A large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging method and system

Abstract

The application discloses a large-focal-depth dual-frequency band terahertz frequency-modulated continuous wave radar imaging method and system; the system comprises a terahertz radiation source, a lens, a beam splitter, a parabolic reflector, a polarization grating, a sample to be measured, a data acquisition card and an upper computer; echo information of a thick sample under different polarizations of dual-frequency bands can be acquired through a large-focal-depth dual-frequency band quasi-optical system, and the range resolution can be greatly improved through a frequency band fusion-expansion Fourier algorithm. Compared with existing terahertz nondestructive testing technologies, the quasi-optical design and algorithm can greatly improve the focal depth and the range resolution of the imaging system, so that the thickness of the test sample and the position flexibility are increased, and the demand for large-bandwidth hardware and the system complexity are reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of terahertz radar imaging, and particularly relates to a dual-band large-focal-depth terahertz frequency-modulated continuous wave radar imaging method and system. BACKGROUND

[0002] Due to the unique penetration and non-ionizing characteristics of terahertz waves, terahertz non-destructive testing technology has been successfully used in the fields of art protection, industrial product quality control, non-destructive testing of packaged integrated circuits (IC), etc. The terahertz frequency-modulated continuous wave imaging technology has the characteristics of high power, miniaturization, low cost, three-dimensional imaging, etc., and has received extensive attention in the field of terahertz non-destructive testing. The existing terahertz real-aperture imaging system is mainly a small focal depth system with a Gaussian beam, which greatly limits the thickness and placement position of the test sample. In addition, due to the limitation of microwave devices, the bandwidth of the signal is greatly limited, thereby restricting the range resolution.

[0003] In order to improve the depth of field of the imaging system, a synthetic aperture imaging method can be used. However, the synthetic aperture needs to be focused by an algorithm, which is more complex than the real aperture imaging method, and the beam energy cannot be focused, thereby limiting the beam penetration depth. In order to improve the range resolution of the radar, on the one hand, higher frequency devices can be used to achieve a higher carrier frequency, thereby improving the bandwidth of the signal under the same baseband bandwidth. However, high-frequency ultra-wideband terahertz devices undoubtedly bring greater implementation difficulty, and the output power is limited. On the other hand, time-frequency analysis algorithms, subspace algorithms, and function fitting algorithms can be used to improve the range resolution, among which typical time-frequency analysis algorithms such as continuous wavelet transform (CWT) have very limited resolution improvement. Typical subspace algorithms include multiple signal classification (MUSIC), etc. Such algorithms need to know the number of harmonic components in the signal, and will destroy the amplitude and lobe width information of the return signal, which is not conducive to the inversion of the sample dielectric constant. Function fitting algorithms include Sinc and Guass function fitting algorithms, etc. Such algorithms essentially make discrete data continuous, and the range resolution improvement is very limited, and it is also not conducive to high-precision non-destructive testing of multi-layer structure samples, so there is an urgent need for a range resolution algorithm without prior knowledge. SUMMARY

[0004] (I) Technical problems solved

[0005] In order to solve the above technical problems, the application provides a large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging method and system, which can obtain 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 distance resolution, the focal depth and the distance resolution of the imaging system through a frequency band fusion-expansion Fourier algorithm, so as to increase the thickness and position flexibility of the test sample, and reduce the demand for large bandwidth hardware and the system complexity.

[0006] (II) Technical solutions

[0007] In order to solve the above technical problems and achieve the application purposes, the application is implemented by the following technical solutions:

[0008] In one aspect, the application provides a large focal depth dual-band terahertz frequency-modulated continuous wave radar imaging system, which comprises:

[0009] A first frequency band terahertz radiation source and a second frequency band terahertz radiation source are arranged to radiate a terahertz linear frequency modulation signal.

[0010] A polarization grating is arranged to split beams of different polarization directions.

[0011] A collimating lens is arranged to collimate and focus the beams.

[0012] A beam splitter is arranged to split the incident and reflected beams.

[0013] A parabolic reflector is arranged to reflect and focus the beams.

[0014] An axicon is arranged.

[0015] A first frequency band terahertz detector and a second frequency band terahertz detector are arranged.

[0016] A data acquisition card is arranged to collect intermediate frequency signals of the dual bands.

[0017] An upper computer is arranged to automatically collect data and form images.

[0018] Further, the first frequency band terahertz radiation source radiates a terahertz beam vertically polarized (parallel to the paper plane), and the second frequency band terahertz radiation source 2 radiates a terahertz beam horizontally polarized (perpendicular to the paper plane).

[0019] Further, the phase center of the terahertz radiation source is located at the focal plane of the collimating lens.

[0020] Further, the plane wave generates a large focal depth terahertz beam after passing through the beam splitter and the axicon, and focuses on the sample to be tested, the echo signal is reflected to the parabolic reflector through the beam splitter, and the focused beam is gathered to the phase center of the first frequency band terahertz detector and the second frequency band terahertz detector through the polarization grating.

[0021] In another aspect, the present application provides a frequency band fusion-extension Fourier algorithm method based on large focal depth dual-band terahertz frequency-modulated continuous wave imaging, specifically comprising the following steps:

[0022] Step one: according to the transmitted signal S T (t) and the echo signal S RF (t), after frequency modulation processing (mixing filter denoising), the two-frequency intermediate frequency echo signal is obtained as S ZF1 (t) and S ZF2 (t).

[0023] Step two: gain adjustment is performed on the second frequency band intermediate frequency signal, and the second frequency band intermediate frequency signal S ZF2 (t) is frequency shifted by means of the frequency shift term exp(j2πKΔτt).

[0024] Step three: the second frequency band intermediate frequency signal S ZF2 (t) is time shifted by means of the time shift term Δf / K, wherein Δf / K=(f2-f1) / K, f1 and f2 are the starting frequencies of the first frequency band and the second frequency band respectively, and K is the frequency modulation slope.

[0025] Step four: the second frequency band intermediate frequency signal constant phase f2Δτ is compensated, and the intermediate frequency signal fusion of the two frequency bands is completed.

[0026] Step five: set the power spectrum vector W (i) initial value and the iteration number i, and calculate the autocorrelation matrix R (i) .

[0027] Step six: calculate the distance vector distribution F (i) , the amplitude spectrum A (i) , and update the power spectrum W (i+1)

[0028] Step seven: when the specified iteration number i is reached, output the high-resolution distance vector distribution F (i)

[0029] (Three) beneficial effects

[0030] The present application proposes a large focal depth terahertz dual-band quasi-optical method and system and a distance vector super-resolution algorithm without prior knowledge. Through the large focal depth dual-band quasi-optical system, the echo information of thick samples under different polarizations in dual bands can be obtained, and the distance vector resolution can be greatly improved through the frequency band fusion-extension Fourier algorithm. Compared with existing terahertz nondestructive testing technology, the quasi-optical design and algorithm proposed in the present application can greatly improve the focal depth and distance vector resolution of the imaging system, thereby increasing the test sample thickness and position flexibility, reducing the demand for large bandwidth hardware and system complexity. BRIEF DESCRIPTION OF DRAWINGS

[0031] 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 embodiments of the application and together with the description serve to explain the application. In the drawings:

[0032] Figure 1 is a structure diagram of a dual-band large focal depth terahertz frequency-modulated continuous wave radar imaging system according to an embodiment of the application;

[0033] Figure 2 is a principle diagram of a terahertz frequency-modulated continuous wave radar large focal depth beam generation according to an embodiment of the application;

[0034] Figure 3 is a flowchart of a frequency band fusion-expansion Fourier algorithm according to an embodiment of the application.

[0035] Reference signs:

[0036] 1. first frequency band terahertz radiation source 2. second frequency band terahertz radiation source 3. polarization grating 4. collimating lens 5. beam splitter 6. axicon 7. sample to be measured 8. parabolic reflector 9. polarization grating 10. first frequency band terahertz detector 11. second frequency band terahertz detector 12. data acquisition card 13. host computer DETAILED DESCRIPTION

[0037] The embodiments of the present disclosure will be described in detail below with reference to the drawings.

[0038] The above embodiments are only a part of the embodiments of the present disclosure, and not all the embodiments. The present disclosure can also be implemented or applied by other different specific embodiments, and the details in the specification can be modified or changed based on different views and applications without departing from the spirit of the present disclosure. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict. Based on the embodiments in the present disclosure, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of the present disclosure.

[0039] It should also be noted that the drawings provided in the following embodiments only schematically illustrate the basic concept of the present disclosure, and only the components related to the present disclosure are shown in the drawings, not the number, shape and size of the components when actually implemented. The actual implementation of each component may be a random change, and the component layout pattern may be more complex.

[0040] The dual-band terahertz frequency-modulated continuous wave radar imaging system provided by the embodiment has the advantages that Figure 1 As shown in the figure, the system comprises:

[0041] A first-band terahertz radiation source 1, a second-band terahertz radiation source 2, a polarization grating 3, 9 for beam splitting of different polarization directions, a collimating lens 4 for collimation and focusing of the beam, a beam splitter 5 for splitting of the incident and reflected beams, an axicon 6, an object to be measured 7, a parabolic reflector 8 for focusing of the reflected beam, a first-band terahertz detector 10, a second-band terahertz detector 11, a data acquisition card 12 for acquisition of the intermediate frequency signals in the dual bands, and an upper computer 13 for automatic data acquisition and imaging.

[0042] The principle of quasi-optics is that the first-band terahertz radiation source 1 radiates a vertically polarized (parallel to the paper plane) terahertz beam, and the second-band terahertz radiation source 2 radiates a horizontally polarized (perpendicular to the paper plane) terahertz beam, which is transmitted through the polarization grating 3 and collimated into a plane wave by the collimating lens 4, wherein the phase center of the terahertz radiation source is located at the focal plane of the collimating lens 4. The plane wave passes through the beam splitter 5 and the axicon 6 to generate a terahertz large-focal-depth beam with a large focal depth and is focused to the object to be measured 7. The return signal is reflected to the parabolic reflector 8 by the beam splitter 5, and the focused beam is collected to the phase center of the first-band terahertz detector 10 and the second-band terahertz detector 11 by the polarization grating 9. Then, the intermediate frequency return signals in the dual bands can be obtained by the data acquisition card.

[0043] Based on the dual-band terahertz frequency-modulated continuous wave radar imaging system, the embodiment provides a large-focal-depth terahertz beam generation principle as shown in the figure, and the spatial light intensity distribution I(ρ,z) in the cylindrical coordinate system is as follows: Figure 2

[0044]

[0045] 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 to the axicon, k=2π / λ is the wave number in free space, ω0 is the incident Gaussian beam radius, Z max is the focal depth of the beam, J0 is the zero-order Bessel function, and α0 is the half-beam angle of the beam.

[0046] The embodiment of the application also provides a large-focal-depth dual-band terahertz frequency-modulated continuous wave radar imaging intermediate frequency signal data fusion method, and the algorithm principle is as shown in the figure, and the method specifically comprises the following steps: Figure 3

[0047] Step 1: Obtain the intermediate frequency signals of the dual-band frequency-modulated continuous wave radar, and after frequency modulation processing (mixing filtering), the signal forms of the intermediate frequency return signals in the dual bands are as follows:

[0048] ​​

[0049]

[0050] Where A1 and A2 are the amplitudes of the echo signal of the first frequency band and the echo signal of the second frequency band respectively, f1 is the starting frequency of the first frequency band, f2 is the starting frequency of the second frequency band, τ1 is the echo delay of the first frequency band caused by the target, and τ2 is the echo delay of the second frequency band.

[0051] Step 2: The intermediate frequency signal gain of the two frequency bands needs to be adjusted so that the echo amplitudes of the two frequency bands are roughly equal, that is, A2A c =A1, in addition, the echo delay difference between the two frequency bands Δτ=τ1-τ2 is a constant (quadratic term and can be ignored), with the help of the frequency shift term exp(j2πKΔτt), the intermediate frequency signals of the two frequency bands can be rewritten as:

[0052] S ZF1 (t)=A1·exp[j2π(f1τ1+Ktτ1)] (4)

[0053] S ZF2-C (t) = A2A c ·exp[j2π(f2(τ1-Δτ)+Kt(τ2+Δτ))]

[0054] =A1·exp[j2π(f2τ1+Ktτ1-f2Δτ)] (5)

[0055] Step 3: From formulas (4) and (6), we know that the intermediate frequency signals of the two frequency bands differ only in phase. Therefore, with the help of the time shift term Δf / K, where Δf / K = (f2-f1) / K is a constant term, the intermediate frequency signal of the second frequency band after time shift is:

[0056]

[0057] Step 4: From formulas (4) and (6), we can see that at this time, the intermediate frequency signals of the two frequency bands only have a constant phase difference f2Δτ. After compensating for it, the intermediate frequency fusion signal S(t) of the two frequency bands can be obtained.

[0058] Step 5: Calculate the power spectrum vector W of the Fourier transform of the fusion signal S(t) (1) , and set it to W (i) The initial value of , set the number of iterations i, and calculate the autocorrelation matrix R (i) .

[0059]

[0060] Among them, N is the number of extrapolated points of time domain data, E is the basis function e-jωt constituted nuclear matrix.

[0061] Step six: calculate the range profile F (i) , discrete amplitude spectrum A (i) , and update the power spectrum weight W (i+1)

[0062] F (i) = S (R (i) ) -1 EW (i) (8)

[0063]

[0064] W (i+1) = diag (|A (i) | 2 ) (10)

[0065] Step seven: bring W (i+1) into step five to start the next iteration until the specified number of iterations i is reached, and output the high-resolution range profile F (i) .

[0066] At this point, the high-resolution range profile of the thick test sample under a large bandwidth and multiple polarizations can be obtained simultaneously, and the mechanical scanning can be combined to obtain the high-precision and high-resolution three-dimensional imaging result of the thick test sample.

[0067] The above-described embodiments are only to describe the preferred embodiments of the present application, and not to limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements of the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A large focal depth dual-band terahertz frequency modulated continuous wave radar imaging method, characterized in that: It includes the frequency band fusion-extended Fourier algorithm, which specifically includes the following steps: Step 1: According to the transmission signal and echo signal After mixing, filtering and denoising, the intermediate frequency echo signals of the two frequency bands are obtained as and ; Step 2: Gain adjustment is performed on the intermediate frequency signal of the second frequency band, using the frequency shift term , for the second frequency band intermediate frequency signal Perform frequency shift, where the echo delay difference between the two frequency bands is is a constant, is the first frequency band echo delay caused by the target, and is the echo delay of the second frequency band; Step 3: With the help of time shift term , for the second frequency band intermediate frequency signal Perform time shift, where , and are the starting frequencies of the first and second frequency bands respectively, and K is the frequency modulation slope; Step 4: Constant phase of the second frequency band intermediate frequency signal Perform compensation to complete the fusion of the intermediate frequency signals of the two frequency bands; Step 5: Set the power spectrum vector Initial value and number of iterations , and calculate the autocorrelation matrix ; Step 6: Calculate distance distribution , amplitude spectrum , and update the power spectrum ; Step 7: Bring it into step 5 to start the next iteration until the specified number of iterations is reached , output high-resolution distance distribution .

2. The large focal depth dual-band terahertz frequency modulated continuous wave radar imaging method according to claim 1, characterized in that: The signal forms of the intermediate frequency echo in step 1 are: ； ； Where A1 and A2 are the amplitudes of the echo signal of the first frequency band and the echo signal of the second frequency band respectively; is the starting frequency of the first frequency band, is the starting frequency of the second frequency band.

3. The large focal depth dual-band terahertz frequency modulated continuous wave radar imaging method according to claim 2, characterized in that: The frequency shift method in step 2 is specifically as follows: Adjust the intermediate frequency signal gain of the two frequency bands so that the echo amplitudes of the two frequency bands are roughly equal, that is, , with the help of the frequency shift term , the two-band intermediate frequency signals can be rewritten as: ； ； Among them, S ZF2-C is 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 3, characterized in that: The step three also includes: The intermediate frequency signal of the second frequency band after time shift is: ； in, is the time-shift term, is a constant term.

5. The large focal depth dual-band terahertz frequency modulated continuous wave radar imaging method according to claim 1, characterized in that: The step five also includes: Calculate fusion signal Fourier transformed power spectrum vector .

6. The large focal depth dual-band terahertz frequency modulated continuous wave radar imaging method according to claim 5, characterized in that: The step five also includes: Autocorrelation matrix is calculated as follows: ； in, is the number of extrapolation points for time domain data, is the basis function The kernel matrix.

7. The large focal depth dual-band terahertz frequency modulated continuous wave radar imaging method according to claim 6, characterized in that: The step six also includes: The distance distribution is calculated according to the following formula , discrete amplitude spectrum , and update the power spectrum weights : ； ； 。 8. A large focal depth dual-band terahertz frequency modulated continuous wave radar imaging system, characterized in that: The system is used to execute a large-focus-depth dual-band terahertz frequency-modulated continuous wave radar imaging method according to any one of claims 1 to 7, comprising: a first-band terahertz radiation source and a second-band terahertz radiation source for radiating terahertz linear frequency-modulated signals; a polarization wire grid for splitting beams in different polarization directions; a collimating lens for collimating and focusing the beam; a beam splitter for splitting incident and reflected beams; a parabolic reflector for focusing the reflected beam; an axicon; a first-band terahertz detector and a second-band terahertz detector; a data acquisition card for acquiring dual-band intermediate frequency signals; and a host computer for automated data acquisition and imaging.

9. The large focal depth dual-band terahertz frequency modulated continuous wave radar imaging system according to claim 8, characterized in that: The first frequency band terahertz radiation source radiates a vertically polarized terahertz beam, and the second frequency band terahertz radiation source 2 radiates a horizontally polarized terahertz beam.

10. The large focal depth dual-band terahertz frequency modulated continuous wave radar imaging system according to claim 9, characterized in that: The phase center of the terahertz radiation source is located at the focal plane of the collimating lens; after the plane wave passes through the beam splitter and the axicon, it produces a terahertz beam with a large focal depth and focuses on the sample to be measured. The echo signal is reflected by the beam splitter to the parabolic reflector, and the focused beam is gathered by the polarization wire grid to the phase center of the first-band terahertz detector and the second-band terahertz detector.

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