Heterogeneous fusion nondestructive testing imaging system based on dual-frequency terahertz solid-state source

Abstract

The invention discloses a heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source. The system is used for solving the problems that in the prior art, the aspects of collaborative optimization of penetrating power and resolution, improvement of imaging speed, efficient utilization of light energy, multi-frequency signal separation and the like still have obvious defects. Relates to the technical field of terahertz nondestructive testing and optical imaging, and comprises 110 GHz and 340 GHz orthogonal polarization terahertz sources, a wire grating polarization beam combiner, a beam expanding optical assembly, a 64 * 64-element terahertz area array detector, a piezoelectric ceramic micro-displacement platform, an electric control adjustable attenuator, an electronic control and synchronization unit and an image fusion processing unit. Coaxial illumination is realized through cross-polarization beam combination, signal aliasing is avoided in combination with time division multiplexing acquisition, high-frequency under-sampling is overcome through sub-pixel micro-scanning and super-resolution reconstruction, a low-frequency penetration image and a high-frequency detail image are finally fused, and rapid full-field imaging with high penetration depth and high spatial resolution is realized.

Description

Technical Field

[0001] This invention relates to the field of terahertz nondestructive testing and optical imaging technology, and in particular to a heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source. Background Technology

[0002] Terahertz waves, located in the electromagnetic spectrum region between microwaves and infrared light, possess unique advantages such as non-ionization, strong penetration, and transparency to various non-polar materials. In recent years, they have shown broad application prospects in fields such as non-destructive testing, security inspection, biomedical imaging, and industrial quality control. In particular, in the detection of internal defects in non-metallic materials such as composite materials, layered structures, ceramics, and foams, terahertz imaging technology has become an important non-destructive testing method due to its superior spatial resolution compared to microwaves and superior penetration compared to infrared. However, existing terahertz imaging systems still face several key technical bottlenecks in practical applications.

[0003] Current mainstream terahertz imaging systems mainly include single-frequency continuous-wave imaging systems and mechanical scanning imaging systems. Single-frequency systems typically use a fixed-frequency solid-state source (such as 100 GHz or 300 GHz) for illumination. Although the structure is simple, it is limited by the physical characteristics of terahertz waves: low-frequency bands (such as 100 GHz) have strong penetrating power and can penetrate thicker samples, but their long wavelengths result in low spatial resolution, making it difficult to identify micron-sized cracks or delamination; while high-frequency bands (such as above 300 GHz) can provide higher spatial resolution, but the penetration depth is significantly reduced due to enhanced material absorption, easily causing signal attenuation or even complete loss. Therefore, single-frequency systems cannot simultaneously meet the dual requirements of "high penetration depth" and "high spatial resolution," limiting their applicability in the detection of complex multilayer structures.

[0004] On the other hand, traditional terahertz imaging mostly relies on mechanical grating scanning, which involves acquiring signals point by point using a point source and point detector in conjunction with a two-dimensional displacement platform, and then reconstructing the image. Although this method can achieve a high signal-to-noise ratio, the imaging speed is extremely slow, usually taking several minutes to several hours to complete an image, which cannot meet the real-time and high-throughput requirements of modern industrial online inspection. Although full-field imaging schemes based on terahertz area array cameras have emerged in recent years, the large pixel size of terahertz detectors (such as a typical pixel pitch of 1.5 mm) leads to insufficient spatial sampling rate, especially in the high-frequency band where undersampling is more likely to occur due to the shorter wavelength, which restricts the actual resolution of the system. In addition, area array imaging is often affected by the uneven energy distribution of Gaussian beams, causing illumination artifacts with a bright center and dark edges in the field of view, which further reduces image consistency and detection reliability.

[0005] More importantly, if attempts are made to integrate the advantages of different frequency bands by introducing multi-frequency illumination, existing systems often lack effective common-aperture optical designs; traditional beam combining methods, such as semi-transparent and semi-reflective beam splitters, result in at least 50% energy loss, reducing the system's signal-to-noise ratio; at the same time, mainstream terahertz array detectors (such as microbolometers) are broadband response types and cannot distinguish different frequency components at the physical level; if two frequencies are illuminated simultaneously, the output of the terahertz array detector will produce signal aliasing, making it impossible to separate the information of each frequency band, making it difficult to implement multi-frequency fusion strategies; although frequency selection can be achieved through narrowband filters, such components are expensive, have high insertion loss, and are difficult to integrate efficiently with solid-state sources, which is not conducive to system miniaturization and engineering deployment.

[0006] In summary, existing terahertz nondestructive testing imaging technologies still have significant shortcomings in areas such as the synergistic optimization of penetration capability and resolution, improvement of imaging speed, efficient utilization of light energy, and separation of multi-frequency signals. There is an urgent need for a new imaging architecture that can achieve efficient coaxial transmission of high and low frequency terahertz waves, time-sequential acquisition, and intelligent image fusion without relying on mechanical scanning. This would overcome the performance limitations of a single frequency band and meet the comprehensive needs of industrial scenarios for fast, high-resolution, and deep-penetrating nondestructive testing. Summary of the Invention

[0007] This invention addresses the shortcomings of existing terahertz nondestructive testing technologies, such as the difficulty in balancing penetration depth and spatial resolution in single-frequency imaging systems, the low efficiency of mechanical scanning methods, signal aliasing in broadband response terahertz array detectors under multi-frequency illumination, and undersampling in high-frequency bands due to excessive pixel spacing. It proposes a heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source. This system achieves a balance between high penetration capability and high spatial resolution without introducing mechanical scanning structures by combining orthogonal polarization beam combining, time-division multiplexing acquisition, and dual-frequency image fusion, while meeting the requirements of industrial online inspection for imaging speed and image quality.

[0008] This invention provides a heterogeneous fusion nondestructive testing imaging system based on dual-frequency terahertz solid-state sources, comprising a first terahertz solid-state source, a second terahertz solid-state source, a wire-grid polarizer, a beam expander optical assembly, a sample stage, a terahertz imaging lens, a terahertz area array detector, a piezoelectric ceramic micro-displacement platform, an electrically adjustable attenuator, an electronic control and synchronization unit, and an image processing and fusion unit. The first terahertz solid-state source is a 110 GHz frequency harmonic source with an output power of 400 mW and its polarization direction is set to horizontal polarization; the second terahertz solid-state source is a 340 GHz frequency harmonic source with an output power of 50 mW and its polarization direction is set to vertical polarization. The two terahertz beams are collimated into parallel beams by a Teflon (PTFE) collimating lens and a high-density polyethylene (HDPE) collimating lens, respectively, before being incident on the wire-grid polarizer.

[0009] Furthermore, the wire-grid polarizer is positioned at a 45° angle on the principal optical axis, exhibiting high transmittance for horizontally polarized light and high reflectivity for vertically polarized light, thereby combining the 110 GHz and 340 GHz beams into a coaxially propagating composite beam. This composite beam then enters the beam-expanding optics and is expanded into a uniform illumination area with a diameter of approximately 100 mm, illuminating the non-metallic material sample placed on the test stage. The transmitted or reflected terahertz signal is focused by a large-aperture terahertz imaging lens onto the focal plane of a 64×64 element terahertz microbolometer camera. This terahertz array detector is a broadband response type with a noise equivalent power (NEP) of 1. The pixel pitch is 1.5 mm.

[0010] Specifically, to address the issue that broadband response terahertz array detectors cannot physically distinguish between signals of different frequencies, the electronic control and synchronization unit performs high-speed alternating modulation on the two terahertz solid-state sources: S1, in odd-numbered frames, the first terahertz solid-state source is turned on and the second terahertz solid-state source is turned off; S2, in even-numbered frames, the second terahertz solid-state source is turned on and the first terahertz solid-state source is turned off; S3, the terahertz array detector is synchronously triggered to acquire image data at the corresponding frequency frame by frame. Through the above time-division multiplexing mechanism, the terahertz array detector receives only a single frequency component signal in each frame, avoiding signal aliasing caused by simultaneous illumination of multiple frequencies.

[0011] Furthermore, to address the spatial undersampling problem in the 340 GHz high-frequency band caused by a pixel pitch (1.5 mm) greater than half a wavelength (approximately 0.44 mm), the system integrates a piezoelectric ceramic micro-displacement platform at the bottom of the terahertz array detector. This platform can perform 2×2 or 3×3 sub-pixel micro-scanning operations in 0.75 mm steps (i.e., 0.5 pixel pitch) in both the X and Y directions. Specifically, S4, within each frequency channel, the controller drives the piezoelectric ceramic micro-displacement platform to sequentially complete micro-displacements at four or nine positions; S5, one frame of image is acquired at each position; S6, multiple low-resolution images are input into the super-resolution reconstruction algorithm module to generate a high-resolution image with an equivalent pixel pitch of 0.75 mm or smaller. This overcomes the physical limitations of the original pixels and improves the spatial resolution capability in the high-frequency band.

[0012] An electrically controlled adjustable attenuator is incorporated into the 110 GHz optical path. This device is dynamically adjusted by an electronic control and synchronization unit based on preset parameters or real-time feedback of sample thickness information. S7: When the sample thickness is large or absorption is strong, the output power of the 110 GHz source is reduced to prevent the terahertz array detector from entering the saturation region due to high-power illumination. S8: When the sample is thin or absorption is weak, a higher output power is maintained to ensure the signal-to-noise ratio. This measure ensures that the terahertz array detector always operates within its linear response range, guaranteeing consistent image quality.

[0013] Finally, the image processing and fusion unit registers the low-frequency penetration image and the high-frequency detail image after super-resolution reconstruction, and then fuses them using wavelet transform or multi-scale Laplacian pyramid algorithm. S9: Extract deep structural information from the low-frequency image as the base layer; S10: Extract edge and texture details from the high-frequency image as the enhancement layer; S11: Superimpose the two images according to the energy matching principle to generate a comprehensive image that combines deep penetration capability and high-resolution surface details.

[0014] The beneficial effects of this invention are:

[0015] By constructing a dual-frequency coaxial illumination optical path, the system effectively penetrates thick samples using the 110 GHz band, while simultaneously acquiring micron-level defect features using the 340 GHz band, significantly improving the defect detection rate. Electronic modulation and time-division multiplexing strategies replace traditional mechanical scanning, reducing imaging time from minutes to frames, making it suitable for real-time inspection scenarios in industrial production lines. Using a wire grid polarizer as a beam combiner reduces energy loss by approximately 50% compared to traditional semi-transparent, semi-reflective beam splitters, improving overall light energy utilization. Sub-pixel micro-scanning and super-resolution reconstruction technologies effectively overcome undersampling issues caused by excessive pixel spacing in high-frequency bands. The entire system uses solid-state electronic components, eliminating the need for expensive narrowband filters, resulting in a compact structure and controllable cost, demonstrating excellent engineering practicality and industrialization prospects. Attached Figure Description

[0016] Figure 1 This is a structural principle block diagram of the present invention;

[0017] The attached figures are labeled as follows:

[0018] 1a. First terahertz solid-state source; 1b. Second terahertz solid-state source; 2. Grid polarizer; 3. Beam expander optical assembly; 4. Sample stage; 5. Terahertz imaging lens; 6. Terahertz array detector; 7. Piezoelectric ceramic micro-displacement platform; 8. Electrically controlled adjustable attenuator; 9. Electronic control and synchronization unit; 10. Image processing and fusion unit. Detailed Implementation

[0019] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The illustrative embodiments and descriptions herein are used to explain the present invention, but are not intended to limit the present invention.

[0020] like Figure 1 As shown: This invention provides a heterogeneous fusion nondestructive testing imaging system based on dual-frequency terahertz solid-state sources. Its overall structure includes a first terahertz solid-state source 1a, a second terahertz solid-state source 1b, a wire grid polarizer 2, a beam expander optical assembly 3, a sample stage 4, a terahertz imaging lens 5, a terahertz area array detector 6, a piezoelectric ceramic micro-displacement platform 7, an electrically adjustable attenuator 8, an electronic control and synchronization unit 9, and an image processing and fusion unit 10. All components are fixedly connected via a precision optomechanical structure, and their operation sequence is uniformly coordinated by the electronic control and synchronization unit 9, ensuring that the entire system achieves high-penetration, high-resolution, and high-speed terahertz nondestructive testing imaging without mechanical scanning.

[0021] In actual operation, the first terahertz solid-state source 1a uses a 110 GHz frequency doubling module with an output power of 400 mW. Its emitted terahertz waves are shaped into a parallel beam by a Teflon (PTFE) collimating lens, with the polarization direction set to horizontal polarization. The second terahertz solid-state source 1b uses a 340 GHz frequency doubling module with an output power of 50 mW. Its emitted terahertz waves are also shaped into a parallel beam by a high-density polyethylene (HDPE) collimating lens, with the polarization direction set to vertical polarization. The two parallel beams are incident on the wire-grid polarizer 2 along different paths. This polarizer is mounted at a 45° angle on the main optical axis, and its structural characteristics determine that it has high transmittance for horizontally polarized light and high reflectivity for vertically polarized light. Therefore, the 110 GHz horizontally polarized light is directly transmitted through the wire-grid polarizer 2, while the 340 GHz vertically polarized light is reflected. The two beams are spatially combined into a coaxially propagating composite terahertz beam, thus constructing a dual-frequency, co-aperture illumination optical path.

[0022] The composite beam then enters the beam expander optics 3, which consists of a pair of aspherical terahertz lenses to expand the beam diameter to approximately 100 mm to cover the target area on the test stage 4. The test sample is typically a common industrial material such as a non-metallic composite material, a foam sandwich structure, or a ceramic substrate, and is placed at the center of the test stage 4. When the composite terahertz beam irradiates the sample, some energy is absorbed or scattered by the sample, while the remaining transmitted signal, carrying information about the internal structure, is focused by the large-aperture terahertz imaging lens 5 onto the focal plane of the terahertz array detector 6. This terahertz array detector 6 is a 64×64 element microbolometer array with broadband response characteristics and a noise equivalent power (NEP) of 1. The pixel pitch is 1.5 mm.

[0023] Because the terahertz array detector 6 is broadband, it cannot physically distinguish between 110 GHz and 340 GHz signals. Simultaneous illumination of both frequencies would lead to signal aliasing. Therefore, the electronic control and synchronization unit 9 implements high-speed alternating modulation of the two terahertz sources: S1, in odd-numbered frames, the first terahertz solid-state source 1a is turned on and the second terahertz solid-state source 1b is turned off; S2, in even-numbered frames, the second terahertz solid-state source 1b is turned on and the first terahertz solid-state source 1a is turned off; S3, the terahertz array detector 6 is synchronously triggered to acquire image data at the corresponding frequency frame by frame. Through this time-division multiplexing mechanism, the terahertz array detector 6 receives only a single frequency component signal in each frame, effectively avoiding multi-frequency signal aliasing. Simultaneously, it eliminates the need for a mechanical scanning device, significantly improving imaging efficiency and reducing the single-image acquisition time to milliseconds, meeting the real-time requirements of industrial online inspection.

[0024] To address the spatial undersampling issue in the 340 GHz high-frequency band caused by the pixel pitch (1.5 mm) being greater than half its wavelength (approximately 0.44 mm), the system integrates a piezoelectric ceramic micro-displacement platform 7 at the bottom of the terahertz array detector 6. This platform can perform 2×2 or 3×3 sub-pixel micro-scanning operations in 0.75 mm steps (i.e., 0.5 pixel pitch) in both the X and Y directions. Specifically, S4, within each frequency channel, the controller drives the piezoelectric ceramic micro-displacement platform 7 to sequentially complete micro-displacements at four or nine positions; S5, one frame of image is acquired at each position; S6, multiple frames of low-resolution images are input into the super-resolution reconstruction algorithm module to generate a high-resolution image with an equivalent pixel pitch of 0.75 mm or smaller. For example, in the 340 GHz channel, the original image can only resolve defects at a scale of approximately 1.5 mm due to pixel limitations. After 2×2 micro-scanning and super-resolution reconstruction, the equivalent resolution is improved to approximately 0.75 mm, significantly enhancing the ability to identify minute defects such as microcracks and delamination.

[0025] Furthermore, considering that the first terahertz solid-state source 1a has an output power of up to 400 mW, which could easily lead to saturation or even damage of the terahertz array detector 6 when detecting thin samples or low-absorbing materials, an electrically controlled adjustable attenuator 8 is added to the 110 GHz optical path. This attenuator is dynamically adjusted by the electronic control and synchronization unit 9 based on preset parameters or real-time feedback of sample thickness information. S7: When the sample thickness is large or the absorption is strong, the output power of the 110 GHz source is reduced to prevent the terahertz array detector 6 from entering the saturation region due to high-power illumination; S8: When the sample is thin or the absorption is weak, a higher output power is maintained to ensure the signal-to-noise ratio. This measure ensures that the terahertz array detector 6 always operates within its linear response range, guaranteeing the consistency and stability of image quality.

[0026] Finally, the image processing and fusion unit 10 registers the low-frequency penetration image and high-frequency detail image after super-resolution reconstruction, and fuses them using wavelet transform or multi-scale Laplacian pyramid algorithm. S9: Extract deep structural information from the low-frequency image as the base layer; S10: Extract edge and texture details from the high-frequency image as the enhancement layer; S11: Superimpose the two images according to the energy matching principle to generate a comprehensive image that combines deep penetration capability and high-resolution surface details. For example, when detecting aerospace composite skin-honeycomb sandwich structures, the 110 GHz image can clearly present the overall morphology and internal voids of the honeycomb core layer, while the 340 GHz image can accurately identify micron-level cracks or debonding areas on the skin surface. The fused image simultaneously contains both types of key information, significantly improving the defect detection rate and interpretation accuracy.

[0027] In practical industrial applications, this system can be deployed at the end of the production line or at quality inspection stations for rapid, non-destructive screening of components. For example, in lithium battery separator inspection, the 110 GHz band can penetrate multi-layer polymer structures to identify internal bubbles or uneven thickness, while the 340 GHz band can capture surface scratches or micropore defects. In ceramic packaged device inspection, low-frequency images reveal the direction of internal cracks, while high-frequency images locate areas of poor surface sintering. The entire system requires no replacement of the light source or adjustment of the optical path; imaging modes can be switched simply through electronic modulation, making it easy to operate and requiring low maintenance costs.

[0028] It is worth noting that the wire grid polarizer 2, as a beam combiner, has a beam combining efficiency far exceeding that of traditional semi-transparent and semi-reflective beam splitters. The latter typically results in energy loss of over 50%, while in this scheme, 110 GHz light is transmitted almost without loss, and 340 GHz light is reflected efficiently, with a theoretical beam combining efficiency approaching 100%. This fully utilizes the valuable 400 mW and 50 mW of terahertz energy, improving the system's signal-to-noise ratio and imaging contrast. Furthermore, the entire system employs solid-state electronic components, has no moving parts (except for the micro-displacement platform), has a compact structure, good vibration resistance, and is suitable for long-term stable operation in factory environments.

[0029] In summary, this invention constructs a dual-frequency coaxial optical path through orthogonal polarization beam combining, solves the signal aliasing problem of the terahertz array detector 6 by combining a time-division multiplexing acquisition strategy, introduces sub-pixel micro-scanning and super-resolution reconstruction to overcome the physical limitations of pixels, and supplements this with dynamic power regulation to ensure the linear operating area of ​​the terahertz array detector 6. Finally, it generates a comprehensive image with both deep penetration and high-resolution details through multi-scale image fusion. This technical solution achieves high-efficiency, high-precision, and highly adaptable terahertz non-destructive testing without adding mechanical scanning structures, and has significant engineering practical value and industrialization prospects.

[0030] The technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made in accordance with the technical solutions of the present invention fall within the protection scope of the present invention.

Claims

1. A heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source, characterized in that, It includes a first terahertz solid-state source (1a), a second terahertz solid-state source (1b), a wire grid polarizer (2), a beam expander optical assembly (3), a sample stage (4), a terahertz imaging lens (5), a terahertz array detector (6), a piezoelectric ceramic micro-displacement platform (7), an electrically adjustable attenuator (8), an electronic control and synchronization unit (9), and an image processing and fusion unit (10). The first terahertz solid-state source (1a) outputs a horizontally polarized 110 GHz terahertz wave, and the second terahertz solid-state source (1b) outputs a vertically polarized 340 GHz terahertz wave. The two terahertz waves are collimated by collimating lenses to form parallel beams and then incident on a grating polarizer (2) set at a 45° angle on the main optical axis. The grating polarizer (2) has high transmission for horizontally polarized light and high reflection for vertically polarized light, so that the two beams are combined into a coaxially propagating composite beam. The composite beam is expanded by the beam expander optical component (3) and then illuminates the sample placed on the sample stage (4). The transmitted or reflected signal is focused by the terahertz imaging lens (5) onto the terahertz array detector (6). The electronic control and synchronization unit (9) controls the first terahertz solid source (1a) and the second terahertz solid source (1b) to turn on alternately and synchronously triggers the terahertz array detector (6) to acquire image data of the corresponding frequency frame by frame. The piezoelectric ceramic micro-displacement platform (7) is connected to the bottom of the terahertz array detector (6) and is used to perform sub-pixel-level micro-displacement in the X and Y directions. The electrically controlled adjustable attenuator (8) is disposed in the optical path of the first terahertz solid-state source (1a); the image processing and fusion unit (10) is used to register and fuse images of different frequency channels.

2. The heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source as described in claim 1, characterized in that, The electronic control and synchronization unit (9) turns on the first terahertz solid-state source (1a) and turns off the second terahertz solid-state source (1b) in odd-numbered frames, turns on the second terahertz solid-state source (1b) and turns off the first terahertz solid-state source (1a) in even-numbered frames, and synchronously triggers the terahertz array detector (6) to acquire images frame by frame.

3. The heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source as described in claim 2, characterized in that, The piezoelectric ceramic micro-displacement platform (7) performs 2×2 or 3×3 sub-pixel micro-scanning in each frequency channel, with a micro-displacement step size of 0.75 mm, which corresponds to half of the 1.5 mm pixel pitch of the terahertz array detector (6).

4. The heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source as described in claim 3, characterized in that, The image processing and fusion unit (10) inputs multiple frames of images acquired by the piezoelectric ceramic micro-displacement platform (7) in the same frequency channel into the super-resolution reconstruction algorithm module to generate a high-resolution image with an equivalent pixel spacing of 0.75 mm or less.

5. The heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source as described in claim 1, characterized in that, The electronically controlled adjustable attenuator (8) is dynamically adjusted by the electronic control and synchronization unit (9) according to preset parameters or real-time feedback of sample information to control the power of the first terahertz solid source (1a) incident on the sample.

6. The heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source as described in claim 1, characterized in that, The terahertz array detector (6) is a 64×64 element microbolometer array with a noise equivalent power of 1. The pixel pitch is 1.5 mm.

7. The heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source as described in claim 1, characterized in that, The beam-expanding optical component (3) consists of a pair of aspherical terahertz lenses, used to expand the composite beam into a uniform illumination area with a diameter of 100 mm.

8. The heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source as described in claim 1, characterized in that, The image processing and fusion unit (10) uses wavelet transform or multi-scale Laplacian pyramid algorithm to fuse the 110 GHz image and the 340 GHz image after super-resolution reconstruction to generate a comprehensive image containing deep structural information and surface detail information.

9. The heterogeneous fusion nondestructive testing imaging system based on a dual-frequency terahertz solid-state source as described in claim 8, characterized in that, The image processing and fusion unit (10) uses the 110 GHz image as the base layer and the 340 GHz image as the enhancement layer during the fusion process, and superimposes them according to the energy matching principle.

Images