Abstract

Unstable and bistable behavior has been predicted recently in multimode homogeneously broadened lasers as a function of the detuning between the cavity and the atomic frequencies. As an example of a device for which most required physical conditions for the appearance of this instability are satisfied, we have analyzed the behavior of a low-pressure (10–20 Torr) CO₂ laser. The operating parameters (relaxation rates for the population inversion, 10⁴s⁻¹, and for the polarization, 10⁸s⁻¹) restrict the accessible range of theoretically expected phenomena to a stable intensity output without evidence of stable self-pulsing. However, mode–mode competition is responsible for bistability in both the intensity and the operating frequency on varying the detuning parameter. This behavior is confirmed by additional observations of the transient that accompanies the jump between the two stable branches. We have also studied the effect of varying the cavity losses and discovered bistability also in the vicinity of the threshold for laser action. This kind of behavior is not predicted by the plane wave Maxwell-Bloch equations but can be explained in terms of a radial gain profile. Both effects—mode–mode competition and transverse—lead to a variety of bistable behaviors depending on gas pressure and discharge current.

© 1985 Optical Society of America