A terahertz continuous wave beam calibration method and imaging device for high dielectric materials

Abstract

The present invention discloses a terahertz continuous wave beam calibration method and imaging device for high dielectric materials. The imaging device mainly comprises: a terahertz continuous wave transceiver module, a terahertz optical path scanning module, and a main control host module. The calibration method mainly comprises the following steps: first, the terahertz transceiver module is initialized and set to form a Gaussian beam that is approximately a ray, and a parameter S that can characterize the reflectivity is obtained. 11 and transmittance parameter S 21 ; Then, the parameter S of the terahertz wave during vertical total reflection is measured 11 S 11全 ; The terahertz beam scans the object in three dimensions and records the S at each position 11 and S 21 ; Use S 11 The maximum value in the dielectric constant is combined with the relationship between the dielectric constant and the reflection coefficient to deduce the dielectric constant of the material; 21 The jump of S is close to the actual outline of the object; the reflectivity, refraction angle and optical path difference of each position of the object are calculated using the above information, and then S is corrected. 21 '; Combined with scanning correction S 21 ' and tomography algorithms to reconstruct the internal structure of the imaged object.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz continuous wave tomography, and in particular relates to a terahertz continuous wave beam calibration method and an imaging device for high dielectric materials. Background Art

[0002] The terahertz (THz) band refers to a frequency band from 100GHz to 10THz, which has not yet been widely used. Its spectrum is distributed between millimeter waves and infrared light. Since terahertz waves are between electronics and optics, they have many special properties. For example, many biological macromolecules have absorption peaks in the terahertz band, and the organisms themselves also generate energy in the terahertz band. Therefore, characteristic resonance and absorption can be used to identify organisms; since the photon energy of terahertz waves is very low and they are non-ionizing electromagnetic waves, they will not cause damage to biological irradiation, so non-destructive testing of organisms can be performed; due to their high frequency characteristics, terahertz waves have strong penetration ability for non-metallic non-polar materials, so transmission imaging can be performed. In summary, terahertz has broad application prospects in physical sciences, life sciences, electronic information, and national defense science and technology.

[0003] Terahertz imaging technology started in the mid-1990s and has been developed for 20 years. Compared with X-ray imaging, terahertz imaging does not cause harm to the human body and can provide better contrast for soft materials. Compared with ultrasonic imaging, it is completely non-contact, has a wider frequency band, carries more information in the signal, and is more suitable for testing materials with severe sound wave attenuation. Compared with visible light and infrared imaging, terahertz radiation has stronger penetration into objects such as foam, ceramics, plastics, polymer materials, and magnetic materials, and is sometimes even better than ultrasound, while X-rays penetrate these materials and their internal defects too much and cannot be clearly distinguished; the operating frequency band of terahertz imaging is generally between 100GHz and 3THz, while microwave imaging is mostly between 40 and 90GHz. The higher terahertz frequency increases its imaging resolution by 10 to 50 times, thereby being able to detect tiny defects that microwave imaging cannot distinguish. In summary, terahertz imaging technology has demonstrated unique advantages with its safety, effectiveness and high resolution, and is suitable for detecting non-metallic materials. It is a new type of non-destructive testing supplement.

[0004] Terahertz tomography is similar to tomography based on X-rays. By utilizing the properties of terahertz waves, such as their ability to penetrate dielectrics, and using tomographic reconstruction algorithms, two-dimensional and three-dimensional imaging of the internal structure of an object can be achieved.

[0005] At present, terahertz tomography technology is mostly based on X-ray CT technology. However, the propagation mode of X-rays cannot be fully applied to terahertz waves. When X-rays pass through matter, they propagate in a straight line, and almost no reflection and refraction occur. However, when terahertz waves penetrate matter, they will produce reflection and refraction on the surface of the matter. When they are incident at different incident angles, their reflectivity is different and the deflection caused by refraction is also different, which puts forward higher requirements for image reconstruction and beam detection. For the above reasons, in the early research of terahertz CT, it was found that when imaging complex samples with high dielectric constants (such as turkey bones and polytetrafluoroethylene), their internal structure could not be displayed, which was caused by strong reflection and refraction at the boundary. Therefore, in view of the characteristics of terahertz waves themselves, when imaging high dielectric constant objects, the refraction offset and reflection loss of the samples will greatly affect the imaging quality. Due to the existence of refraction and reflection problems, terahertz tomography is currently mostly studied using low refractive index samples, which greatly limits its application and development. Summary of the invention

[0006] The technology of the present invention solves the problem: to overcome the shortcoming of poor imaging effect of terahertz continuous wave tomography under high dielectric materials, to provide a correction method for terahertz continuous wave beam, and to divide the error into reflection loss and refraction optical path difference, and to use S in S parameter 11 The relative dielectric constant of the object can be obtained by the relationship between the parameters and the relative dielectric constant and the reflection coefficient. Then, the general outline of the object can be inverted by judging whether the terahertz continuous wave beam produces attenuation. The size of the incident angle can be obtained by using the relationship between the boundary and the beam. According to the relationship between the reflection coefficient and the relative dielectric constant and the angle, and the relationship between the refraction angle and the incident angle and the dielectric constant, the light path transmission coefficient S without refraction and reflection can be calculated theoretically. 21 ′, thereby performing beam calibration.

[0007] The technical solution of the present invention is: a terahertz continuous wave beam calibration method and imaging device for high dielectric materials, the imaging device comprising:

[0008] The terahertz continuous wave transceiver module is used to generate and receive terahertz continuous wave signals and form a Gaussian beam that is regarded as a ray model. It is also used to process the transmitted and received terahertz continuous wave signals to obtain the S parameters S required for calibration and imaging. 11 With S 21 , where S 11 It describes the ratio of the terahertz continuous wave signal strength received by the transmitter to the terahertz continuous wave signal strength emitted, S 21 It describes the ratio of the terahertz continuous wave signal strength received by the receiving end to the terahertz continuous wave signal strength transmitted.

[0009] The terahertz continuous wave optical path scanning module is used to move the object in three dimensions, thereby forming a three-dimensional scanning of the object by the light beam.

[0010] The main control host module is used to control the three-dimensional movement of the object, correct the scanned parameters, and use the imaging algorithm to reconstruct the image of the internal structure of the object.

[0011] Another aspect of the present invention provides a terahertz continuous wave beam calibration method for high dielectric constant materials as follows:

[0012] S1. Initialize and set the terahertz continuous wave transceiver module so that its optical path can form a Gaussian beam that can be approximated as a ray at the imaging object.

[0013] S2. Place a flat plate in the imaging area that can fully reflect the THz continuous wave beam perpendicular to the optical path of the THz continuous wave and measure the parameter S at this time. 11 , and denoted as S 11全 .

[0014] S3. Translate and rotate the imaging target object, so that the terahertz continuous wave beam performs a three-dimensional scan of the imaging target object and records the parameters S at each position 11 and parameter S 21 .

[0015] S4. 11 The maximum value in is denoted as S 11max , and assumes that it is the reflection coefficient of the terahertz continuous wave beam vertically incident on the surface of the target object. The relationship between the vertical reflection coefficient and the dielectric constant can be used to calculate the dielectric constant of the target object, and the S of (1) can be used to calculate the dielectric constant of the target object. 11全 The actual dielectric constant is calibrated. The specific calculation method of the actual dielectric constant is as follows:

[0016]

[0017] S5. Record the S 21 Divided into two categories, S 21 Equal to S 21max is denoted as a, and the rest are S 21 Denoted as b, for each angle, a range width of b can be obtained. By subtracting half of the beam width on both sides of this range width, the actual range width of the target object can be obtained. Then the actual range widths of each angle are overlapped and their intersection is taken to obtain a circumscribed polygon of a certain cross-section of the target object to be measured. The number of its sides is related to the angle. In theory, the more angles recorded, the more sides the circumscribed polygon has, and the more accurate the restored image of this cross-section of the object to be measured will be.

[0018] S6. After obtaining the external contour and dielectric constant of the target object, the actual attenuation value of the object is calculated and corrected using the reflection coefficient formula and the refraction angle formula. The correction method is as follows:

[0019]

[0020] Among them, Γ ⊥ is the reflection loss, L is the actual optical path, L′ is the optical path without refraction, Γ ⊥ The calculation formula is:

[0021]

[0022] Among them, θ i is the angle of incidence.

[0023] S7. Finally, using the corrected S 21 ′ Combined with imaging algorithms, the three-dimensional image of the internal structure of the object is reconstructed.

[0024] In step S2, S is measured 11全 The purpose is: Since different terahertz continuous wave tomography systems have different optical paths, and the loss of terahertz continuous waves in the air cannot be ignored, 1-S 11全 That is the path loss of the terahertz continuous wave during reflection, so using S 11全 In step S4, the relative dielectric constant calculation formula may be calibrated.

[0025] In step S5, S 21 The physical meaning of the point marked as a is that the terahertz continuous wave is received by the receiving end without loss at the object to be tested. 21 , so S here 21 is the maximum value, and S 21 The physical meaning of the point denoted as b is that at least a part of the terahertz continuous wave beam passes through the object to be measured, resulting in loss. Therefore, S 21 Less than S 21max .

[0026] The advantages and innovations of the present invention are: a method for calibrating the beam of terahertz continuous wave tomography is proposed, and since the correction work is performed using S parameters, no other external equipment is required. At the same time, the method can be used to invert the contour information of the imaging object and estimate the size of the dielectric constant of the object. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a flow chart of the method of the present invention;

[0028] Figure 2 is a schematic diagram of the imaging system;

[0029] Figure 3 It is a schematic diagram of the profile scanning of the present invention;

[0030] Figure 4 is a schematic diagram of error correction;

[0031] Figure 5 This is a schematic diagram illustrating the function of the cylinder example. DETAILED DESCRIPTION

[0032] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0033] like Figure 2 As shown, according to Figure 2 The system diagram shown and Figure 1 The specific implementation steps of the present invention are as follows:

[0034] (1) Place a metal surface perpendicular to the terahertz continuous wave beam on a three-dimensional translation and rotation stage, turn on the vector network analyzer, and measure the network parameter S at this time 11 , denoted as S 11全 .

[0035] (2) Place the object to be tested on the three-dimensional translation and rotation stage, move it along the X-axis at a certain step length, and record the grid S at each step length point. 11 and S 21 .

[0036] (3) Rotate a certain angle and repeat step (2).

[0037] (4) In all recorded S 11 Take the maximum value among them and record it as S 11max , default S 11max The terahertz continuous wave beam is incident vertically on the object to be measured, and the measured reflection coefficient. The dielectric constant of the object can be calculated from the relationship between the vertically incident reflection coefficient and the dielectric constant. The specific formula is as follows:

[0038]

[0039] where ε r is the relative dielectric constant of the object to be measured.

[0040] (5) Considering the loss of terahertz continuous wave in air, the S measured by total reflection of metal in (1) 11全 By modifying the formula in (4), we can obtain:

[0041]

[0042] (6) Figure 3 As shown, the recorded S 21Divided into two categories, S 21 Equal to S 21max is denoted as a, and the rest are S 21 Denoted as b, for each angle when b just jumps to a and the two points where a just jumps to b, record the distance between the two bs to get a range width W1, subtract half the beam width R on both sides of this range width to get the actual range width W2 of the object, measure the actual range width W2 of each angle and then overlap them to take their image intersection, and a circumscribed polygon of a cross-section contour of the object to be measured can be restored. The number of its sides is related to the number of measured angles. In theory, the more angles are measured, the more sides the circumscribed polygon has, and the more accurate the restored image contour of this cross-section of the object to be measured will be.

[0043] (7) Figure 4 As shown in the figure, after restoring the cross-sectional image profile of the object to be measured, the incident angle θ can be calculated using the relationship between the beam and the cross-sectional profile. i , and then the ε calculated in (5) r By the following formula:

[0044]

[0045] Calculate Figure 4 Medium refraction angle θ t The size of the incident angle θ i , refraction angle θ t , and the cross-sectional profile, the refractive path length L and the non-refractive path length L′ can be obtained through geometric calculation, and the reflection loss can be obtained through the calculation formula of the reflection coefficient:

[0046]

[0047] So the final corrected expression is:

[0048]

[0049] Using the obtained S 21 ′ describes the attenuation value actually generated by the terahertz continuous wave in the object, and combines the tomography algorithm to reconstruct the image of the internal structure of the object.

[0050] Figure 5The figure is a schematic diagram for explaining the role of the cylinder example, in which the circular terahertz tomography target object on the left is a schematic diagram of the imaging of a cross-section of the cylinder. Since the incident angle of the terahertz continuous wave beam at the boundary is close to 90 degrees, the reflection coefficient and the refraction angle are too large, so that the receiving module cannot receive the terahertz continuous wave signal. The imaging algorithm will misjudge the attenuation here as extremely strong, resulting in an edge effect in the imaging pattern. At this time, the cylinder may be misjudged as a "cup". The method of this patent can reduce or eliminate the edge effect and improve the image quality to achieve the effect of the circle on the right.

[0051] The present invention is not limited to the above-mentioned optimal implementation mode. Anyone can derive other various forms of products under the inspiration of the present invention. However, no matter what changes are made in the shape or structure, all technical solutions that are the same or similar to those of the present application fall within the protection scope of the present invention.

Claims

1. A terahertz continuous wave beam calibration method for high dielectric materials, characterized in that: The imaging device includes: The terahertz continuous wave transceiver module includes a transmitter and a receiver, which are used to transmit and receive terahertz continuous wave signals respectively, and form a Gaussian beam regarded as a ray model. At the same time, it is used to process the transmitted and received terahertz continuous wave signals respectively to obtain the parameters S required for Gaussian beam calibration and imaging of the terahertz continuous wave. 11 With parameter S 21 , where S 11 It is the ratio of the terahertz continuous wave signal strength received by the transmitter to the terahertz continuous wave signal strength transmitted, S 21 It is the ratio of the terahertz continuous wave signal strength received by the receiving end to the terahertz continuous wave signal strength transmitted; A terahertz optical path scanning module is used to move the target object in three dimensions, thereby forming a terahertz Gaussian beam to scan the target object in three dimensions; The main control host module is used to control the three-dimensional movement of the target object and the scanned parameters S 11 With parameter S 21 Perform correction processing and use imaging algorithms to reconstruct images of the internal structure of the object; The method comprises the following steps: S1. Initialize and set the terahertz continuous wave transceiver module so that its optical path can form a Gaussian beam that can be approximated as a ray at the imaging target object; S2. Place a plate that can fully reflect the terahertz wave in the imaging area perpendicular to the optical path of the terahertz continuous wave and measure the parameter S at this time. 11 , and denoted as S 11全 ; S3. Scan the imaging target object in three dimensions and record the parameters S corresponding to each position. 11 and parameter S 21 ; S4. 11 The maximum value in is denoted as S 11max , and assumes that it is the reflection coefficient generated by the terahertz continuous wave beam incident vertically on the surface of the target object. The relationship between the vertical reflection coefficient and the dielectric constant can be used to calculate the dielectric constant of the target object, and the S in S2 can be used to calculate the dielectric constant of the target object. 11全 The actual dielectric constant is calibrated. The specific calculation method of the actual dielectric constant is as follows: S5. Record the S 21 Divided into two categories, S 21 Equal to S 21max is denoted as a, and the rest are S 21 Denoted as b, a range width of b can be obtained for each angle. By subtracting half of the beam width on both sides of this range width, the actual range width of the target object can be obtained. Then the actual range widths of each angle are overlapped and their intersection is taken to obtain a circumscribed polygon of a cross section of the target object to be measured; S6. After obtaining the external contour and dielectric constant of the target object, the actual attenuation value of the object is calculated and corrected using the reflection coefficient formula and the refraction angle formula. The correction method is as follows: Among them, Γ ⊥ is the reflection loss, L is the actual optical path, L′ is the optical path without refraction, Γ ⊥ The calculation formula is: Among them, θ i is the angle of incidence; S7. Finally, using the corrected S 21 ′ Combined with imaging algorithms, the three-dimensional image of the internal structure of the object is reconstructed.

Images