Orbital Angular Momentum Of Electromagnetic Waves And Its Applications

Since the 1990s,orbital angular momentum(OAM)of electromagnetic(EM)fields has attracted more and more attention.The EM waves carrying OAM have a helical phase front.One can generate OAM carrying beams with different modes by different phase shiftings.By using different phase shifting methods,different kinds of OAM antennas can be designed.Theoretically,OAM beams have an infinite number of modes and these OAM modes are orthogonal to each other.This dissertation focuses on the generation of OAM of EM waves and its applications.The main contributions of this work include:1.The angular momentum(AM)of EM fields is analyzed,which can be decomposed into the spin angular momentum(SAM)and the OAM.Note that the SAM is associated to the polarization of EM waves.The calculation methods of OAM modes are presented and used to analyze the OAM properties.Moreover,radiation patterns and OAM modes of OAM beams with multipath effects are investigated.Since the AM of EM fields can be used to represent the torque of the beams,the minimum of AM refers to the radiation center.Thus,the AM of EM fields can be used to calculate the radiation center of an antenna.The calculation results show good agreement with the actual position of the antenna.2.The theory of increasing channel capacity by OAM waves is investigated using the spatial diversity of OAM beams.Through the calculation of OAM modes,it is demonstrated that a 2-element antenna array cannot be used to generate OAM beams.According to the spatial diversity of OAM beams,a wireless communication system with 2-element antenna arrays as transmitter/receiver is designed,which can be used for spatial-division multiplexing/demultiplexing without signal processing.The proposed wireless link can be used to transmit two different digital data streams.It is shown that the channel crosstalk is less than-12 dB.3.The delay in space is proposed to generate OAM beams.A helicoidal array is designed to generate different OAM modes by using the delay in space.In addition,crossed array is proposed by using delay in space and delay in phase,which can be used to generate OAM beams in four directions.In our experiment,the helicoidal array and the crossed array with l=1 and l=2 are fabricated and measured.Simulated and measured return loss,radiation patterns and packet-lost-rates show that delay in space can be used to generate OAM beams.Delay in space is a new degree of freedom to generate OAM beams.4.A broadband OAM multiplexing is proposed.A broadband phase-shifting network is used to generate broadband OAM beams with different modes.A broadband dual-polarized dual-OAM-mode antenna is designed and fabricated,which can transmit OAM beams with l=1 and l=-1 on each of two polarizations from 2.1-2.7 GHz.A wireless link is proposed by using the broadband dual-polarized dual-OAM-mode antenna with four independent channels.The wireless link has a distance of 2.5 meters.The channel crosstalk is less than-13.3 dB.When the signal-to-noise ratio is 24 dB,a 4.8-Gbit/s data rate with bit-error-rate less than 3.5×10-3 can be obtained.The spectral efficiency of the proposed link is about 8 bit/s/Hz.5.An OAM mode reconfigurable antenna is proposed.The OAM mode,l=1 or l=-1,of the proposed antenna can be controlled by a remote controller.Simulated and measured return loss,near-field patterns,far-field patterns and OAM mode orthogonality show that the proposed antenna can be used to generate l=1 or l=-1 OAM beam.Moreover,by mapping l=1 to symbol 0,l=-1 to symbol 1,an OAM mode digital data encoding is realized by the proposed antenna.6.The rotational Doppler effect is experimentally demonstrated by spinning a polarization-independent metasurface which can be used to generate the OAM.From the reflection phase point of view,a propeller is equivalent to an OAM metasurface.A spinning propeller has a transversal rotational speed and a translational velocity which can generate rotational Doppler shift and conventional Doppler shift,respectively.We have demonstrated the feasibility of measuring the rotational speed of a propeller by using opposite values of OAM carrying beam illuminating on a spinning propeller.Through the comprehensive investitagtion of the OAM of EM waves,the theory and methods for its applications have been enriched,which establishes the foundation for engineering applications.