Optimal Diffractive Focusing of Vortex Beams
The research aimed to find an optimal circular pupil shape for focusing paraxial beams carrying orbital angular momentum (OAM). The solution is a combination of Fresnel phase structure and an OAM-dependent Bessel radial modulation. This analysis establishes a fundamental performance limit for diffractive focusing of aperture-limited vortex beams, showing that the optimal mask is amplitude-weighted.