Orbital-angular-momentum dependent speckles for spatial mode sorting and multiplexed data transmission

Abstract: Characterizing the orbital angular momentum (OAM) of a vortex beam is critically important for OAM-encoded data transfer. However, in typical OAM-based applications where vortex beams transmit through diffusers, the accompanying scattering effect tends to be either deliberately prevented, or characterized and then modulated actively based on complex wavefront shaping and interferometry techniques. Here, we aim to investigate the characteristics of blurred speckles obtained after a vortex beam transmits through a ground glass diffuser. It is theoretically and experimentally demonstrated that a cross-correlation annulus can be identified by implementing the cross-correlation operation between speckle patterns corresponding to vortex beams with different OAM values. Besides, it is worth noting that, the size of the cross-correlation annulus is determined by the absolute value of the topological charge difference between the two corresponding vortex beams. Based on this mechanism, the OAM modes can be easily sorted from the incoherently measured OAM-dependent speckles as well as their cross-correlation. Furthermore, to make full use of the orthogonal feature of the OAM-dependent speckles, demultiplexing of OAM-encoded data transfer is verified using a ground glass diffuser. Both 8-bit grayscale and 24-bit RGB OAM-encoded data transfers are carried out in experiments with superior error rates. We can conclude that the OAM-dependent speckles can be not only utilized as a competitive candidate for the OAM mode sorting function in a simple way but also provide an efficient method for the demultiplexing of OAM-encoded data transfer in a practical application.