s.
A Bell Telephone System advertisement, published in Newsweek and Time the same week as the press conference, enthused: 'The possibilities are breath-taking. Light beams might be transmitted through long pipes, or could someday be just what are needed for communications in space - for example between space ships.'
It was an 830nm semiconductor laser that proved Bell's marketing prescience, in November 2001, when the first inter-satellite data link was established between the European Space Agency's Artemis spacecraft and the French space agency's Spot 4 satellite. Laser communications in space had become a reality.
From the early 1960s, developments came thick and fast: in 1962, the first semiconductor laser and the carbon dioxide laser, which was used in industry for cutting and welding, arrived. In 1963, ion lasers using mercury vapour appeared. And in 1966, a liquid laser using fluorescent dye was developed. By the mid-1970s, rare-gas excimer lasers used in semiconductor lithography and eye surgery had been added to the list, as had free-electron lasers which now have the widest frequency range of any laser type: from microwaves, through the optical spectrum, to soft X-rays, first demonstrated in 1985. According to historian Dennis Cheek, 'by the end of the 20th century, over 1,000 types of laser had been developed'.
As for applications, Tom Baer, executive director of the Stanford Photonics Research Center, says that 'well over 50,000' laser patents have been issued since its invention, 'placing it in good company with other innovations such as the computer, the LCD, and fibre optics'.
Asked which applications of the laser he considered the most important or interesting, Townes says: 'I'm most pleased with the medical applications, [but] for my own purposes, the scientific applications have been most important. I presently use them for astronomy.' And what of his feelings for that ubiquitous lecturing tool? 'Yes, I like using a laser pointer'.