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

Tutorial on laser linewidths