Gain-feedback approach to optical instabilities in sodium vapor.

The gain-feedback approach to lasing and optical instabilities has been applied to sodium vapor driven by a nearly resonant intense field. The observed lasing frequencies agree with the two-beam-coupling gain curve calculated for a Doppler-broadened two-level medium. Rayleigh-gain lasing is seen for no external cavity with use of counterpropagating beams, and Raman-gain lasing is seen in a ring cavity.