"Micro-focusing" effect in far-diffraction zone
for fractal-like scatterers
I.S. Peretochkin, D.A. Zimnyakov
Department of Physics, Saratov State University, Russia,
Blagonravov Mechanical Engineering Institute (Russian Academy of Sciences)
Characterization of the scattering structures can be carried out using the statistical analysis of near- and far-zone non-stationary speckle patterns produced by coherent beam scanning of the object under study. Dependencies of the second-order normalized statistical moment (contrast) of intensity fluctuation point (for the case of broad collimated beam illumination and near-zone detection), or between scatterer and illuminating beam waist plane (of focused illuminating beam and Fraunhoffer zone detection are used) demonstrate the presence of maxima and minima. Appearance of such peculiarities can be interpreted as manifestation of "micro-focusing" effect [1] and used for the estimations of scattering structure parameters. Relationships between maxima parameters (amplitudes and positions) and structure characteristics (standard deviation and correlation length of phase fluctuation) had been obtained earlier [1] for scatterers with Gaussian phase statistics and Gaussian-type autocorrelation function. In present work "micro-focusing" effect in the far diffraction zone has been studied for different types of fractal and subfractal [2] scatterers; one-dimensional Weierstrass function [3] has been examined as scattering structure model. Computer simulation had been used to obtain the dependencies of the speckle contrast on defocusing parameter for Gaussian illuminating beams.
Obtained results can be useful from the viewpoint of scattering structures diagnostics in technical and biomedical applications.