Maser

For the Italian commune, see Maser, Italy.

A Hydrogen RF discharge, the first element inside a Hydrogen Maser (see description below), courtesy NASA/JPL-Caltech.

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 [link]. 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.

Contents

1 History
2 Technology

2.1 Some common types of masers

3 Uses

3.1 Hydrogen maser

4 Astrophysical masers
5 Terminology
6 Masers in science fiction
7 See also
8 Footnotes
9 References
10 External links

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 energised 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

Ammonia maser
Hydrogen maser

Gas masers

Rubidium maser

Solid State masers

Ruby maser

The dual noble gas maser [link] 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 Atomic 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: 1.420405751 GHz.

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) and the frequency is extremely stable but can not be changed. 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 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 [link], to "molecular amplification by stimulated emission of radiation." Molecular is used here in the sense of kinetic theory, where the base element of a kinetic system is a molecule, even if it happens to be monatomic. This should not be confused with the usage of the term in the molecular sciences, where it refers to a bound state comprising two or more atoms.

Initially, visible light oscillators based on stimulated emission were called optical masers, but this terminology has become uncommon. It is more conventional now to refer to devices that emit in the X-ray through infrared portions of the spectrum as lasers, and devices that emit in the microwave region and below as masers. There is some debate over whether maser or laser is the correct generic term for all devices that produce coherent electromagnetic waves through stimulated emission.

Distinct names were originally proposed 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 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. The maser tank is also present as a unit in Outpost 2. It is one of the weaker units.
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.
In the anime series Gundam Seed, maser cannons are often equipped on amphibious mobile suits "in lieu of beam weapons", which function poorly underwater. Within the context of the series, masers are described as sound waves focused in laser-like fashion.
The Chiss race in Star Wars use a weapon known as a [[Wikicities:c:StarWars:Charric|charric]], which fires a maser-guided particle discharge.
The Darkstars exo-mantle's principal weapons were maser units that could fire energy blasts with pinpoint presicion.

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.

External links

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