Green's Function Theory of Multimode Cavities

Abstract

Schwinger has applied his generalized quantum action principle and the method of thermodynamic Green's functions to an harmonic oscillator. He has shown how this technique describes the buildup of cavity oscillations in a simple model of the maser. This paper applies Schwinger's technique to multimode cavities. The energy, autocorrelation (or coherence), and spectral distribution of individual cavity modes in the steady state, their rate of buildup and their response to external signals have been calculated. Conditions for the buildup and for the steady state of laser action are stated. It has been shown that the steady-state laser radiation in each cavity mode can be described in terms of spontaneous and induced emissions, the latter one containing a coherent and an incoherent part.