Maser (source code)
= Maser
{wiki}
A maser uses <stimulated emission> to amplify <microwaves>. With <population inversion> and feedback it can sustain a <coherent wave field> as an <oscillator>. The name abbreviates "microwave amplification by stimulated emission of radiation".<Production of coherent radiation by atoms and molecules by Charles Hard Townes>[pp. 60–62, Basic maser principles]{ref}
Why make <molecules> supply an <oscillator>? Their <energy levels> offer a reproducible <frequency> reference, while <stimulated emission> lets successive <molecules> reinforce one <coherent wave field>. In his 1964 lecture, <Charles Hard Townes> traced his motivation to <microwave spectroscopy>: resolving molecular structure required useful coherent sources.<Production of coherent radiation by atoms and molecules by Charles Hard Townes>[p. 58, opening discussion of microwave spectroscopy]{ref} The interesting balance is between molecular amplification and escaping <energy>: even a beam with <population inversion> fails to oscillate if cavity losses win. This is the microwave counterpart of <laser threshold>.
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