GAIN/FEEDBACK APPROACH TO OPTICAL INSTABILITIES IN SODIUM VAPOR

We use the gain-feedback approach1 to explain a number of instabilities observed in sodium vapor with the laser frequency vL tuned to the defocusing side of line center at v0. The gain curve, which includes motion of the atoms, predicts two regions of possible gain: Rayleigh gain for frequencies v just above vL and Raman gain for v ≅ vL-vR, where vR is the on-resonance Rabi frequency; see Fig. 1.2 Lasings based on these two gain mechanisms have been seen in two different feedback configurations. The new observations are: Rayleigh-gain lasing with no external mirrors using the distributed feedback of counterpropagating pump beams, Rayleigh-gain lasing without a foreign gas, and Raman-gain lasing in a ring cavity. The gain curve for moving atoms2 is shown in Figure 1 and differs substantially from that for stationary atoms.3