OAM-based physical-layer security enabled by hybrid free-space optical-Terahertz technology

In order to address security issues of optical networks, quantum key distribution (QKD) has been proposed. However, most of the research efforts in QKD are based on a spin angular momentum (also known as a photon polarization), which represents a fragile source of quantum information for transmission over single-mode fiber (SMF), so that the secure key rates are very low, and at the same time the transmission distance is limited. In this paper, we follow a different strategy. The orbital angular momentum (OAM) modes, which are related to the azimuthal dependence of the wavefront, are orthogonal among others so that this additional degree-of-freedom can be used to improve the physical-layer security (PLS) in both wireless and optical networks. Spatial light modulators (SLMs) are routinely used to generate OAM modes in optical domain, in particular in free-space optical (FSO) communications. 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 PLS of either FSO or wireless communications.