Mode Expansion of the Diffraction Field of a Wave Beam by an Annular Aperture

The diffraction field of a wave beam by an annular aperture is obtained by using the Kirchhoff approximation. This field is then represented as a sum of beam mode functions which have arbitrary beam wave parameters. The expansion coefficients which represent the mode coupling coefficients have simple forms and theoretical or numerical discussions on them can be done with ease. As an example, some discussions on the mode coupling coefficient between dominant modes are made and numerical results are shown