Observation of chromatic effects near a white-light vortex

Using a spatial light modulator to modify the phase of a light beam we create a spatially coherent, white-light beam containing an optical vortex. All the spectral components are helically phased; hence the beam carries an orbital angular momentum that is an integer multiple of (h) over bar per photon. A low-dispersion prism, positioned after the modulator, ensures that the vortices associated with each spectral component are co-axial. In addition, deliberate introduction of slight spectral dispersion means that the vortices associated with each wavelength no longer overlap. Subsequent examination near the beam axis reveals the chromatic effects predicted by Berry (2002 New J. Phys. 4 66; 74).