| With the exploration and utilization of marine resources and increasing requirements of people for high-capacity,high-speed as well as high-security information transmission,the transmission medium of information has extended from traditional optical fibers to free space(atmosphere,sea steam,and underwater).The carriers of information have extended from the classical degrees of freedom such as optical amplitude,phase,polarization(spin angular momentum)and frequency to the optical orbit angular momentum(OAM).However,when the vortex(or OAM)carried by the vortex carrier is transmitted in the sea water,the vortex mode is bound to encounter the disturbance of the sea water turbulence.Therefore,it is necessary to study the disturbance mechanism of seawater turbulence on the vortex mode to optimize the underwater wireless optical communication system.Considering that local wave energy propagates without diffractive in uniform medium,it can be used to solve the problem of low transmittance of light wave in seawater.In this paper,Bessel-Gaussian localized waves(BGLW)are used as the information carrier to study the propagation rules of these waves in seawater turbulence.The main research contents and results of this paper are as follows:1.By establishing the detection probability model of the signal vortex mode,the influences of seawater turbulence and BGLW related parameters on the detection probability of the signal vortex mode are studied.It is concluded that the detection probability of signal vortex modes increase with the increase of the half-modulated pulse width of the input pulse,rate of dissipation of kinetic energy per unit mass of fluid,turbulent inner scale,but they increase with the decrease of the Bessel cone angle,quantum number of vortex mode and the dissipation rate of the mean-squared temperature.At the same time,we find that the eddy model is more sensitive to salinity fluctuation than temperature fluctuation in weak ocean turbulence.These results show that the BGLW is more suitable for the wireless optical communication based on the vortex mode than the traditional vortex wave.2.Based on the power spectrum of the outer scale exponential fluctuation of weak oceanic turbulence,the channel capacity of weak ocean turbulence is studied by using the Bessel-Gaussian vortex local wave.The effects of seawater turbulence and beam parameters on channel capacity are studied by the channel capacity model.It is concluded that the larger anisotropy factor,the higher channel capacity of the link.The smaller internal scale of turbulence,the more vortexes the beam intersected in the channel,and the smaller channel capacity.Moreover,the influence of salinity on channel capacity is greater than that of temperature.This work provides a theoretical basis for higher capacity ocean optical communication using BGLW.3.Using the modified Rytov theory and the approximate ocean refractive index spectrum,the scintillation index of beam horizontal path in strong ocean turbulence is derived.And the analytical expression of channel capacity of a BGLW with OAM in the strong ocean turbulence is obtained.Based on the modified Rytov approximation,we further discuss the propagation of channel capacity of Bessel-Gaussian vortex beam in strong turbulent channel.In a series of numerical simulations,we study the effects of beam parameters and turbulence parameters on the channel capacity of signal vortex mode,and we find that there exists an optimal size of signal detection aperture under specific turbulence and optical communication system conditions.The channel capacity of signal vortex mode can be improved by the larger beam waist radius,smaller Bessel cone Angle and the best receiving aperture.In strong oceanic turbulence,the influence of salinity fluctuation on channel capacity of vortex mode is larger than that of temperature fluctuation.These results indicate that the channel capacity of strong oceanic turbulence communication link can be optimized by selecting appropriate beam parameters. |
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