Maser

A maser is any amplifier that produces coherent electromagnetic waves due to stimulated emission.

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Historically the term came from the acronym "microwave amplification by stimulated emission of radiation." This led to the invention of optical lasers. However, now that materials have been made to mase across a wide spectrum of electromagnetic radiation, not just the microwave band, this type of maser is sometimes characterized as a microwave laser or microwave maser. This is to differentiate from, for example, radio masers or radio lasers which operate in the radio frequency range (and are most often referred to as simply masers; or more rarely as rasers).

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Thus the term maser can also be taken to mean the more general "molecular amplification by stimulated emission of radiation". http://nobelprize.org/physics/laureates/1964/townes-lecture.pdf Under this definition, an optical laser is a specific type of the more general maser (see below for details).

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Amplifier: For guitar amplifier, go to Instrument amplifier...

Coherent: REDIRECT coherence...

Electromagnetic waves: REDIRECT Electromagnetic radiation...