After a brief discussion on the quality factor of a parallel-plate interferometer, threshold conditions of oscillation for a beam maser, a solid-state maser, and a gas maser are calculated from a unified theory. Operation of a gas maser on the line broadened by both collision and the Doppler effect is discussed. The rate of producing population inversion is calculated in terms of gas pressure, quantum efficiency, saturation parameter, and the intensity of excitation. Finally, the optimum condition is discussed.
Koichi Shimoda, "Threshold Condition of Masers for Higher Frequencies," Appl. Opt. 1, 303-307 (1962)