stimulated emission

maser

[mey-zer]

Device that produces and amplifies electromagnetic radiation in the microwave range of the spectrum. The first maser was built in 1951 by Charles H. Townes. Its name is an acronym for “microwave amplification by stimulated emission of radiation.” The wavelength produced by a maser is so constant and reproducible that it can be used to control a clock that will gain or lose no more than a second over hundreds of years. Masers have been used to amplify faint signals returned from radar and communications satellites, and have made it possible to measure faint radio waves emitted by Venus, giving an indication of the planet's temperature. The maser was the principal precursor of the laser.

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Device that produces an intense beam of coherent light (light composed of waves having a constant difference in phase). Its name, an acronym derived from “light amplification by stimulated emission of radiation,” describes how its beam is produced. The first laser, constructed in 1960 by Theodore Maiman (born 1927) based on earlier work by Charles H. Townes, used a rod of ruby. Light of a suitable wavelength from a flashlight excited (see excitation) the ruby atoms to higher energy levels. The excited atoms decayed swiftly to slightly lower energies (through phonon reactions) and then fell more slowly to the ground state, emitting light at a specific wavelength. The light tended to bounce back and forth between the polished ends of the rod, stimulating further emission. The laser has found valuable applications in microsurgery, compact-disc players, communications, and holography, as well as for drilling holes in hard materials, alignment in tunnel drilling, long-distance measurement, and mapping fine details.

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Self-Amplified Spontaneous (or Stimulated) Emission (SASE) is a process within a Free electron laser (FEL) by which a laser beam is created by the high-energy electron beam. The lasing starts up from the random microbunching (i.e., shot noise) on the electron beam instead of being coherently produced by an input seed laser source. This source is fully transversely coherent at saturation, but, because the radiation starts up from random noise at many radiation wavelengths, the longitudinal coherence of the radiation is less than that of the amplifier case but better than that of spontaneous radiation.

The SPring-8 FEL in Japan has demonstrated this concept and it is the principle that will be employed by the Linac Coherent Light Source (LCLS) and the European x-ray free electron laser.

References

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