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