Phase correlation between four-wave mixing and optical fields in double Λ-type atomic system.

We study the spectral features and phase of four-wave mixing (FWM) light according to the relative phase-noise of the optical fields coupled to a double Λ-type atomic system of the 5S_1/2-5P_1/2 transition of ⁸⁷Rb atoms. We observe that the spectral shape of the FWM spectrum is identical to that of the two-photon absorption (TPA) spectrum due to two-photon coherence and that it is independent of the relative phase-noise of the pump light. From these results, we clarify that the two-photon coherence plays a very important role in the FWM process. Furthermore, we measure the relative linewidth of the FWM signal to the probe and pump lasers by means of a beat interferometer. We confirmed that the phase of the FWM signal is strongly correlated with that of the pump laser under the condition of phase-locked probe and coupling lasers for two-photon coherence.