māˈzər, device for creation, amplification, and transmission of an intense, highly focused beam of high-frequency radio waves. The name maser is an acronym for microwave amplification by stimulated emission of radiation, microwaves being radio waves of short wavelength, or high frequency. The maser is an oscillator in which the basic frequency control arises from an atomic resonance rather than a resonant electronic circuit. The waves produced by the maser are coherent, that is, all of the same frequency, direction, and phase relationship, while the waves produced by most sources of electromagnetic radiation are emitted in all directions over a wide range of frequencies and have all possible phase relationships. Maser radiowaves are much closer to an ideal single-frequency source than those of ordinary radio transmitters. As a result, the maser output can be transmitted over fairly large distances with relatively little loss. The principle of the maser was conceived of in the early 1950s, based on the developments of the quantum theory, and the first maser was operated in 1954 by C. H. Townes, J. P. Gordon, and H. J. Zeiger. In 1960 the first optical maser was developed by T. H. Maiman (the optical maser is now called a laser ). Beginning in 1965 a number of masers have been found in space; the first such natural laser discovered lies in the Great Nebula of Orion and is driven by the hydroxyl (OH) molecule. Masers have been developed to operate at many different wavelengths, so that the original designation "microwave" is no longer strictly accurate. In the maser, electromagnetic radiation is produced by stimulated emission; an atom or molecule in an excited state (i.e., a state of increased energy) emits a photon of a specific frequency when struck by a second photon of the same frequency. The emitted photon and the bombarding photon emerge in phase and in the same direction. For such emissions to take place in sufficient numbers to produce a steady source of radiation, many atoms or molecules must first be "pumped" to the higher energy state. The first maser used molecules of ammonia gas, which oscillate at a characteristic natural frequency between two energy states. Paramagnetic ions in crystals have also been used as the source of coherent radiation for a maser. A maser may be used as an amplifier or as an oscillator, the latter application requiring a higher power level. One of the most useful types of maser is based on transitions in atomic hydrogen occurring at a frequency of 1,421 megahertz. The hydrogen maser provides a very sharp, constant oscillating signal, and thus serves as a time standard for an atomic clock. The process also occurs in instellar space and a number of such natural masers have been discovered, including one based upon the hydroxyl (OH) group in the Great Nebula of Orion.

See M. Bertolloti, Masers and Lasers (1983).

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