Study On The Effects Of Oceanic Turbulence On Laser Beam Propagation Characteristics

Nowadays,demands for high-speed and large-capacity in underwater communication become difficult to be supported by using the traditional underwater communication technology such as underwater acoustic communication(UAC)as a result of the continuous exploration of human activities in the underwater environment monitoring,scientific data collection and disaster prevention.Compared with the UAC,underwater optical wireless communication(UOWC)has attracted extensive attention because of its high-bandwidth,low-time-delay,good confidentiality and strong anti-electromagnetic interference capability.Despite the above-mentioned advantages,the propagation of UOWC is still hampered by absorption,scattering,and optical turbulence(i.e.,refractive-index random fluctuations)in seawater.Most of the previous investigations into the channel models of UOWC were concerned the effects of the absorption and scattering,but less considered the oceanic turbulence.Thus,this paper focuses on the effect of the oceanic turbulence on laser beam.As in the seawater,the random fluctuations in the index of refraction cause the effects on the laser beam related to its irradiance and phase fluctuations resulting in optical propagation phenomena such as irradiance scintillation and beam wander,which have a significant impact on the quality of the received optical signal and the performance of UOWC systems.This special optical channel transmission characteristic makes it difficult to design the UOWC systems in a certain extent.In this context,it is necessary to analyze the characteristics of laser beam propagating through oceanic turbulence.To this end,the main research works are listed as follows:(1)The extended oceanic refractive-index spectrum(ORIS)model.The approximate oceanic refractive-index spectrum(AORIS)is firstly extended to a new approximate oceanic refractive-index spectrum(NAORIS),which considers the effects of the finite outer scale as well as the eddy diffusivity ratio of salinity to temperature.The proposed NAORIS model can accurately describe the stochastic diffusivities of the temperature and salinity in the actual underwater environment.(2)Beam-wander analysis in weak oceanic turbulence.Based on the Rytov theory and the NAORIS,the exact closed-form expressions for beam wander variance of Gaussian beam with the finite and infinite outer scale are derived.By using the numerical method,the effects of the oceanic parameters on the collimated and focused Gaussian beams are studied.(3)Scintillation-index analysis in weak turbulent ocean.In terms of the Rytoy theory and the NAORIS,the closed-form expressions for the scintillation index of a horizontally propagating plane and spherical waves and their influence on the performance of UOWC systems are presented.By use of the numerical simulation,the effects of the oceanic parameters on the scintillation index and the UOWC systems are analyzed.(4)Development of scintillation-index model under moderate-to-strong oceanic irradiance fluctuations.On the basis of the extended Rytoy method and the NAORIS,the analytical expressions for scintillation index of the plane wave and spherical wave propagating a horizontal path are firstly developed that are valid in moderate-to-strong oceanic turbulence.The corresponding computer simulation is designed to verify the correctness of the scintillation index model.By means of the predicted scintillation index and the GammaGamma distribution model,the effect of arbitrary oceanic turbulence on the performance of UOWC systems are investigated.