Design of diffractive lenses that generate optical nulls without phase singularities

We exploit a technique, based on nonlinear optimization, to design diffractive lenses that focus optical nulls without any phase singularities. To ensure ease of fabrication, these lenses are composed of concentric circular zones. Furthermore, we show that this technique is readily extended to multiple wavelengths and can be used to improve tolerance to fabrication errors. (C) 2009 Optical Society of America