Abstract

Homogeneously broadened lasers are considered as intrinsically single mode monostable devices [1] where intensity and frequency are defined by the cavity mode that lies nearest to the center of the atomic line. Here we show theoretically and experimentally that this heuristic picture falls when the interaction between different cavity modes becomes relevant. In fact, for sufficiently high detuning between cavity and atomic resonance frequencies (δ_AC), self pulsing of the laser arise from the simultaneous operation in two modes (phase instability) even if it is operating near threshold (C ⪯ 1) and in the so-called good cavity limit (k ≪ γ_⊥, γ_‖). Using parameter values which are typical of many solid state and molecular gas lasers (γ_‖, k ≪ γ_⊥), the theory predicts a bistable behavior in both the output intensity and frequency, as δ_AC is varied (Fig. 1). These theoretical results have been confirmed by experiments carried out on a CO₂ laser where the distance between modes was controlled by changing the losses inside the cavity and the detuning δ_AC was varied by a fine tuning of the distance between mirrors. The output intensity versus δ_AC is shown in Fig. 2 for two different values of losses and gas pressure. Direct comparison between Fig. 2b) and Fig. 1 shows a good qualitative agreement. Measurements of the transients during the jump of Fig.2b) confirmed the mode-mode competition interpretation.

© 1985 Optical Society of America