A semiclassical, nonlinear theory of a single-mode laser is developed
so that the transition around threshold can be studied. The held is ex
pressed in the frequency domain which allows us to emphasize the role
of the Fabry-Perot cavity. A generalized Airy function is obtained for
the laser Line shape: it contains the source line shape and the empty
cavity line shape. It describes the laser both above-and below thresh
old including the transition region where there is an abrupt increase
of intensity and decrease of the laser linewidth. The general argument
s are illustrated by detailed numerical calculations for the 3.39-mu m
line of the He-Ne laser. The intensity of the spontaneous emission so
urce which plays a central role is computed from first principles.