Generalized expression for the natural width of the radiation spectrum of quantum oscillators

A simple method for calculating the linewidth of sources of stimulated coherent radiation caused by spontaneous noise and thermal fluctuations has been proposed in the framework of the two-level model of the active medium. An expression for the spectral width has been derived beyond the adiabatic approximation under an arbitrary relation between relaxation times in a self-sustained oscillation system, taking into account a finite spectral band of spontaneous radiation. The result is applicable for a wide class of coherent radiation sources.