Investigation of the competition between two axial modes in a laser with a homogeneously broadened line

An experimental investigation was made of the competition between two axial modes emitted by a helium–neon laser with a homogeneously broadened λ = 3.39 μ line. The 3s₂–3p₄ line of Ne was used as a model of a homogeneously broadened transition. The region of stable two-frequency emission was investigated as a function of the laser parameters. A study was also made of the transition for the two- to the single-frequency emission, which occurred when the competing modes converged. A qualitative explanation of the observed effects was provided on the basis of a simple physical model, in which use was made of the formation of a dip in a homogeneously broadened line under the influence of the saturation field. The results obtained should apply qualitatively to other lasers with homogeneous line provided T₂ << T₁ (T₁ and T₂ are, respectively, the longitudinal and the transverse relaxation times) and provided the spatial distribution of the inversion is not affected by the "standing" light waves of the competing modes.