For the Italian commune, see Maser, Italy.

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.^[1] 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,^[2] 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
• Ammonia maser
• Hydrogen maser
• Gas masers
• Rubidium maser
• Solid State masers
• Ruby maser
• Brandon maser

The dual noble gas maser is an example of a masing medium which is nonpolar.^[3]

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) 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

Main article: Astrophysical maser

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,^[4] 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.^[5]

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.^[6] 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, masers are 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.

See also

• Laser
• List of laser types

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

• arXiv.org search for "maser"
• Noble gas Maser
• The Red Shift (Hydrogen Maser Clock) Experiment