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Acronym for Microwave Amplification by Stimulated Emission of Radiation. Similar to a laser, but operating in the microwave region of the electromagnetic spectrum.

The first maser was built by Charles H. Townes, J. P. Gordon and H. J. Zeiger 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.

Masers are used as high precision frequency references, for example as an atomic clock. They are also used as electronic amplifiers in radio telescopes.

For more information about Frequency reference masers, see atomic clock.

Telescopic MASERs use arrays of chromium atoms in an insulating aluminum oxide crystal as amplifiers, pumping the energy in at a different radio frequency. That is, they use polished strips of synthetic ruby.

As the input signal comes in, a gold comb (it can't corrode and change shape!) distributes it along the strip of polished ruby. As the radio wave moves through the crystal, it knocks electrons to different orbits. As the electrons wiggle into their new, lower orbits, closer to their atoms' nuclei, they add to the wave that knocked them down. A radio wave is made by a wiggling electric charge or magnetic field.

The comb-fingers are spaced so that the wanted radio waves add together as they move down the crystal. This means that unwanted radio waves don't add together, and are therefore filtered out.

The MASERs are cooled by boiling liquid Helium to only about 4 degrees above a perfect lack of motion (absolute zero). This reduces the noise from electrons, nuclei, and other charged particles that can be bounced around by the molecular motion of heat.

The atoms are pumped away from their nuclei by longer, ignorable radio waves put onto the ruby from a different comb.