Theoretical Study On Generation And Detection Of Acoustic Vortex Beams Based On Structural Diffraction

Orbital Angular Momentum(OAM)carried by vortex beams is widely used in the information industry due to its new freedom of space independent of time and frequency.In the information age and high requirements for high-speed,large-capacity communication quality,people have proposed to apply vortex beams that carry orbital angular momentum to communication systems.Because the value of orbital angular momentum is theoretically infinite,and the modes are orthogonal to each other,the potential of applying vortex beams to communication systems is unlimited.At this stage,the application of acoustic vortices in communication systems is still in its infancy.Based on the above,this paper designs a new spiral structure that can generate acoustic vortex beams with different OAM and the structure can also be used to detect acoustic vortex beams.At the same time,three structures that can efficiently detect the topological charge of acoustic vortex are also designed.The main research contents are divided into the following points:(1)A Fermat spiral grating structure capable of generating higher-order and composite acoustic vortex waves is proposed.According to the analysis of diffraction theory,the plane acoustic beams pass through Fermat spiral gratings with different number of arms,due to constructive and destructive interference between transmitted acoustic beams,the required high-order acoustic vortices and composite acoustic vortices can be generated in the far field.And using the Fermat spiral structure,the topological charge of the acoustic vortex beam can be detected.Through theoretical and simulation results,it can be known that the topological charge of the generated acoustic vortex is flexible and controllable,and its purity is calculated to be above 71%.(2)Methods of detecting the topological charge of the acoustic vortex by diffraction pattern are proposed.Firstly,a circular hole array interference screen structure is proposed.Starting from the theoretical analysis,the principle that the circular hole array can detect acoustic vortex is clarified.The circular hole array designed in this paper is uniformly distributed in a circular shape,and the number of circular holes in the circular hole array must be at least 4 or more,and the number of circular holes minus 1 is the maximum topological charge which can detect.Acoustic vortex beams carrying different orbital angular momentum show different regular distributions through the same circular hole array;vortex beams carrying same orbital angular momentum also show different regular distributions through different circular hole arrays.The topological charge is determined according to the distribution law of the diffraction pattern.Of course,the detection range of this method is not infinite.When the number of circular holes gradually increases,the diffraction pattern will no longer show a regular distribution.Finally,by designing reasonable parameters and simulation environment,the COMSOL simulation software is used for verification.Then two annular apertures are proposed that can detect the topological charge of the acoustic vortex.The two annular structures designed are annular triangular aperture and annular ellipse aperture.Firstly,the theoretical analysis was used to explain the principle that this kind of annular aperture can detect acoustic vortices.Then a theoretical model was established to obtain the results of formula calculations.Finally,the COMSOL simulation software was used to verify the results of theoretical calculations.By comparison,it is found that the calculation result of the formula is the same as the simulation result.By observing the typical characteristics of the diffraction pattern,the topological charge carried by the acoustic vortex wave is discriminated.