CN114839619A - Large-focal-depth dual-band terahertz frequency modulation continuous wave radar imaging method and system - Google Patents
Large-focal-depth dual-band terahertz frequency modulation continuous wave radar imaging method and system Download PDFInfo
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- CN114839619A CN114839619A CN202210543407.3A CN202210543407A CN114839619A CN 114839619 A CN114839619 A CN 114839619A CN 202210543407 A CN202210543407 A CN 202210543407A CN 114839619 A CN114839619 A CN 114839619A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/89—Lidar systems specially adapted for specific applications for mapping or imaging
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
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Abstract
The invention discloses a large-focal-depth double-frequency-band terahertz frequency modulation continuous wave radar imaging method and system; the system comprises: the system comprises a terahertz radiation source, a lens, a beam splitter, a parabolic reflector, a polarization wire grid, a sample to be detected, a data acquisition card and an upper computer; echo information of the thick sample under different polarizations of the two frequency bands can be obtained through the large-focal-depth dual-band quasi-optical system, and the range resolution can be greatly improved through a frequency band fusion-extended Fourier algorithm. Compared with the existing terahertz nondestructive testing technology, the quasi-optical design and algorithm provided by the invention can greatly improve the focal depth and the distance-direction resolution of an imaging system, thereby increasing the thickness and position flexibility of a test sample, and reducing the requirement on large-bandwidth hardware and the system complexity.
Description
Technical Field
The invention relates to the technical field of terahertz radar imaging, in particular to a method and a system for dual-band large-focal-depth terahertz frequency-modulated continuous wave radar imaging.
Background
Due to the unique penetrability and non-ionization characteristics of terahertz waves, terahertz nondestructive testing technology has been successfully applied to the fields of artwork protection, industrial product quality control, packaged Integrated Circuit (IC) nondestructive testing and the like. The terahertz frequency modulation continuous wave imaging technology has the characteristics of high power, miniaturization, low cost, three-dimensional imaging and the like, and is widely concerned in the field of terahertz nondestructive testing. The existing terahertz real aperture imaging system is a small focal depth system mainly based on a Gaussian beam, so that the thickness and the placement position of a test sample are greatly limited, and in addition, due to the limitation of a microwave device, the bandwidth of a signal is limited to a great extent, so that the distance resolution is limited.
In order to improve the depth of field of the imaging system, a synthetic aperture imaging mode can be adopted. However, the synthetic aperture needs an algorithm to focus the beam, which is more complicated than the real aperture imaging method, and the beam energy cannot be focused, thereby limiting the beam penetration depth. In order to improve the radar range resolution, on one hand, a higher-order frequency multiplier can be used for realizing a higher-frequency carrier frequency, so that the bandwidth of a signal is improved under the same baseband bandwidth. However, the high-frequency ultra-wideband terahertz device undoubtedly brings greater implementation difficulty, and meanwhile, the output power is limited. On the other hand, the distance resolution can be improved by a time-frequency analysis algorithm, a subspace algorithm and a function fitting algorithm, wherein the typical time-frequency analysis algorithm is Continuous Wavelet Transform (CWT), but the improvement of the resolution of the algorithm is very limited. Typical subspace algorithms include multiple signal classification (MUSIC), etc., which require the number of harmonic components in a known signal and destroy the amplitude and lobe width information of an echo signal, thus being unfavorable for the inversion of the dielectric constant of a sample. The function fitting algorithm comprises algorithms such as Sinc and Guass function fitting, the essence of the algorithm is that discrete data are changed continuously, the distance-to-resolution ratio is improved in a limited way, and the algorithm is not beneficial to high-precision nondestructive detection of a multilayer structure sample, so that a distance-to-super-resolution algorithm without prior knowledge is urgently needed.
Disclosure of Invention
Technical problem to be solved
In order to solve the technical problems, the invention provides a large-focal-depth dual-band terahertz frequency modulation continuous wave radar imaging method and a system, the method can acquire echo information of a thick sample under different polarizations of dual bands through a large-focal-depth dual-band quasi-optical system, and can greatly improve the focal depth and the distance resolution of the imaging system through a band fusion-extended Fourier algorithm, so that the thickness and the position flexibility of a test sample are improved, and the requirement on large-bandwidth hardware and the system complexity are reduced.
(II) technical scheme
In order to solve the technical problems and achieve the purpose of the invention, the invention is realized by the following technical scheme:
on one hand, the invention provides a large-focal-depth dual-band terahertz frequency modulation continuous wave radar imaging system, which comprises:
the terahertz radiation source comprises a first frequency band terahertz radiation source and a second frequency band terahertz radiation source, and is used for radiating terahertz linear frequency modulation signals;
a polarization wire grating for splitting beams with different polarization directions;
a collimating lens for collimating and focusing the beam;
a beam splitter for splitting the incident and reflected beams;
a parabolic mirror for focusing the reflected beam;
an axicon;
a first frequency band terahertz detector and a second frequency band terahertz detector;
the data acquisition card is used for acquiring the dual-band intermediate frequency signals;
and the upper computer is used for automatically acquiring and imaging data.
Further, the first-frequency-band terahertz radiation source radiates a vertically polarized (parallel to the paper surface) terahertz beam, and the second-frequency-band terahertz radiation source 2 radiates a horizontally polarized (perpendicular to the paper surface) terahertz beam;
further, the phase center of the terahertz radiation source is located at the focal plane of the collimating lens;
further, the plane wave generates a terahertz light beam with a large focal depth after passing through the beam splitter and the axicon and is focused to a sample to be detected, an echo signal is reflected to the parabolic reflector through the beam splitter, and the focused light beam is focused to phase centers of the first frequency band terahertz detector and the second frequency band terahertz detector through the polarization wire grid.
On the other hand, the invention provides a frequency band fusion-extended Fourier algorithm method based on large focal depth dual-frequency band terahertz frequency modulation continuous wave imaging, which specifically comprises the following steps:
the method comprises the following steps: from the transmitted signal S T (t) and echo signal S RF (t) obtaining a two-frequency band intermediate frequency echo signal S after frequency modulation processing (frequency mixing filtering denoising) ZF1 (t) and S ZF2 (t)。
Step two: the gain adjustment is carried out on the intermediate frequency signals of the second frequency band, and the intermediate frequency signals S of the second frequency band are subjected to the gain adjustment by means of a frequency shift term exp (j2 pi K delta tau) ZF2 (t) frequency shifting.
Step three: by means of the time shift term Δ f/K, the intermediate frequency signal S is processed in the second frequency band ZF2 (t) is time-shifted, where Δ f/K ═ f 2 -f 1 )/K，f 1 And f 2 The initial frequencies of the first frequency band and the second frequency band are respectively, and K is the frequency modulation slope.
Step four: constant phase f of intermediate frequency signal in second frequency band 2 And (4) compensating the delta tau to complete the fusion of the intermediate frequency signals of the two frequency bands.
Step five: setting a power spectral vector W (i) Initial value and iteration times i, and calculating autocorrelation matrix R (i) 。
Step six: calculating the distribution F of the distance directions (i) Amplitude spectrum A (i) And updating the power spectrum W (i+1)
Step seven: the specified iteration times i are reached, and high-resolution distance distribution is outputF (i)
(III) advantageous effects
The invention provides a terahertz dual-band quasi-optical method and system with large focal depth and a distance super-resolution algorithm without priori knowledge. Compared with the existing terahertz nondestructive testing technology, the quasi-optical design and algorithm provided by the invention can greatly improve the focal depth and the distance-direction resolution of an imaging system, thereby increasing the thickness and position flexibility of a test sample, and reducing the requirement on large-bandwidth hardware and the system complexity.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the application and together with the description serve to explain the application and not to limit the application. In the drawings:
FIG. 1 is a structural diagram of a dual-band large focal depth terahertz frequency modulation continuous wave radar imaging system according to an embodiment of the application;
FIG. 2 is a schematic diagram of generation of a terahertz frequency-modulated continuous wave radar large focal depth beam according to an embodiment of the application;
fig. 3 is a flowchart of a frequency band fusion-extended fourier algorithm according to an embodiment of the present application.
Reference numerals:
1. the terahertz radiation source comprises a first frequency band terahertz radiation source 2, a second frequency band terahertz radiation source 3, a polarized wire grid 4, a collimating lens 5, a beam splitter 6, an axicon 7, a sample to be detected 8, a parabolic reflector 9, a polarized wire grid 10, a first frequency band terahertz detector 11, a second frequency band terahertz detector 12, a data acquisition card 13 and an upper computer
Detailed Description
The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.
The embodiments of the present disclosure are described below with specific examples, and other advantages and effects of the present disclosure will be readily apparent to those skilled in the art from the disclosure in the specification. It is to be understood that the described embodiments are merely illustrative of some, and not restrictive, of the embodiments of the disclosure. The disclosure may be embodied or carried out in various other specific embodiments, and various modifications and changes may be made in the details within the description without departing from the spirit of the disclosure. It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict. All other embodiments, which can be derived by a person skilled in the art from the embodiments disclosed herein without making any creative effort, shall fall within the protection scope of the present disclosure.
It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present disclosure, and the drawings only show the components related to the present disclosure rather than the number, shape and size of the components in actual implementation, and the type, amount and ratio of the components in actual implementation may be changed arbitrarily, and the layout of the components may be more complicated.
As shown in fig. 1, the dual-band terahertz frequency modulation continuous wave radar imaging system provided in the embodiment of the present invention includes:
a first frequency band terahertz radiation source 1 and a second frequency band terahertz radiation source 2; a polarizing wire grid 3, 9 for splitting beams of different polarization directions; a collimating lens 4 for collimating and focusing the beam; a beam splitter 5 for splitting the incident and reflected beams; an axicon 6; an article 7 to be tested; a parabolic mirror 8 for focusing the reflected beam; a first frequency band terahertz detector 10 and a second frequency band terahertz detector 11; the data acquisition card 12 is used for acquiring the dual-band intermediate frequency signals; and the upper computer 13 is used for automatic data acquisition and imaging.
The quasi-optical principle is as follows: the first frequency band terahertz radiation source 1 radiates a vertically polarized terahertz wave beam (parallel to a paper surface), the second frequency band terahertz radiation source 2 radiates a horizontally polarized terahertz wave beam (perpendicular to the paper surface) which is transmitted by the polarized wire grid 3 and collimated into a plane wave by the collimating lens 4, and the phase center of the terahertz radiation source is positioned at the focal plane of the collimating lens 4. The plane wave generates a terahertz large focal depth light beam with large focal depth after passing through the beam splitter 5 and the axicon 6 and is focused on a sample 7 to be detected, an echo signal is reflected to the parabolic reflector 8 through the beam splitter 5, and the focused light beam is focused to phase centers of the first frequency band terahertz detector 10 and the second frequency band terahertz detector 11 through the polarization wire grid 9. And then, acquiring by a data acquisition card to obtain a dual-band intermediate frequency echo signal.
Based on a dual-band terahertz frequency modulation continuous wave radar imaging system, the large-focal-depth terahertz light beam generation principle provided by the embodiment is shown in fig. 2, and the spatial light intensity distribution I (rho, z) under a cylindrical coordinate system is as follows:
wherein z is the direction of the optical axis, ρ is the radial distance from the optical axis, E is the total energy of the beam incident on the axicon, k 2 π/λ is the wave number in free space, ω is 0 Is the incident Gaussian beam radius, Z max Is the beam focal depth, J 0 Is a zero order Bessel function, alpha 0 Is the half apex angle of the beam.
The embodiment of the invention also provides an intermediate frequency signal data fusion method for imaging by the large-focal-depth dual-band terahertz frequency modulation continuous wave radar, the algorithm principle is shown in figure 3, and the method specifically comprises the following steps:
the method comprises the following steps: obtaining intermediate frequency signals of a dual-band frequency modulation continuous wave radar, and obtaining signal forms of dual-band intermediate frequency echoes after frequency modulation processing (frequency mixing filtering) as follows:
wherein A1 and A2 are the amplitudes of the echo signals of the first frequency band and the second frequency band respectively, f 1 In a first frequency rangeStarting frequency, f 2 Is the starting frequency, τ, of the second frequency band 1 For the echo delay of the first frequency band caused by the target, and 2 and delaying the echo of the second frequency band.
Step two: the gain of the intermediate frequency signal of the two frequency bands needs to be adjusted to make the echo amplitudes of the two frequency bands approximately equal, namely, A is made 2 A c ＝A 1 In addition, the echo delay difference between two frequency bands is delta tau 1 -τ 2 Is a constant (quadratic term)
And
negligible), by means of the frequency shift term exp (j2 π K Δ τ t), the two-band intermediate frequency signal can be rewritten as:
S ZF1 (t)＝A 1 ·exp[j2π(f 1 τ 1 +Ktτ 1 )] (4)
S ZF2-C (t)＝A 2 A c ·exp[j2π(f 2 (τ 1 -Δτ)+Kt(τ 2 +Δτ))]
＝A 1 ·exp[j2π(f 2 τ 1 +Ktτ 1 -f 2 Δτ)] (5)
step three: as can be seen from equations (4) and (6), the intermediate frequency signals of the two frequency bands differ only in phase, and therefore by means of the time shift term Δ f/K, where Δ f/K is (f ═ f) 2 -f 1 ) the/K is a constant term, and the intermediate frequency signal of the second frequency band after time shift is as follows:
step four: it can be known from the formulas (4) and (6) that only a constant phase difference f exists between the intermediate frequency signals of the two frequency bands 2 Delta tau, and compensating the delta tau to obtain a two-frequency-band intermediate frequency fusion signal S (t).
Step five: computing a power spectral vector W of a Fourier transform of a fusion signal S (t) (1) And set it as W (i) Initial value ofSetting iteration times i and calculating an autocorrelation matrix R (i) 。
Wherein N is the extrapolation number of time domain data, E is the basis function E -jωt And (5) forming a kernel matrix.
Step six: calculating the distribution F of the distance directions (i) Discrete amplitude spectrum A (i) And updating the power spectrum weight W (i+1)
F (i) ＝S(R (i) ) -1 EW (i) (8)
W (i+1) ＝diag(|A (i) | 2 ) (10)
Step seven: w is to be (i+1) Substituting the step five to start the next iteration until reaching the specified iteration number i, and outputting a high-resolution distance distribution F (i) 。
Therefore, high-resolution distance distribution of the thick test sample under large-bandwidth multi-polarization can be obtained simultaneously, and a high-precision and high-resolution three-dimensional imaging result of the thick test sample can be obtained by matching with mechanical scanning.
The above-mentioned embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements made to the technical solution of the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the protection scope defined by the claims of the present invention.
Claims (10)
1. A large-focal-depth dual-band terahertz frequency modulation continuous wave radar imaging method is characterized by comprising a frequency band fusion-extended Fourier algorithm and specifically comprising the following steps of:
the method comprises the following steps: from the transmitted signal S T (t) and echoSignal S RF (t) after frequency mixing filtering and denoising, obtaining a two-frequency band intermediate frequency echo signal S ZF1 (t) and S ZF2 (t)。
Step two: the gain adjustment is carried out on the intermediate frequency signals of the second frequency band, and the intermediate frequency signals S of the second frequency band are subjected to the gain adjustment by means of a frequency shift term exp (j2 pi K delta tau) ZF2 (t) frequency shifting.
Step three: by means of the time shift term Δ f/K, the intermediate frequency signal S is processed in the second frequency band ZF2 (t) is time-shifted, where Δ f/K ═ f 2 -f 1 )/K，f 1 And f 2 The initial frequencies of the first frequency band and the second frequency band are respectively, and K is the frequency modulation slope.
Step four: constant phase f of intermediate frequency signal in second frequency band 2 And (4) compensating the delta tau to complete the fusion of the intermediate frequency signals of the two frequency bands.
Step five: setting a power spectral vector W (i) Initial value and iteration times i, and calculating autocorrelation matrix R (i) 。
Step six: calculating the distribution F of the distance directions (i) Amplitude spectrum A (i) And updating the power spectrum W (i+1)
Step seven: w is to be (i+1) Substituting the step five to start the next iteration until reaching the specified iteration number i, and outputting a high-resolution distance distribution F (i) 。
2. The large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging method according to claim 1, wherein the signal forms of the first intermediate frequency echoes are respectively as follows:
wherein A is 1 、A 2 The amplitudes of the first frequency band echo signal and the second frequency band echo signal are respectively; f. of 1 Is the starting frequency of the first frequency band, f 2 Is the starting frequency, τ, of the second frequency band 1 For the echo delay of the first frequency band caused by the target, and 2 and delaying the echo of the second frequency band.
3. The large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging method according to claim 1, wherein the frequency shifting method in the second step is specifically:
adjusting the gain of the intermediate frequency signal of the two frequency bands to make the echo amplitudes of the two frequency bands approximately equal, namely, making A 2 A c ＝A 1 The echo delay difference between two frequency bands is delta tau 1 -τ 2 By means of the frequency shift term exp (j2 π K Δ τ t), the two-band intermediate frequency signal can be rewritten as:
S ZF1 (t)＝A 1 ·exp[j2π(f 1 τ 1 +Ktτ 1 )]
S ZF2-C (t)＝A 2 A c ·exp[j2π(f 2 (τ 1 -Δτ)+Kt(τ 2 +Δτ))]
＝A 1 ·exp[j2π(f 2 τ 1 +Ktτ 1 -f 2 Δτ)]
wherein S is ZF2-C The intermediate frequency signal of the second frequency band after frequency shift.
4. The large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging method according to claim 1, wherein the third step further comprises:
the second frequency band intermediate frequency signal after time shift is:
wherein, Δ f/K is time shift term, Δ f/K ═ f 2 -f 1 ) and/K is a constant term.
5. The large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging method according to claim 1, wherein the fifth step further comprises:
calculating a power spectral vector W of a Fourier transform of a fusion signal S (t) (1) 。
6. The large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging method according to claim 5, wherein the fifth step further comprises:
autocorrelation matrix R (i) The calculation method of (c) is as follows:
wherein N is the extrapolation number of time domain data, E is the basis function E -jωt And (5) forming a kernel matrix.
7. The large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging method according to claim 1, wherein the sixth step further comprises:
calculating the distance distribution F according to the following formula (i) Discrete amplitude spectrum A (i) And updating the power spectrum weight W (i+1) ：
F (i) ＝S(R (i) ) -1 EW (i)
W (i+1) ＝diag(|A (i) | 2 )。
8. The utility model provides a big focal depth double-frenquency terahertz frequency modulation continuous wave radar imaging system, its characterized in that includes: the terahertz radiation source comprises a first frequency band terahertz radiation source and a second frequency band terahertz radiation source, and is used for radiating terahertz linear frequency modulation signals; a polarization wire grating for splitting beams with different polarization directions; a collimating lens for collimating and focusing the beam; a beam splitter for splitting the incident and reflected beams; a parabolic mirror for focusing the reflected beam; an axicon; a first frequency band terahertz detector and a second frequency band terahertz detector; the data acquisition card is used for acquiring the dual-band intermediate frequency signals; and the upper computer is used for automatically acquiring and imaging data.
9. The large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging system according to claim 8,
the first frequency band terahertz radiation source radiates vertically polarized terahertz beams, and the second frequency band terahertz radiation source 2 radiates horizontally polarized terahertz beams.
10. The large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging system according to claim 9,
the phase center of the terahertz radiation source is positioned at the focal plane of the collimating lens; the plane wave generates a terahertz large focal depth light beam with large focal depth after passing through the beam splitter and the axicon and is focused to a sample to be detected, an echo signal is reflected to the parabolic reflector through the beam splitter, and the focused light beam is focused to phase centers of the first frequency band terahertz detector and the second frequency band terahertz detector through the polarization wire grid.
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| CN115144869A (en) * | 2022-09-05 | 2022-10-04 | 北京理工大学 | Curved surface object detection method and device based on terahertz radar |
| CN115712126A (en) * | 2023-01-09 | 2023-02-24 | 北京理工大学 | Terahertz frequency modulation continuous wave quasi-optical array three-dimensional imaging system and method |
| CN116106254A (en) * | 2023-04-07 | 2023-05-12 | 北京理工大学 | Terahertz three-dimensional tomography system and method |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN115144869A (en) * | 2022-09-05 | 2022-10-04 | 北京理工大学 | Curved surface object detection method and device based on terahertz radar |
| CN115144869B (en) * | 2022-09-05 | 2022-11-29 | 北京理工大学 | Curved surface object detection method and device based on terahertz radar |
| CN115712126A (en) * | 2023-01-09 | 2023-02-24 | 北京理工大学 | Terahertz frequency modulation continuous wave quasi-optical array three-dimensional imaging system and method |
| CN116106254A (en) * | 2023-04-07 | 2023-05-12 | 北京理工大学 | Terahertz three-dimensional tomography system and method |
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