Macroscopic parameters and line shapes of a gas laser

The theory of line shapes in monomode weak-gain gas lasers which we have developed recently shows that asymmetries of Lamb dips are governed by opposite lens effects which arise from population and saturation inhomogeneities. The competition between these effects depends on the geometrical characteristics of the laser. It is our aim in this paper to illustrate experimentally this fact with the 3.39-μm line of a He-Ne laser. To get a strongly diverging beam we have chosen a quasihemispheric resonator whose length (57 cm) almost equals the radius of curvature of the concave mirror ($R=60\mathrm{}$ cm). The amplifying tube is about 20 cm long and is placed on the side of either the plane or the concave mirror, thus allowing us to vary the saturation inhomogeneity. In the first case the inhomogeneity is stronger and causes the maximum of intensity to be on the low-frequency side of the line, while in the second case population and saturation inhomogeneities are of the same order. We have repeated the experiment with two tubes having different internal diameters (8 and 4 mm) in order to vary the population effects. In the case of the narrower tube one observes a gradual passage from a low- to a high-frequency type of asymmetry when the discharge current is increased. This observation is explained by an increase of the lens effect due to saturation which overcomes the one due to population. Applying our previously developed theoretical methods, we find the results in qualitative agreement with these experiments.