Instability of counterpropagating beams in a two-level-atom medium

Self-oscillations are predicted for a simple nonlinear optical system consisting of two counterpropagating waves in a two-level medium. Oscillations at frequencies of $T_1^{\mathrm{-}1}$ and the Rabi frequency are expected for certain limits of system parameters. The physical origin of these instabilities is traced to parametric amplification in the medium. The possibility of experimental verification is discussed.