FOCUSING OF ELECTRIC-DIPOLE WAVES IN THE DEBYE AND KIRCHHOFF APPROXIMATIONS

We compare the Debye and Kirchhoff approximations for focusing of elec tromagnetic waves, with special emphasis on the focusing of converging electric-dipole waves. Numerical solutions for the spectral amplitude of the incident converging wave and for the focused field are provide d in both approximations for a variety of Fresnel numbers and f-number s. Comparisons of numerical solutions in Kirchhoff and Debye approxima tions show that for systems of Fresnel numbers larger than 20 and f-nu mbers less than 0.5, both approximations give indistinguishable result s for focused electric-dipole waves; At low Fresnel numbers, the Kirch hoff approximation gives the best results irrespective of the f-number , as in the scalar case. The focal shift depends mainly on the Fresnel number, but at low Fresnel numbers, a reduction of the f-number gives an increase of the relative focal shift.