2. Key Laboratory of Underwater Information and Control, China Shipbuilding Industry Corporation 705 Research Institute, Xi'an, Shaanxi 710077, China
Overview: The orbital angular momentum (OAM) is carried to Hank-Bessel (HB) vortex beam, and the HB vortex beam has non-diffracting nature and self-focusing properties, for instance, it does not change without diffracting propagation. Lateral intensity distribution can be reconstructed when the HB beam encounter obstacles. With the development of underwater wireless optical communication (UOWC) technology, the OAM-carrying beam is used to study high-capacity and ultra-high-speed underwater wireless optical communication. Different OAM modes are orthogonal to each other, and the channel capacity of the underwater wireless optical communication link can be improved by using the orbital angular momentum spatial multiplexing technique. Consequently, HB vortex beams can be used as the carriers to increase the channel capacity of information transmission. However, due to the rotation of the earth, the OAM mode crosstalk of the vortex beam is caused by the anisotropic ocean turbulence when the beam is transmitted in ocean. The effects include beam point jitter, intensity and phase fluctuation and damage beam pattern. Thereby, the detection probability of transmitting OAM is reduced, and the error rate of the underwater wireless optical communication link is increased. Therefore, in this paper, the spiral phase spectrum of the HB vortex beam in an anisotropic ocean turbulent channel is studied. Firstly, based on the Rytov approximation theory, the cross-spectral density of HB beams in anisotropic ocean turbulence is analyzed, and the influence of anisotropic ocean turbulence on HB beam propagation is studied. An OAM crosstalk model of HB beam in anisotropic ocean turbulence is established by analyzing the spiral phase spectrum of HB beams in anisotropy ocean turbulence. The relationship between mode crosstalk and equilibrium parameters, temperature variance dissipation rate, dynamic energy dissipation rate is discussed, and compared with the transmission characteristics of HB beams in isotropic ocean turbulence. The results show that the detection probability of the emission mode is decreased and the spiral phase spectrum is expanded due to the ocean turbulence. Furthermore, with the increases of anisotropy factor, the influence of ocean turbulence on the detection probability of HB beam becomes smaller. Meanwhile, with the increase of the temperature variance dissipation rate and the equilibrium parameter, and the decrease of the dynamic energy dissipation rate, the influence of ocean turbulence on the orbital angular momentum transmission is increased. In the same way, with the increase of the temperature variance dissipation rate and the equilibrium parameter, and the decrease of the dynamic energy dissipation rate, the spatial coherence length in oceanic turbulence decreases is increased. Moreover, OAM mode detection probability, the crosstalk probability and the spiral phase spectrum of the HB beam are more negatively affected by ocean turbulence dominated by saliniy fluctuations.