Sjm. Kuppens et al., INCOMPLETE INVERSION AND DOUBLE-VALUED FUNDAMENTAL LINEWIDTH OF INFRARED HENE AND HEXE LASERS, IEEE journal of quantum electronics, 32(3), 1996, pp. 383-387
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-Town
es behavior strong deviations from the inverse power dependence are ob
served, 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 t
he inversion, Our analysis shows that typically N-sp has a value rangi
ng 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.