Improvement in orbital angular momentum mode sorting of optical vortices by using polarization gratings

Abstract

Optical vortices (OVs) are known as light waves with helical wavefronts, which are characterized by their azimuthal mode indices (topological charges) corresponding to orbital angular momenta (OAMs). Recently, mode division multiplexing (MDM) based on mode orthogonality is expected to enhance transmission capacity in telecommunications remarkably. The key technology for such OAM-based MDM is the mode decomposition with respect to topological charges, namely OAM mode sorting. Among various kinds of candidates, the OAM sorter based on log-polar transformation in optics [1] , [2] attracts great attentions since its insertion loss is low enough to support the decomposition on single photon level, and it also supports an inverse transformation. However, the log-polar transformation method has the serious disadvantage of non-negligible crosstalks among the neighboring modes. In order to overcome this problem, a beam copying technique by using specially-designed diffractive elements was demonstrated [3] . Although this novel technique can greatly reduce the crosstalks, but it spoils the advantage of the low-loss inverse transformation.