Sub-Doppler light amplification in a coherently pumped atomic system

We report the observation of steady-state amplification of a weak probe laser without population inversion in the bare states in a Doppler-broadened Lambda-type Rb atomic system. The probe amplification was observed with the Rabi frequency of a coupling laser much smaller than the Doppler width and can be viewed as due to the dressed-state inversion. A 10% single-pass amplification of a weak probe laser was obtained, and the measured linewidth of the gain spectral feature approaches the Rb natural linewidth. A theoretical analysis based on a three-level Doppler-broadened Lambda system is presented, and qualitative agreement with the experiment results is achieved.