Microwave imaging system based on liquid crystal programmable metasurface

Abstract

The invention discloses a microwave imaging system based on a liquid crystal programmable metasurface, and belongs to the technical field of microwave imaging. The system comprises a transmitting end, an imaging target and a receiving end, the transmitting end comprises a feed source, a liquid crystal programmable metasurface and a bias voltage control module, and the receiving end comprises a receiving antenna and a data processing module. A microwave signal emitted by the feed source is modulated by the liquid crystal programmable metasurface to generate a plurality of random wave beams in a space, namely a low-correlation measurement mode, an imaging target is irradiated, the signal is transmitted to the receiving antenna after being scattered by the imaging target, and the data processing module obtains a scattering coefficient of an imaging area through echo signal inversion. Therefore, target reconstruction is realized, and the problems of high cost, complex structure, low efficiency and the like of a traditional microwave imaging system are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave imaging, and in particular relates to a microwave imaging system based on a liquid crystal programmable metasurface. Background Art

[0002] With the rapid development of wireless communication and sensor technology, microwave imaging systems have shown broad application prospects in military reconnaissance, security inspection, medical diagnosis and other fields. Traditional microwave imaging systems mostly rely on mechanical scanning or phased array technology to achieve directional scanning of the beam, but these methods often have limitations such as large size, high cost and low efficiency.

[0003] With the emergence of new technologies such as computational imaging and metasurfaces, microwave imaging technology has been further promoted. Computational imaging detects targets through different radiation signals and achieves perception through indirect inversion calculation, which can greatly reduce the hardware complexity and cost of the imaging system. The flexible modulation capability of metasurfaces for electromagnetic waves provides a flexible hardware foundation for designing various measurement modes in computational imaging. Existing microwave metasurface imaging systems are mainly based on two types of metasurfaces, namely, frequency selective metasurfaces composed of frequency-agile metasurface units and programmable metasurfaces composed of metasurface units with controllable phase-frequency response. The former requires a wide frequency band working characteristic to generate enough effective measurement modes, which is difficult to design and has low spectrum utilization; the latter mostly uses integrated PIN diodes, varactor diodes and other semiconductor switches to achieve encoding, which has limitations such as few switchable states, large high-frequency loss, and high cost, making it difficult to achieve low-correlation measurement modes and high-precision microwave imaging.

[0004] A Chinese patent application with application number 202310057727.2 and invention name “A microwave imaging system and imaging method based on a folded feed active transmission array” discloses a microwave imaging system and algorithm, which uses PIN diodes to implement a 1-bit programmable transmission array to generate random beams to detect the imaging area. It has fewer coding states, a complex system structure, and high costs.

[0005] The prior art “A K-band, varactor-tuned dynamic metasurface aperture for microwave computational imaging” discloses a one-dimensional metasurface for microwave imaging, which uses varactor diodes to achieve grayscale modulation. Compared with the binary metasurface using PIN diodes, it can improve the diversity of measurement modes, but requires mechanical scanning to achieve two-dimensional and three-dimensional imaging, which affects the imaging efficiency. Summary of the invention

[0006] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a microwave imaging system and analytical calculation method based on liquid crystal programmable metasurface, which adopts liquid crystal multi-bit programmable metasurface, combines compressed sensing method, and utilizes electrically adjustable liquid crystal material to solve the problems of high cost, complex structure and low efficiency of traditional microwave imaging system.

[0007] The technical problem proposed by the present invention is solved in this way:

[0008] A microwave imaging system based on a liquid crystal programmable metasurface comprises a transmitting end, an imaging target 4 and a receiving end; the transmitting end comprises a feed source 1, a liquid crystal programmable metasurface 2 and a bias voltage control module 3, and the receiving end comprises a receiving antenna 5 and a data processing module 6;

[0009] The microwave signal radiated by the feed source 1 reaches the liquid crystal programmable metasurface 2, which modulates and reflects the microwave signal to generate a variety of random beams in space, forming a low-correlation measurement mode; the modulated reflected random beam illuminates the imaging target 4, and the imaging target 4 scatters the random beam to generate an echo signal; the receiving antenna 5 receives the echo signal and sends it to the data processing module 6; the data processing module 6 inverts the echo signal to obtain the scattering coefficient of the imaging area where the imaging target 4 is located, thereby realizing the reconstruction of the imaging target 4.

[0010] Furthermore, the liquid crystal programmable metasurface 2 includes M×N metasurface units arranged in a two-dimensional periodic pattern, where M and N are both positive integers greater than or equal to 2; the liquid crystal programmable metasurface 2 includes, from top to bottom, a first metal layer 21, a first dielectric layer 22, a second metal layer 23, a liquid crystal layer 24, a third metal layer 25, a second dielectric layer 26, and a fourth metal layer 27;

[0011] The first metal layer 21 is located on the upper surface of the first dielectric layer 22, and is composed of M×N metal patches 211 arranged in a two-dimensional periodic manner; the second metal layer 23 is etched with M×N rectangular slots 231 arranged in a two-dimensional periodic manner on the basis of fully covering the lower surface of the first dielectric layer 22, and the long side direction of the rectangular slot 231, the polarization direction of the feed source 1 and the polarization direction of the receiving antenna 5 are consistent; the center positions of the metal patches 211 and the rectangular slots 231 correspond one to one;

[0012] The first dielectric layer 22 and the second dielectric layer 26 are parallel to each other and spaced apart; the liquid crystal layer 24 is filled between the lower surface of the first dielectric layer 22 and the upper surface of the second dielectric layer 26, and a sealing frame is provided around the edges of the liquid crystal layer 24. The sealing frame, the first dielectric layer 21 and the second dielectric layer 26 are bonded together to form a sealed liquid crystal box to prevent the liquid crystal layer 24 from leaking;

[0013] The third metal layer 25 is located on the upper surface of the second dielectric layer 26 and is composed of M×N two-dimensionally periodically arranged bent metal delay lines 251 ; the fourth metal layer 27 serves as a metal floor and fully covers the lower surface of the second dielectric layer 26 .

[0014] Furthermore, the bias voltage control module 3 includes an FPGA control circuit, a soft strip line, M×N pins and M×N bias lines. The FPGA control circuit is connected to each pin through a soft strip line, and is respectively connected to each bent metal delay line 251 in the third metal layer 25 through the corresponding bias line; the FPGA control circuit outputs a voltage to each pin through a wave control plate, and then applies it to the corresponding bent metal delay line 251, controls the bias voltage of the liquid crystal in the corresponding area, deflects the liquid crystal molecules, realizes the regulation of the dielectric constant of the liquid crystal, and then controls the reflection phase encoding state of each metasurface unit.

[0015] Furthermore, a 3-bit encoding method is used to quantize the reflection phase of the metasurface unit into 8 states with an interval of 45°.

[0016] Furthermore, the liquid crystal programmable metasurface 2 and the imaging target 4 are placed relative to each other, and each metasurface unit in the liquid crystal programmable metasurface 2 presents a certain coding state in a random manner; the feed source 1 and the receiving antenna 5 use corner cone horn antennas, which are placed offset relative to the liquid crystal programmable metasurface 2 and the imaging target 4, respectively.

[0017] Utilizing the above-mentioned microwave imaging system based on liquid crystal programmable metasurface, the present invention also provides a microwave imaging analytical calculation method based on liquid crystal programmable metasurface, comprising the following steps:

[0018] Step 1. Calculate the incident electric field from the feed source to the liquid crystal programmable metasurface

[0019] The incident electric field fed to the center of the liquid crystal metasurface unit

[0020]

[0021] Among them, r, θ and They represent the distance, pitch angle and azimuth angle of each liquid crystal metasurface unit center when the feed equivalent phase center is taken as the origin, j is the imaginary unit, k is the wave number, λ is the working wavelength, and are the cosine function power exponents of the E-plane and H-plane radiation patterns of the feed, respectively; represents the unit vector in the direction of the pitch angle θ, The azimuth is The unit vector of the direction;

[0022] Decompose the incident electric field at the center of the metasurface unit into tangential electric fields in the x-direction and the y-direction and

[0023] Step 2. Calculate the electric field reflected by the liquid crystal programmable metasurface modulated to the imaging area where the imaging target is located

[0024] The full-wave simulation software is used to model and simulate the various coding states of the metasurface unit. Using periodic boundary conditions, the number of Floquet port modes is set to 2 to obtain a pair of orthogonal TE and TM excitation modes, and the reflection coefficient Γ of different coding states of the metasurface unit is extracted. ε ;

[0025] Calculate the tangential components of the electric field reflected by the metasurface unit in the x and y directions and

[0026]

[0027] Perform inverse Fourier transform on the tangential components of the reflected electric field in the x and y directions of the metasurface unit to obtain the spectral function F in the x and y directions. x and F y :

[0028]

[0029] Among them, x and y represent the independent variables in the x-direction and y-direction respectively;

[0030] Calculation of the electric field reflected from a liquid crystal programmable metasurface to the imaging area

[0031]

[0032] Step 3. Calculate the electric field scattered by the imaging target to the receiving antenna

[0033] Calculate the electric field scattered by the imaging target to the receiving antenna

[0034]

[0035] in, is the receiving antenna radius vector, is the normal vector of the imaging plane, is the scattered wave vector, is the imaging target radius vector, f is the scattering coefficient of the imaging target, and dσ represents the differential component of the target imaging area;

[0036] Step 4. Obtain the measurement signal

[0037] The measured signal at the receiving antenna consists of the scattered signal and the noise signal:

[0038]

[0039] Where n is additive Gaussian noise;

[0040] Step 5. Calculate the electric field incident on the imaging area by the receiving antenna

[0041] Calculate the electric field incident on the imaging area by the receiving antenna

[0042]

[0043] Among them, r′, θ′ and They represent the distance, elevation and azimuth of each imaging grid center when the equivalent phase center of the receiving antenna is taken as the origin. and are the cosine function power exponents of the E-plane and H-plane radiation patterns of the receiving antenna respectively; represents the unit vector in the direction of the pitch angle θ′, The azimuth is The unit vector of the direction;

[0044] Step 6. Obtain the measurement matrix

[0045] The product of the liquid crystal programmable metasurface and the electric field radiated by the receiving antenna in the imaging area calculated in step 2 and step 5 is used as the measurement matrix H I×J Elements:

[0046]

[0047] Among them, H ij Denotes the measurement matrix H I×J The element in the i-th row and j-th column in the image is: i is the index of the measurement mode of the liquid crystal programmable metasurface, 1≤i≤I, I is the total number of measurement modes; j is the index of the imaging grid in the imaging area, 1≤j≤J, J is the total number of imaging grids in the imaging area; and They represent the electric fields of the liquid crystal programmable metasurface and the receiving antenna incident on the j-th imaging grid in the imaging area in the i-th measurement mode;

[0048] Step 7. Construct the imaging equation and solve the scattering coefficient distribution of the imaging area

[0049] The imaging process is simplified into a linear model, which can be expressed as:

[0050] g I×1 =H I×J ×f J×1 +n I×1

[0051] Among them, g I×1is the measurement signal vector obtained by the receiving antenna, and its i-th element g i represents the measurement signal of the liquid crystal programmable metasurface at the receiving antenna in the i-th measurement mode; H I×J is the measurement matrix composed of different measurement modes; f J×1 is the scattering coefficient distribution of the imaging area, and its jth element f is the unknown quantity to be solved. j represents the scattering coefficient of the jth imaging grid in the imaging area; n I×1 is an additive noise vector, whose i-th element n i represents the additive Gaussian noise of the liquid crystal programmable metasurface in the i-th measurement mode;

[0052] Solving the above equation, we can obtain the scattering coefficient distribution of the imaging area.

[0053] Furthermore, when the number of measurement modes I is less than the number of imaging grids J, that is, the number of equations is less than the number of unknown quantities to be solved, the compressed sensing method is used to solve the scattering coefficient distribution of the imaging area.

[0054] The beneficial effects of the present invention are:

[0055] Compared with the microwave imaging system in the prior art, the microwave imaging system based on liquid crystal programmable metasurface described in the present invention uses liquid crystal programmable metasurface to realize more coding states and generate measurement modes with lower correlation, and operates in the high-frequency microwave millimeter wave frequency band, which is conducive to achieving high-precision and high-resolution imaging, and has the advantages of low cost, easy control, and saving spectrum resources; the analytical calculation method corresponding to the imaging system described in the present invention is combined with the compressed sensing algorithm, which saves the time and computing power consumed by full-wave simulation of the imaging system, and can efficiently verify the imaging effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 A schematic diagram of the structure of the imaging system of the present invention;

[0057] Figure 2 A schematic diagram of the three-dimensional structure of a liquid crystal programmable metasurface in the imaging system of the present invention;

[0058] Figure 3 Schematic diagram of the simulation results of the reflection coefficient of different coding states of the liquid crystal programmable metasurface unit at 30 GHz in the imaging system described in the embodiment;

[0059] Figure 4 Different coding sequences and corresponding normalized directional patterns of the liquid crystal programmable metasurface in the imaging system described in the embodiment, wherein (a) is a schematic diagram of the first coding sequence, (b) is a normalized directional pattern corresponding to the first coding sequence, (c) is a schematic diagram of the second coding sequence, and (d) is a normalized directional pattern corresponding to the second coding sequence;

[0060] Figure 5 It is a flowchart of the imaging analysis calculation method corresponding to the imaging system of the present invention;

[0061] Figure 6 It is a simulation reconstruction result diagram corresponding to the imaging system described in the embodiment, wherein (a) is a plurality of discrete point targets, (b) is a heart-shaped target, (c) is a “UESTC” letter target, (d) is the imaging result distribution of a plurality of discrete point targets, (e) is the imaging result distribution of a heart-shaped target, and (f) is the imaging result distribution of a “UESTC” letter target. DETAILED DESCRIPTION

[0062] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0063] This embodiment provides a microwave imaging system based on a liquid crystal programmable metasurface, and its structural composition framework schematic diagram is shown in FIG. Figure 1 As shown, it includes a transmitting end, an imaging target 4 and a receiving end; the transmitting end includes a feed source 1, a liquid crystal programmable metasurface 2 and a bias voltage control module 3, and the receiving end includes a receiving antenna 5 and a data processing module 6.

[0064] The microwave signal radiated by the feed source 1 reaches the liquid crystal programmable metasurface 2, which modulates and reflects the microwave signal to generate a variety of random beams in space, forming a low-correlation measurement mode; the modulated reflected random beam illuminates the imaging target 4, and the imaging target 4 scatters the random beam to generate an echo signal; the receiving antenna 5 receives the echo signal and sends it to the data processing module 6; the data processing module 6 inverts the echo signal to obtain the scattering coefficient of the imaging area where the imaging target 4 is located, thereby realizing the reconstruction of the imaging target 4.

[0065] Schematic diagram of the three-dimensional structure of the liquid crystal programmable metasurface 2 Figure 2 As shown, it includes M×N metasurface units arranged in a two-dimensional periodic pattern, where M and N are both positive integers greater than or equal to 2; the liquid crystal programmable metasurface 2 includes, from top to bottom, a first metal layer 21, a first dielectric layer 22, a second metal layer 23, a liquid crystal layer 24, a third metal layer 25, a second dielectric layer 26 and a fourth metal layer 27.

[0066] The first metal layer 21 is located on the upper surface of the first dielectric layer 22, and is composed of M×N metal patches 211 arranged in a two-dimensional periodic pattern. The second metal layer 23 is etched with M×N rectangular slots 231 arranged in a two-dimensional periodic pattern on the basis of fully covering the lower surface of the first dielectric layer 22. The long side direction of the rectangular slot 231, the polarization direction of the feed source 1 and the polarization direction of the receiving antenna 5 are consistent. The center positions of the metal patches 211 and the rectangular slots 231 correspond one to one.

[0067] The first dielectric layer 22 and the second dielectric layer 26 are parallel to each other and spaced apart; the liquid crystal layer 24 is filled between the lower surface of the first dielectric layer 22 and the upper surface of the second dielectric layer 26, and a sealing frame is provided around the edges of the liquid crystal layer 24. The sealing frame, the first dielectric layer 21 and the second dielectric layer 26 are bonded together to form a sealed liquid crystal box to prevent the liquid crystal layer 24 from leaking.

[0068] The third metal layer 25 is located on the upper surface of the second dielectric layer 26 and is composed of M×N two-dimensionally periodically arranged bent metal delay lines 251 ; the fourth metal layer 27 serves as a metal floor and fully covers the lower surface of the second dielectric layer 26 .

[0069] The bias voltage control module 3 includes an FPGA control circuit, a soft strip line, M×N pins and M×N bias lines. The FPGA control circuit is connected to each pin through a soft strip line, and is respectively connected to each bent metal delay line 251 in the third metal layer 25 through the corresponding bias line; the FPGA control circuit outputs a voltage to each pin through a wave control plate, and then applies it to the corresponding bent metal delay line 251, controls the bias voltage of the liquid crystal in the corresponding area, deflects the liquid crystal molecules, thereby realizing the regulation of the dielectric constant of the liquid crystal, and then controls the reflection phase of each metasurface unit, that is, the coding state; this embodiment adopts a 3-bit coding method to quantize the reflection phase of the metasurface unit into 8 states with an interval of 45°.

[0070] In this embodiment, the liquid crystal programmable metasurface 2 and the imaging target 4 are placed opposite to each other, and each metasurface unit in the liquid crystal programmable metasurface 2 presents a certain coding state in a random manner; the feed source 1 and the receiving antenna 5 use corner cone horn antennas, which are respectively placed offset relative to the liquid crystal programmable metasurface 2 and the imaging target 4 to reduce the impact of occlusion.

[0071] The metasurface scale M=N=16 described in this embodiment, the unit period is 4mm, the first dielectric layer 22 and the second dielectric layer 26 are both made of glass substrates with a relative dielectric constant of 4.81, an area of ​​64mm×64mm, and a thickness of 0.55mm; all metal layers are made of copper with a thickness of 0.003mm, the side length of the metal patch 211 is 1.9mm, the length of the rectangular gap 231 is 0.85mm, and the width is 0.3mm. The lengths of the bent metal delay line 251 from the inside to the outside are 1.5mm, 1mm, 2mm, 2mm, 2.5mm, and 1.5mm, respectively, and the width is 0.1mm; the thickness of the liquid crystal layer 24 is 0.03mm, and the relative dielectric constant adjustment range is 2.5 to 3.4.

[0072] The commercial electromagnetic simulation software CST Studio Suite is used to model and simulate the liquid crystal programmable metasurface unit described in this embodiment. The reflection coefficient is as follows: Figure 3As shown, the reflection phases of the eight coding states differ by 45° sequentially at an operating frequency of 30 GHz. Compared with the two states of 1-bit coding, it has a higher degree of freedom and can generate more complex and diverse beams and measurement modes, thereby improving imaging resolution.

[0073] The commercial electromagnetic simulation software CST Studio Suite was used to model and simulate the liquid crystal programmable metasurface described in this embodiment. A standard horn antenna was used as the feed source. The normalized directional patterns corresponding to different coding sequences are shown in the figure. Figure 4 As shown; Figure 4 (a) is a schematic diagram of the first coding sequence, Figure 4 (b) is the normalized directional diagram corresponding to the first coding sequence. Figure 4 (c) is a schematic diagram of the second coding sequence, Figure 4 (d) is the normalized pattern corresponding to the second coding sequence. The results show that changing the reflected phase coding sequence can generate rich and diverse spurious beams with low correlation, thereby constructing multiple low-correlation measurement modes for imaging.

[0074] Using the above-mentioned microwave imaging system based on liquid crystal programmable metasurface, this embodiment also provides a microwave imaging analytical calculation method based on liquid crystal programmable metasurface, and its flow chart is as follows: Figure 5 As shown, the following steps are included:

[0075] Step 1. Calculate the incident electric field from the feed source to the liquid crystal programmable metasurface

[0076] Calculate the incident electric field from the feed source to the center of the liquid crystal metasurface unit based on cosine function fitting

[0077]

[0078] Among them, r, θ and They represent the distance, pitch angle and azimuth angle of each liquid crystal metasurface unit center when the feed equivalent phase center is taken as the origin, j is the imaginary unit, k is the wave number, λ is the working wavelength, and are the cosine function power exponents of the E-plane and H-plane radiation patterns of the feed, respectively, obtained by fitting; represents the unit vector in the direction of the pitch angle θ, The azimuth is The unit vector of the direction;

[0079] Applying the conversion formula between spherical coordinate system and rectangular coordinate system, the incident electric field at the center of the metasurface unit is decomposed into the tangential electric field in the x-direction and the y-direction. and

[0080] Step 2. Calculate the electric field reflected by the liquid crystal programmable metasurface modulated to the imaging area where the imaging target is located

[0081] The full-wave simulation software is used to model and simulate the different coding states of the metasurface unit. The periodic boundary conditions are used and the number of Floquet port modes is set to 2 to obtain a pair of orthogonal TE and TM excitation modes. The reflection coefficient Γ of the different coding states of the metasurface unit is then calculated. ε To perform the extraction:

[0082]

[0083] in, It represents the reflection coefficient when the incident electric field is in TE mode and the reflected electric field is in TE mode. represents the reflection coefficient when the incident electric field is in TM mode and the reflected electric field is in TE mode, It represents the reflection coefficient when the incident electric field is in TE mode and the reflected electric field is in TM mode. It represents the reflection coefficient when the incident electric field is in TM mode and the reflected electric field is in TM mode.

[0084] Further calculations are performed to obtain the tangential components of the reflected electric field in the x and y directions of the metasurface unit. and

[0085]

[0086] Perform inverse Fourier transform on the tangential components of the reflected electric field in the x and y directions of the metasurface unit to obtain the spectral function F in the x and y directions. x and F y :

[0087]

[0088] Among them, x and y represent the independent variables in the x-direction and y-direction respectively;

[0089] According to the plane wave angular spectrum theory, the electric field reflected from the liquid crystal programmable metasurface to the imaging area is calculated.

[0090]

[0091] Step 3. Calculate the electric field scattered by the imaging target to the receiving antenna

[0092] According to the backscattering theory of ideal scatterers, the electric field scattered by the imaging target to the receiving antenna is calculated.

[0093]

[0094] in, is the receiving antenna radius vector, is the normal vector of the imaging plane, is the scattered wave vector, is the imaging target radius vector, f is the scattering coefficient of the imaging target, and dσ represents the differential component of the target imaging area.

[0095] Step 4. Obtain the measurement signal

[0096] The measured signal at the receiving antenna consists of the scattered signal and the noise signal:

[0097]

[0098] Where n is additive Gaussian noise, which is randomly generated according to the signal-to-noise ratio.

[0099] Step 5. Calculate the electric field incident on the imaging area by the receiving antenna

[0100] Calculate the electric field incident on the imaging area by the receiving antenna based on cosine function fitting

[0101]

[0102] Among them, r′, θ′ and They represent the distance, elevation and azimuth of each imaging grid center when the equivalent phase center of the receiving antenna is taken as the origin. and are the cosine function power exponents of the E-plane and H-plane radiation patterns of the receiving antenna, respectively, obtained by fitting; represents the unit vector in the direction of the pitch angle θ′, The azimuth is The unit vector of the direction.

[0103] Step 6. Obtain the measurement matrix

[0104] According to the first-order Born approximation, the product of the electric field radiated by the liquid crystal programmable metasurface and the receiving antenna in the imaging area calculated in steps 2 and 5 is used as the element of the measurement matrix H:

[0105]

[0106] Among them, H ij represents the element in the i-th row and j-th column of the measurement matrix H; i is the index of the measurement mode of the liquid crystal programmable metasurface, 1≤i≤I, and I is the total number of measurement modes; j is the index of the imaging grid in the imaging area, 1≤j≤J, and J is the total number of imaging grids in the imaging area; and They respectively represent the electric field of the liquid crystal programmable metasurface and the receiving antenna incident on the j-th imaging grid in the imaging area in the i-th measurement mode.

[0107] Step 7. Construct imaging equation

[0108] The imaging process is simplified into a linear model, which can be expressed as:

[0109] g I×1 =H I×J ×f J×1 +n I×1

[0110] Among them, g I×1 is the measurement signal vector obtained by the receiving antenna, and its i-th element g i represents the measurement signal of the liquid crystal programmable metasurface at the receiving antenna in the i-th measurement mode; H I×J is the measurement matrix composed of different measurement modes; f J×1 is the scattering coefficient distribution of the imaging area, that is, the unknown quantity that needs to be solved, and its jth element f j represents the scattering coefficient of the jth imaging grid in the imaging area; n I×1 is an additive noise vector, whose i-th element n i represents the additive Gaussian noise of the liquid crystal programmable metasurface in the i-th measurement mode.

[0111] Step 8. Solve the imaging equation and reconstruct the target image

[0112] When the number of measurement modes I is less than the number of imaging grids J, that is, the number of equations is less than the number of unknowns, the imaging equation is an ill-conditioned equation and cannot be solved directly by matrix inversion. According to the theory of compressed sensing, if the target image is sparse, it can be reconstructed by random subsampling at a sampling rate far lower than the Nyquist rate. The compressed sensing algorithm can be used to solve the underdetermined equations and restore the more complex target image with fewer measurements.

[0113] The technical effects of the present invention are further described in detail below in conjunction with simulation experiments of the embodiments of the present invention.

[0114] Without loss of generality, it is assumed that the operating frequency of the metasurface is f = 30 GHz, the number of metasurface units is M × N = 16 × 16; the imaging area is located at a distance of d = 1 m from the metasurface and is divided into 30 × 30 discrete grids with a side length of 15 mm, and the number of measurement modes is I = 150. In order to verify the effectiveness of the imaging system and imaging method of the present invention, the imaging system and imaging method of the present invention are respectively Figure 6 (a) shows multiple discrete point targets, Figure 6 (b) The heart-shaped target and Figure 6(c) The "UESTC" target is imaged, where the white part indicates a scattering coefficient of 1 and the black part indicates a scattering coefficient of 0. The imaging results are distributed as follows: Figure 6 (d) Figure 6 (e) Figure 6 As shown in (f), the image reconstruction position is consistent with the original target. This result shows that the system designed by the present invention can be used for microwave computational imaging.

[0115] The present invention is described exemplarily above in conjunction with the accompanying drawings, but the specific implementation of the present invention is not limited to the above-mentioned manner. As long as various non-substantial improvements are made using the method concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A microwave imaging system based on a liquid crystal programmable metasurface, characterized in that: It comprises a transmitting end, an imaging target (4) and a receiving end; the transmitting end comprises a feed source (1), a liquid crystal programmable metasurface (2) and a bias voltage control module (3), and the receiving end comprises a receiving antenna (5) and a data processing module (6); A feed source (1) radiates a microwave signal to a liquid crystal programmable metasurface (2), which modulates and reflects the microwave signal to generate a variety of random beams in space, thereby forming a low-correlation measurement mode; the modulated reflected random beams illuminate an imaging target (4), and the imaging target (4) scatters the random beams to generate echo signals; a receiving antenna (5) receives the echo signals and sends them to a data processing module (6); the data processing module (6) inverts the echo signals to obtain a scattering coefficient of an imaging region where the imaging target (4) is located, thereby realizing reconstruction of the imaging target (4).

2. The microwave imaging system based on liquid crystal programmable metasurface according to claim 1, characterized in that: The liquid crystal programmable metasurface (2) comprises M×N metasurface units arranged in a two-dimensional periodic pattern, where M and N are both positive integers greater than or equal to 2; the liquid crystal programmable metasurface (2) comprises, from top to bottom, a first metal layer (21), a first dielectric layer (22), a second metal layer (23), a liquid crystal layer (24), a third metal layer (25), a second dielectric layer (26), and a fourth metal layer (27); The first metal layer (21) is located on the upper surface of the first dielectric layer (22) and is composed of M×N metal patches (211) arranged in a two-dimensional periodic pattern; the second metal layer (23) is etched with M×N rectangular slots (231) arranged in a two-dimensional periodic pattern on the basis of fully covering the lower surface of the first dielectric layer (22); the long side direction of the rectangular slot (231), the polarization direction of the feed source (1) and the polarization direction of the receiving antenna (5) are consistent; The center positions of the metal patch (211) and the rectangular gap (231) correspond one to one; The first dielectric layer (22) and the second dielectric layer (26) are parallel to each other and spaced apart; the liquid crystal layer (24) is filled between the lower surface of the first dielectric layer (22) and the upper surface of the second dielectric layer (26); a sealing frame is provided around the edges of the liquid crystal layer (24); the sealing frame, the first dielectric layer (21) and the second dielectric layer (26) are bonded together to form a sealed liquid crystal box to prevent leakage of the liquid crystal layer (24); The third metal layer (25) is located on the upper surface of the second dielectric layer (26) and is composed of M×N bent metal delay lines (251) arranged in a two-dimensional periodic pattern; the fourth metal layer (27) serves as a metal floor and fully covers the lower surface of the second dielectric layer (26).

3. The microwave imaging system based on liquid crystal programmable metasurface according to claim 2, characterized in that: The bias voltage control module (3) comprises an FPGA control circuit, a soft strip line, M×N pins and M×N bias lines. The FPGA control circuit is connected to each pin via the soft strip line and is respectively connected to each bent metal delay line (251) in the third metal layer (25) via the corresponding bias line. The FPGA control circuit outputs a voltage to each pin via a wave control plate, and then applies the voltage to the corresponding bent metal delay line (251), controls the bias voltage of the liquid crystal in the corresponding area, deflects the liquid crystal molecules, realizes the regulation of the liquid crystal dielectric constant, and then controls the reflection phase encoding state of each metasurface unit.

4. The microwave imaging system based on liquid crystal programmable metasurface according to claim 3, characterized in that: A 3-bit encoding method is used to quantize the reflection phase of the metasurface unit into 8 states with an interval of 45°.

5. The microwave imaging system based on liquid crystal programmable metasurface according to claim 1, characterized in that: The liquid crystal programmable metasurface (2) and the imaging target (4) are placed relative to each other, and each metasurface unit in the liquid crystal programmable metasurface (2) presents a certain coding state in a random manner; the feed source (1) and the receiving antenna (5) use a pyramidal horn antenna, which are respectively placed in an offset feed relative to the liquid crystal programmable metasurface (2) and the imaging target (4).

6. A microwave imaging analytical calculation method based on a liquid crystal programmable metasurface, implemented based on the microwave imaging system of the liquid crystal programmable metasurface described in claim 1, characterized in that: The following steps are involved: Step 1. Calculate the incident electric field from the feed source to the liquid crystal programmable metasurface The incident electric field fed to the center of the liquid crystal metasurface unit Among them, r, θ and They represent the distance, pitch angle and azimuth angle of each liquid crystal metasurface unit center when the feed equivalent phase center is taken as the origin, j is the imaginary unit, k is the wave number, λ is the working wavelength, and are the cosine function power exponents of the E-plane and H-plane radiation patterns of the feed, respectively; represents the unit vector in the direction of the pitch angle θ, The azimuth is The unit vector of the direction; Decompose the incident electric field at the center of the metasurface unit into tangential electric fields in the x-direction and the y-direction and Step 2. Calculate the electric field reflected by the liquid crystal programmable metasurface modulated to the imaging area where the imaging target is located The full-wave simulation software is used to model and simulate the various coding states of the metasurface unit. Using periodic boundary conditions, the number of Floquet port modes is set to 2 to obtain a pair of orthogonal TE and TM excitation modes, and the reflection coefficient Γ of different coding states of the metasurface unit is extracted. ε ; Calculate the tangential components of the electric field reflected by the metasurface unit in the x and y directions and Perform inverse Fourier transform on the tangential components of the reflected electric field in the x and y directions of the metasurface unit to obtain the spectral function F in the x and y directions. x and F y : Among them, x and y represent the independent variables in the x-direction and y-direction respectively; Calculation of the electric field reflected from a liquid crystal programmable metasurface to the imaging area Step 3. Calculate the electric field scattered by the imaging target to the receiving antenna Calculate the electric field scattered by the imaging target to the receiving antenna in, is the receiving antenna radius vector, is the normal vector of the imaging plane, is the scattered wave vector, is the imaging target radius vector, f is the scattering coefficient of the imaging target, and dσ represents the differential component of the target imaging area; Step 4. Obtain the measurement signal The measured signal at the receiving antenna consists of the scattered signal and the noise signal: Where n is additive Gaussian noise; Step 5. Calculate the electric field incident on the imaging area by the receiving antenna Calculate the electric field incident on the imaging area by the receiving antenna Among them, r′, θ′ and They represent the distance, elevation and azimuth of each imaging grid center when the equivalent phase center of the receiving antenna is taken as the origin. and are the cosine function power exponents of the E-plane and H-plane radiation patterns of the receiving antenna respectively; represents the unit vector in the direction of the pitch angle θ′, The azimuth is The unit vector of the direction; Step 6. Obtain the measurement matrix The product of the liquid crystal programmable metasurface and the electric field radiated by the receiving antenna in the imaging area calculated in step 2 and step 5 is used as the measurement matrix H I×J Elements: Among them, H ij Denotes the measurement matrix H I×J The element in the i-th row and j-th column in the image is: i is the index of the measurement mode of the liquid crystal programmable metasurface, 1≤i≤I, I is the total number of measurement modes; j is the index of the imaging grid in the imaging area, 1≤j≤J, J is the total number of imaging grids in the imaging area; and They represent the electric fields of the liquid crystal programmable metasurface and the receiving antenna incident on the j-th imaging grid in the imaging area in the i-th measurement mode; Step 7. Construct the imaging equation and solve the scattering coefficient distribution of the imaging area The imaging process is simplified into a linear model, which can be expressed as: g I×1 =H I×J ×f J×1 +n I×1 Among them, g I×1 is the measurement signal vector obtained by the receiving antenna, and its i-th element g i represents the measurement signal of the liquid crystal programmable metasurface at the receiving antenna in the i-th measurement mode; H I×J is the measurement matrix composed of different measurement modes; f J×1 is the scattering coefficient distribution of the imaging area, and its jth element f is the unknown quantity to be solved. j represents the scattering coefficient of the jth imaging grid in the imaging area; n I×1 is an additive noise vector, whose ith element n i represents the additive Gaussian noise of the liquid crystal programmable metasurface in the i-th measurement mode; Solving the above equation, we can obtain the scattering coefficient distribution of the imaging area.

7. The microwave imaging analytical calculation method based on liquid crystal programmable metasurface according to claim 6, characterized in that: When the number of measurement modes I is less than the number of imaging grids J, that is, the number of equations is less than the number of unknown quantities to be solved, the compressed sensing method is used to solve the scattering coefficient distribution of the imaging area.

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