Continuous-wave off-resonance rings and continuous-wave on-resonance enhancement

Continuous-wave (cw) off-resonance rings and w on-resonance enhancement (CORE) are observed as a result of an optical beam propagating through sodium vapor. They are interpreted in terms of diffraction-free encoding (DFE) followed by free-space propagation. They are discussed with respect to the three regimes of DFE, namely, pure phase encoding, on-resonance pure amplitude encoding leading to stripping, and the intermediate case involving both phase and amplitude encodings. First there are far-field interference rings displaying pure phase encoding. They are observed on both sides of resonance, as expected by theory. Second, there is CORE, which is a Fresnel diffraction manifestation resulting from the stripping of the beam profile by the nonlinear medium, and in the far field, the corresponding Fraunhofer patterns. CORE is observed in an inhomogeneously broadened system, and the role of Doppler broadening is theoretically studied. Finally, there are off-resonance rings involving both phase and amplitude encodings. They are numerically simulated and discussed with respect to DFE predictions.