Scintillation in the Fresnel Region Behind a Sub-fractal Diffuser

The results of a numerical computation of contrast versus propagation distance in the Fresnel region behind a diffuser with fractal slope are presented and discussed. Both corrugated and isotropic cases are examined for a range of values of the diffuser spectral index and it is found that, in the absence of a finite outer scale, the asymptotic value of the contrast at large distances is greater than that of gaussian speckle. The contrast plot is monotonic for ‘smoother’ scatterers but develops a weak peak as the fractal/sub-fractal marginal case is approached. The effect of a finite outer scale is examined briefly, and a comparison is made with published experimental data.