Large aperture terahertz spot diameter calibration device and method based on metal tag

Abstract

The present application relates to the field of terahertz imaging, and discloses a large-aperture terahertz spot diameter calibration device and method based on a metal label; a computer controls a stepper motor to move along the negative direction of the Y axis, so that the terahertz spot maintains a preset distance from the square copper sheet and the metal label, only the skin surface generates a terahertz reflection signal, and the skin displacement Y is 0; the stepper motor is continuously moved, and a terahertz reflection signal of the square copper sheet appears; the stepper motor is continuously moved, the terahertz spot is tangent to the edge of the metal label, and the skin displacement is Y1; the stepper motor is continuously moved, and the skin displacement at this time is recorded as Y2; the diameter of the terahertz spot in the Y direction is calculated according to the formula d=Y2-Y1-WLabel; the computer controls the stepper motor to move along the Z axis direction, and the steps are repeated, so that the diameter of the terahertz spot in the Z direction is measured and calculated; and the present application solves the problem that there is a lack of a simple and rapid spot diameter measurement method in the prior art.

Description

Technical Field

[0001] This invention relates to the field of terahertz imaging, and more particularly to a device and method for calibrating the diameter of a large-aperture terahertz spot based on a metal tag. Background Technology

[0002] In the field of terahertz imaging, accurately measuring the diameter of the terahertz spot at the focal point is crucial. This parameter directly affects the overall performance of the terahertz imaging system, especially determining the imaging resolution. Currently, conventional terahertz systems, such as those used in non-destructive testing, mostly employ reflective measurement methods and often use 1-inch TPX lenses for terahertz wave focusing. For example, at a terahertz wave frequency of 0.2 THz and a focal length of approximately 10 cm, the terahertz beam waist diameter at the focal point is typically about 5 mm. Generally, when the lens aperture D remains constant while the focal length f increases, the terahertz spot diameter at the focal point also increases; its specific size is closely related to the terahertz wave frequency, lens focal length, and aperture. Furthermore, terahertz waves experience path loss during propagation in air, which is positively correlated with both frequency and transmission distance: as the transmission distance increases or the frequency rises, the terahertz wave intensity gradually weakens, leading to a decrease in the peak frequency of the terahertz wave at the focal point. This frequency reduction, combined with the increase in focal length, further contributes to the enlargement of the terahertz spot diameter at the focal point, thus significantly reducing the system's imaging resolution. This effect limits the effective range of terahertz imaging and restricts its application in a wider range of scenarios.

[0003] Currently, off-axis parabolic mirrors are commonly used for reflective focusing in terahertz spot measurements, but the spot diameter at the focal point is typically large. Especially when using small-aperture (e.g., 2-inch) off-axis parabolic mirrors, if the focal length reaches 20 cm or even 30 cm, the terahertz spot diameter at the focal point can increase to over ten centimeters, far exceeding the spot size corresponding to a system with the same aperture and a focal length of 10 cm. Such large-aperture terahertz spots are difficult to measure directly using conventional terahertz cameras, while a simple and rapid method for determining the spot diameter is urgently needed in engineering applications such as terahertz imaging.

[0004] This invention proposes a large-aperture terahertz spot diameter calibration device and method based on metal tags. This scheme utilizes the intensity and delay characteristics of terahertz pulse reflection in different media, requiring only the measurement of terahertz waveforms at two typical locations to achieve accurate spot diameter calibration. This invention has significant engineering application value in reflective terahertz imaging, non-destructive testing, and hidden point measurement. Summary of the Invention

[0005] The present invention aims to provide a device and method for calibrating the diameter of a large-aperture terahertz light spot based on a metal tag, so as to solve the problem of the lack of a simple and rapid method for measuring the diameter of light spots in the prior art.

[0006] To achieve the above objectives, the present invention provides the following method:

[0007] The large-aperture terahertz spot diameter calibration device based on metal tags provided by this invention includes:

[0008] The components include: probe, metal tag, square copper sheet, skin surface, femtosecond laser system, stepper motor, controller, and computer.

[0009] The femtosecond laser system is used to irradiate the photoconductive antenna to generate terahertz waves; the probe has a built-in off-axis profile system for emitting terahertz waves and receiving reflected terahertz signals; the metal tag is used to reflect terahertz waves, the square copper sheet is used to reduce background noise, and the skin surface is used to attach the square copper sheet and the metal tag; the stepper motor is electrically connected to the controller, the controller is electrically connected to the computer, and the control program in the computer controls the stepper motor to move, realizing the overall movement of the skin surface, the square copper sheet, and the metal tag; the computer is also used for the data acquisition and storage of terahertz reflection signals.

[0010] Preferably, the metal tag has a size of 2cm × 2cm, and the square copper sheet has a size of 5cm × 5cm; the metal tag is pasted on the upper surface of the square copper sheet, and the square copper sheet is fixed to the skin surface; the structure of the metal tag is as follows: a 20nm thick Gr adhesive layer is set on a 0.5mm thick JGS2 quartz substrate, and a 100nm thick gold film is plated on the Gr adhesive layer.

[0011] Preferably, the stepper motor is connected to the skin surface via a 3D printed adapter, and the skin surface is fixed to the square copper sheet and the square copper sheet to the metal label with double-sided adhesive. The stepper motor drives the skin surface to move along the Y-axis and Z-axis, thereby driving the square copper sheet and the metal label to move synchronously along the Y-axis and Z-axis.

[0012] Preferably, the control program in the computer is a LabVIEW three-dimensional motion control program, and the generation and detection of the terahertz wave are both achieved through a photoconductive antenna.

[0013] This invention discloses a method for calibrating the diameter of a large-aperture terahertz beam based on a metal tag and a large-aperture terahertz beam diameter calibration device, characterized in that the method includes:

[0014] S1: The stepper motor is controlled by computer to move along the negative Y-axis so that the terahertz light spot maintains a preset distance from the square copper sheet and metal label. At this time, only the skin surface generates a terahertz reflection signal. This position is recorded as skin displacement Y=0.

[0015] S2: Control the stepper motor to move along the negative Y-axis until the terahertz light spot is tangent to the edge of the square copper sheet. At this time, the terahertz reflection signal of the square copper sheet appears, and the amplitude of the terahertz reflection signal of the skin surface begins to decrease.

[0016] S3: Move the stepper motor along the negative Y-axis until the terahertz light spot is tangent to the edge of the metal tag. The metal tag begins to generate a terahertz reflection signal. Record the skin displacement at this time as Y1.

[0017] S4: Move the stepper motor along the negative Y-axis until the terahertz light spot is tangent to the other edge of the metal label, and record the skin displacement at this time as Y2;

[0018] S5: Calculate the diameter of the terahertz spot in the Y direction according to the formula d=Y2−Y1−WLabel, where WLabel is the width of the metal label;

[0019] S6: Control the stepper motor to move along the Z-axis using computer control, repeat steps S1-S5, and measure and calculate the diameter of the terahertz spot in the Z-direction.

[0020] Preferably, the method further includes a step of measuring the thickness of the metal tag, specifically: acquiring the terahertz reflection signal of the square copper sheet and the terahertz reflection signal of the metal tag, obtaining the time-domain difference between the two terahertz reflection signals, and then applying the formula:

[0021] HLabel = (t2 - t1) × V × 1 / 2;

[0022] Calculate the thickness of the metal tag; where (t2-t1) is the time difference between the reflected signal from the square copper sheet and the metal tag, t1 is the time corresponding to the reflected pulse from the metal tag, t2 is the time corresponding to the reflected signal from the square copper sheet, HLabel is the thickness of the metal tag, and V is the propagation speed of the terahertz reflected signal.

[0023] Preferably, the propagation speed of the terahertz reflected signal is the speed of light, and the width WLabel of the metal label is 2cm.

[0024] Preferably, the method further includes a step of measuring the thickness of the square copper sheet, specifically: acquiring the terahertz reflection signal of the square copper sheet and the terahertz reflection signal of the skin surface, obtaining the time-domain difference between the two terahertz reflection signals, and then applying the formula:

[0025] HCu=(t4-t3)×V×1 / 2;

[0026] Calculate the thickness of the square copper sheet; where (t4-t3) is the time difference between the reflected signal from the skin surface and the square copper sheet, t3 is the time corresponding to the reflected signal from the square copper sheet, t4 is the time corresponding to the reflected signal from the skin surface, HCu is the thickness of the square copper sheet, and V is the propagation speed of the terahertz reflected signal.

[0027] Preferably, in steps S3 and S4, the amplitude of the terahertz reflection signal depends on the size of the irradiation area of ​​the terahertz light spot on the corresponding surface; when the terahertz light spot is tangent to the edge of the metal tag, the skin surface, the square copper sheet, and the metal tag will all generate terahertz reflection signals.

[0028] The beneficial effects of this invention are as follows: By utilizing the intensity and delay characteristics of terahertz pulses reflected in different media, this invention allows for accurate calibration of the spot diameter by measuring the terahertz waveforms at only two typical locations. This invention has significant engineering application value in reflective terahertz imaging, non-destructive testing, and hidden point measurement. Large-aperture terahertz spots are difficult to measure directly using conventional terahertz cameras; however, this invention can meet the need for simple and rapid spot diameter determination in engineering applications such as terahertz imaging. Attached Figure Description

[0029] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0030] Figure 1 This is a schematic diagram of the structure of a large-aperture terahertz spot diameter calibration device based on a metal tag provided in an embodiment of the present invention;

[0031] Figure 2 This is a schematic diagram of the spot position when the skin displacement is Y=0, provided in an embodiment of the present invention.

[0032] Figure 3 This is a schematic diagram showing the position of the light spot when it reaches the edge of the copper sheet, provided in an embodiment of the present invention.

[0033] Figure 4 This is a schematic diagram showing the position of the light spot when it reaches the right edge of the metal label, as provided in an embodiment of the present invention.

[0034] Figure 5 A schematic diagram illustrating the actual measurement of the reflected signals of the metal label, copper sheet, and skin when the light spot reaches the edge of the metal label, as provided in an embodiment of the present invention.

[0035] Figure 6This is a schematic diagram showing the position of the light spot when it reaches the left edge of the metal tag, as provided in an embodiment of the present invention.

[0036] Figure 7 This is a schematic diagram illustrating the principle of spot diameter measurement provided in an embodiment of the present invention;

[0037] Figure 8 This is a schematic diagram showing the position of the light spot covering the light spot and the square copper sheet, provided in an embodiment of the present invention.

[0038] Figure 9 A schematic diagram of the amplitude of the metal tag and the square copper sheet provided in an embodiment of the present invention;

[0039] Figure 10 A schematic diagram showing the position of the light spot covering half of the skin and half of the square copper sheet, as provided in an embodiment of the present invention;

[0040] Figure 11 This is a schematic diagram of the amplitude of the reflected signal from the square copper sheet and the skin provided in an embodiment of the present invention;

[0041] Figure 12 This is a schematic flowchart of a large-aperture terahertz spot diameter calibration method based on metal tags provided in an embodiment of the present invention.

[0042] Attached reference numerals: 1-Computer, 2-Controller, 3-Stepper motor, 4-Skin surface, 5-Square copper sheet, 6-Metal tag, 7-Probe, 8-Terahertz spot. Detailed Implementation

[0043] To enable those skilled in the art to better understand the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] The terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this invention are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, apparatus, product, or end that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or ends.

[0045] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0046] Currently, off-axis parabolic mirrors are commonly used for reflective focusing in terahertz spot measurements, but the spot diameter at the focal point is typically large. Especially when using small-aperture (e.g., 2-inch) off-axis parabolic mirrors, if the focal length reaches 20 cm or even 30 cm, the terahertz spot diameter at the focal point can increase to over ten centimeters, far exceeding the spot size corresponding to a system with the same aperture and a focal length of 10 cm. Such large-aperture terahertz spots are difficult to measure directly using conventional terahertz cameras, while a simple and rapid method for determining the spot diameter is urgently needed in engineering applications such as terahertz imaging.

[0047] This invention proposes a large-aperture terahertz spot diameter calibration device and method based on metal tags. This scheme utilizes the intensity and delay characteristics of terahertz pulse reflection in different media, requiring only the measurement of terahertz waveforms at two typical locations to achieve accurate spot diameter calibration. This invention has significant engineering application value in reflective terahertz imaging, non-destructive testing, and hidden point measurement.

[0048] The present invention aims to provide a device and method for calibrating the diameter of a large-aperture terahertz light spot based on a metal tag, so as to solve the problem of the lack of a simple and rapid method for measuring the diameter of light spots in the prior art.

[0049] like Figure 1 As shown, a specific embodiment of the present invention provides a large-aperture terahertz spot diameter calibration device based on a metal tag, comprising:

[0050] 7. Probe, 6. Metal tag, 5. Square copper sheet, 4. Skin surface, 4. Femtosecond laser system, 3. Stepper motor, 2. Controller and 1. Computer;

[0051] The femtosecond laser system is used to irradiate the photoconductive antenna to generate terahertz waves; the probe 7 has a built-in off-axis profiling system for emitting terahertz waves and receiving the reflected terahertz signals.

[0052] The metal tag 6 is used to reflect terahertz waves, the square copper sheet 5 is used to reduce background noise, and the skin surface 4 is used to attach the square copper sheet 5 and the metal tag 6; the size of the metal tag 6 is 2cm×2cm, and the size of the square copper sheet 5 is 5cm×5cm; the metal tag 6 is pasted on the upper surface of the square copper sheet 5, and the square copper sheet 5 is fixed on the skin surface 4; the structure of the metal tag 6 is as follows: a 20nm thick Gr adhesive layer is set on a 0.5mm thick JGS2 quartz substrate, and a 100nm thick gold film is plated on the Gr adhesive layer;

[0053] Stepper motor 3 is electrically connected to controller 2, and controller 2 is electrically connected to computer 1. The control program in computer 1 controls the movement of stepper motor 3 through controller 2, realizing the overall movement of skin surface 4, square copper sheet 5, and metal label 6. Computer 1 is also used for the data acquisition and storage of terahertz reflection signals. Stepper motor 3 and skin surface 4 are connected by a 3D printed adapter. Skin surface 4 and square copper sheet 5, as well as square copper sheet 5 and metal label 6, are fixed with double-sided adhesive. Stepper motor 3 drives skin surface 4 to move along the Y-axis and Z-axis, thereby driving square copper sheet 5 and metal label 6 to move synchronously along the Y-axis and Z-axis. The control program in computer 1 is a LabVIEW three-dimensional motion control program. The generation and detection of terahertz waves are both realized through photoconductive antennas.

[0054] like Figure 12 As shown, a specific embodiment of the present invention provides a method for calibrating the diameter of a large-aperture terahertz light spot based on a metal tag, the method comprising:

[0055] S1: The stepper motor 3 is controlled by computer 1 to move along the negative Y-axis so that the terahertz light spot 8 maintains a preset distance from the square copper sheet 5 and the metal label 6. At this time, only the skin surface 4 generates a terahertz reflection signal. This position is recorded as the skin displacement Y=0.

[0056] In this embodiment of the invention, the propagation speed of the terahertz reflected signal is the speed of light, and the width WLabel of the metal label 6 is 2cm.

[0057] S2: Control the stepper motor 3 to move along the negative Y-axis until the terahertz spot 8 is tangent to the edge of the square copper sheet 5. At this time, the terahertz reflection signal of the square copper sheet 5 appears, and the amplitude of the terahertz reflection signal of the skin surface 4 begins to decrease.

[0058] S3: Move stepper motor 3 along the negative Y-axis until the terahertz light spot 8 is tangent to the edge of the metal tag 6. The metal tag 6 begins to generate a terahertz reflection signal. Record the skin displacement at this time as Y1.

[0059] S4: Move stepper motor 3 along the negative Y-axis until the terahertz spot 8 is tangent to the other edge of the metal label 6, and record the skin displacement at this time as Y2.

[0060] In this embodiment of the invention, in steps S3 and S4, the amplitude of the terahertz reflection signal depends on the size of the irradiation area of ​​the terahertz spot 8 on the corresponding surface; when the terahertz spot 8 is tangent to the edge of the metal tag 6, the skin surface 4, the square copper sheet 5, and the metal tag 6 will all generate terahertz reflection signals.

[0061] S5: Calculate the diameter of the terahertz spot 8 in the Y direction according to the formula d=Y2−Y1−WLabel, where WLabel is the width of the metal label 6.

[0062] S6: Control the stepper motor 3 to move along the Z-axis using computer 1, repeat steps S1-S5, and measure and calculate the diameter of the terahertz spot 8 in the Z-direction.

[0063] In this embodiment of the invention, a step for measuring the thickness of the metal tag 6 is also included, specifically: acquiring the terahertz reflection signal of the square copper sheet 5 and the terahertz reflection signal of the metal tag 6, obtaining the time-domain difference between the two terahertz reflection signals, and calculating the difference according to the formula:

[0064] HLabel = (t2 - t1) × V × 1 / 2;

[0065] Calculate the thickness of the metal tag 6; where (t2-t1) is the time difference between the reflected signals of the square copper sheet 5 and the metal tag 6, t1 is the time corresponding to the reflected pulse of the metal tag 6, t2 is the time corresponding to the reflected signal of the square copper sheet 5, HLabel is the thickness of the metal tag 6, and V is the propagation speed of the terahertz reflected signal; it also includes the measurement step of the thickness of the square copper sheet 5, specifically: acquiring the terahertz reflected signal of the square copper sheet 5 and the terahertz reflected signal of the skin surface 4, obtaining the time domain difference between the two terahertz reflected signals, according to the formula:

[0066] HCu=(t4-t3)×V×1 / 2;

[0067] Calculate the thickness of the square copper sheet 5; where (t4-t3) is the time difference between the reflected signals from the skin surface 4 and the square copper sheet 5, t3 is the time corresponding to the reflected signal from the square copper sheet 5, t4 is the time corresponding to the reflected signal from the skin surface 4, HCu is the thickness of the square copper sheet 5, and V is the propagation speed of the terahertz reflected signal.

[0068] Example 1

[0069] like Figure 1As shown, the measurement system mainly consists of a probe 7, a metal tag 6, a square copper sheet 5, a skin surface 4, a femtosecond laser system (as a terahertz wave source), a stepper motor 3, a controller 2, and a computer 1. In the system, the femtosecond laser system irradiates the photoconductive antenna to generate terahertz waves. The probe 7 has a built-in off-axis parabolic system for emitting and receiving reflected terahertz signals. The metal tag 6 reflects terahertz waves; the copper sheet reduces background noise; and the skin surface 4 is used to attach the copper sheet and tag, preventing distortion. The stepper motor 3 is connected to the computer 1 via the controller 2, and its movement is controlled by a LabVIEW program on the computer 1, thereby achieving the overall three-dimensional displacement of the skin, copper sheet, and tag. The computer 1 is also responsible for data acquisition, storage, and motion control.

[0070] The specific structure and function of each part of the system are as follows: the tag size is 2 cm × 2 cm, the copper sheet size is 5 cm × 5 cm, and the skin surface 4 is relatively large, so it will not be described in detail here. The tag is pasted on the upper surface of the copper sheet, and the copper sheet is fixed to the skin surface 4. The tag structure consists of a 0.5 mm thick JGS2 quartz substrate with a 20 nm thick Gr adhesive layer coated with a 100 nm thick gold film. The stepper motor 3 is connected to the skin surface 4 through a 3D printed adapter to fix the skin surface 4 to the screw holes of the motor (not shown in the figure). A three-dimensional motion control program written in LabVIEW drives the tag and the copper sheet to move to different positions of the light spot. The probe 7 emits terahertz waves to the tag, the copper sheet, and the skin surface 4, and receives the time-domain signals reflected from each surface. The computer 1 displays and records the waveforms. The generation and detection of the terahertz waves are both achieved using photoconductive antennas. Since the terahertz spot 8 covers different areas on different surfaces, and the reflectivity of each surface to terahertz waves varies, the amplitude of the obtained reflected signal is also different.

[0071] The following explains the direction of motor movement and the fixing method of each layer of media: Stepper motor 3 can drive the skin surface 4 to move along the Y-axis and Z-axis, thereby measuring the diameter of the light spot in these two directions through reflected terahertz pulses. The skin, copper sheet, and label are fixed together with double-sided tape; moving stepper motor 3 achieves the overall displacement of the three components. At the start of the measurement, stepper motor 3 moves along the negative Y-axis, at which point the light spot is farther from the label and copper sheet. Figure 2 As shown. At this position, only the skin surface 4 generates a reflected signal, and the signal amplitude is the largest. This position is recorded as displacement Y=0. Continuing to move along the negative Y-axis, before the light spot touches the copper surface, only the skin surface 4 generates a reflected signal, and its amplitude remains unchanged.

[0072] like Figure 3As shown, when the light spot reaches the edge of the copper sheet, a weak reflection signal from the copper surface begins to appear, while the amplitude of the reflection signal from skin surface 4 begins to decrease. At this point, the reflection signal is still mainly contributed by skin surface 4. Continuing to move along the negative Y-axis, the amplitude of the copper sheet reflection signal gradually increases, while the amplitude of the skin surface 4 reflection signal decreases accordingly.

[0073] like Figure 4 As shown, the light spot continues to move along the negative Y-axis. When it reaches the edge of the label, the label begins to generate a reflected signal (with a small amplitude). At this time, the skin, copper sheet, and label all have reflected signals. The skin displacement at this position is recorded as Y1. The amplitude of the reflected signal on each surface mainly depends on the area irradiated by the terahertz light spot 8 on that surface.

[0074] Regarding the explanation of signal time delay, such as Figure 5 As shown: In the experiment, the copper sheet thickness was 0.2 mm, and the theoretical time delay of the reflected signal between the copper sheet and the skin was 2 × 0.2 mm × 3.33 ps = 1.332 ps (where 3.33 ps is the time required for a terahertz wave to travel 1 mm in air). This result is consistent with... Figure 5 The measured delay of approximately 4 ps is basically consistent with the expected value. The error of a few picoseconds mainly stems from the terahertz wave being incident on the skin surface at a small angle and not perpendicularly, causing the actual optical path to be slightly larger than the estimated value. The tag thickness is 1.5 mm, and the theoretical delay of the reflected signal between it and the substrate is 2 × 1.5 mm × 3.33 ps = 9.99 ps. This value is consistent with... Figure 6 The delay of approximately 11 ps between the first peak (Label) and the second peak (Cu) is consistent.

[0075] Continuing to move along the negative Y-axis, the amplitude of the skin reflection signal further decreases, while the amplitude of the label and copper sheet reflection signals gradually increases, such as... Figure 6 As shown. When the label is at the edge of the light spot, the skin displacement is recorded as Y2. If the movement continues along the Y direction, the light spot will completely leave the label, and only the skin and copper sheet will generate reflected signals. The diameter d of the terahertz light spot 8 can be calculated using the formula d=Y2−Y1−WLabel, where WLabel is the label size (2 cm). A schematic diagram of the measurement principle is shown below. Figure 7 As shown. Similarly, the spot diameter in the Z direction can also be measured by moving along the Z direction.

[0076] like Figure 7 As shown, the red circle represents the terahertz wave spot, and the yellow square represents the high-reflectivity terahertz label. Y1 indicates that the label is just in contact with the terahertz wave spot, Y2 indicates that the label's terahertz wave spot is just away from the terahertz wave spot, and d represents the diameter of the terahertz wave spot. The formula for calculating the diameter of the terahertz spot 8 is: d = Y2 - Y1 - WLabel. Where WLabel is the width of the label, and d is the width of the terahertz spot 8 in the Y direction.

[0077] Tag thickness is measured using the time-domain difference between the reflected signals from the copper sheet and the tag, and the speed of light. Figure 8 As shown, when the skin moves to the point where the light spot covers approximately half of the label and half of the copper sheet, the label reflects terahertz waves weakly while the copper sheet reflects them strongly. Consequently, the label's reflected signal is weak, and the amplitudes of the label's reflected signal and the copper sheet's reflected signal differ significantly, with a time delay in the appearance of the amplitudes.

[0078] like Figure 9 The diagram shows the time delay distribution of the reflected signals between the tag and the copper sheet. The intensity values ​​of these two reflected signals are meaningless and only related to the area of ​​the terahertz spot 8 on the irradiated tag and copper sheet. Since the tag is attached to the copper sheet, it is closer to the probe 7, and the amplitude of the tag's reflected signal appears first. The tag thickness is calculated using the formula HLabel = (t2-t1)*V*(1 / 2), where (t2-t1) is the time difference between the two reflected signals. Here, t1 is the time corresponding to the pulse reflected by the tag, t2 is the time corresponding to the signal reflected by the skin, and HLabel is the thickness of the tag. V is the propagation speed of the terahertz reflected signal, which is considered to be the speed of light (the same applies below). The reason for the number (1 / 2) is mainly because terahertz waves are measured by reflection, and the optical path envelope includes the optical path of the incident terahertz wave and the optical path of the reflected terahertz wave.

[0079] The copper sheet thickness is measured using the time-domain difference and speed of light between the reflected signals from the copper sheet and the skin. The system uses a 5x5cm copper sheet superimposed on the skin, with a thickness of 0.2 mm. Figure 10 As shown, when the skin moves to the point where the light spot covers half of the area on the copper sheet and the skin, the reflectivity of the copper sheet and the skin to terahertz waves is different, and the amplitude of the reflected signal from the skin and the reflected signal from the copper sheet are different.

[0080] like Figure 11 As shown, the thickness of the copper sheet can be calculated from the time delay difference between the terahertz signals reflected by the copper sheet and the skin. Since the copper sheet is attached to the skin and is closer to probe 7, the amplitude of the copper sheet's reflected signal appears first, and there is a time delay between the amplitudes of the two reflected signals. The formula for calculating the copper sheet thickness is HCu = (t4-t3)*V*(1 / 2). Here, HCu is the thickness of the copper sheet, t3 is the time corresponding to the copper sheet's reflected signal, and t4 is the time corresponding to the skin's reflected signal.

[0081] The beneficial effects of this invention are as follows: By utilizing the intensity and delay characteristics of terahertz pulses reflected in different media, this invention allows for accurate calibration of the spot diameter by measuring the terahertz waveforms at only two typical locations. This invention has significant engineering application value in reflective terahertz imaging, non-destructive testing, and hidden point measurement. Large-aperture terahertz spots are difficult to measure directly using conventional terahertz cameras; however, this invention can meet the need for simple and rapid spot diameter determination in engineering applications such as terahertz imaging.

[0082] The above descriptions are merely embodiments of the present invention. Commonly known technical solutions or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A large-aperture terahertz spot diameter calibration device based on metal tags, characterized in that, The device includes: The probe (7), metal tag (6), square copper sheet (5), skin surface (4), femtosecond laser system, stepper motor (3), controller (2) and computer (1); The femtosecond laser system is used to irradiate the photoconductive antenna to generate terahertz waves; the probe (7) has a built-in off-axis profile system for emitting terahertz waves and receiving the reflected terahertz signals; the metal tag (6) is used to reflect terahertz waves, the square copper sheet (5) is used to reduce background noise, and the skin surface (4) is used to attach the square copper sheet (5) and the metal tag (6); the stepper motor (3) is electrically connected to the controller (2), the controller (2) is electrically connected to the computer (1), and the control program in the computer (1) controls the movement of the stepper motor (3) through the controller (2) to realize the overall movement of the skin surface (4), the square copper sheet (5) and the metal tag (6); the computer (1) is also used for the data acquisition and storage of terahertz reflection signals.

2. The large-aperture terahertz spot diameter calibration device based on a metal tag according to claim 1, characterized in that: The metal tag (6) has a size of 2cm×2cm, and the square copper sheet (5) has a size of 5cm×5cm. The metal tag (6) is pasted on the upper surface of the square copper sheet (5), and the square copper sheet (5) is fixed on the skin surface (4). The structure of the metal tag (6) is as follows: a 20nm thick Gr adhesive layer is set on a 0.5mm thick JGS2 quartz substrate, and a 100nm thick gold film is plated on the Gr adhesive layer.

3. The large-aperture terahertz spot diameter calibration device based on a metal tag according to claim 1, characterized in that: The stepper motor (3) is connected to the skin surface (4) via a 3D printed adapter. The skin surface (4) is fixed to the square copper sheet (5), and the square copper sheet (5) is fixed to the metal label (6) via double-sided adhesive. The stepper motor (3) drives the skin surface (4) to move along the Y-axis and Z-axis, thereby driving the square copper sheet (5) and the metal label (6) to move synchronously along the Y-axis and Z-axis.

4. The large-aperture terahertz spot diameter calibration device based on a metal tag according to claim 1, characterized in that: The control program in the computer (1) is a LabVIEW three-dimensional motion control program, and the generation and detection of the terahertz wave are both achieved through a photoconductive antenna.

5. A method for calibrating the diameter of a large-aperture terahertz light spot based on a metal tag, using the device described in any one of claims 1-4, characterized in that... The method includes: S1: The stepper motor is controlled by computer to move along the negative Y-axis so that the terahertz light spot maintains a preset distance from the square copper sheet and metal label. At this time, only the skin surface generates a terahertz reflection signal. This position is recorded as skin displacement Y=0. S2: Control the stepper motor to move along the negative Y-axis until the terahertz light spot is tangent to the edge of the square copper sheet. At this time, the terahertz reflection signal of the square copper sheet appears, and the amplitude of the terahertz reflection signal of the skin surface begins to decrease. S3: Move the stepper motor along the negative Y-axis until the terahertz light spot is tangent to the edge of the metal tag. The metal tag begins to generate a terahertz reflection signal. Record the skin displacement at this time as Y1. S4: Move the stepper motor along the negative Y-axis until the terahertz light spot is tangent to the other edge of the metal label, and record the skin displacement at this time as Y2; S5: Calculate the diameter of the terahertz spot in the Y direction according to the formula d=Y2−Y1−WLabel, where WLabel is the width of the metal label; S6: Control the stepper motor to move along the Z-axis using computer control, repeat steps S1-S5, and measure and calculate the diameter of the terahertz spot in the Z-direction.

6. The method for calibrating the diameter of a large-aperture terahertz light spot based on a metal tag according to claim 5, characterized in that: It also includes a step for measuring the thickness of the metal tag, specifically: acquiring the terahertz reflection signal of the square copper sheet and the terahertz reflection signal of the metal tag, obtaining the time-domain difference between the two terahertz reflection signals, and then applying the formula: HLabel = (t2 - t1) × V × 1 / 2; Calculate the thickness of the metal tag; where (t2-t1) is the time difference between the reflected signal from the square copper sheet and the metal tag, t1 is the time corresponding to the reflected pulse from the metal tag, t2 is the time corresponding to the reflected signal from the square copper sheet, HLabel is the thickness of the metal tag, and V is the propagation speed of the terahertz reflected signal.

7. The method for calibrating the diameter of a large-aperture terahertz light spot based on a metal tag according to claim 6, characterized in that: The propagation speed of the terahertz reflected signal is the speed of light, and the width WLabel of the metal label is 2cm.

8. The method for calibrating the diameter of a large-aperture terahertz light spot based on a metal tag according to claim 5, characterized in that: It also includes a step for measuring the thickness of the square copper sheet, specifically: acquiring the terahertz reflection signal of the square copper sheet and the terahertz reflection signal of the skin surface, obtaining the time-domain difference between the two terahertz reflection signals, and then applying the formula: HCu=(t4-t3)×V×1 / 2; Calculate the thickness of the square copper sheet; where (t4-t3) is the time difference between the reflected signal from the skin surface and the square copper sheet, t3 is the time corresponding to the reflected signal from the square copper sheet, t4 is the time corresponding to the reflected signal from the skin surface, HCu is the thickness of the square copper sheet, and V is the propagation speed of the terahertz reflected signal.

9. The method for calibrating the diameter of a large-aperture terahertz light spot based on a metal tag according to claim 5, characterized in that: In steps S3 and S4, the amplitude of the terahertz reflection signal depends on the size of the irradiation area of ​​the terahertz light spot on the corresponding surface. When the terahertz light spot is tangent to the edge of the metal tag, the skin surface, the square copper sheet, and the metal tag will all generate terahertz reflection signals.

Images