Research On Propagation Characteristics Of A Novel Auto-focusing Vortex Beam With Orbital Angular Momentum In Turbulence

In recent years,the demand for bandwidth and performance of communication networks in society has been increasing.However,the traditional communication multiplexed technology can't meet the demands.Theoretically,Orbital Angular Momentum(OAM)multiplexed technology has a great advantage.What's more,compared with traditional acoustic waves and microwaves,it is easier to produce OAM beams in the light wave band.When the laser beam passes through the turbulent channel,the beam quality decreases.In atmospheric turbulence,the fluctuation of temperature causes the refractive index of the laser beam to fluctuate randomly,while in ocean turbulence,the refractive index of the laser beam is affected by temperature and salinity,which cause the change of the light intensity and the phase.With this change,the turbulence effects such as beam spread occur,which affects the performance of the Wireless Optical Communications(WOC)communication system.In order to solve the above problems,this paper designs two kinds of new self-focusing vortex beams with OAM by adopting the strategy of source light source control,and study the propagation characteristics,the resistance to ocean turbulence and other advantages by simulating in the atmosphere and ocean turbulence mediums,respectively.Firstly,considering the problem of using compound lens to focus in wireless optical communication system and the problem that the transmission distance is not easy to regulate,a new type of non-diffraction self-focusing vortex beam array with Non-Zero Radial Initial Velocity AVBA is proposed,also known as spatial controllable AVBA(SCAVBA),comparing with Ring Airy Vortex Beam(RAVB)and Airy Vortex Beam Array(AVBA).By applying phase screen simulation method,the focus distance,beam quality,beam spread,OAM distribution and so on are studied and analyzed.Finally,simulated results revealed that not only does SCAVBA have certain resistance to atmospheric turbulence,but also when the velocity is negative,it can also be transmitted over long distances,and at close range when positive.With the advantage of resistance to atmospheric turbulence,the negative initial velocity beam spread will be rapidly reduced at a certain distance,the power proportion of OAM target modal is comparable to the AVAB vortex beam,the positive velocity beam spread is small,and the power proportion of OAM is concentrated in the target mode.In other words,by simple control of the initial velocity of parameter items,SCAVBA vortex beams can be applied in WOC scenes where distance control is required frequently.Secondly,faced with the bad performance of WOC communication link and the problem of aimless movement occurred in the oceanic communication scenario,which will result in the transmission distance change,a new type of non-diffraction self-focusing vortex beam array with Non-Zero Radial Initial Velocity AGVBA is proposed,also known as spatial controllable AGVBA(SCAGVBA).In order to regulate the SCAGVBA easily,the Gaussian factor was introduced and at the same time,SCAGVBA I and SCAGVBA II were obtained.To verify the propagation characteristics of SCAGVBA type vortex beam,the following indicators in ocean turbulence medium are analyzed,such as focusing distance,beam spread,OAM distribution,OAM purity and so on by comparing with SCAVBA.Finally,simulated results demonstrated that(1)SCAGVBA type vortex beam is better than SCAVBA vortex beam,as reflected in,when the initial velocity is negative,the focusing distance is longer than SCAVBA,the trend of beam spread shows a large downward,the power proportion of OAM target modal is relatively large and OAM purity is great in the propagation process.(2)when the Gaussian factor is large,it can improve the quality of SCAGVBA type vortex beam,which is reflected in the concentration of the main ring energy,the greater decrease in beam spread,the balance between beam spread and beam drift,and the higher ratio in the power proportion of OAM target mode and OAM purity.(3)Compared with SCAGVBA I and SCAGVBA II,the properties of the two beams are mostly the same,except that the SCAGVBA II has a longer propagation distance at the same negative initial velocity.The results showed above contribute to improve the resistance to ocean turbulence performance of WOC communication system,and provide a guidance for the study of Underwater Wireless Optical Communications(UWOC)communication links with variable distance.