Abstract

Nondegenerate four-wave mixing (FWM) has been used for wavelength conversion of optical information.¹ In such experiments, two beams F-P (frequency ω₁) intersect in a nonlinear medium with a small crossing angle θ (≈ 10 mrad), while a third beam B {frequency ω₂) is nearly counterpro- pagating to F [Fig. 1(a)]. This can be viewed as real-time holography with frequency conversion.¹ However, the physics differ considerably when the medium is transparent at frequency ω₁, as there is no optically induced grating. This occurs when FWM is based on resonant two-photon, ω₁ + ω₂, absorption (frequencies ω₁ and ω₂ being resonant with a cascade three-level system and ω₁ coupling unpopulated excited states). One interesting feature is that the two-photon coherence is nearly Doppler-free and does not depend much on the θ angle.

© 1987 Optical Society of America