Radial profile of neon 1s5atoms in a He-Ne active discharge and their lens effect on lasing at 6401 Å

A laser interferometer was used to determine the spatial distribution of the neon
1s_{5}
metastable states in an auxiliary active discharge, identical to the one used for a He-Ne laser at 6401 Å. The resulting distribution was a parabola, which crossed zero at the discharge tube walls. Measurements of the 6401-Å spot size were used to determine the axial metastable density from the calculation of the gas lens effect associated with this radial distribution. Excellent agreement was obtained between the two methods. The results show that the lens effect of these metastables is responsible for the anomalous appearance of lasering at 6401 Å in near-hemispherical cavities. For such resonators, the effective radius of curvature of the spherical mirror is increased while the equivalent mirror spacing is decreased; thus, the diffraction losses at 6401 Å are significantly reduced. Preliminary results are also given to show that the lens effect is more pronounced for the higher order modes.