The schematic diagram of cascade correlation-enhanced Raman scattering. The output intensities of Stokes fields are dependent on the initial conditions. (a) Experimental setup. The experimental setup is simple and the same with usual spontaneous Raman scattering in additional to a flat mirror. (b) The pump field P₁ leads to spontaneous emission of the Stokes field S₁, accompanied with the generation of atomic spin wave S_a1. (c) The atomic spin wave S_a1 stays in the cell, the S₁ and P₁ fields propagate out together and both are reflected back to the atomic medium by a flat mirror M. (d) Two correlation-enhanced Raman scattering Stokes fields S₁ and S₂. Panels (a)–(d) form the first cycle and panels (e)–(f) form the second cycle. The cycles are repeated constantly in a period, and the intensity of Stokes field S₂ is coherent enhancement until it is saturated. The dashed curves (green) denote the correlation between the Stokes field and atomic spin wave.