Bezier curve lens for terahertz high-resolution large-depth-of-field imaging

Abstract

The invention discloses a Bessel curve lens for terahertz high-resolution large-depth-of-field imaging. The outline of the Bessel curve lens is an axisymmetric curved surface formed by rotating a second-order Bessel curve around a central shaft by 360 degrees; the rotational symmetry curved surface of the Bessel curve lens is of an aspheric surface structure, has the characteristic of gradient curvature, and can regulate and control the spatial distribution of terahertz waves to generate terahertz jet flow. The positions of the Bezier curve control points are adjusted to be freely and flexibly designed, the Bezier curve lens obtained at different control point positions can generate over-jet jets with different effective lengths, focal lengths, working distances and full width at half maximum, the imaging requirements of different scenes can be met, and the practicability is high. The over-jet jet flow generated by the Bezier curve lens can achieve a longer effective length while maintaining a narrower full width at half maximum.

Description

Technical Field

[0001] The present invention belongs to the technical field of Bessel curve lenses, and particularly relates to a Bessel curve lens for terahertz high-resolution large-depth-of-field imaging. Background Art

[0002] Terahertz (THz) waves occupy a very special position in the electromagnetic spectrum. It refers to electromagnetic waves with frequencies in the range of 0.1 - 10 THz (1 THz = 10^12 Hz). Its long wavelength band is adjacent to microwaves, and its short wavelength band is adjacent to infrared rays. Due to its special position in the electromagnetic spectrum, terahertz waves have some unique properties that infrared and microwaves do not have simultaneously. Such as low energy: referring to low single-photon energy and no light damage to biological tissues; transient nature: the typical pulse width is on the picosecond scale, with high time resolution and signal-to-noise ratio; broadband nature: most terahertz sources can emit broadband signals ranging from GHz to dozens of THz, which can be used for spectral detection. High penetrability: terahertz waves have strong penetration ability for organic substances and non-polar substances. Due to the above characteristics, terahertz waves have broad application prospects in the fields of imaging, medical diagnosis, communication, biochemistry, security detection, etc.

[0003] There have been various methods to improve the resolution of terahertz imaging. THz near-field imaging, THz confocal scanning imaging, THz image restoration processing, and metamaterial superlenses, etc., severely limit the application scope of high-resolution terahertz imaging technology due to problems such as complex equipment, large losses in energy and spectral bandwidth, and difficulty in information extraction.

[0004] Existing lenses for generating terahertz jet effects are mostly spherical, hemispherical, cubic, conical, etc. However, the effective length of the terahertz jets generated by the above structures is very short (usually less than 3λ). Although the full width at half maximum of the terahertz jets can reach 0.4λ - 0.5λ for high-resolution imaging, there is almost no depth of field. During imaging, the sample must be closely attached to these lenses, increasing the risk of sample contamination and posing high requirements for experimental operation and accuracy. In addition, there are already conical lenses designed with spherical harmonic functions. Although they can generate terahertz jets with relatively long effective lengths, the full width at half maximum at the focus reaches dozens or even hundreds of wavelengths and cannot be used for high-resolution imaging.

[0005] As described above, first, compared with the Bessel curve lens, the lens shapes such as spherical, hemispherical, cubic, and conical are relatively fixed and difficult to produce large changes. Subsequently, the terahertz jets generated by these lenses are very short. When imaging, the sample must be close to these lenses, increasing the risk of sample contamination and posing high requirements for experimental operations and precision. Then, lenses such as spherical, hemispherical, cubic, and conical have certain requirements for processing accuracy and usually cannot be fabricated using low-cost 3D printing technology. Secondly, changing parameters such as the material and size of these lenses has limited effects on adjusting the effective length, focal length, working distance, and full width at half maximum (FWHM) of the terahertz jets generated by the lenses, and cannot significantly change the effective length, focal length, working distance, and FWHM of the terahertz jets, making it difficult to meet practical applications. In addition, although the conical lens designed using the spherical harmonic function can generate terahertz jets with relatively long effective lengths, the FWHM is relatively wide and cannot be used for high-resolution imaging. Summary of the Invention

[0006] In order to overcome the deficiencies of the above-mentioned prior art, the purpose of the present invention is to provide a Bessel curve lens for terahertz high-resolution large-depth-of-field imaging. Through the free and flexible design by adjusting the positions of the Bessel curve control points, the Bessel curve lenses obtained with different control point positions can generate terahertz jets with different effective lengths, focal lengths, working distances, and FWHMs, which can meet the imaging requirements of different scenarios and have strong practicability. The terahertz jets generated by the Bessel curve lens can achieve a long effective length while maintaining a narrow FWHM.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is:

[0008] A Bessel curve lens for terahertz high-resolution large-depth-of-field imaging, the contour of the Bessel curve lens is an axially symmetric surface formed by rotating a second-order Bessel curve around the central axis (y-axis) by 360°; the axially symmetric surface of the Bessel curve lens is an aspherical structure with a gradually changing curvature characteristic, which can regulate the spatial distribution of terahertz waves and generate terahertz jets.

[0009] The Bessel curve is designed using the Bessel curve formula, and the Bessel curve formula is as follows:

[0010] B(t) = (1 - t) 2 P0 + 2t(1 - t)P1 + t 2 P2 (t ∈ [0, 1])

[0011] where t is time, P0 is the starting point, P2 is the ending point, and P1 is the control point. By adjusting the coordinates (x1, y1) of P1, the curvature distribution on the lens surface can be dynamically changed;

[0012] Fix the starting point P0 at the origin (0, 0), fix the end point P2 on the x-axis at a distance R from the origin, and only adjust the coordinates (x1, y1) of P1. Gradually increase the x1 and y1 coordinate values of P1 from 0 to the maximum value R in the curve drawing software; when adjusting P1, fix the x1 coordinate and only change the y1 coordinate, or fix the y1 coordinate and only change the x1 coordinate; after each adjustment of P1, export and save the obtained curve coordinate data; subsequently, read out the curve coordinate data in the drawing software to draw the curve, and then rotate it 360° around the central axis (y-axis) to obtain the three-dimensional model of the corresponding Bessel curve lens.

[0013] The said R is less than 10 wavelengths, and x1 and y1 are less than R.

[0014] Import the STL format file of the three-dimensional model into the simulation software to perform numerical simulation of the terahertz wave irradiating the Bessel curve lens, and obtain the terahertz jet generated by the Bessel curve lens;

[0015] According to the simulation results, repeatedly adjust the position of the control point P1 to optimize the Bessel curve lens until the terahertz jet generated by the Bessel curve lens has a longer effective length while maintaining a narrow full width at half maximum, ensuring that the imaging has a relatively high resolution and a large depth of field.

[0016] Place the Bessel curve lens on the propagation path of the terahertz wave to generate the terahertz jet effect for terahertz imaging.

[0017] An imaging method for a Bessel curve lens for terahertz high-resolution and large-depth-of-field imaging includes the following steps;

[0018] Step 1: Select a terahertz light source:

[0019] First, select a suitable terahertz light source according to the imaging requirements;

[0020] Step 2: Design a terahertz imaging system and build an imaging optical path:

[0021] Design a suitable terahertz imaging system, and sequentially set a terahertz light source, a first lens, a Bessel curve lens, an imaging sample, a second lens, a third lens, and a detector in the imaging optical path;

[0022] The first lens is used to collimate the divergent terahertz wave generated by the terahertz light source, convert it into a collimated parallel beam, and improve the directivity of the incident wave; the second lens is used to collimate the transmitted terahertz wave of the imaging sample to avoid signal distortion and ensure that the phase and amplitude information of the signal received by the detector accurately reflect the characteristics of the sample; the third lens is used to focus the collimated transmitted terahertz wave onto the detector to improve the signal-to-noise ratio of the detector.

[0023] Step 3: Select a suitable imaging sample:

[0024] Prepare a suitable imaging sample. The thickness of the imaging sample should be less than or equal to the effective length of the terahertz jet generated by the Bessel curve lens used in the imaging optical path to meet the requirements of large depth-of-field imaging;

[0025] Step 4: Design and manufacture a Bessel curve lens:

[0026] Design a Bessel curve using the Bessel curve formula, import the curve into a drawing software and rotate it 360 degrees around the central axis (y-axis) to obtain a three-dimensional model of the Bessel curve lens. Subsequently, select a suitable 3D printing material and use 3D printing technology to manufacture the Bessel curve lens, which is used to couple terahertz waves;

[0027] Step 5: Place the Bessel curve lens in the imaging optical path and generate a terahertz jet effect:

[0028] Place the manufactured Bessel curve lens in the designed imaging optical path. After the incident terahertz wave irradiates on the Bessel curve lens, due to the curvature distribution on the surface of the Bessel curve lens, the Poynting vector of the incident terahertz wave deflects, and a terahertz jet effect is generated after focusing behind the Bessel curve lens, thereby regulating the terahertz light field distribution;

[0029] Step 6: Detect and record terahertz signals:

[0030] Select a suitable detector, such as a pyroelectric detector, a CCD camera, etc., to detect the transmitted terahertz signal of the imaging sample, and record the detected terahertz signal through a data acquisition device. The detected signal includes information on amplitude and phase;

[0031] Step 7: Display and analyze terahertz images:

[0032] Display and analyze the recorded terahertz signals. Use image processing and analysis techniques to extract and interpret the characteristics and structural information of the sample to be measured.

[0033] In the said Step 1, the terahertz light source includes a terahertz laser and a terahertz pulse generator.

[0034] In the optical path of the said Step 2, the lens is made of a material with low absorption and a refractive index not exceeding 2 in the terahertz band. Select PE or HDPE materials. Select a detector according to the imaging method. For point-scanning imaging, select a pyroelectric detector and read the amplitude using an oscilloscope;

[0035] For fast imaging, select a CCD camera as the detector and read the acquired data in the supporting computer software; then, build the imaging optical path according to the designed system and calibrate the optical path.

[0036] The said Step 4 is specifically:

[0037] First, design a Bezier curve using the Bezier curve formula. The Bezier curve formula is as follows:

[0038] B(t) = (1 - t) 2 P0 + 2t(1 - t)P1 + t 2 P2 (t ∈ [0, 1])

[0039] Fix the starting point P0 at the origin (0, 0), and fix the ending point P2 at a distance R from the origin on the x-axis. Only adjust the coordinates (x1, y1) of P1, where R is generally less than 10 wavelengths, and x1 and y1 are less than R. When adjusting P1, fix the x1 coordinate and only change the y1 coordinate; or fix the y1 coordinate and only change the x1 coordinate.

[0040] After each adjustment of P1, export and save the obtained curve coordinate data. Subsequently, read the curve coordinate data in a drawing software to draw the curve, and then rotate it 360° around the central axis (y-axis) to obtain the 3D model of the Bezier curve lens.

[0041] Import the STL format file of the 3D model into a simulation software to perform numerical simulation of terahertz wave irradiation on the Bezier curve lens, obtain the terahertz jet generated by the Bezier curve lens, and analyze the changing trends of the focal length, maximum intensity, effective length, and full width at half maximum of the jet. Subsequently, select a suitable 3D printing material and use 3D printing technology to manufacture the Bezier curve lens, which will be used to couple terahertz waves.

[0042] Specifically, step 7 is as follows:

[0043] Preprocess the data through noise suppression and geometric calibration to improve the signal-to-noise ratio. Subsequently, draw the terahertz signal through a plotting function, reconstruct the terahertz image by combining the scanning position and signal delay, and then extract and analyze the sample features through edge detection and region segmentation algorithms.

[0044] Advantages of the present invention:

[0045] 1. First, the Bezier curve lens is designed based on the second-order Bezier curve. There is a ready-made formula, and the design process is simple and easy. Moreover, it can be fabricated using 3D printing technology, with low lens cost and easy production.

[0046] 2. Second, compared with the terahertz jet effect generated by traditional lenses, the Bezier curve lens designed in the present invention has a rotationally symmetric surface that is an aspherical structure with a gradually changing surface curvature. By adjusting the coordinates (x1, y1) of P1, the curvature and thickness of the lens can be changed, enabling the regulation of the spatial distribution of terahertz waves and generating a terahertz jet with a longer effective length, so that the sample does not need to be in close contact with the lens, reducing the probability of contaminating the sample and the difficulty of experimental operations.

[0047] 3. Additionally, by importing the three-dimensional model of the Bessel curve lens designed in step 4 into a simulation software for numerical simulation of terahertz plane wave incidence, repeat step 4 according to the simulation results, continuously adjust the position of the control point P1 to optimize the Bessel curve lens until the terahertz jet generated by the Bessel curve lens has a longer effective length while maintaining a narrow full width at half maximum. An appropriate control point position can make the terahertz jet generated by the Bessel curve lens maintain an effective length of approximately 10λ while controlling the full width at half maximum within 0.419λ, which can significantly expand the imaging depth of field at high resolution and obtain a larger imaging result while maintaining a high resolution.

[0048] 4. Finally, by adjusting the position of the Bessel curve control points, the characteristics such as the focal length, effective length, and full width at half maximum of the terahertz jet generated by the Bessel curve lens can be precisely regulated, enabling the terahertz imaging system to have the ability to adapt to different scenario requirements and expanding the applicability of terahertz high-resolution and large-depth-of-field imaging by utilizing the terahertz jet effect. Description of the Drawings

[0049] Figure 1 (a) is a schematic diagram of the Bessel curve; Figure 1 (b) is a three-dimensional structure schematic diagram of the Bessel curve lens.

[0050] Figure 2 is a schematic diagram of the experimental configuration for terahertz high-resolution and large-depth-of-field imaging with the Bessel curve lens.

[0051] Figure 3 is a schematic diagram of the terahertz jet generated by the Bessel curve lens with the P1 coordinates of x1 = 3 mm and y1 = 4 mm

[0052] Figure 4 are the numerical simulation results of the terahertz jet effect generated by the Bessel curve lens at different y1 coordinate positions of the control points. (a) P1 coordinates are x1 = 3 mm and y1 = 2 mm; (b) P1 coordinates are x1 = 3 mm and y1 = 3 mm; (c) P1 coordinates are x1 = 3 mm and y1 = 5 mm; (d) P1 coordinates are x1 = 3 mm and y1 = 6 mm.

[0053] Figure 5 are the numerical simulation results of the terahertz jet effect generated by the Bessel curve lens at different x1 coordinate positions of the control points. (a) P1 coordinates are x1 = 1 mm and y1 = 4 mm; (b) P1 coordinates are x1 = 2 mm and y1 = 4 mm; (c) P1 coordinates are x1 = 4 mm and y1 = 4 mm; (d) P1 coordinates are x1 = 5 mm and y1 = 4 mm. Detailed Implementation Manner

[0054] The present invention will be further described in detail below with reference to the accompanying drawings.

[0055] The present invention designs a lens based on the linear design of the second-order Bezier curve for realizing terahertz high-resolution and large-depth-of-field imaging.

[0056] First, the Bezier curve is designed using the Bezier curve formula, and the Bezier curve formula is as follows:

[0057] B(t) = (1 - t) 2 P0 + 2t(1 - t)P1 + t 2 P2 (t ∈ [0, 1])

[0058] The starting point P0 is fixed at the origin (0, 0), and the ending point P2 is fixed at a distance R from the origin on the x-axis. Only the coordinates (x1, y1) of P1 are adjusted, where R is generally less than 10 wavelengths, and x1 and y1 are less than R. The adjustment of P1 can fix the x1 coordinate and only change the y1 coordinate, or fix the y1 coordinate and only change the x1 coordinate; after each adjustment of P1, the obtained curve coordinate data is exported and saved; subsequently, in the drawing software, the curve coordinate data is read out and the curve is drawn, and then it is rotated 360° around the central axis (y-axis) to obtain the three-dimensional model of the corresponding Bezier curve lens.

[0059] By importing the STL format file of the three-dimensional model into the simulation software to perform numerical simulation of terahertz wave irradiating the Bezier curve lens, the terahertz jet generated by the Bezier curve lens can be obtained, and the changing trends of the focal length, maximum intensity, effective length, and full width at half maximum of the jet can be analyzed. According to the simulation results, the position of the control point P1 is repeatedly adjusted to optimize the Bezier curve lens until the terahertz jet generated by the Bezier curve lens has a longer effective length while maintaining a narrower full width at half maximum. Subsequently, the lens can be obtained by processing a material with low absorption and high transmittance in the terahertz band through 3D printing or other means.

[0060] By fixing the x1 coordinate and only changing the y1 coordinate; or fixing the y1 coordinate and only changing the x1 coordinate to adjust the position of the Bezier curve control point P1, a series of Bezier lenses with different surface shapes can be obtained. The terahertz jets generated by these lenses have different effective lengths, focal lengths, working distances, and full widths at half maximum, and can meet the imaging requirements in different scenarios.

[0061] The Bezier curve lens proposed by the present invention can generate a terahertz jet with a longer effective length and a narrower full width at half maximum for realizing terahertz high-resolution and large-depth-of-field imaging.

[0062] First, a second-order Bezier curve needs to be drawn, which is determined by the second-order Bezier curve formula. The obtained Bezier curve is as shown in the appendix Figure 1(a). Next, the drawn Bezier curve needs to be rotated 360 degrees around the central axis (y-axis) in the drawing software to obtain the 3D model of the Bezier curve lens, and its 3D structure is as shown in Figure 1 (b). Then, select a material with low absorption in the terahertz band and use 3D printing technology to fabricate the lens. Finally, place the Bezier curve lens on the propagation path of the terahertz wave to generate the terahertz jet effect for terahertz imaging. Attached Figure 2 shows the schematic diagram of the optical path, illustrating how to use the Bezier curve lens for high-resolution and large-depth-of-field terahertz imaging.

[0063] To complete the terahertz high-resolution and large-depth-of-field imaging process proposed in this invention, the experimental operations should include the following steps:

[0064] Step 1: Select a terahertz light source:

[0065] First, a suitable terahertz light source needs to be selected according to the imaging requirements. Common light sources include terahertz lasers and terahertz pulse generators, etc.

[0066] Step 2: Design a terahertz imaging system and build the imaging optical path:

[0067] Design a suitable terahertz imaging system, reasonably select optical elements (such as lenses, gratings, etc.), detectors (such as pyroelectric detectors, CCD cameras, etc.) and data acquisition devices. Then, build the imaging optical path according to the designed system and calibrate the optical path.

[0068] Step 3: Select a suitable imaging sample:

[0069] Prepare a suitable imaging sample, which needs to have a certain thickness to meet the requirements of large-depth-of-field imaging.

[0070] Step 4: Design and manufacture a Bezier curve lens:

[0071] Use the Bezier curve formula to design a Bezier curve, import the curve into the drawing software and rotate it 360 degrees around the central axis (y-axis) to obtain the 3D model of the Bezier curve lens. Subsequently, select a suitable 3D printing material and use 3D printing technology to manufacture the Bezier curve lens, which will be used to couple terahertz waves.

[0072] Step 5: Place the Bezier curve lens in the imaging optical path and generate the terahertz jet effect:

[0073] Place the manufactured Bezier curve lens in the designed imaging optical path, in front of the imaging sample to generate the terahertz jet effect, thereby regulating the terahertz light field distribution.

[0074] Step 6: Detect and record terahertz signals:

[0075] Select a suitable detector, such as a pyroelectric detector, a CCD camera, etc., to detect the transmitted terahertz signal of the sample, and record the detected terahertz signal through a data acquisition device. The detected signal may include information such as amplitude and phase.

[0076] Step 7: Display and analyze the terahertz image:

[0077] Display and analyze the recorded terahertz signal, and use image processing and analysis techniques to extract and interpret the characteristics and structural information of the sample to be measured.

[0078] These steps will ensure the smooth progress of the experiment and obtain terahertz imaging results with high resolution and large depth of field. In addition, the characteristics of the terahertz jet generated by the Bessel curve lens proposed by the present invention need to be described and explained in detail.

[0079] As shown in the appendix Figure 3 The terahertz jet generated by the Bessel curve lens has the following characteristics:

[0080] (1) Focal length: The focal length is defined as the distance from one side of the plane of the Bessel curve lens to the position of the maximum intensity at the focal point along the z-axis direction. The focal point position depends on the positions of the Bessel curve control points. Increasing x1 or increasing y1 will cause the focal point position to move closer to the lens, which allows adjusting the focal point position as needed to meet the requirements of specific applications.

[0081] (2) Effective length: The effective length refers to the distance between the two positions where the maximum light intensity of the focal spot drops to 1 / e of the maximum value. It describes the spatial range of the terahertz jet and is related to the depth of field of the imaging system.

[0082] (3) Full width at half maximum: The full width at half maximum refers to the full width at half peak of the focal point and is related to the resolution of the imaging system.

[0083] To illustrate the influence of adjusting the positions of the second-order Bessel curve control points on the properties of the generated terahertz jet, while keeping other structural parameters fixed (i.e., the lens radius is 10λ, the refractive index of the lens material is 1.49, and the surrounding environment is air with a refractive index of 1. λ represents the wavelength, which is determined by the formula λ = c0 / f, where c0 = 3×10 8 m / s is the speed of light), the positions of the Bessel curve control points are changed.

[0084] First, adjust the position of the y1 coordinate. The appendix Figure 4 shows that the x1 coordinate of the Bessel curve control point is fixed at 3 mm, and the y1 coordinate is increased from 2 mm to 6 mm in increments of 1 mm. Second, adjust the position of the x1 coordinate. The appendix Figure 5It shows that the y1 coordinate is fixed at 4 mm, and the x1 coordinate increases from 1 mm to 5 mm in increments of 1 mm. Among them, the result with the x1 coordinate of 3 mm and the y1 coordinate of 4 mm is as shown in the appendix Figure 3 The light field distribution of the terahertz jet generated by the Bessel curve lens with different control point coordinates.

[0085] From the appendix Figure 4 and the appendix Figure 5 It can be clearly seen that after changing the position of the control point of the Bessel curve lens, the light field distribution of the terahertz jet has changed significantly, indicating that the Bessel curve lenses with different control point positions have different modulation capabilities for terahertz waves. Further analysis shows that as the position of the control point of the Bessel curve lens changes, the focal length of the terahertz jet can vary in the range of 2.664λ to 13.317λ, and the effective length can be modulated in the range of 4.021λ to 26.802λ. When the control point position is set reasonably, the terahertz jet generated by the Bessel curve lens can reduce the full width at half maximum to 0.419λ while maintaining an effective length of 10.642λ, indicating that the Bessel curve lens can be used in terahertz high-resolution large-depth-of-field imaging.

[0086] From the above discussion, it can be seen that during the process of changing the position of the control point of the Bessel curve lens, the focal length, effective length, and full width at half maximum of the terahertz jet can be adjusted, and there is a large range of changes. In practical applications, it is necessary to select the appropriate control point position and 3D printing material according to the specific application scenario and requirements. In short, the above simulation results fully prove the feasibility of the Bessel curve lens proposed in the present invention in realizing terahertz high-resolution large-depth-of-field imaging.

[0087] Different processing methods: In addition to using 3D printing technology for the lens proposed in the present invention, the lens can also be fabricated by mechanical processing.

[0088] Different manufacturing materials: In addition to 3D printing materials, the manufacturing materials also include materials with appropriate refractive indices and low absorption in the terahertz band, such as PP, PE, PS, etc.

[0089] Designing lenses using Bessel curves of different orders: In addition to the second-order Bessel curve mentioned in the present invention, there are other Bessel curves of different orders that can be used to design lenses.

[0090] Adjusting the positions of more Bessel curve control points: In addition to adjusting the position of one control point of the Bessel curve mentioned in the present invention, the positions of multiple control points can be adjusted simultaneously to further optimize the terahertz jet effect generated by the lens, thereby expanding the performance and application of the imaging system.

[0091] The present invention designs a Bessel curve lens using a second-order Bessel curve. The present invention lies in designing a Bessel curve lens using a second-order Bessel curve, which is different from traditional lenses such as spherical, cubic, conical, frustum-shaped lenses, etc.

[0092] Application of the Bessel curve lens in terahertz high-resolution large-depth-of-field imaging: By applying the Bessel curve lens designed in the present invention to a terahertz high-resolution large-depth-of-field imaging system, precise control of terahertz jets can be achieved, thereby improving the resolution and depth of field of the imaging system.

[0093] Manufacturing the Bessel curve lens using 3D printing technology: The present invention applies 3D printing technology to manufacture the designed Bessel curve lens, which is of low cost and can be mass-produced.

[0094] The present invention changes the focal length, effective length, and full width at half maximum of the terahertz jets generated by the Bessel curve lens by adjusting the positions of the control points of the second-order Bessel curve: By adjusting the positions of the control points of the second-order Bessel curve, the present invention can achieve adjustment of the focal length, effective length, and full width at half maximum of the terahertz jets. While maintaining a long effective length, a narrower full width at half maximum can be achieved, which enables the terahertz imaging system to achieve high-resolution large-depth-of-field imaging.

Claims

1. A Bessel curve lens for terahertz high-resolution large-depth-of-field imaging, characterized in that The contour of the Bessel curve lens is an axisymmetric surface formed by rotating a second-order Bessel curve around the central axis by 360°. The axisymmetric surface of the Bessel curve lens is an aspherical structure with a gradient curvature characteristic, which can regulate the spatial distribution of terahertz waves and generate terahertz jets.

2. The Bessel curve lens for terahertz high-resolution large-depth-of-field imaging according to claim 1, characterized in that, Design the Bessel curve using the Bessel curve formula. The Bessel curve formula is as follows: B(t) = (1 - t) 2 P0 + 2t(1 - t)P1 + t 2 P2 (t ∈ [0, 1]) where t is time, P0 is the starting point, P2 is the ending point, and P1 is the control point. By adjusting the coordinates (x1, y1) of P1, the curvature distribution of the lens surface is dynamically changed. Fix the starting point P0 at the origin (0, 0), and fix the ending point P2 at a distance R from the origin on the x-axis. Only adjust the coordinates (x1, y1) of P1. In the curve drawing software, gradually increase the x1 and y1 coordinate values of P1 from 0 to the maximum value R. When adjusting P1, fix the x1 coordinate and only change the y1 coordinate, or fix the y1 coordinate and only change the x1 coordinate. After each adjustment of P1, export and save the obtained curve coordinate data. Subsequently, read the curve coordinate data in the drawing software and draw the curve, and then rotate it 360° around the central axis to obtain the three-dimensional model of the corresponding Bessel curve lens.

3. The Bessel curve lens for terahertz high-resolution large-depth-of-field imaging according to claim 2, wherein, The R is less than 10 wavelengths, and x1 and y1 are less than R.

4. A Bessel curve lens for terahertz high-resolution large-depth-of-field imaging according to claim 2, characterized in that, Import the STL format file of the three-dimensional model into the simulation software to perform numerical simulation of terahertz wave irradiation on the Bessel curve lens, and obtain the terahertz jets generated by the Bessel curve lens. According to the simulation results, repeatedly adjust the position of the control point P1 to optimize the Bessel curve lens until the terahertz jets generated by the Bessel curve lens have a long effective length while maintaining a narrow full width at half maximum, ensuring that the imaging has a relatively high resolution and a large depth of field.

5. The Bessel curve lens for terahertz high-resolution large-depth-of-field imaging according to claim 1, wherein Place the Bessel curve lens on the propagation path of the terahertz wave to generate the terahertz jet effect for terahertz imaging.

6. An imaging method of a Bessel curve lens for terahertz high-resolution and large-depth-of-field imaging, characterized in that, It includes the following steps: Step 1: Select a terahertz light source: Select a suitable terahertz light source according to the imaging requirements. Step 2: Design a terahertz imaging system and build an imaging optical path: Sequentially set a terahertz light source, lens 1, Bessel curve lens, imaging sample, lens 2, lens 3, and detector in the imaging optical path. Lens 1 is used to collimate the divergent terahertz waves generated by the terahertz light source and convert them into collimated parallel beams to improve the directivity of the incident waves. Lens 2 is used to collimate the transmitted terahertz waves of the imaging sample to avoid signal distortion and ensure that the phase and amplitude information of the signals received by the detector accurately reflect the characteristics of the sample. Lens 3 is used to focus the collimated transmitted terahertz waves onto the detector to improve the signal-to-noise ratio of the detector. Step 3: Select a suitable imaging sample: The thickness of the imaging sample should be less than or equal to the effective length of the terahertz jets generated by the Bessel curve lens used in the imaging optical path to meet the requirements of large-depth-of-field imaging. Step 4: Design and manufacture a Bessel curve lens: Design the Bessel curve using the Bessel curve formula, import the curve into the drawing software and rotate it 360 degrees around the central axis to obtain the three-dimensional model of the Bessel curve lens. Subsequently, select a suitable 3D printing material and use 3D printing technology to manufacture the Bessel curve lens, which is used to couple terahertz waves. Step 5: Place the Bessel curve lens in the imaging optical path and generate the terahertz jet effect: Place the fabricated Bessel curve lens in the designed imaging optical path. After the incident terahertz wave irradiates on the Bessel curve lens, due to the curvature distribution on the surface of the Bessel curve lens, the Poynting vector of the incident terahertz wave deflects, and the terahertz jet effect is generated after focusing behind the Bessel curve lens, thereby regulating the terahertz light field distribution; Step 6: Detect and record the terahertz signal: Select a suitable detector to detect the transmitted terahertz signal of the imaging sample, and record the detected terahertz signal through a data acquisition device. The detected signal includes information on amplitude and phase; Step 7: Display and analyze the terahertz image: Display and analyze the recorded terahertz signal, and use image processing and analysis techniques to extract and interpret the characteristics and structural information of the sample to be measured.

7. An imaging method of a Bessel curve lens for terahertz high-resolution large-depth-of-field imaging according to claim 6, characterized in that, In the said Step 1, the terahertz light source includes a terahertz laser and a terahertz pulse generator.

8. An imaging method of a Bessel curve lens for terahertz high-resolution and large-depth-of-field imaging according to claim 6, characterized in that, In the optical path of the said Step 2, the lens is made of a material with low absorption and a refractive index not exceeding 2 in the terahertz band. Select PE or HDPE materials. Select a detector according to the imaging method. Select a pyroelectric detector for point scanning imaging, and use an oscilloscope to read the amplitude; For fast imaging, select a CCD camera as the detector, and read the collected data in the supporting computer software; then, build the imaging optical path according to the designed system and calibrate the optical path.

9. The imaging method of a Bessel curve lens for terahertz high-resolution and large depth of field imaging according to claim 6, characterized in that, The said Step 4 is specifically: First, design a Bessel curve using the Bessel curve formula. The Bessel curve formula is as follows: B(t) = (1 - t) 2 P0 + 2t(1 - t)P1 + t 2 P2 (t ∈ [0, 1]) Fix the starting point P0 at the origin (0, 0), fix the ending point P2 on the x-axis at a distance R from the origin, and only adjust the coordinates (x1, y1) of P1, where R is generally less than 10 wavelengths, and x1 and y1 are less than R. When adjusting P1, fix the x1 coordinate and only change the y1 coordinate; or fix the y1 coordinate and only change the x1 coordinate; Export and save the obtained curve coordinate data every time P1 is adjusted; subsequently, read the curve coordinate data in a drawing software to draw the curve, and then rotate it 360° around the y-axis to obtain the three-dimensional model of the Bessel curve lens; Import the STL format file of the three-dimensional model into a simulation software to perform numerical simulation of the terahertz wave irradiating the Bessel curve lens, obtain the terahertz jet generated by the Bessel curve lens, and analyze the changing trends of the focal length, maximum intensity, effective length, and full width at half maximum of the jet; subsequently, select a suitable 3D printing material and use 3D printing technology to fabricate the Bessel curve lens, and the Bessel curve lens will be used to couple terahertz waves.

10. The imaging method of a Bessel curve lens for terahertz high-resolution large-depth-of-field imaging according to claim 6, characterized in that, The said Step 7 is specifically: Preprocess the data through noise suppression and geometric calibration to improve the signal-to-noise ratio. Subsequently, draw the terahertz signal through a plotting function, combine the scanning position and signal delay to reconstruct the terahertz image, and then extract and analyze the sample characteristics through edge detection and region segmentation algorithms.

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