Topological defects in moire fringes with spiral zone plates

We present a study of spatial structures created by superposition of spiral zone plates used for generating optical beams with phase singularities. Moire fringes are observed that show topological defects similar to those appearing in interference patterns of optical vortices. A brief theoretical discussion is included that supports the similarities between the two phenomena. Our results may lead to interesting applications to digital information processing by optical means. (C) 2003 Optical Society of America.