The atomic-iodine hyperfine structure is shown to produce anomalous dispersion effects on its hyperfine laser transition at 1.315 µm whenever reasonable small-signal gains are available (~1% cm). This dispersion effect is linearly dependent on the iodine inversion density. Such an anomalous dispersion effect may produce strong phase-induced mode-media interactions for all the lower-gain iodine hyperfine transitions in any low pressure (<50 Torr) atomic-iodine laser. Fortunately, the highest-gain hyperfine transition, <i>F</i>′ = 3 → <i>F</i>″ = 4, has the smallest amount of additional phase shift (or refractivity) introduced by this anomalous dispersion. All the other transitions experience much larger anomalous dispersion effects. This condition should act as an internal frequency discriminator, forcing the iodine to lase on the highest-gain hyperfine transition.
L. A. Schlie, "Anomalous dispersion effects in low-pressure atomic-iodine lasers at 1.315 µm," J. Opt. Soc. Am. 71, 1080-1093 (1981)