UNFOLDING OF AN UNSTABLE SINGULARITY POINT INTO A RING

It is well known that phase singularities, in general lines in space, are topologically stable features of a wave field. An exception is a p ointlike singularity, which is unstable and deforms into a ring or dis appears when a small perturbation is applied. Recently, Nye showed how such an event can be understood as an unfolding of a higher-order dis location [J. Opt. Soc. Am. A (to be published)]. We present an optical implementation of this model and show experimentally that the focal r egion of a lens contains points of zero intensity on the optical axis that deform into rings when a small amount of spherical aberration is applied to the system. (C) 1998 Optical Society of America.