Performance And Scattering Characteristics Study Of Underwater Orbital Angular Momentum Optical Communication System

China has vast maritime territory and rich underwater resources,which are of great significance to its economy,security,and geostrategic interests.In the exploration of underwater resources,Underwater Wireless Optical Communication(UWOC)has tremendous potential to provide high data rate communication.Orbital Angular Momentum(OAM)beams have shown great potential in improving UWOC transmission rates and channel capacity due to their unique spiral phase distribution,attracting widespread attention from researchers.However,when light is transmitted in underwater channels,the UWOC system is affected by Absorption and Scattering(AS)of photons by water,which causes signal power attenuation and limits the transmission distance of the signal.The scattering of light in water not only causes signal power attenuation and reduces signal-to-noise ratio but also causes OAM beam phase disturbance,resulting in mode crosstalk.At the same time,changes in seawater salinity and temperature result in phenomena such as light intensity fluctuations,spot displacement,and beam divergence,which deteriorate the performance of the UWOC-OAM system.Therefore,this paper proposes to consider both the absorption and scattering effects of seawater and ocean turbulence,construct a complete underwater optical communication channel model,and use the Finite Difference Time Domain(FDTD)method to study the scattering characteristics of OAM beams and particles’ interactions.The main research contents and achievements are as follows:1.The performance of UWOC-OAM system in the visible light band is explored based on the Beer-Lambert law.Firstly,the optical attenuation characteristics of underwater channels are analyzed,and the effects of chlorophyll concentration and incident light wavelength on the attenuation coefficient are studied.Then,a random phase screen is used to simulate the turbulence effect in the ocean,and the Signal-to-Noise Ratio(SNR)and channel capacity based on the UWOC-OAM system are studied.The results show that the increase in topological charge,ocean turbulence intensity,and chlorophyll concentration will reduce the system SNR and channel capacity.The incident light with a longer wavelength has better turbulence resistance and better system performance at close distances.However,with the increase in distance,the stronger water attenuation effect results in a greater decrease in received SNR and channel capacity.Therefore,for OAM multiplexing communication systems,the transmission distance and appropriate wavelength need to be considered to achieve optimal performance.2.Using the Electric Field Monte Carlo(EMC)method to construct the Transmission Matrix(TM),this study simulates the underwater AS channel model and combines it with the ocean random phase screen to realize segmented underwater AS and turbulence(Absorption,Scattering,and Turbulence,AST)channel model simulation,while studying the comprehensive effects of absorption,scattering,and turbulence on the underwater channel.The study shows that in the absence of turbulence,the higher the water quality attenuation coefficient,and the increase in transmission distance and topological charge number will lead to a decrease in OAM detection probability.However,under the AST channel,the decrease in incident light wavelength and the increase in incident light waist radius,turbulence intensity,and salinity humidity equilibrium parameter will cause an increase in the incident light mode crosstalk.The simulation results provide a reference basis for OAM underwater optical communication research in real environments.3.The scattering phase function distribution of OAM beams after single-particle scattering and the volume scattering function(VSF)distribution of OAM beams after multiple-particle scattering were investigated using FDTD Solutions software and array theory.The study shows that for single spherical particles,increasing the topological charge reduces the lateral scattering intensity,increasing the particle size causes more fluctuation in the scattering intensity,and increasing the refractive index of the particle results in higher scattering intensity.For single cylindrical particles,cylinders with a diameter-to-length ratio of 1 have higher and less fluctuating scattering intensity,and the average scattering intensity is lowest when the incident light is in the red light band compared to the violet and green light bands.For multiple-particle scattering,OAM vortex beams with different topological charges exhibit similar propagation characteristics in multi-particle channels.Compared with the full electromagnetic simulation method,FDTD software simulation combined with array theory significantly reduces the computational cost of OAM vortex beam propagation in multi-particle channels.The study of scattering phase function distribution can provide guidance for modeling complex channels in UWOC systems by understanding the scattering characteristics and angular distribution of different particle materials in light signals.