We show that the dip in the intensity line shape of a single-mode laser can appear at definite experimental
conditions in cases both of nonhomogeneously broadened line as well as homogeneously broadened line of the
atomic transition. In both cases the dip is explained by the nonlinear character of the phenomenon. The
saturation of the population difference of the involved atomic energy levels has its maximum in the vicinity of
the line center. Therefore the saturation of the amplification is at maximum near the atomic line center, and, as a consequence, the lasing power is reduced.
The shift of the molecular resonance peak in the He-Ne-CH4 laser connected with combined action of transverse transit-time effect and the nonreciprocity of the wave-fronts curvature of counterpropagating waves in the cavity was investigated. (Summary only available)