The Development of the Maser and Laser Leads to Widespread Commercial and Research Applications

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The Development of the Maser and Laser Leads to Widespread Commercial and Research Applications

Overview

In 1953 Charles Hard Townes (1915- ) produced the first working maser. Masers generate and amplify beams of coherent microwave radiation through stimulation of excited energy states in resonant atomic or molecular systems. MASER is an acronym for this process of microwave amplification by stimulated emission of radiation. Maser principles were extended by Townes and Arthur Leonard Schawlow (1921- ) to the optical portion of the electromagnetic spectrum in 1958 when they published the first detailed proposal for building a laser (light amplification by stimulated emission of radiation). Masers have important though limited applications while lasers are more widely used in research and industry.

Background

In 1951 Charles Townes realized that Albert Einstein's (1879-1955) theory of stimulated emission could be exploited to generate and amplify microwave radiation. According to quantum theory, atoms or molecules exist in certain discrete energy states. The lowest energy level is the ground state, with higher levels being excited states. Moving from one state to another requires absorption or emission of precise amounts of energy in the form of photons of light. Since the wavelength of light determines its energy, photons must have specific wavelengths and no others. When atoms in the ground state absorb photons, they move into excited states. Excited atoms can spontaneously emit this extra energy as a photon or, as Einstein noted in 1916, emission may be accomplished by stimulation from a photon with the same energy. This stimulated emission results in two photons (amplification) of the same wavelength that can then stimulate other atoms. However, since most atoms are in the ground state, more photons will be absorbed than emitted.

Substances containing more atoms in ground states than excited states are said to have "inverted populations." Though rare in nature, inverted populations do occur. Stimulation in such substances produces photon cascades. Townes's key insight was to realized that inverted populations could be created artificially by isolating an ensemble of excited atoms. When placed in a resonating cavity and stimulated by electromagnetic waves of the appropriate wavelength, this unstable ensemble becomes self-oscillating and generates a coherent beam of monochromatic radiation. In 1953, after two years of work with James P. Gordon and H. J. Zeiger, Townes successfully produced a working maser. Various design improvements followed, after which masers were quickly adapted for research, commercial, and military applications.

The precise frequencies generated by masers have made possible better atomic clocks. Maser amplifiers also have extremely low noise and high sensitivity, which enables detection of weak signals in radio and radar astronomy. The most important application arising from applications in astronomy has been Arno Penzias (1933- ) and Robert Wilson's (1936- ) discovery of the cosmic background radiation predicted by Big Bang theories. They were awarded the 1978 Nobel Prize in physics for their research. Masers are also used for military radar, microwave spectroscopy, and microwave satellite communications.

In 1957 Townes turned his attention to creating an optical maser or laser. As the name suggests, the laser operates with visible light instead of microwaves. Townes and Arthur Schawlow, having earlier collaborated on the classic Microwave Spectroscopy (1955), decided to work together on the optical maser. Their "Infrared and Optical Masers" paper, published in the December 1958 issue of Physical Review, provided the first detailed theoretical description of a laser and initiated the race to build the first working laser.

All lasers consist of (1) an active medium; (2) a pump; and (3) resonating cavity. The active medium is a collection of atoms or molecules that are raised to excited states. This is typically achieved by optical pumping of electromagnetic radiation with the appropriate wavelength into the resonating cavity. Continued pumping initiates stimulated emission. The resonating cavity usually consists of a pair of mirrors (one only partially reflective) know as a Fabry-Perot etalon. The photon beam is amplified as it is reflected back and forth between the mirrors. When amplification rises above a certain level, the beam passes through the partially reflective mirror.

While at Hughes Research Laboratories in 1960, Theodore H. Maiman (1927- ) constructed the first working laser. For the active medium he chose a synthetic ruby rod. The resonator consisted of silver mirrors applied to the rod ends. Optical pumping was achieved by placing the laser rod inside the coil of a helical quartz-xenon flashlamp. The bright lamp pulse stimulated the rod to emit a short ruby florescence. Maiman's ruby laser work was quickly duplicated and employed for various industrial and research purposes.

Impact

While many different types of lasers have been and can be constructed by adopting different lasing materials, pumps, or resonators, relatively few have found widespread use. This is due to trade-offs among efficiency, ease of use, reliability, and cost. For example, the second working laser was also an optically pumped solid-state laser with a uranium lasing medium instead of synthetic ruby. However, because of its low efficiency and cryogenic cooling requirements, the uranium laser has yet to find practical applications.

The high-power output and room temperature operating conditions of Maiman's ruby laser made it the most widely used during the 1960s. One of its first applications was optical ranging, the most dramatic example of which was the determination of the distance between Earth and the Moon to within an inch. Optical ranging has also been used for military targeting and land surveying.

The first working gas laser was the heliumneon laser (1960). This was the first laser to emit a continuous beam, which for many applications is preferable to the pulsed operation of the ruby laser. The red helium-neon laser was produced a year later and, after overtaking the ruby laser in the late sixties, became the most commonly used laser up until the mid-1980s. Red helium-neon lasers are still sold by the hundreds of thousands. Their applications range from supermarket scanners for reading product bar codes to aligning construction and laboratory equipment.

The next important laser development was the CO₂ gas laser (1963). Early gas lasers had limited power, but the CO₂ laser operated in the 10 kilowatt range and is still one of the most efficient lasers. This device made possible full-scale laser welding and machining operations. Its low atmospheric absorption also made it an ideal candidate for battlefield weapons development.

Another important solid state laser is the Nd:YAG (neodymium ions embedded in a yttrium aluminum garnet crystal) laser first successfully demonstrated in 1964. The versatility of the YAG laser has made it the most common crystal solid state laser today. It emits over a range of different wavelengths and has a range of beam durations including continuous emission. YAG lasers are employed in many aspects of materials processing such as cutting, trimming, welding, marking, and laser annealing of electronic components.

These lasers have also made possible new alloys. Traditional alloying methods for surface coating have been plagued by various problems including inefficient use of key materials, structural weaknesses due to excessive heating, and discontinuities between alloy coatings and interior metal. The high power density and short pulse duration of lasers makes it possible to heat only the surface coating and small portions of the underlying metal, thus conserving resources and avoiding excessive heating. A greater degree of control is also exercised over the surface properties, allowing one to select those characteristics best suited to the design requirements. However, laser alloying is most fundamentally different from traditional coating practices in the elimination of the near-surface discontinuities. Laser alloys are continuous extensions of the interior metals, which makes for a more effective coating.

Tunable dye lasers were first operated in 1965. Their wavelengths can be continuously varied across a given range. This feature has made them important for many high-precision spectroscopic experiments. Schawlow's Stanford research group extensively used such lasers and developed advanced techniques to reveal spectra and give improved values for fundamental constants. For this work Schawlow won a share of the 1981 Nobel Prize in physics.

Semiconductor diode lasers were first built in 1962, but it was not until 1975 that the first continuous beam semiconductor diode laser, capable of operating at room temperatures, reached the commercial market. With tens of millions sold each year, they are now the most common lasers. They are used to read compact discs and CD-ROMs, for laser printing and optical data retrieval, and for amplifying attenuated laser signals in fiber optic communication cables.

Other lasers include multi-level devices that generate extremely high peak powers in short pulses for use in thermonuclear fusion research. The development of chemical and x-ray lasers has focused on their potential as high-energy sources for military weapons. Eximer lasers provide powerful sources of pulsed ultraviolet radiation and have various commercial applications. Finally, development of free-electron lasers was heavily funded as part of the Reagan administration's Strategic Defense Initiative because of their potential for generating extremely high power signals at infrared wavelengths. Small-scale free-electron lasers are used in basic research and medicine.

STEPHEN D. NORTON