Definitions

maser

maser

[mey-zer]
maser, 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).

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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A maser is a device that produces coherent electromagnetic waves through amplification due to stimulated emission. Historically the term came from the acronym "Microwave Amplification by Stimulated Emission of Radiation", although modern masers emit over a broad portion of the electromagnetic spectrum. This has led some to replace "microwave" with "molecular" in the acronym, as suggested by Townes. When optical coherent oscillators were first developed, they were called optical masers, but it has become more common to refer to these as lasers. See the section on terminology below for more on this.

History

Theoretically, reflecting principles previously discussed by Joseph Weber at the June 1952 conference of the Institute of Radio Engineers, the principle of the maser was described by Nikolay Basov and Alexander Prokhorov from Lebedev Institute of Physics at an All-Union Conference on Radio-Spectroscopy held by USSR Academy of Sciences in May 1952. They subsequently published their results in October 1954. Independently, Charles H. Townes, J. P. Gordon, and H. J. Zeiger built the first maser at Columbia University in 1953. The device used stimulated emission in a stream of energized ammonia molecules to produce amplification of microwaves at a frequency of 24 gigahertz. Townes later worked with Arthur L. Schawlow to describe the principle of the optical maser, or laser, which Theodore H. Maiman first demonstrated in 1960. For their research in this field Townes, Basov, and Prokhorov were awarded the Nobel Prize in Physics in 1964.

Technology

The maser is based on the principle of stimulated emission proposed by Albert Einstein in 1917. When atoms have been put into an excited energy state, they can amplify radiation at the proper frequency. By putting such an amplifying medium in a resonant cavity, feedback is created that can produce coherent radiation.

Some common types of masers

  • Atomic beam masers
  • Gas masers
    • Rubidium maser
  • Solid State masers
    • Ruby maser

The dual noble gas maser is an example of a masing medium which is nonpolar.

Uses

Masers serve as high precision frequency references. These "atomic frequency standards" are one form of atomic clock. They are also used as electronic amplifiers in radio telescopes.

Hydrogen maser

Today, the most important type of maser is the hydrogen maser which is currently used as an atomic frequency standard. Together with other types of atomic clocks, they constitute the "Temps Atomique International" or TAI. This is the international time scale, which is coordinated by the Bureau International des Poids et Mesures, or BIPM.

It was Norman Ramsey and his colleagues who first realized this device. Today's masers are identical to the original design. The maser oscillation relies on stimulated emission between two hyperfine levels of atomic hydrogen. Here is a brief description of how it works:

  • First, a beam of atomic hydrogen is produced. This is done by submitting the gas at low pressure to an RF discharge (see the picture on this page).
  • The next step is "state selection"—in order to get some stimulated emission, it is necessary to create a population inversion of the atoms. This is done in a way that is very similar to the famous Stern-Gerlach experiment. After passing through an aperture and a magnetic field, many of the atoms in the beam are left in the upper energy level of the lasing transition. From this state, the atoms can decay to the lower state and emit some microwave radiation.
  • A high quality factor microwave cavity confines the microwaves and reinjects them repeatedly into the atom beam. The stimulated emission amplifies the microwaves on each pass through the beam. This combination of amplification and feedback is what defines all oscillators. The resonant frequency of the microwave cavity is exactly tuned to the hyperfine structure of hydrogen: 1420 405 751.768 Hz.
  • A small fraction of the signal in the microwave cavity is coupled into a coaxial cable and then sent to a coherent receiver.
  • The microwave signal coming out of the maser is very weak (a few pW). The frequency of the signal is fixed but extremely stable. The coherent receiver is used to amplify the signal and change the frequency. This is done using a series of phase-locked loops and a high performance quartz oscillator.

Astrophysical masers

Stimulated microwave and radio wave emission is observed in astronomy, and this is usually called "masing", even in the absence of the resonant feedback that would be required for a true maser. Technically this form of stimulated emission is called superradiant emission, and it is closely associated with lasing and masing. Such emission is observed from molecules such as water (H2O), hydroxyl radicals (OH), methanol (CH3OH), formaldehyde (CH2O), and silicon monoxide (SiO).

Maser-like stimulated emission also occurs in nature in interstellar space. Water molecules in star-forming regions can undergo a population inversion and emit radiation at 22 GHz, creating the brightest spectral line in the radio universe. Some water masers also emit radiation from a vibrational mode at 96 GHz.

Terminology

The meaning of the term maser has changed slightly since its introduction. Initially the acronym was universally given as "microwave amplification by stimulated emission of radiation," which described devices which emitted in the microwave region of the electromagnetic spectrum. The principle of stimulated emission has since been extended to more devices and frequencies, and so the original acronym is sometimes modified, as suggested by Charles H. Townes, to "molecular amplification by stimulated emission of radiation." Some have asserted that Townes's efforts to extend the acronym in this way were primarily motivated by the desire to increase the importance of his invention, and his reputation in the scientific community.

When the laser was developed, Townes and Schawlow and their colleagues at Bell Labs pushed the use of the term optical maser, but this was largely abandoned in favor of laser, coined by their rival Gordon Gould. In modern usage, devices that emit in the X-ray through infrared portions of the spectrum are typically called lasers, and devices that emit in the microwave region and below are commonly called masers, regardless of whether they emit microwaves or other frequencies.

Gould originally proposed distinct names for devices that emit in each portion of the spectrum, including grasers (gamma ray lasers), xasers (x-ray lasers), uvasers (ultraviolet lasers), lasers (visible lasers), irasers (infrared lasers), masers (microwave masers), and rasers (RF masers). Most of these terms never caught on, however, and all have now become (apart from in science fiction) obsolete except for maser and laser.

Masers in science fiction

Masers often appear as weapons in science fiction movies and novels. Their characteristics often differ from those of real masers, however, and it is doubtful whether a practical maser weapon such as these can actually be made.

Some notable science fiction appearances of masers:

  • Masers are the most recognizable weapon in the Godzilla series and Toho's other monster movies. "Maser tanks" are often deployed against monsters. These fire a bolt of electricity, presumably created by amplified microwaves. Maser tanks have also appeared in various video games and in other science fiction movies.
  • Masers are common in anime and Japanese-inspired animated science fiction stories. They have appeared in the Transformers, Gundam SEED, GaoGaiGar, Code Geass and others. In Gundam SEED, Phonon Masers are weapons described as sound waves focused in laser-like fashion, rather than microwave devices.
  • Masers are used predominantly as weaponry, both from spaceships and by ground troops in Peter F. Hamilton's Night's Dawn universe, as well as appearing in his Commonwealth saga.
  • Masers are frequently used as the assault weapon-of-choice by military forces in William Shatner's fictional novel series "Quest for Tomorrow".
  • In the Star Wars Expanded Universe, masers (called "charrics") are the primary weapon for the Chiss race, on their fighters and for handheld rifles.
  • The DANGI Maser is a prominent and very lethal weapon in a popular (and free) scenario for Marathon called Marathon Rubicon
  • In Carl Sagan's novel Contact, the main character, Ellie Arroway, does her thesis project on developing a "ruby maser".
  • In the video game Ratchet and Clank: Size Matters, there is a weapon called the Laser Tracer. When fully upgraded to level 4, it becomes the Optical Maser Array.
  • In the Star Control series history, the Androsynth defeated the humans' defenses with the use of MASER technology.
  • The Monkeylord unit in Supreme Commander is armed with a "Microwave Laser."
  • In David Brin's Uplift Universe, masers are used not as weapons, but as communication devices.
  • In the ABC television program Alias (a member of the Spy-fi genre), the season 3 episode Prelude featured a maser attached to a Chinese satellite, able to pinpoint assassination targets from space.
  • In the Halo universe masers are used for communication on spacecraft.
  • In the MMORPG EVE Online, masers are a variation of Energy Turrets primarily used by the Amarrian race.
  • In the webcomic Starslip Crisis, masers are a common form of starship weaponry.
  • In the video game Phantasy Star Online, there is a weapon called a "Maser Beam."
  • In the TV series Lost In Space, "Maser Beams" are used for teleportation in several episodes.
  • In the Japanese anime Code Geass, some knightmare frames have this technology incorperated into swords to cut more efectively through things.

See also

Footnotes

References

  • J.R. Singer, Masers, John Whiley and Sons Inc., 1959.
  • J. Vanier, C. Audoin, The Quantum Physics of Atomic Frequency Standards, Adam Hilger, Bristol, 1989.
  • Cartoon Megas XLR 2005

External links

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