  The emission linewidth in active medium emerges due to homogeneous and
inhomogeneous broadening. We demonstrate that in lasers with inhomogeneous
broadening there is a critical pump rate, above which the special mode forms.
This mode consists of locked-in oscillations of cavity mode and of the active
particles with different transition frequencies. Below the critical value of
the pump rate, the radiation spectrum of the laser has a Gaussian profile,
provided that inhomogeneous broadening is dominant. Above the critical value of
pump rate, the special mode mostly determines the laser radiation spectrum. As
the result, the spectrum attains Lorentz shape characteristic for homogeneous
broadening. We demonstrate that the formation of the special mode precedes
lasing and that the critical pump rate plays the role of lasing prethreshold.
We obtain expressions for the threshold and generation frequency of single-mode
laser where both homogeneous and inhomogeneous broadening are present.
