The Propagation Properties Of OAM Carried By Vortex Beams In Oceanic Turbulence

The incessant deployment of various bandwidth intensive services and applications in ocean,such as the large-scale sensor networks and high definition video/image transmission,demands for high speed wireless data communication.Although the traditional underwater acoustic communication(UAC)has the high technology maturity,it is limited by the low capacity and the large time latency because of the use of the low-speed sound-wave.As for the underwater optical communications(UOWC),which employs the blue–green optical waves as the communication carrier proffers potential large bandwidth.However,it still cannot meet the current demand for large-capacity and high-speed communications in the ocean.Therefore,the task of increasing the capacity of UOWC system is urgent.To further enhance the capacity of UOWC systems,previous studies suggested the use of multiplexing technology that transmits the data simultaneously through multiple orthogonal channels.The most attractive approach is to enhance the capacity of UOWC systems by adopting orbital angular momentum(OAM)multiplexing technology.This is due to the fact that the different OAM modes carried by vortex beams are mutually orthogonal,which can greatly enhance the channel capacity of UOWC system without a corresponding increase in spectral bandwidth.Nevertheless,it is shown that the orthogonality of the OAM modes can be severely distorted by oceanic turbulence.Turbulence causes the wavefront distortion and induces the crosstalk between the OAM modes.Thus,study on OAM modes carried by vortex beams through oceanic turbulence is of theoretical importance and practical value.Based on this,this paper conducts research from the following two aspects:Firstly,this paper studies the propagation properties of partially coherent Laguerre-Gaussian(LG)beams under weak oceanic turbulence: On the basis of the power-law spectrum of the oceanic refractivity fluctuations proposed by Yi,the mode probability density and the mode detection probability of the orbital angular momentum modes carried by the partially coherent LG beams have been derived.As compared with the previous results on Nikishov's spectrum,this paper's results present a lower mode detection probability,which corresponds to the stronger oceanic turbulence predicted by Yi's spectrum.Secondly,this paper studies the propagation properties of LG beams under moderate to strong oceanic turbulence: The analytic expression of the signal OAM mode detection probability is derived based on the extended Rytov theory.This paper's results show that turbulence with smaller dissipation rate of mean-squared temperature,larger dissipation rate of turbulent kinetic energy per unit mass of fluid and smaller temperature-salinity balance parameter can improve the detection probability.The LG beam with a smaller azimuthal mode order,larger radial mode order and longer wavelength can be less affected by oceanic turbulence.The results also indicate that Bessel Gauss beams possess much better resistant ability to oceanic turbulence than LG beams in the moderate to strong turbulent ocean because of their non-diffraction and self-healing characteristics.This paper's results can be beneficial for the design of the practical OAM-based UOWC system.