Nondegenerate parametric self-oscillation via multiwave mixing in coherentatomic media

We demonstrate an efficient nonlinear process in which Stokes and anti-Stok es components are generated spontaneously in a multilevel medium coherently prepared by resonant, counterpropagating fields. A medium of this kind com bines large nonlinear gain and efficient intrinsic feedback, thereby allowi ng for very weak optical fields to induce nonlinear processes. Mirrorless s elf-oscillation induced by microwatts of light power (nanojoules of pulse e nergy) is observed.