Abstract

From the diffraction integral equation, the axial irradiance distribution in the near region, i.e., in the region between the aperture and the Fresnel region, of Gaussian beams through a circular aperture is presented. It is shown that when the waist radius of a Gaussian beam is very large and axial irradiance oscillates with the alternate maximum and minimum and the value of the minimum is not zero. As the observer moves toward the aperture, the range of the irradiance oscillation becomes small, while the period of oscillation decreases dramatically. When the waist radius decreases, the range of the axial irradiance oscillation decreases continuously and the modulation of the irradiance becomes small. When the waist radius is very small, the axial irradiance in positions far from the aperture no longer oscillates and decreases monotonically with the increase of the axial distance, while in places near the aperture, the axial irradiance is the same as that of the center of an aperture. Therefore it is different from the incidence of a uniform and spherical beam and agrees with similar observations in the Fresnel region.

© 1989 Optical Society of America