Laser linewidths: tutorial

In this tutorial, the physical origins and mathematical analyses of laser linewidths are reviewed. The semi-classical model is based on an equation for light-mode amplitude that includes random source terms, one term each process affects (stimulated spontaneous emission, stimulated absorption, facet material loss). Although terms classical, their assigned strengths consistent with laws quantum optics. Analysis shows linewidth proportional to sum (positive) all gain loss processes. Three-level semiconductor lasers have broader than comparable four-level lasers, because absorption emission compensates it both contribute linewidth.