A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, the word "maser" is derived from the original upper-case acronym MASER, which stands for "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 the usage using "molecular" to replace "microwave" for contemporary linguistic accuracy.
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. Wikipedia. Maser
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