Degenerate multi-wave mixing inside the 4f coherent imaging system

A new contribution in the domain of experimental methods related to the nonlinear refraction characterization is introduced in this paper. By using different objects composed of different numbers of circular apertures at the entry of a 4f coherent imaging system and matched field stops (blocking geometrical image of the object) at the output, we can obtain diffracted beams beyond the stop areas. Presence of these beams is related to the intensity distribution pattern creating a combination of circular and sinusoidal gratings in the nonlinear material placed near the focal plane of focusing lens. The self-diffracted spectrum on this induced pattern generates diffracted beams appearing in the image plane. The three wave mixing process was combined with Z-scan. When the nonlinear material is moved around the focus we observe the variations of the diffraction efficiency defined as a new physical quantity to measure. The numerical simulation using FFT beam propagation method in angular spectrum were performed in order to determine the nonlinear refractive index. A good agreement between experiment and theory validated our approach.