Compact diffractive optics for high-resolution sorting of orbital angular momentum beams

A novel optical architecture is designed and fabricated in order to overcome the limits of the traditional sorter based on log-pol optical transformation for the demultiplexing of optical beams carrying orbital angular momentum (OAM) of light. The proposed configuration simplifies the alignment procedure and significantly improves the compactness of the optical device. In addition, the integration of an optical fan-out provides higher resolution on OAM modes separation. Since the miniaturization level and the optical configuration require to operate beyond the paraxial approximation, a rigorous formulation of transformation optics in the non-paraxial regime is developed and applied for the first time. Samples have been fabricated as 256-level phase-only diffractive optics with high-resolution electron-beam lithography and tested for the demultiplexing of optical vortex superposition in a 8-bit free-space optical link. The results confirm the expected high efficiency and resolution in OAM sorting.