Research On Several Key Issues Of Technologies Of Orbital Angular Momentum In Wireless Optical Communications

With the development of the information-based society,requirements for large capacity communications increase a lot.With the advantages of both microwave communications and fiber communications,wireless optics communications(WOC)have high communication speed without constraint of fibers and spectrum applications,which attracts many researches and applications in regions like emergency communications,satellite communications and submarine communications.Vortex beams carry orbital angular momentum(OAM),which are orthogonal to each others and can be any value in theory.As novel technologies in multiplexing and encoding,OAM can increase the channel capacity and spectra effeceincy,which attracts many reseachers.However,wireless channels,such as atmospheric and underwater channels,will break the orthogonality of OAM modes and lead to the crosstalk.In order to study the changing rules of OAM modes in WOC and the effects of the OAM communication systems,this paper aimed at theoretical studying of the OAM technologies in typical WOC channels,including relevant theortical derivations,simulatical anaylsis and experimental researches.The main contents in this paper are stated below:(1)Aimed at the problems of tedium of expression of OAM spectrums in weak atmospheric and oceanic turbulence,this paper propose a simplied method of Laguerre-Gaussian(LG)beams with radial index equal to 0;deriving the simplied expressions of OAM spectrum in weak atmospheric and oceanic turbulence,and study the accuracies rules;studies show that in most of the situations,overall errors are lower than 1%;accuracies increase with larger OAM modes and weaker turbulence.This paper propose the ideal of using argument of x,analytically explain the changing rules of OAM spectrum:with weak atmospheric turbulence channel as an example,explain the phenomenens that OAM spectrum disperse more violent with larger OAM states,stronger turbulence,more far distance and shorter wavelength.Aimed at the features in oceanic turbulence,using the argument x to analyse the changing rules of OAM spectrum with temperature-salinity balance parametes,dissipation of kinetic energy and dissipation rate of temperature variance.The expressions derived in this paper eliminate infinite integral compared with the prime fomula,reduce the calculation amounts;and it is easy to analyse with the simplied expressions.(2)This paper expand the weak turbulence conditions to the moderate-to-strong turbulence conditions,derive the expressions of OAM spectrum under moderate-to-strong terrestrial atmospheric turbulence and marine atmospheric turbulence.Then derive the simplied expressions of them and study the accuracies.Studies show that under moderate-to-strong terrestrial atmospheric turbulence,overall errors are lower than 10%when the OAM modes are small and lower than 2%when the OAM modes are large;accuracies increase with larger OAM states,while have less relationship with the turbulence intensity.This paper analytically study the changing rules of OAM spectrum with turbulence intensity,distance,OAM modes and wavelength.(3)This paper study the OAM mode crosstalk caused by the displacements of vortex beams,build the model of LG beams with angular deflection and derive the expression of OAM spectrum under angular deflection;based on the simplied method,this paper derive the simplied expression and study the accuracies.Then study the changing rules of OAM spectrum with the angles of angular deflection,obtain the variation law.Based on the simplied expression,this paper derive the expressions of BER and capacity under angular deflection.Then,based on the expression of BER,derive the expression of offset tolerant(OT).Studies show that with larger OAM states and longer distance,the OT will be less.This work provide the method of calculating the OAM spectrum under angular deflection.(4)Unlike the atmospheric channels,underwater channels disturb the OAM modes more severe.Convolutional Neural Networks(CNN)are robust to the noise in image recognition.This paper aim at short-to-moderate underwater WOC condition,propse using CNN-OAM mode sorters in recognition OAM superposition intensity patterns underwater.This paper study the recognition accuracies and robustness in simulated oceanic channels by simulations.Studies show that this project have high accuracies in short-to-moderate oceanic turbulence.This method also have higher accuracies compared to the traditional conjugate phase mode sorting method.Then,this paper studied the performance of CNN-OAM with 16-ary under emulated seawater and oceanic turbulenc in laboratory.Experiments study the performance of CNN-OAM in short clear seawater,turbid seawater and emulated oceanic turbulence.The paper also study the accuracies performance with different OAM mode sets,OAM superposition types,OAM mode spacing and coding orders.And propose several methods to increase the accuracies.(5)This paper propose a sheme of OAM based modulating retro-reflector(MRR)combining with CNN-OAM mode sorters.And study the applications of this scheme underwater.Using method of random phase screens,this paper study the accuracies of CNN-OAM-based MRR under tens meters of simulated oceanic turbulence,and transmit images with this scheme,which verify the feasibility of this scheme under short-to-moderate oceanic turbulence on simulation.This paper also experimentally study the feasibility and accuracies under short emulated clear seawater and oceanic turbulence.This work firstly propose the idea of OAM-based MRR scheme,which should be beneficial to the conditions like communications between underwater vehicles and information obtaining from underwater sensors.