Folding metasurface structure and method for generating diffraction-free Bessel beam

Abstract

The invention belongs to the technical field of radio frequency communication, and discloses a folded metasurface structure and method for generating a diffraction-free Bessel beam, the folded metasurface structure comprises a horn feed source, a transmission type metasurface and a reflection type metasurface, the horn feed source is located at the uppermost layer, the transmission type metasurface is located below the horn feed source, the reflection type metasurface is located at the lowermost layer, and the horn feed source is located below the transmission type metasurface. And an electrode disposed below the transmissive metasurface. The horn feed source is a standard 14 GHz rectangular feed source horn and radiates spherical waves, and the aperture size of the horn feed source is 28 * 35 mm. The transmission type metasurface is composed of four layers of F4BM-2 dielectric plates, and the height of each layer is 1.035 mm. Five layers of metal patches, namely four layers of snowflake-shaped metal patches and one metal patch etched with snowflake-shaped gaps, are laminated between the dielectric plates and are used for enhancing the transmission efficiency of electromagnetic waves. The reflection type metasurface unit is composed of an opening circular patch, and an opening circular ring and a Jerusalem type gap are etched in the middle of the reflection type metasurface unit and used for achieving 360-degree phase coverage.

Description

Technical Field

[0001] The present invention belongs to the field of radio frequency communication technology, and in particular relates to a folded metasurface structure and method for generating a diffraction-free Bessel beam. Background Art

[0002] In the field of radio frequency, F. Tamburini and others conducted a communication experiment of vortex electromagnetic waves in Venice, proving that orbital angular momentum is another degree of freedom independent of polarization, phase, and assignment, which can effectively improve communication capacity and efficiency. Scholars have conducted extensive research on this, using spiral phase plates, reflector antennas, uniform circular arrays, and metasurfaces to generate vortex electromagnetic waves, intending to use their inherent modal orthogonality to further improve communication capacity and meet the needs of current wireless communications. However, vortex electromagnetic waves have phase singularities, and their hollow characteristics limit their application in long-distance wireless communications, which requires further research.

[0003] The paper "N. Kou, S. Yu, and L. Li, Generation of high-order Besselvortex beam carrying orbital angular momentum using multilayer amplitude-phase modulated surfaces in radio frequency domain, Applied Physics Express, vol. 10, no. 1, p. 016701, Jan. 2017" uses multilayer transmission metasurfaces to simultaneously control phase and amplitude and generates 83 The undiffraction-free Bessel beam is generated. However, its amplitude and phase cannot be independently controlled, and since the horn feed needs to be placed at the optimal focal ratio, the antenna profile is 17.35 .

[0004] The paper “H Liu, H. Xue, Y. Liu, Q. Feng, and L. Li, Generation of high-order bessel orbital angular momentum vortex beam using a single-layer reflective metasurface, IEEE Access, vol. 8, pp. 126 504–126 510, 2020” achieved 36 MHz at 10 GHz using a reflective metasurface. Non-diffracting Bessel beam, but the profile is also 11.93 So big.

[0005] Reference "Y. Yang, Y. Zhu, W. Xie, L. Bu, Y. Zang, and X. Liu, "High-efficiency ultrathin metasurfaces with simultaneous control of completephase, amplitude, and polarization." Optics express, vol. 31 2, pp. 3134–3142, 2022" selected chiral metasurface units to achieve independent control of amplitude and phase, and the total cross-sectional height of a single-layer metasurface is only 4.6 , but the diffraction-free distance only reaches 37.4 .

[0006] Based on the above literature, in order to further utilize the new degrees of freedom provided by orbital angular momentum to increase communication capacity, the hollow problem must be urgently solved. At the same time, miniaturization and low profile should be taken into consideration.

[0007] Through the above analysis, the problems and defects of the existing technology are: it is impossible to independently control the two physical quantities of amplitude and phase at the same time, which reduces the efficiency of modulating electromagnetic waves and the purity of the generated mode; because the feed source is placed above the reflective metasurface, a blocking effect is caused, which brings errors to the amplitude and phase compensation; theoretically, an infinitely large antenna aperture is required to produce a diffraction-free Bessel beam, but the existing design has a high profile, which is not conducive to miniaturization, and the generated Bessel beam with orbital angular momentum is short. Summary of the invention

[0008] In view of the problems existing in the prior art, the present invention provides a folded metasurface structure and method for generating a diffraction-free Bessel beam.

[0009] The present invention is implemented as follows: a folded metasurface structure for generating a diffraction-free Bessel beam includes a horn feed, a transmissive metasurface and a reflective metasurface, wherein the horn feed is located at the top layer, the transmissive metasurface is located below the horn feed, and the reflective metasurface is located at the bottom layer and is arranged below the transmissive metasurface.

[0010] Furthermore, the horn feed is a standard 14 GHz rectangular feed horn, radiating spherical waves, and its aperture size is 28×35 mm.

[0011] Furthermore, the transmission metasurface is composed of four layers of F4BM-2 dielectric plates, each with a height of 1.035 mm. Five layers of metal patches are laminated between the dielectric plates, including four layers of snowflake-shaped metal patches and one metal patch etched with snowflake-shaped gaps. The transmission unit amplitude covers 0-1 amplitude and is insensitive to the incident angle, which is used to improve the transmission efficiency of electromagnetic waves.

[0012] Furthermore, the reflective metasurface unit consists of an open circular patch with an open ring and Jerusalem-type gap etched in the middle, which is insensitive to the incident angle and is used to achieve 360° phase coverage.

[0013] Furthermore, according to the principle of ray tracing, the electromagnetic wave is reflected once in the transmission metasurface and the reflection metasurface, reducing the original cross-section to half the height. The spherical electromagnetic wave emitted from the feed source adjusts the amplitude required by the Bessel beam through the lower reflection metasurface, and then performs phase compensation through the upper transmission metasurface to convert the spherical wave into a plane wave. The vortex phase required for orbital angular momentum is added to finally emit a diffraction-free Bessel beam.

[0014] Furthermore, each unit of the metasurface used for amplitude modulation satisfies: , in, is the amplitude of the modulated metasurface unit, It is The location of the unit, is the electric field amplitude of the speaker feed, is the nth-order Bessel function, is the transverse wave constant.

[0015] Furthermore, the metasurface used for phase modulation mainly needs to compensate for three items: , in, is the modulated metasurface phase, is the compensating phase required to convert a spherical wave into a plane wave, is the phase required to generate OAM is the phase required to generate the Bessel beam: , , , in, is the compensation phase required to convert a spherical wave into a plane wave in a rectangular coordinate system; x and y represent the coordinates in the rectangular coordinate system; is the wave vector in free space; is the x-coordinate of the (i, j)th hypersurface unit; is the x-coordinate value of the feed source; is the y coordinate of the (i, j)th hypersurface unit; is the y coordinate value of the feed source; is the z coordinate of the coordinate center; is the z coordinate value of the feed source; is the distance between the metasurface unit and the feed source; is the mode number of orbital angular momentum; is the wavelength of the center frequency; is the base angle of the equivalent axicon; is the focal length of the metasurface.

[0016] The present invention also provides a method for generating a diffraction-free Bessel beam with a folded metasurface structure, characterized in that a horn feed, a transmission-type metasurface and a reflection-type metasurface utilize folded metasurface technology to generate a diffraction-free Bessel beam, the horn feed generates a primary electromagnetic wave, the transmission-type metasurface performs preliminary modulation on the electromagnetic wave, the reflection-type metasurface further modulates the electromagnetic wave and completes the formation of the Bessel beam, wherein the horn feed is located at the top layer, the transmission-type metasurface is located below the horn feed, and the reflection-type metasurface is located at the bottom layer and is arranged below the transmission-type metasurface.

[0017] Furthermore, a standard 14 GHz rectangular feed horn is used as the horn feed to radiate spherical waves, and its aperture size is 28×35 mm.

[0018] Furthermore, the transmission metasurface is composed of four layers of F4BM-2 dielectric plates, each with a height of 1.035 mm. Five layers of metal patches are laminated between the dielectric plates to improve the transmission efficiency of electromagnetic waves.

[0019] Furthermore, the reflective metasurface is composed of open circular patches with an open ring and Jerusalem-type gap etched in the middle to achieve 360° phase coverage.

[0020] Furthermore, according to the principle of ray tracing, the electromagnetic wave is reflected once in the two layers of metasurfaces, the transmission metasurface and the reflection metasurface, reducing the original cross-section to half the height. The spherical electromagnetic wave emitted from the feed source adjusts the amplitude required by the Bessel beam through the lower reflective metasurface, and then performs phase compensation through the upper transmission metasurface, converting the spherical wave into a plane wave, and adding the vortex phase required for orbital angular momentum, finally emitting a non-diffraction Bessel beam.

[0021] First, in view of the technical problems existing in the above-mentioned prior art, some creative technical effects are brought about after solving the problems. The specific description is as follows: 1. The present invention generates 110 Long-distance non-diffracting Bessel beam with an overall profile of only 3.25 , which to some extent solves the problem of vortex electromagnetic wave divergence angle in wireless communications.

[0022] 2. The present invention uses the principle of ray tracing. After the electromagnetic wave is emitted from the horn feed source, it is reflected by the lower metasurface and then transmitted from the upper transmissive metasurface. Two metasurfaces with different functions form a folded antenna by independently adjusting the phase and amplitude, thereby achieving the purpose of reducing the profile.

[0023] 3. The reflective metasurface unit can cover the amplitude of 0-1, and when modulating the amplitude, the phase is relatively stable, with fluctuations not exceeding 10°. The unit characteristics are relatively stable at 12-15GHz and are insensitive to the incident angle of 50°.

[0024] 4. The transmission metasurface unit can cover 360° phase and be stable above 0.8 amplitude. The unit characteristics are stable at 13-16GHz and are insensitive to 30° incident angle.

[0025] 5. The antenna of the present invention has a simple structure, has no complicated feeding structure, is manufactured using PCB technology, and is economical and applicable.

[0026] Second, considering the technical solution as a whole or from the perspective of the product, the technical effects and advantages of the technical solution to be protected by the present invention are described in detail as follows: 1. The upper transmission metasurface of the present invention is composed of four layers of F4BM-2 dielectric plates and five layers of metal patches. The two types of metal patches are snowflake-shaped and square metal patches etched with snowflake gaps. The amplitude of the transmission unit covers 0-1 and is insensitive to the incident angle.

[0027] 2. The lower reflective metasurface unit of the present invention is composed of an open circular patch with an open circular ring and a Jerusalem-type slit etched in the middle, with a phase coverage of 360° and insensitive to the incident angle.

[0028] 3. In order to avoid the blocking effect caused by the feed source, the present invention digs out a 28×35mm rectangle in the center of the upper transmission metasurface to place the horn feed source, while also improving the degree of integration and reducing the overall antenna profile.

[0029] 4. The metasurface antenna of the present invention can also be used in other frequency bands.

[0030] Third, the technical solution of the present invention uses two metasurfaces to achieve simultaneous and independent control of the modulation of the amplitude and phase of electromagnetic waves. The overall cross-section of the folded metasurface structure antenna designed according to the principle of ray tracing is only 3.25 , which meets the design requirements of antenna miniaturization and is conducive to antenna integration. The technical solution of the present invention is to effectively improve the phase singularity and divergence problems inherent in the vortex electromagnetic wave, generate 110 Long-range non-diffraction Bessel beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 is a schematic diagram of a folded metasurface structure for generating a non-diffraction Bessel beam provided by an embodiment of the present invention; Figure 2 is a schematic diagram of a transmissive metasurface provided by an embodiment of the present invention; Figure 3 Schematic diagram of a reflective metasurface and a unit provided in an embodiment of the present invention; Figure 4 is a schematic diagram of the phase and amplitude characteristics of the transmission type metasurface unit provided by an embodiment of the present invention. At 14 GHz, as the length of the metal patch s increases from 1 mm to 4.4 mm, a complete 360° phase shift can be covered, and the transmission amplitude is maintained at about 0.88; Figure 5 Schematic diagram of the phase and amplitude characteristics of the reflective metasurface unit provided by an embodiment of the present invention. At 14 GHz, as the rotation angle of the metal patch increases from 0° to 45°, the full 0-1 amplitude can be covered, and the reflection phase is maintained at ±10°; Figure 6 It is a graph of the transverse electric field, electric field phase, assignment and mode purity results of the metasurface emitting a diffraction-free Bessel beam provided by an embodiment of the present invention at propagation distances z=800, 1000, and 2000 mm. Figure 6 (bd) Its amplitude distribution is donut-shaped with a hole in the center. Figure 6 The phase distribution in (eg) is spiral-shaped. Figure 6 The mode purity of the orbital angular momentum of (hj) mode 2 at 800, 1000, and 2000 mm is 0.84, 0.81, and 0.8, respectively. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0034] Example 1: Laboratory-level Bessel beamforming device 1) Speaker feed configuration: A standard 14 GHz rectangular horn feed with an aperture size of 28 × 35 mm is used to generate the primary spherical wave, providing the source wave for Bessel beam forming.

[0035] 2) Transmissive metasurface design: A transmissive metasurface was constructed, consisting of four layers of F4BM-2 dielectric plates, with each layer having a height of 1.035 mm.

[0036] A layer of snowflake-shaped metal patch is pressed between each dielectric plate, and an additional metal patch etched with snowflake-shaped gaps is added to the top layer, for a total of five layers of metal patches to enhance the efficiency of electromagnetic wave transmission.

[0037] 3) Reflective metasurface construction: A reflective metasurface is designed, in which each unit consists of an open circular patch with an open ring and a Jerusalem-type gap etched in the middle to achieve 360° phase coverage to adjust the phase of the reflected wave.

[0038] 4) Beam forming and detection: After the electromagnetic wave is emitted from the horn feed source, it undergoes preliminary phase and amplitude modulation by the transmissive metasurface, and then is further phase-adjusted and reflected by the reflective metasurface.

[0039] Appropriate detection equipment, such as electromagnetic wave field intensity detectors, are set up to monitor and verify the non-diffraction characteristics and stability of the Bessel beam at different locations.

[0040] Example 2: Bessel beam application in outdoor long-distance communication system 1) Speaker feed customization: According to the frequency band requirements of the outdoor communication system, a high-power rectangular horn feed of the corresponding frequency (such as 14GHz) is customized, and the aperture size is adjusted according to the actual application environment.

[0041] 2) Transmission metasurface adjustment: In view of the special needs of outdoor environments, the material and thickness of the dielectric plate of the transmission-type metasurface need to be optimized according to environmental factors (such as temperature and humidity) to ensure maximum electromagnetic wave transmission efficiency.

[0042] 3) Reflective metasurface optimization: The design of the reflective metasurface unit is optimized according to the communication distance and environmental conditions to ensure that the phase of the electromagnetic wave can be effectively adjusted to form a stable Bessel beam.

[0043] 4) System integration and testing: The horn feed, transmission type and reflection type metasurfaces are integrated into the outdoor long-distance communication system. Through field testing, the metasurface parameters are adjusted to ensure the optimal transmission performance of the Bessel beam.

[0044] A receiving device is set up at the communication terminal to test the coverage and signal stability of the Bessel beam to ensure that the communication quality meets the requirements.

[0045] The two embodiments provided in the present invention respectively demonstrate how to implement and utilize the generation of Bessel beams in a laboratory environment and in practical applications, covering the entire process from design, construction to testing, and providing a specific implementation solution.

[0046] In view of the problems existing in the prior art, the present invention provides a folded metasurface structure for generating a diffraction-free Bessel beam. The present invention is described in detail below in conjunction with the accompanying drawings.

[0047] The folded metasurface structure for generating a diffraction-free Bessel beam designed in an embodiment of the present invention is composed of a horn feed, a transmission metasurface, and a reflection metasurface from top to bottom. Figure 1 shown.

[0048] The horn feed is a standard 14GHz rectangular feed horn that radiates spherical waves, and its aperture size is 28×35mm.

[0049] The transmission metasurface is composed of four layers of F4BM-2 dielectric plates, each with a height of 1.035 mm. Five layers of metal patches are laminated between the dielectric plates. Figure 2 As shown, there are four layers of snowflake-shaped metal patches and a metal patch etched with snowflake-shaped gaps, which are used to enhance the transmission efficiency of electromagnetic waves.

[0050] The reflective metasurface unit consists of an open circular patch, such as Figure 3 As shown, an open ring and Jerusalem-type gap are etched in the middle to achieve 360° phase coverage.

[0051] According to the principle of ray tracing, the electromagnetic wave is reflected once in the two layers of transmission and reflection metasurfaces, reducing the original cross-section to half the height. The spherical electromagnetic wave emitted from the feed source is adjusted to the amplitude required by the Bessel beam through the lower reflective metasurface. Then, the upper transmission metasurface performs phase compensation, converts the spherical wave into a plane wave, and adds the vortex phase required for orbital angular momentum, finally emitting a non-diffraction Bessel beam.

[0052] Each unit of the metasurface used for amplitude modulation should satisfy: (1), in, is the amplitude that the metasurface unit satisfies, It is The location of the unit, is the electric field amplitude of the speaker feed, is the nth-order Bessel function, is the transverse wave constant.

[0053] The metasurface used to modulate phase mainly needs to compensate for three items: , in, is the compensating phase required to convert a spherical wave into a plane wave, is the phase required to generate OAM is the phase required to generate the Bessel beam, as shown in equations (3)-(5): (3), (4), (5), in, is the compensation phase required to convert a spherical wave into a plane wave in a rectangular coordinate system; x and y represent the coordinates in the rectangular coordinate system; is the wave vector in free space; is the x-coordinate of the (i, j)th hypersurface unit; is the x-coordinate value of the feed source; is the y coordinate of the (i, j)th hypersurface unit; is the y coordinate value of the feed source; is the z coordinate of the coordinate center; is the z coordinate value of the feed source; is the distance between the metasurface unit and the feed source; is the mode number of orbital angular momentum; is the wavelength of the center frequency; is the base angle of the equivalent axicon; is the focal length of the metasurface.

[0054] Here is the detailed working principle of the method of folded metasurface structure to generate non-diffraction Bessel beams: The structure uses folded metasurface technology to generate non-diffraction Bessel beams, which mainly include three parts: horn feed, transmission metasurface and reflection metasurface. The horn feed generates primary electromagnetic waves, the transmission metasurface performs preliminary modulation on the electromagnetic waves, and the reflection metasurface further modulates the electromagnetic waves and completes the formation of the Bessel beam.

[0055] Horn feed: As the electromagnetic wave transmitter of the system, the horn feed emits a spherical wave with a center frequency of 14GHz. Its rectangular aperture size is 28×35mm, which ensures the directionality and stability of the beam emission.

[0056] Transmissive metasurfaces: The transmission metasurface is composed of four layers of F4BM-2 dielectric plates, each with a thickness of 1.035 mm. Snowflake-shaped metal patches are sandwiched between the dielectric plates to enhance the transmittance of electromagnetic waves and perform preliminary amplitude modulation. The design of the transmission metasurface aims to optimize the phase and amplitude of electromagnetic waves when they pass through, creating conditions for generating Bessel beams.

[0057] Reflective metasurface: The unit structure of the reflective metasurface is designed to further modulate the electromagnetic waves after passing through the transmissive metasurface, including open circular patches, open rings, and Jerusalem-type gaps to achieve all-round phase coverage. This design enables the electromagnetic waves to obtain all the required phase modulation when reflected on this layer of metasurface, including the phase required to convert spherical waves into plane waves, add orbital angular momentum, and form Bessel beams.

[0058] Bessel beam formation: According to the principle of ray tracing, the electromagnetic wave is reflected once in the two layers of the transmission and reflection metasurfaces, so that the height of the original cross-section is reduced to half. The spherical wave emitted by the horn feed source is first amplitude modulated by the reflective metasurface to form the amplitude distribution required for the Bessel beam. Then, phase compensation is performed by the transmission metasurface, including plane wave conversion, orbital angular momentum addition, and phase modulation required for Bessel beam formation. Finally, the system emits a non-diffraction Bessel beam with a specific orbital angular momentum.

[0059] Amplitude and phase modulation: Amplitude modulation satisfies a specific mathematical relationship to ensure that the electric field amplitude corresponding to each metasurface unit is related to its position, the electric field amplitude of the horn feed source, the Bessel function and the transverse wave constant. Phase modulation includes three main parts: the compensation phase for the conversion from spherical wave to plane wave, the vortex phase required to generate orbital angular momentum, and the specific phase modulation required to form the Bessel beam.

[0060] like Figure 4 As shown, the phase and amplitude characteristics of the transmission metasurface unit at 14 GHz can cover a full 360° phase shift as the length of the metal patch s increases from 1 mm to 4.4 mm, and the transmission amplitude remains at around 0.88.

[0061] like Figure 5 As shown, the phase and amplitude characteristics of the reflective metasurface unit can cover the full 0-1 amplitude at 14 GHz as the rotation angle of the metal patch increases from 0° to 45°, and the reflection phase remains at ±10°.

[0062] like Figure 6 As shown, the transverse electric field, electric field phase, assignment and mode purity results of the diffraction-free Bessel beam emitted by the metasurface at propagation distances z=800, 1000, and 2000 mm. Figure 6 The amplitude distribution of the middle bd is a donut shape with a hole in the center. Figure 6 The phase distribution in eg is spiral. Figure 6 The mode purity of the orbital angular momentum of hj mode 2 at 800, 1000, and 2000 mm is 0.84, 0.81, and 0.8, respectively.

[0063] In the description of the present invention, unless otherwise specified, "plurality" means two or more than two; the orientations or positional relationships indicated by the terms "upper", "lower", "left", "right", "inner", "outer", "front end", "rear end", "head", "tail", etc. are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", "third", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0064] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modification, equivalent substitution and improvement made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A folded metasurface structure for generating a non-diffracting Bessel beam, characterized in that: It includes a horn feed, a transmissive metasurface and a reflective metasurface. The horn feed is located at the uppermost layer, the transmissive metasurface is located below the horn feed, and the reflective metasurface is located at the lowermost layer and is arranged below the transmissive metasurface.

2. The folded metasurface structure for generating a non-diffraction Bessel beam according to claim 1, characterized in that: The horn feed is a standard 14GHz rectangular feed horn that radiates spherical waves, and its aperture size is 28×35mm.

3. The folded metasurface structure for generating a non-diffraction Bessel beam according to claim 1, characterized in that: The transmission-type metasurface is composed of four layers of F4BM-2 dielectric plates, each with a height of 1.035 mm. Five layers of metal patches are laminated between the dielectric plates, including four layers of snowflake-shaped metal patches and one metal patch etched with snowflake-shaped gaps, which are used to improve the transmission efficiency of electromagnetic waves.

4. The folded metasurface structure for generating a non-diffraction Bessel beam according to claim 1, characterized in that: The reflective metasurface unit consists of an open circular patch with an open ring and Jerusalem-type gap etched in the middle to achieve 360° phase coverage.

5. The folded metasurface structure for generating non-diffraction Bessel beams according to claim 1, characterized in that: According to the principle of ray tracing, the electromagnetic wave is reflected once in the two layers of metasurfaces, the transmission metasurface and the reflection metasurface, reducing the original cross-section to half the height. The spherical electromagnetic wave emitted from the feed source adjusts the amplitude required by the Bessel beam through the lower reflective metasurface, and then performs phase compensation through the upper metasurface to convert the spherical wave into a plane wave. The vortex phase required for the orbital angular momentum is added to finally emit a diffraction-free Bessel beam.

6. The folded metasurface structure for generating non-diffraction Bessel beams according to claim 1, characterized in that: Each unit of the metasurface used for amplitude modulation satisfies: ， in, is the amplitude that each unit satisfies, It is The location of the unit, is the electric field amplitude of the speaker feed, is the nth-order Bessel function, is the transverse wave constant.

7. The folded metasurface structure for generating non-diffraction Bessel beams according to claim 1, characterized in that: The metasurface used to modulate phase mainly needs to compensate for three items: , in, is the modulated metasurface phase is the compensating phase required to convert a spherical wave into a plane wave, is the phase required to generate OAM is the phase required to generate the Bessel beam, is the ijth: , , , in, is the compensation phase required to convert a spherical wave into a plane wave in a rectangular coordinate system; x and y represent the coordinates in the rectangular coordinate system; is the wave vector in free space; is the x-coordinate of the (i, j)th hypersurface unit; is the x-coordinate value of the feed source; is the y coordinate of the (i, j)th hypersurface unit; is the y coordinate value of the feed source; is the z coordinate of the coordinate center; is the z coordinate value of the feed source; is the distance between the metasurface unit and the feed source; is the mode number of orbital angular momentum; is the wavelength of the center frequency; is the base angle of the equivalent axicon; is the focal length of the metasurface, It is The location of the unit.

8. A method for generating a folded metasurface structure with a non-diffraction Bessel beam, characterized in that: The horn feed, transmission metasurface and reflective metasurface use folded metasurface technology to generate diffraction-free Bessel beams. The horn feed generates primary electromagnetic waves, the transmission metasurface performs preliminary modulation on the electromagnetic waves, and the reflective metasurface further modulates the electromagnetic waves and completes the formation of the Bessel beam. The horn feed is located at the top layer, the transmission metasurface is located below the horn feed, and the reflective metasurface is located at the bottom layer and is arranged below the transmission metasurface.

9. The method for generating a folded metasurface structure with a non-diffraction Bessel beam as claimed in claim 8, characterized in that: The transmissive metasurface is made up of four layers of F4BM-2 dielectric plates, each with a height of 1.035 mm. Five layers of metal patches are laminated between the dielectric plates to enhance the transmission efficiency of electromagnetic waves.

10. The method for generating a folded metasurface structure with a non-diffraction Bessel beam according to claim 8, characterized in that: The reflective metasurface is composed of an open circular patch with an open ring and Jerusalem-type gap etched in the middle to achieve 360° phase coverage; According to the principle of ray tracing, the electromagnetic wave is reflected once in the two layers of metasurfaces, the transmission metasurface and the reflection metasurface, reducing the original cross-section to half the height. The spherical electromagnetic wave emitted from the feed source adjusts the amplitude required by the Bessel beam through the lower reflective metasurface, and then performs phase compensation through the upper metasurface to convert the spherical wave into a plane wave. The vortex phase required for the orbital angular momentum is added to finally emit a diffraction-free Bessel beam.

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