Statistical model for the weak turbulence-induced attenuation and crosstalk in free space communication systems with orbital angular momentum

Scintillation of the orbital angular momentum of a Bessel Gaussian beam and its application on multi-parameter multiplexing

The scintillation of the orbital angular momentum (OAM) of a Bessel Gaussian beam was derived based on the Rytov method to characterize the performance of the OAM communication. Moreover, a multi-parameter demultiplexing method was also proposed which could decode the OAM state, the amplitude and two additional beam width information dimensions. The advantages of the OAM states as the communication carrier over the beam intensity were that the minimum scintillation of the fundamental mode was smaller, and its corresponding radius also diverged slower. The coefficient of variation of the decoding amplitude was approximated to the square root of the radial minimum scintillation, and it provided an estimated decoding precision for the input sample selection. This study not only provided theoretical basis for communication link design, but also had a promising application on the large capacity beam multiplexing in free-space laser communication.

Modeling and performance analysis of oblique underwater optical communication links considering turbulence effects based on seawater depth layering

Underwater wireless optical communication (UWOC) has been introduced to support emerging high-speed and low latency underwater communication applications. Most of the current studies on UWOC assume that the water temperature and salinity are constant, which can be justified only for horizontal links. In fact, as the temperature and salinity of seawater change with increasing depth, the seawater at different depths is bound to exhibit different optical properties. This implies that for the same link length, the communication system with the transmitter and receiver at different depths, will exhibit different performances. This paper first proposes an oblique optical link model considering turbulence effects, which is based on the layering of temperature and salinity with depth in realistic ocean water. Subsequently, the performance of the optical communication system with vertical and oblique links is analysed by adopting the oceanic power spectrum and seawater data from different ocean areas measured by the global ocean observation buoy, Argo. Our simulation shows that the performance of the underwater optical communication system is worse when the optical transmitter is located at the mixed layer than at the thermocline. When the transmitter is at the thermocline, the communication quality of the system will be worse at environments that temperature and salinity vary more slowly. When the tilt angle of the optical link in the vertical direction is less than 10°, the oblique link can be treated as a vertical link with the same link length.

Mitigating Vortex Splitting by Controlling the Wavefront Isophase Line Curvature of Vector Autofocusing Airy Vortex Beams in Free Space

The match between the orbital angular momentum (OAM) transmission and receipt is a prerequisite for vortex beams as a carrier of the wireless optical communication system in free space. However, the vortex splitting induced by atmospheric turbulence results in an offset in the average OAM measured after propagating over the free space optical communication link. Therefore, how to reduce the vortex splitting effect to improve the stability of orbital angular momentum propagation is studied in this paper. First, a new parameter (m) is introduced to modulate the radial distribution of the polarization state of the autofocusing Airy vortex beams (AAVBs). Second, to control the optical field distribution in focal plane, the wavefront isophase line curvature of the AAVBs is reconstructed by using the Pancharatnam-Berry phase generated in the polarization conversion process. Finally, by comparing the vortex splitting rates of the AAVBs with different wavefrontisophase line curvatures under different turbulence environments, it can be found that the vortex splitting in free space can be effectively mitigated by choosing an appropriate wavefront isophase line curvature. In addition, the mitigation effect is more obvious in the atmosphere with stronger turbulence. This study provides a new feasible method to control the phase structure of vortex beams and promises potential applications in the OAM-based free space optical communication system.

Security enhancement for adaptive optics aided longitudinal orbital angular momentum multiplexed underwater wireless communications

The frozen-wave-based longitudinal orbital angular momentum multiplexing (LOAMM) system developed in [IEEE Photonics J.10, 7900416 (2018)10.1109/JPHOT.2017.2778238] has the potential to overcome the crosstalk effects induced by turbulence. In this paper, we propose a defocus measurement aided adaptive optics (DMA-AO) technique for turbulence compensation in a LOAMM underwater wireless optical communication (UWOC) system to investigate the enhancement of physical layer security. Relying on a phase retrieval algorithm and probe beam, three amplitude-only measurements obtained from different back focus planes can realize phase reconstruction of distorted OAM beams. Moreover, the so-called mixture generalized gamma-Johnson S_B (GJSB) distribution is proposed to characterize the probability density function (PDF) of reference-channel irradiance of OAM. The GJSB allows for obtaining closed-form and analytically tractable expression for the probability of strictly positive secrecy capacity (SPSC) in a single input single output (SISO) system. Furthermore, the average secrecy capacity (ASC) and probability of SPSC for a multiple input multiple output (MIMO) system are investigated. Compared to the traditional OAM multiplexing system based on Laguerre-Gaussian (LG) beams, the LOAMM system with a probe beam assisted DMA-AO technique has potential advantages for improving the security performance in UWOC.