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The transmission line matrix formalism so useful for describing the transfer properties of microwave networks is extended to the electromagnetic fields associated with optical masers. The spontaneous emission noise of the optical maser is examined and shown to be amenable to a thermal description. Taking the point of view, well accepted at microwave frequencies, that a weakly nonlinear oscillator is a saturated amplifier of noise, the power and linewidth of the noise radiation emitted by the optical maser is calculated using the transmission line formalism. The significant parameters for any optical maser are shown to be the frequency, the single-pass gain of the maser medium, the effective mirror reflectivity and the population ratio. The pre-oscillation characteristics of the maser are examined and the reason for the extremely sharp oscillation threshold of the gas masers is discussed. Some observations concerning semiconductor optical masers are also made.
E. I. Gordon (1964) studied this question.