| The rapid development of the big data era has brought many challenges to the communication system.In order to meet the growing demand for communication capacity,the research work in military and environmental fields such as human production and life,resource exploration and pollution detection has been continuously extended from atmospheric space to ocean.The application of underwater wireless optical communication is becoming more and more popular due to its high data bit rate advantage over traditional acoustic communication,which can meet the communication needs of high capacity,low latency and high confidentiality.However,the transmission performance of underwater wireless optical communication systems is very susceptible to underwater turbulence,absorption and scattering effects,which lead to the attenuation of optical signals and beam expansion,thus hindering the propagation of optical waves over long distances in underwater media.Therefore,studying the transmission characteristics of vortex beams carrying orbital angular momentum in turbulent absorbing seawater is essential to improve the performance of underwater wireless optical communication links.To this end,the main direction of this thesis is to analyze the transmission performance of the beam by constructing a transmission model for weak to strong turbulent environments starting from the optimal parameters of the generalized Bessel-Gaussian(GBG).The main research contents and innovations are as follows: generalized Bessel-Gaussian1.The receiving probability spectrum and the bit error probability model of the GBG vortex beam carrying the OAM mode are established.The optimal light source parameters of the GBG vortex beam are investigated.The underwater transmission invariance of the GBG vortex is analyzed by numerical simulation,and the optimal aperture angle of the GBG vortex beam is obtained.We find that:(1)The transmission invariance of the orbital angular momentum(OAM)mode of the GBG vortex is well maintained even in weak turbulent absorbing seawater,but the anti-diffraction loss ability of beam decreases with the increase of the topological charge.(2)Even if the aperture angle deviates from its optimal position by a small amount,the receiving probability and the bit error probability are significantly affected,and the Durning-ring radius should be minimized within the experimentally allowed range.2.The effects of weak turbulent absorbing seawater and GBG vortex beam structure parameters on the OAM transmission are investigated at the optimal aperture angle of the beam.The conclusions are as follows:(1)According to the signal transmission distance,the transmission efficiency of OAM signal can be optimized by selecting the transceiver system with high signal-to-noise ratio,optimal aperture angle,low-order OAM topological charge,long-wavelength beam and the minimum Durnin-ring radius allowed by the experiment.(2)The bit error rate increases with the ratio of the contribution of the ratio of temperature and salinity to the refractive index spectrum,the virtual refractive index of seawater,and the turbulent outer scale,and increases with the decrease of the turbulent inner scale.3.According to the modified Rytov method theory,the wave structure function and scintillation index theoretical model of plane waves under strong ocean turbulence are firstly established,and the rationality of the modified wave structure function is verified.On this basis,the general model of spatial coherence length is derived.Next,the receiving probability and bit error rate model of GBG vortex beams carrying OAM modes under strong turbulence are constructed.Under the optimal aperture angle of GBG vortex beam,the effects of turbulence intensity,turbulence parameters and beam parameters on the bit error rate of GBG transmission link are analyzed.It is concluded that(1)The BER increases linearly in the weak to strong turbulence region;(2)When a certain turbulence intensity is reached,the seawater absorption effect will be greater than the wavelength effect,so the carrier wavelength should be adjusted according to the absorption parameter to reduce the impact of the absorption effect on the link and ensure that the underwater communication link is in the best condition;(3)Under strong ocean turbulence,salinity fluctuation has a greater influence on the signal transmission of GBG vortex beam than temperature fluctuation.This work provides a theoretical basis for the study of wireless optical communication links under strong ocean turbulence. |
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