Linewidth modifications due to nonorthogonality, inhomogeneity, and directionality of longitudinal laser modes

In a laser cavity, deviations from uniformity, whether explicitly present in index and gain or implicitly due to outcoupling, influence the linewidth. An explicit expression for the longitudinal modification factor is derived in the WKB approximation using a nonorthogonal quasi-mode decomposition. Both enhancement and reduction can occur, depending on the specific inhomogeneous distributions of index and gain. Dramatic noise-enhancement figures of 100 or more are predicted for a single-mode laser with strongly localized gain. The author emphasizes the classical rather than the quantum mechanical nature of the modifications, even when the laser operates in the quantum regime