Spontaneous emission in non-Hermitian optical systems: Distributed-feedback semiconductor lasers.

Abstract

We develop a general theory for noise fluctuations in open optical systems with gain or loss based on an expansion in terms of nonorthogonal quasimodes. Spatial variation of the index of refraction and of the gain are taken into account. We show that below threshold, i.e., when the system is linear, this method is formally equivalent to using Green's matrices. We calculate the modes and the fluctuation spectra above threshold for a distributed-feedback laser including gain saturation effects.