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A hydrogen radio frequency discharge, the first element inside a hydrogen maser (see description below)

A maser () is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The word "maser" is derived from the acronym MASER: "microwave amplification by stimulated emission of radiation". The lower-case usage arose from technological development having rendered the original definition imprecise, because contemporary masers emit electromagnetic waves not just at microwave frequencies, but rather across a broader band of the electromagnetic spectrum. Hence, the physicist Charles H. Townes suggested using "molecular" to replace "microwave" for contemporary linguistic accuracy.[1]

When the coherent optical oscillator was first imagined in 1957, it was originally called the "optical maser." However, this was ultimately changed to laser for "Light Amplification by Stimulated Emission of Radiation." Gordon Gould is credited with creating this acronym in 1957.


  • History 1
  • Technology 2
    • Some common types of masers 2.1
    • Twenty-first century developments 2.2
  • Uses 3
    • Hydrogen maser 3.1
  • Astrophysical masers 4
  • Terminology 5
  • See also 6
  • References 7
  • Further reading 8
  • External links 9


The theoretical principles describing the operation of a maser were first described by Nikolay Basov and Alexander Prokhorov from Lebedev Institute of Physics at an All-Union Conference on Radio-Spectroscopy held by the USSR Academy of Sciences in May 1952. The results were subsequently published in October 1954.

A precursor of the maser was a hydrogen device built and tested by the physicists Theodor V. Ionescu and Vasile Mihu in 1946.

Independently, Charles H. Townes, James P. Gordon, and H. J. Zeiger built the first ammonia maser at Columbia University in 1953. This device used stimulated emission in a stream of energized ammonia molecules to produce amplification of microwaves at a frequency of about 24.0 gigahertz. Townes later worked with Arthur L. Schawlow to describe the principle of the optical maser, or laser, which Theodore H. Maiman created the first working model of in 1960.

For their research in the field of stimulated emission, Townes, Basov and Prokhorov were awarded the Nobel Prize in Physics in 1964.


The maser is based on the principle of stimulated emission proposed by Albert Einstein in 1917. When atoms have been induced 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

Twenty-first century developments

In 2012, a research team from the National Physical Laboratory and Imperial College London developed a way to make a solid-state maser operate at room temperatures by using pentacene-doped p-Terphenyl as the amplifier medium.[3][4] This development could possibly lead to a renewal of the maser technology into a wide range of applications, including communications and space exploration.[5]


Masers serve as high precision frequency references. These "atomic frequency standards" are one of the many forms of atomic clocks. They are often used as low-noise microwave amplifiers in radio telescopes. Masers are being used by a few countries as directed-energy weapons.

Hydrogen maser

A hydrogen maser.

As of 2012, the most important type of maser is the hydrogen maser which is currently used as an atomic frequency standard. Together with other kinds of atomic clocks, these help make up the International Atomic Time ("Temps Atomique International" or "TAI" in French). This is the international time scale, which is coordinated by the International Bureau of Weights and Measures.

Norman Ramsey and his colleagues first conceived of the maser as a timing standard. More recent masers are practically identical to their original design. Maser oscillations rely on the stimulated emission between two hyperfine energy levels of atomic hydrogen. Here is a brief description of how they work:

  • First, a beam of atomic hydrogen is produced. This is done by submitting the gas at low pressure to a high-frequency radio wave 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 Q factor (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 tuned to the frequency of the hyperfine energy transition of hydrogen: 1,420,405,752 hertz.[6]
  • A small fraction of the signal in the microwave cavity is coupled into a coaxial cable and then sent to a coherent radio receiver.
  • The microwave signal coming out of the maser is very weak (a few picowatts). The frequency of the signal is fixed and 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

Maser-like stimulated emission has also been observed in nature from interstellar space, and it is frequently called "superradiant emission" to distinguish it from laboratory masers. Such emission is observed from molecules such as water (H2O), hydroxyl radicals (OH), methanol (CH3OH), formaldehyde (CH2O), and silicon monoxide (SiO). Water molecules in star-forming regions can undergo a population inversion and emit radiation at about 22.0 GHz, creating the brightest spectral line in the radio universe. Some water masers also emit radiation from a vibrational mode at a frequency of about 96 GHz.

Extremely powerful masers, associated with active galactic nuclei, are known as megamasers and are up to a million times more powerful than stellar masers.


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 and concept of stimulated emission has since been extended to more devices and frequencies. Thus the original acronym is sometimes modified, as suggested by Charles H. Townes,[1] 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.[7]

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.[8] 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.

During the early 1960s, the Jet Propulsion Laboratory developed a maser to provide ultra-low-noise amplification of S-band microwave signals received from deep space probes. This maser used deeply refrigerated hydrogen to chill the amplifier down to a temperature of four kelvin. Amplification was achieved by exciting a ruby comb with a 12.0 gigahertz klystron. In the early years, it took days to chill, and remove the impurities from, the hydrogen lines. Refrigeration was a two-stage process with a large Linde unit on the ground, and a crosshead compressor within the antenna. The final injection was at 3000 pounds per square inch through a six thousandths of an inch, micrometer adjustable, entry to the chamber. The whole system noise temperature looking at cold sky (2.7 kelvins in the microwave band) was 17 kelvins. This gave such a low noise figure that the Mariner IV space probe could send still pictures from Mars back to the Earth even though the output power of its radio transmitter was only 15 watts, and hence the total signal power received was only -169 decibels with respect to a milliwatt (dBm).

See also


  1. ^ a b Charles H. Townes – Nobel Lecture
  2. ^ The Dual Noble Gas Maser, Harvard University, Department of Physics
  3. ^ Brumfiel, G. (2012). "Microwave laser fulfills 60 years of promise". Nature.  
  4. ^ Palmer, Jason (16 August 2012). Maser' source of microwave beams comes out of the cold"'". BBC News. Archived from the original on 2012. Retrieved 23 August 2012. 
  5. ^ Microwave Laser Fulfills 60 Years of Promise
  6. ^ Time and Frequency From A to Z: H
  7. ^ Taylor, Nick (2000). LASER: The inventor, the Nobel laureate, and the thirty-year patent war. New York: Simon & Schuster.  
  8. ^ Taylor, Nick (2000). LASER: The inventor, the Nobel laureate, and the thirty-year patent war. New York: Simon & Schuster. pp. 66–70.  

Further reading

  • 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

  • search for "maser"
  • Noble gas Maser
  • "The Hydrogen Maser Clock Project". Harvard-Smithsonian Center for Astrophysics. Archived from the original on 2006-10-10. 
  • Bright Idea: The First Lasers
  • Invention of the Maser and Laser, American Physical Society
  • Shawlow and Townes Invent the Laser, Bell Labs
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