Amplification of the intracavity dark-state field by a four-wave mixing process

We demonstrate both experimentally and theoretically that the intracavity electromagnetically induced transparency dark-state field can be significantly amplified with the characteristic three-peak structure unchanged by a four-wave mixing process in an optical ring cavity containing four-level double-Λ Doppler-broadened rubidium atoms. The intracavity dispersion and absorption/gain properties are used to qualitatively explain this cavity resonance structure. The amplification is saturated at a relatively high pump laser power, and the dark-state peak position is shifted due to the ac-Stark effect.