Study On The Generation And Detection Of Perfect Vortex Beams In The Terahertz Band

In recent years,structured beams represented by Airy beams,Bessel beams,and vortex beams,have received much attention.Vortex beams have a spiral phase wavefront structure.The orbital angular momentum carried by vortex beams can provide a new spatial degree of freedom for encoding information to improve the capacity and spectral efficiency of communication systems.Meanwhile,Terahertz(THz)Science and Technology is attracting increasing attention as a continuously developing frontier discipline.THz waves,which possess many unique properties,have great prospects for applications over many fields,such as security screening,material identification,ultra-high-speed wireless communication,etc.THz vortex beams,combining THz waves and vortex beams,have both large bandwidth and an additional degree of freedom for data multiplexing.It has the potential to greatly increase the capacity and spectral efficiency of communication systems.However,the ring radius of THz vortex beams increases with increasing the topological charge(TC),which will make it difficult to couple THz vortex beams with different TC into a fixed-aperture fiber or waveguide.To overcome this difficulty,perfect vortex beams(PVBs)whose ring radius independent of the TC,have been proposed and extensively studied in the optical band.Currently,the study of PVBs in the THz band is absent.Thus,the primary study of PVBs is the study of their generation and detection.This thesis focuses on the generation and detection of PVBs in the THz band.A series of phase elements are designed to generate and detect PVBs by phase modulation of THz waves.The simulations are performed using numerical simulation methods based on angular spectrum theory and the experiments are carried out utilizing the 3D printing technology.The main research work can be summarized as follows.Firstly,a new method for detecting the TC of THz vortex beams is proposed.This method is known as the focal hyperbolic lens detection method.At 0.1 THz,the THz vortex beams pass through the focal hyperbolic lens and the diffraction fringes are then formed at the focal plane of the lens.The magnitude and sign of the TC can be determined from the number and direction of the diffraction fringes,respectively.The experimentally measured TC values reached 5.This detection method using a single lens simplifies the detection system of THz vortex beams.This work provides a feasible detection scheme for future THz communication systems based on vortex beams.Secondly,the generation and detection of PVBs have been realized for the first time in the THz band.For generation,the THz circular PVBs are generated by the optimal phase elements at 0.1 THz.It is verified that the THz circular PVBs have perfect vortex properties,i.e.,the ring radius does not change with the TC.To meet the requirements of various application scenarios,the superimposed,elliptical and elliptical superimposed optimal phase elements are designed by superposition,scale and their combined transformation at 0.3 THz.The THz circular superimposed,elliptical and elliptical superimposed PVBs are generated,and their perfect vortex properties are verified experimentally.For detection,the carrying TC is detected using the focal hyperbolic lens detection method.Simple and effective identification of the TC is achieved.These THz PVBs with different TCs have the potential to be coupled into a circular or elliptical fixed-aperture fiber or waveguide and then applied to circular or elliptical fiber or waveguide communication systems based on THz vortex beams.Thirdly,this paper realized the generation and detection of quasi-PVBs for the first time in the THz band.These quasi-PVBs can carry more TCs.At 0.3 THz,the THz quasi-PVBs carrying individual and multiplexed TCs are generated using a spiral axicon and a multiplexed spiral axicon,respectively.For the THz quasi-PVBs carrying an individual TC,their quasi-perfect vortex properties are verified,i.e.,the ring radius becomes slightly larger with increasing the TC.For the THz quasi-PVBs carrying a multiplexed TC,the ring radius remains essentially constant with changing the multiplexed TC.Subsequently,the individual and multiplexed TCs carried by the THz quasi-PVBs are detected using the focal hyperbolic lens detection method and the modified coordinate transformation method,respectively.Further,the generation and detection of one-dimensional(1D)and two-dimensional(2D)THz quasi-PVBs arrays are numerically simulated based on the study of THz PVBs.The 1D and 2D THz quasi-PVBs arrays are generated using 1D and 2D integrated encoding holographic grating,respectively.The TC of these vortex beam arrays is detected using a focal hyperbolic lens.These quasi-PVBs with different TCs are promising to be coupled into single or multiple fixed-aperture fibers or waveguides for enabling single-ring or multi-ring fiber or waveguide communication based on THz vortex beams.