Experimental Evidence of Radiation Amplification Without Population Inversion in Sodium Atoms

We have demonstrated a radiation amplification without population inversion in sodium according to a four-level model, which was recentlyproposed by Nor-ducci et al. The amplified radiation signal at 589.0nm has been observed when an intense coherent driving field at 589.6nm was on and an incoherent pumping process from the ground state 3S_1/2 to 3P_3/2 excited state was operating.