Transmission Characteristics Of Perfect Optical Vortex In Absorbing Ocean Turbulence

With the development of the underwater wireless optical communication network,the underwater wireless optical communication network puts forward higher requirements for the optical information transmission capacity,transmission speed,security performance and high transmission accuracy of the communication link.Because the orbital angular momentum(OAM)mode carried by the vortex beam with high-order topological charge can form an infinite(N)-dimensional orthogonal basis,theoretically,this infinite(N)-dimensional orthogonal basis can meet the requirements of high communication capacity,high transmission speed,high security performance and so on in the current underwater wireless optical communication chain.Therefore,the research on underwater transmission performance of vortex light has become the core focus in this field.However,the signal loss caused by seawater turbulence disturbance and seawater absorption encountered by underwater communication link is one of the core challenges faced by underwater optical communication.At present,one of the effective methods to solve the signal loss caused by seawater turbulence disturbance and seawater absorption is to shape the beam and select the best beam parameters.The main purpose of this paper is to study the transmission effect of perfect vortex beam and perfect Laguerre-Gaussian beam in absorbing ocean turbulence and the optimization of light source parameters from the aspects of link channel capacity,beam wander and spread.The main research contents and achievements of this paper are as follows:1、Based on Rytov approximation,the influence of ocean absorption effect on orbital angular momentum mode transmission carried by perfect optical vortex(POV)is studied from the angle of OAM mode receiving probability and channel information capacity.On the basis of establishing the receiving probability of OAM module carried by POV and the channel capacity of the link,the influence of weak turbulence absorbing seawater channel on the transmission of OAM module carried by POV and the channel capacity of underwater communication link with POV as signal carrier is analyzed by numerical calculation,and the following conclusions are drawn:(1)The receiving probability of OAM module decreases linearly with the increase of virtual refractive index of water body;In the "window wavelength" range,longer signal wavelength,lower OAM quantum number,larger radius of vortex ring and smaller aperture of transceiver can obtain higher OAM signal receiving probability.(3)The transmission performance of OAM signal carried by POV is better than that carried by Laguerre-gauss.2、Based on Rytov approximation,from the point of view of channel capacity of perfect Laguerre-Gauss(PLG)vortex carrier communication link in turbulent absorbing ocean,the influence of turbulent absorbing seawater and PLG structural parameters on OAM mode transmission is studied.Based on the theoretical model of bit error rate and average capacity of PLG beam carrier wireless communication link,the conclusions are drawn by numerical simulation:(1)In the region of "window transmission wavelength",the conclusion that longer signal source wavelength is beneficial to the performance of non-absorbing turbulent channel is no longer valid;(2)The underwater communication link with PLG beam can obtain higher channel capacity with low OAM topological charge and smaller aperture of transmitter and receiver.3、The effects of turbulent seawater absorption on the wander and spread of PLG beams are studied respectively.The long-term broadening of PLG beam in the region of weak to strong turbulence fluctuation is derived by using the cross spectral density under Huygens-Fresnel integral,and the wander of circular perfect Laguerre-Gaussian(CPLG)beam in the region of weak turbulence fluctuation is derived by Rytov approximation.The relationship between long-term beam broadening/beam wander and beam wavelength and seawater absorption is numerically analyzed by using the spectral absorption coefficient of seawater.From the research,we get the following conclusions:(1)Under the given communication link condition,PLG beam has the best beam diameter and self-convergence effect,and the long-term beam spread of PLG beam is less than that of LG beam,while the wander evolution trend of PLG beam with the increase of transmission distance is opposite to that of LG beam with the increase of transmission distance;(2)PLG/CPLG beam has stronger anti-turbulence ability than LG/CLG beam.