Fractional vortex lens

The properties of a fractional vortex lens are discussed and its focal plane intensity distribution is investigated. The azimuthal phase pattern of a fractional topological charge vortex and the quadratic phase variation of the lens are displayed on a spatial light modulator (SLM) that is used in the phase mode. The intensity distribution possesses a low intensity radial opening in contrast to the doughnut pattern formed by a vortex beam with integer charge. The presence of multiple singularities in the radial opening of the doughnut structure is detected interferometrically.