Abstract
It was commonly believed that the spherical wavefront aberration of an optical beam always leads to a lower maximum focused intensity along the optical axis. It was shown recently both theoretically and experimentally that this was only true for high Fresnel number systems. In low Fresnel number focusing systems a properly chosen spherical wavefront aberration can increase the maximum focused intensity. In this paper, we call this 'super focusing'. To investigate the characteristics of the super focusing, an improved axial intensity formula has been newly derived for Gaussian beams with spherical aberrations. Based on this formula, several examples of axial intensity distributions are shown. The cause of the super focusing is considered based on these intensity distributions and the derived formulae.
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