Linewidth and Conditions for Steady Oscillation in Single and Multiple Element Lasers

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Abstract
A derivation is given for the linewidth of an optical maser at steady power, taking into account its peculiar physical configuration and mode structure. Two limiting cases are considered: one in which the number of back and forth reflections between the end plates is not sufficient to establish a mode in the sense of Fox and Li, the other in which such a mode is established. The linewidth for the latter case may be interpreted as the ``theoretical lower limit.'' It is pointed out, however, that this limit could not be attained even if all other sources of noise in the maser system could be eliminated. General conditions are also developed for the oscillation of a system consisting of any combination of laser and mirror elements.