Investigation of the stimulated emission spectrum of a helium–neon laser

A study was made of the dependence of the emission spectrum of an He + Ne²⁰ laser (λ = 0.63 μ) on the resonator tuning in a wide range of gas-mixture pressures and excitation levels. It was found that the stimulated emission of neighboring modes was possible at low pressures of the mixture but not at high pressures. The transition between these two types of emission was sudden, almost independent of the excitation level, and accompanied by frequency pulling. This transition corresponded approximately to the onset of strong coupling in the two-mode regime.