  Fresnel zone plates are diffractive elements that are essential to form
images in many scientific and technological areas, especially where refractive
optics is not available. One of the main shortcomings of Fresnel zone plates,
which limit their utility with broadband sources, is their high chromatic
aberration. Recently presented, Fractal Zone Plates (FZPs) are diffractive
lenses characterized by an extended focal depth with a fractal structure. This
behaviour predicts an improved performance of FZPs as image forming devices
with an extended depth of field and a reduced chromatic aberration. Here we
report the achievement of the first polychromatic images obtained with a FZP
that confirm these predictions. We show that the polychromatic modulation
transfer function (MTF) of a FZP affected by defocus is about two times better
than one corresponding to a Fresnel zone plate. This result opens the
possibility to improve imaging techniques that use conventional Fresnel zone
plates over a wide range of the electromagnetic spectrum, ranging from x-rays
to THz.
