Natural linewidth of semiconductor lasers

A general and simple expression for the natural linewidth of lasers with high, spatially inhomogeneous gain, such as semiconductor lasers, has recently been reported by the author in a short paper. In the present paper, the relation is applied to a number of circuits relevant to semiconductor injection lasers. Its significance is clarified by considering simple lumped circuits. It is further generalised to anisotropic materials which may be nonreciprocal, and the role of dispersion inside and outside the laser cavity is discussed. The theory is restricted to single-mode operation (well above threshold operation), but saturation is neglected.