Theory of the fundamental laser linewidth. II

A recent theory of the fundamental laser linewidth developed by the authors is extended to include the effects of the Henry α parameter. The usual (1+${\mathrm{\alpha }}^2$) enhancement of the linewidth is shown to be increased for arbitrary output coupling by spatial hole burning. A total enhancement factor is derived that multiplies the Schawlow-Townes linewidth and takes into account line broadening due to amplification of quantum noise as well as that due to phase-amplitude coupling.