Investigation of wavefront reconstruction by stimulated backscattering through a two-lens system

Abstract

A two-lens system was used to investigate wavefront reconstruction by stimulated Raman and Brillouin backscattering. The unique feature of the two-lens system was to allow the focus of the two-lens system to be varied continuously from near the second lens, through the image of an object placed between the two lenses onto infinity. Experiments where the lens focus was located beyond the image of the object have not been reported previously. Images reconstructed from stimulated Raman and Brillouin backscattering were obtained and their positions and magnifications were found to be in agreement with the modified holographic model. Experiments show that for all locations of the lens focus (in front of the cell and inside or beyond the cell), Geometrical and power conditions must be satisfied to obtain optimal image reconstruction. The conditions are: (1) The image of the object must be located near the entrance window of the nonlinear medium cell. (2) The incident laser power must be within a finite power interval. The minimum incident laser power for good image reconstruction, Pmin, was found to be dependent on cell length and the arrangements (in front of the cell for the single-lens arrangement and inside or beyond the cell for the two-lens system arrangement). Pmin was significantly lower for the two-lens system arrangement than for the single-lens arrangement. In the single-lens arrangement, as well as in the two-lens system arrangement, Pmin drops rapidly with increasing cell length and approaches saturation gradually. This is explained well by a simple model. When the geometrical and power conditions are satisfied the proposed two-lens system produces good image reconstruction with lower incident power. In addition, the more general two-lens system arrangement with condition "B" satisfied can conclude the object in front of the two lenses. This arrangement had valuable applications in laser amplifiers.