Observation of a stronger focus due to spherical aberration

Recently it was predicted that addition of spherical aberration to a focusing system with a small Fresnel number can increase the value of the maximum intensity on the optical axis. In this paper we report experiments which confirm this prediction. To this end we have measured the three-dimensional intensity distribution in the focal region of a lens with spherical aberration. We also show that this effect cannot be used to increase the intensity at a fixed distance from the lens.