Nonlinear images of gain defects

Abstract

The nonlinear imaging effect for small-scale defects with gain defect, which may be caused by the uneven gain in gain media, is investigated through numerically solving the propagation model with the standard split-step fast-Fouriertransform algorithm. Two cases, i.e. defects only causing gain and defects causing both gain and phase modulation, are considered. It is proved that nonlinear images can also be formed. However, when compared to the nonlinear imaging effect for defects causing attenuation, it is found that: firstly, the intensity at the point corresponding to the center of the scatterer experiences an obvious decreasing just before the nonlinear image is formed, contrary to the case the defect causing attenuation; secondly, this decreasing is enhanced as the gain caused by the defect increases; thirdly, there is a distance several centimeters, even more than ten centimeters, between the nonlinear images for gain defects and those for attenuation defects. For defects which cause both gain and phase modulation, above differences are ignorable, but the nonlinear imaging effect has some new properties: firstly, the beam peak intensity on the exit surface of the nonlinear medium is higher than that in the case the defect only causing phase modulation; secondly, the second-order nonlinear image is increased obviously, but the effect of the defect's gain on the image's peak intensity is limited. Besides, the effect of the distance between the defect and the nonlinear medium and that of the power of incident beam are discussed.