Spectral anomalies at wave-front dislocations

In a recent Letter, Gbur, Visser, and Wolf [Phys. Rev. Lett. 88, 013901 (2002)] predict that remarkable spectral changes can take place in the neighborhood of phase singularities of a diffracted focused wave. We report here the experimental observation of this anomalous spectral behavior and show that this is a general characteristic of optical vortices. Using a high-resolution interferometric technique, we are able to measure directly both the spectral intensity distribution and the spectral phase at the singular points of optical waves.