Emission spectrum of a semiconductor laser with an external resonator

An investigation is made of the influence of an external resonator on the emission spectrum of a single-mode semiconductor laser. It is shown that spontaneous radiation may be the main reason for the excitation of several longitudinal modes of the external resonator. A correlation approximation allowing for the coupling between the amplitude and phase of spontaneous oscillations is used to study the amplitude and phase spectra of fluctuations and the emission spectrum of a semiconductor laser with an external resonator. The conditions for single-mode emission are determined and the limitations imposed on the attainable line narrowing in the single-mode regime are found.