In many photonics and fiber-optics applications, the irradiance distribution in the very near field (z/ D < 0.25) behind a circular or annular aperture is of interest. We present the results of detailed calculations of the irradiance distribution throughout the entire space behind an annular aperture. Included as a special case of the annular aperture is the circular aperture and the opaque circular disk. A log–log plot over many orders of magnitude in axial distance provides particular insight. The behavior throughout the Fresnel and Fraunhofer region is well known; however, we pay particular attention to the behavior in the near field. A variety of subtle effects in the near field are presented and discussed.
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