OAM-Based Hybrid Free-Space Optical-Terahertz Multidimensional Coded Modulation and Physical-Layer Security

To address capacity constraints and physical-layer security (PLS), defined in information-theoretic sense, of both optical and wireless networks in a simultaneous manner, we propose to use the hybrid free-space optical (FSO)-THz technologies employing orbital angular momentum (OAM) modes in both FSO and THz subsystems. The OAM modes, related to azimuthal dependence of the wavefront, are mutually orthogonal so that this additional degree of freedom can be utilized to improve both spectral efficiency and PLS in both optical and wireless networks. Spatial light modulators (SLMs) are routinely used to generate OAM modes in optical domain, in particular in FSO communications links. On the other hand, it has been recently demonstrated that a traveling-wave circular loop antenna, with azimuthal phase distribution along the loop, can be used to generate OAM in the RF domain. Reliability of FSO links is affected by atmospheric turbulence effects, scattering effects, and low-visibility in foggy conditions. On the other hand, RF technologies are not affected by these effects, but are sensitive to rain and snow. In particular, THz technologies, have available bandwidths comparable to a typical wavelength channel in WDM systems. Based on this complementarity, here we propose to use hybrid FSO-THz technologies to significantly improve the spectral efficiency and PLS of both FSO and wireless communications systems and networks.