Terahertz three-dimensional imaging device and method based on near-field probe

Abstract

The invention discloses a terahertz three-dimensional imaging device and method based on a near-field probe, and belongs to the field of terahertz imaging. The device comprises a vector network analyzer, a terahertz S parameter test module, a gradient waveguide near-field probe, a sample to be tested, an XYZ scanning table, a laser displacement sensor and a feedback control unit. Transverse two-dimensional super-resolution imaging is realized by adopting a terahertz near-field probe, and depth information detection of internal defects of a to-be-detected sample is realized by adopting a stepping frequency modulation continuous wave technology.

Description

Technical Field

[0001] The present invention belongs to the field of terahertz imaging, and in particular relates to a terahertz three-dimensional imaging device and method based on a near-field probe. Background Art

[0002] Terahertz has the advantages of strong penetration and has important applications in fields such as terahertz imaging. Compared with terahertz far-field imaging technology, terahertz near-field imaging can break through the diffraction limit and achieve super-resolution imaging. However, the current terahertz near-field imaging technology can only achieve two-dimensional imaging of materials and cannot detect internal defects of materials. Therefore, it is urgent to develop a terahertz near-field three-dimensional imaging detection technology.

[0003] At present, researchers have developed a terahertz near-field imaging method based on waveguide probes. By gradually processing the end of the waveguide, the focusing of the terahertz beam is achieved. At the same time, by controlling the distance between the end of the waveguide probe and the surface of the sample to be tested to a constant value, the sample is always within the near-field range of the sample, thereby achieving terahertz near-field super-resolution imaging.

[0004] The current terahertz near-field imaging technology can only achieve two-dimensional imaging of materials, but cannot detect internal defects of materials. Summary of the invention

[0005] In view of the above technical problems existing in the prior art, the present invention proposes a terahertz three-dimensional imaging device and method based on a near-field probe, which has a reasonable design, overcomes the shortcomings of the prior art, and has good effects.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A terahertz three-dimensional imaging device based on a near-field probe comprises a vector network analyzer, a terahertz S-parameter test module, a gradient waveguide near-field probe, a sample to be tested, an XYZ scanning stage, a laser displacement sensor and a feedback control unit; wherein:

[0008] The vector network analyzer is configured to provide a radio frequency signal RF and a local oscillator signal LO for the terahertz S-parameter test module respectively;

[0009] The terahertz S-parameter test module is configured to frequency-multiply the radio frequency signal RF and the local oscillator signal LO to the terahertz band respectively, a part of the terahertz radio frequency signal is mixed with the terahertz local oscillator signal after passing through the signal separation device inside the terahertz S-parameter test module to obtain the intermediate frequency reference signal REF, and the other part of the terahertz radio frequency signal is transmitted to the terahertz gradient waveguide near-field probe;

[0010] A gradient waveguide near-field probe is configured to focus another portion of the terahertz radio frequency signal;

[0011] A laser displacement sensor is configured to test the distance between the gradient waveguide near-field probe and the sample to be tested, and transmit the test distance to the feedback control unit;

[0012] A feedback control unit configured to control the Z-direction displacement of the XYZ scanning stage;

[0013] An XYZ scanning stage, the XYZ scanning stage is configured to perform two-dimensional movement of the sample to be tested in the XY plane and control the distance between the probe and the sample to be tested in the Z direction;

[0014] The vector network analyzer provides the terahertz S parameter test module with a radio frequency signal and a local oscillator signal respectively. The terahertz S parameter test module multiplies the radio frequency signal RF and the local oscillator signal LO to the terahertz band respectively. A part of the terahertz radio frequency signal passes through the signal separation device inside the terahertz S parameter test module and is mixed with the terahertz local oscillator signal to obtain an intermediate frequency reference signal REF. The other part of the terahertz radio frequency signal is transmitted to the terahertz gradient waveguide near-field probe, and after being bunched at the end of the terahertz gradient waveguide near-field probe, it interacts with the sample to be tested and then returns to the terahertz S parameter test module, and then is mixed with the terahertz reference signal to obtain an intermediate frequency test signal MEA. The intermediate frequency reference signal REF and the intermediate frequency test signal MEA are transmitted to the vector network analyzer for ratio operation to obtain the amplitude information and phase information of the reflected signal of the scanning test point of the sample to be tested.

[0015] For the surface test of the sample to be tested, the single-frequency test mode is selected, and the amplitude information and phase information of the reflected signal are used for scanning and imaging; for the internal defect depth information test of the sample to be tested, the broadband step-sweep test is selected, and the position of the peak value of the defect reflection signal is obtained by inverse Fourier transform to obtain the depth information of the sample to be tested; during the step-scanning process, in order to keep the distance between the gradient waveguide near-field probe and the sample to be tested always within a certain distance range, a laser displacement sensor is used to test the distance between the gradient waveguide near-field probe and the sample to be tested, and the test distance is transmitted to the feedback control unit. The feedback control unit controls the Z-direction displacement of the XYZ scanning stage. When the reflection signal of the current scanning point is tested, the XY scanning stage is stepped to the next test point for testing.

[0016] In addition, the present invention also mentions a terahertz three-dimensional imaging method based on a near-field probe, which adopts a terahertz three-dimensional imaging device based on a near-field probe and specifically includes the following steps:

[0017] Step 1: Provide a radio frequency signal RF and a local oscillator signal LO to the terahertz S-parameter test module through a vector network analyzer;

[0018] Step 2: The terahertz S-parameter test module frequency-multiplies the radio frequency signal RF and the local oscillator signal LO to the terahertz band respectively. A part of the terahertz radio frequency signal is mixed with the terahertz local oscillator signal after passing through the signal separation device inside the terahertz S-parameter test module to obtain the intermediate frequency reference signal REF. The other part of the terahertz radio frequency signal is transmitted to the terahertz gradient waveguide near-field probe;

[0019] Step 3: The signal after being bunched at the end of the terahertz gradient waveguide near-field probe interacts with the sample to be tested and then returns to the terahertz S-parameter test module, and then is mixed with the terahertz reference signal to obtain the intermediate frequency test signal MEA;

[0020] Step 4: The intermediate frequency reference signal REF and the intermediate frequency test signal MEA are transmitted to the vector network analyzer, and a ratio operation is performed by the vector network analyzer to obtain the amplitude information and phase information of the reflected signal of the scanning test point of the sample to be tested;

[0021] Step 5: For the surface test of the sample to be tested, a single-frequency test mode is selected, and the amplitude information and phase information of the reflected signal are used for scanning imaging;

[0022] Step 6: For the internal defect depth information test of the sample to be tested, a broadband step-sweep test is selected, and the peak position of the defect reflection signal is obtained by inverse Fourier transform to obtain the depth information of the sample to be tested;

[0023] Step 7: Use a laser displacement sensor to test the distance between the gradient waveguide near-field probe and the sample to be tested, and transmit the test distance to the feedback control unit;

[0024] Step 8: The feedback control unit controls the Z-direction displacement of the XYZ scanning stage. After the reflection signal of the current scanning point is tested, the XY scanning stage is moved step by step to the next test point for testing.

[0025] Preferably, the XY scanning stage uses a serpentine scanning trajectory for scanning.

[0026] Beneficial technical effects brought by the present invention:

[0027] The present invention adopts a terahertz near-field probe to realize lateral two-dimensional super-resolution imaging, and adopts a stepped frequency modulation continuous wave technology to realize the depth information detection of the internal defects of the sample to be tested. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of a terahertz three-dimensional imaging device based on a near-field probe;

[0029] Figure 2 Schematic diagram of the near-field probe scanning trajectory. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0031] like Figure 1 As shown, a terahertz three-dimensional imaging device based on a near-field probe includes a vector network analyzer, a terahertz S-parameter test module, a gradient waveguide near-field probe, a sample, an XYZ scanning stage, a laser displacement sensor, a feedback control unit, etc.

[0032] The working principle of this technical solution is:

[0033] The mainframe of the vector network analyzer provides the terahertz S parameter test module with RF signal and local oscillator signal respectively. The terahertz S parameter test module multiplies the RF signal RF and the local oscillator signal LO to the terahertz band respectively. A part of the terahertz RF signal is mixed with the terahertz local oscillator signal after passing through the signal separation device to obtain the intermediate frequency reference signal REF. The other part of the terahertz RF signal is transmitted to the terahertz gradient waveguide near-field probe, which interacts with the sample to be tested after being bunched at the end of the probe and returns to the terahertz S parameter test module, and then mixed with the terahertz reference signal to obtain the intermediate frequency test signal MEA; the intermediate frequency reference signal REF and the intermediate frequency test signal MEA are transmitted to the mainframe of the vector network analyzer and then ratio calculation is performed to obtain the amplitude information and phase information of the reflected signal of the scanning test point of the sample to be tested. For the surface test of the sample to be tested, the single frequency test mode is selected, and the amplitude information and phase information of the reflected signal are used for scanning imaging; for the internal defect depth information test of the sample, the broadband step frequency sweep test is selected, and the position of the defect reflection signal peak is obtained by inverse Fourier transform to obtain the depth information of the sample to be tested. During the step-by-step scanning process, in order to keep the distance between the probe and the sample always within a certain distance range, a laser displacement sensor is used to test the distance between the probe and the sample. After the test distance is transmitted to the feedback control unit, the Z-direction displacement of the XYZ scanning stage is controlled. After the reflection signal of the current scanning point is tested, the XY scanning stage moves to the next test point for testing. The XY scanning stage adopts a serpentine scanning trajectory, such as Figure 2 shown.

[0034] The principle of the step frequency modulation continuous wave test sample defect Z-direction depth information is: Assuming that the RF signal RF and the local oscillator signal LO emitted by the vector network host are up-converted N times by the terahertz S parameter test module, the test signal and the reference signal are obtained respectively, and their normalized expressions are:

[0035]

[0036]

[0037] Among them, t i =t-iT r -τ / 2, f i =f0+iΔf.

[0038] The terahertz test signal is focused by the waveguide probe and irradiated onto the sample. It is reflected by a defect with a distance R and a reflection coefficient γ inside the sample and then transmitted to the terahertz module. The normalized reflection signal obtained is:

[0039]

[0040] The terahertz S-parameter test module mixes the terahertz test signal and the terahertz reflection signal with the terahertz reference signal to obtain normalized intermediate frequency test signals. and the normalized IF reference signal for

[0041]

[0042]

[0043] The terahertz S parameter test module transmits the intermediate frequency test signal and the intermediate frequency reference signal to the vector network host, and obtains the normalized orthogonal signal after IQ down-conversion sampling inside the vector network host.

[0044] s I (i) = γcos[-2πNf i (2R / c)] (6);

[0045] s Q (i) = γsin[-2πNf i (2R / c)] (7);

[0046] Its complex exponential form is

[0047] s(i)=γexp[-j2πNf i (2R / c)] (8);

[0048] Its inverse Fourier transform modulus is

[0049]

[0050] When k = MNΔf2R / c, |s(k)| reaches its maximum value, from which we can get the depth of the internal defect of the sample is R = kc / (2MNΔf). On this basis, the Z-direction distance resolution is δ R =c / (2MNΔf), the maximum detection distance is R max =c / (4NΔf).

[0051] Since the near-field waveguide probe used in this application is gradually processed at the end, resulting in a decrease in its antenna gain and a rapid divergence of the beam after leaving the probe end, this patent application can achieve super-resolution imaging on the sample surface and sub-surface; for internal defects of the sample, although it can achieve longitudinal depth information extraction within a certain distance range, its lateral resolution will be lower than the lateral resolution of the sample surface.

[0052] The present invention adopts a terahertz near-field probe to realize lateral two-dimensional super-resolution imaging, and adopts a stepped frequency-modulated continuous wave to realize longitudinal imaging of internal defects of the material.

[0053] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A terahertz three-dimensional imaging device based on a near-field probe, characterized in that: It includes a vector network analyzer, a terahertz S-parameter test module, a gradient waveguide near-field probe, a sample to be tested, an XYZ scanning stage, a laser displacement sensor and a feedback control unit; among them, The vector network analyzer is configured to provide a radio frequency signal RF and a local oscillator signal LO for the terahertz S-parameter test module respectively; The terahertz S-parameter test module is configured to frequency-multiply the radio frequency signal RF and the local oscillator signal LO to the terahertz band respectively, a part of the terahertz radio frequency signal is mixed with the terahertz local oscillator signal after passing through the signal separation device inside the terahertz S-parameter test module to obtain the intermediate frequency reference signal REF, and the other part of the terahertz radio frequency signal is transmitted to the terahertz gradient waveguide near-field probe; A gradient waveguide near-field probe is configured to focus another portion of the terahertz radio frequency signal; A laser displacement sensor is configured to test the distance between the gradient waveguide near-field probe and the sample to be tested, and transmit the test distance to the feedback control unit; A feedback control unit configured to control the Z-direction displacement of the XYZ scanning stage; An XYZ scanning stage is configured to perform two-dimensional movement of the sample to be tested in the XY plane and control the distance between the probe and the sample to be tested in the Z direction; The vector network analyzer provides the terahertz S parameter test module with a radio frequency signal and a local oscillator signal respectively. The terahertz S parameter test module multiplies the radio frequency signal RF and the local oscillator signal LO to the terahertz band respectively. A part of the terahertz radio frequency signal passes through the signal separation device inside the terahertz S parameter test module and is mixed with the terahertz local oscillator signal to obtain an intermediate frequency reference signal REF. The other part of the terahertz radio frequency signal is transmitted to the terahertz gradient waveguide near-field probe, and after being bunched at the end of the terahertz gradient waveguide near-field probe, it interacts with the sample to be tested and then returns to the terahertz S parameter test module, and then is mixed with the terahertz reference signal to obtain an intermediate frequency test signal MEA. The intermediate frequency reference signal REF and the intermediate frequency test signal MEA are transmitted to the vector network analyzer for ratio operation to obtain the amplitude information and phase information of the reflected signal of the scanning test point of the sample to be tested. For the surface test of the sample to be tested, the single-frequency test mode is selected, and the amplitude information and phase information of the reflected signal are used for scanning and imaging; for the internal defect depth information test of the sample to be tested, the broadband step-sweep test is selected, and the position of the peak value of the defect reflection signal is obtained by inverse Fourier transform to obtain the depth information of the sample to be tested; during the step-scanning process, in order to keep the distance between the gradient waveguide near-field probe and the sample to be tested always within a certain distance range, a laser displacement sensor is used to test the distance between the gradient waveguide near-field probe and the sample to be tested, and the test distance is transmitted to the feedback control unit. The feedback control unit controls the Z-direction displacement of the XYZ scanning stage. When the reflection signal of the current scanning point is tested, the XY scanning stage is stepped to the next test point for testing.

2. A terahertz three-dimensional imaging method based on a near-field probe, characterized in that: The terahertz three-dimensional imaging device based on a near-field probe as claimed in claim 1 specifically comprises the following steps: Step 1: Provide a radio frequency signal RF and a local oscillator signal LO to the terahertz S-parameter test module through a vector network analyzer; Step 2: The terahertz S-parameter test module frequency-multiplies the radio frequency signal RF and the local oscillator signal LO to the terahertz band respectively. A part of the terahertz radio frequency signal is mixed with the terahertz local oscillator signal after passing through the signal separation device inside the terahertz S-parameter test module to obtain the intermediate frequency reference signal REF. The other part of the terahertz radio frequency signal is transmitted to the terahertz gradient waveguide near-field probe; Step 3: The signal after being bunched at the end of the terahertz gradient waveguide near-field probe interacts with the sample to be tested and then returns to the terahertz S-parameter test module, and then is mixed with the terahertz reference signal to obtain the intermediate frequency test signal MEA; Step 4: The intermediate frequency reference signal REF and the intermediate frequency test signal MEA are transmitted to the vector network analyzer, and a ratio operation is performed by the vector network analyzer to obtain the amplitude information and phase information of the reflected signal of the scanning test point of the sample to be tested; Step 5: For the surface test of the sample to be tested, a single-frequency test mode is selected, and the amplitude information and phase information of the reflected signal are used for scanning imaging; Step 6: For the internal defect depth information test of the sample to be tested, a broadband step-sweep test is selected, and the peak position of the defect reflection signal is obtained by inverse Fourier transform to obtain the depth information of the sample to be tested; Step 7: Use a laser displacement sensor to test the distance between the gradient waveguide near-field probe and the sample to be tested, and transmit the test distance to the feedback control unit; Step 8: The feedback control unit controls the Z-direction displacement of the XYZ scanning stage. After the reflection signal of the current scanning point is tested, the XY scanning stage is moved step by step to the next test point for testing.

3. The terahertz three-dimensional imaging method based on a near-field probe according to claim 2, characterized in that: The XY scanning stage uses a serpentine scanning trajectory for scanning.

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