Incomplete inversion and double-valued fundamental linewidth of infrared HeNe and HeXe lasers

We have measured the quantum-limited linewidths of small HeNe 3.39 mu m and HeXe 3.51 mu m lasers. In contrast to the expected Schawlow-Townes behavior strong deviations from the inverse power dependence are observed, leading to a double-valued relation between the linewidth and the output power. This phenomenon is analyzed in terms of the increase of spontaneous emission, by a factor N-sp, due to incompleteness of the inversion, Our analysis shows that typically N-sp has a value ranging from 1-10. Combining existing models for the pump power dependence of the level populations with measurements of the small signal gain we are able to explain the observed double-valued linewidth behavior in a quantitative way.