Research On Orbital Angular Momentum Excitation Devices Based On Frequency Selective Surface

The orbital angular momentum(OAM)has become one of the research hotspots in the microwave and radio frequency bands during recent years.When carrying the orbital angular momentum,the electromagnetic wave has a spiral phase wavefront,which can usually be referred to as a vortex beam.Compared to the spin angular momentum(SAM)of electromagnetic waves,SAM only relates to the polarization of electromagnetic waves,and therefore has only three state numbers.The OAM is the result of both the electromagnetic field intensity and phase spatial distribution.According to the gradient of the spiral phase,it can own infinite mode number(topological charge numbers).Due to the orthogonality between different modes,the information transmitted by each channel does not affect each other.Theoretically,in this way,an infinite number of vortex beams can be obtained under the same working frequency,thereby greatly enhance the capacity of the channel.The efficient excitation of OAM mode is the basis of its application in the microwave band.Traditional OAM excitation devices work with insurmountable thickness limits,and at lower frequencies,the component size will be too large,due to the longer wavelength which is not conducive to large-scale use.To solve this problem,in this thesis an X-band orbital angular momentum mode excitation device in developed based on frequency selective surface(FSS).First of all,the design methods of the vortex lenses in optical band and microwave section are analyzed,and then we propose to use the FSS unit to design the vortex lens.Starting from the working mechanism and the basic analysis method,the filtering characteristics of the FSS are analyzed in detail by taking the infinitely long strip grid as an example.At the same time,the necessity of miniaturization is analyzed from the point of the grating lobe and the stability of the incident angle.Secondly,the equivalent circuit method is adopted to analyze the coupling miniaturization frequency selection surface in detail.Through the second-order unit,the performance influencing factors were analyzed.The advantage of this unit compared to traditional FSS is its sub-wavelength size.The unit itself has been miniaturized and has very good angular stability for the incident angle.A fourth-order bandpass corresponding MEFSS unit is designed.The passband contains the X-band and the transmission coefficient in this range is higher than 0.9,and then the metalens generating vertex beam carrying OAM mode is designed accordingly.The lens is divided into eight regions.By adjusting the metal parameters,the cells in the adjacent regions have a phase shift of 45° at 10.6 GHz,and a co-polarized vortex lens with a topological charge number of 1 is achieved.Finally,the above-mentioned MEFSS unit is further improved.By introducing the P-B phase theory based on the rotation of the metal layer,the phase of the crosspolarized component in the transmission wave can be controlled.Two lenses generating vertex beam carrying OAM mode with a topological charge number of 1 and 2 were realized according to two different types of cells.In addition,according to the generalized Snell's law,a 30° deflection is achieved for vertex beam with topological charge of l=2.