Semiclassical dynamics of excess quantum noise

A semiclassical theoretical framework is presented to describe the essential features of the excess quantum noise that occurs in systems with nonorthogonal eigenmodes. Excess noise is shown to be always spectrally colored, instead of white, so that the Petermann excess noise factor is best written as K(omega) instead of K. The consequences of this spectral coloring are analyzed for lasers, both below and above the lasing threshold.