A lateral high-resolution terahertz time-domain spectroscopy imaging method and system

Abstract

The application provides a transverse high-resolution terahertz time-domain spectrum imaging method and system, and relates to the technical field of terahertz imaging. The method comprises the following steps: obtaining a terahertz time-domain pulse sampling signal; performing Fourier transform on the terahertz time-domain pulse sampling signal to obtain a frequency spectrum; performing inverse Fourier transform on the frequency spectrum after high-pass filtering and low-pass filtering processing to obtain a time-domain signal; establishing a point spread function; performing deconvolution operation on the time-domain signal and the time-domain form of the established point spread function to obtain an image reconstruction signal; and determining a transverse high-resolution terahertz time-domain spectrum image according to the image reconstruction signal of each sampling point in a scanning area and the time of flight of the terahertz pulse wave. Through the method, image blurring caused by terahertz beam distortion can be better eliminated, and the transverse resolution of the terahertz imaging system and the quality of the reconstructed image can be improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of terahertz imaging, and particularly relates to a transverse high-resolution terahertz time-domain spectroscopy imaging method and system. BACKGROUND

[0002] The statements in this section merely provide background information related to the present application and do not necessarily constitute prior art that is already known to those skilled in the art.

[0003] Terahertz waves are located between microwaves and near-infrared, with a frequency range of 100 GHz-10 THz and a wavelength of 3 mm-30 μm, also known as far-infrared or submillimeter waves. Compared with other electromagnetic waves, terahertz waves have the characteristics of low photon energy, strong penetration, high thickness resolution, etc., and therefore, terahertz waves are very suitable for security, non-contact, high-resolution non-destructive testing technology.

[0004] Terahertz imaging is a technology that emits a certain intensity of terahertz signal by a terahertz radiation source and irradiates it to a measured object, receives the reflected wave or transmitted wave of the measured object by a terahertz detector, and analyzes and processes the amplitude and phase information detected by the detector through an imaging system to obtain an image of the irradiated object. The terahertz imaging system can detect non-metallic materials including plastics, ceramics, paper, biological tissues, etc., and can effectively perform three-dimensional imaging.

[0005] Most application fields of terahertz imaging technology are sensitive to imaging resolution, and the application value can be improved by improving the resolution of the terahertz imaging system. At present, researchers mainly use algorithm-based and optical-based methods to improve the resolution of the terahertz imaging system. The algorithm-based terahertz imaging system resolution improvement method includes cross-correlation function (convolution function) method, edge shaping method, blind deconvolution method, high-pass error function filtering method and deconvolution method, etc. The optical-based terahertz imaging system resolution improvement method includes wide-aperture aspheric terahertz lens method, solid-state invasive imaging technology, dielectric cube method and wide-bandgap material method.

[0006] However, the algorithm-based terahertz imaging system resolution improvement method does not fully utilize the imaging characteristics of the system itself, which is a data post-processing method and fails to fully utilize the imaging potential of the system; and the optical-based terahertz imaging system resolution improvement method needs to add additional optical devices, which increases the complexity of the system and reduces the signal-to-noise ratio in many cases. SUMMARY

[0007] In order to solve the above problems, the present application provides a transverse high-resolution terahertz time-domain spectroscopy imaging method and system to improve the transverse resolution of the terahertz imaging system and the quality of the reconstructed image.

[0008] To achieve the above object, the present application mainly includes the following aspects:

[0009] In a first aspect, the present application provides a transverse high-resolution terahertz time-domain spectroscopy imaging method, comprising:

[0010] acquiring a terahertz time-domain pulse sampling signal;

[0011] performing Fourier transform on the terahertz time-domain pulse sampling signal to obtain a frequency spectrum, performing inverse Fourier transform on the frequency spectrum after high-pass filtering and low-pass filtering processing to obtain a time-domain signal; establishing a point spread function, and performing deconvolution operation on the time-domain signal and the time-domain form of the established point spread function to obtain an image reconstruction signal;

[0012] determining a transverse high-resolution terahertz time-domain spectroscopy image according to the image reconstruction signal of each sampling point in the scanning area and the time of flight of the terahertz pulse wave.

[0013] In a possible implementation, the sample is scanned by a terahertz time-domain spectroscopy system, and a plurality of groups of terahertz time-domain pulse data are collected at each sampling point, and the terahertz time-domain pulse sampling signal is obtained after averaging.

[0014] In a possible implementation, Fourier transform is performed on the terahertz time-domain pulse sampling signal of each sampling point to obtain a frequency spectrum; a high-pass filter in the frequency domain is used to filter out low-frequency data of the frequency spectrum, and a low-pass filter in the frequency domain is used to filter out high-frequency data of the frequency spectrum, and then inverse Fourier transform is performed to obtain a time-domain signal; wherein the turning frequency of the high-pass filtering is lower than the turning frequency of the low-pass filtering.

[0015] In a possible implementation, the high-pass filtering cutoff frequency is 0.3 THz, and the low-pass filtering cutoff frequency is 1.5 THz.

[0016] In a possible implementation, the point spread function is expressed as:

[0017]

[0018] wherein z represents the radial position coordinate of the optical axis, f represents the frequency, I ref represents the reference beam intensity, p represents the beam radius of the z plane, NA represents the numerical aperture, k represents a factor related to the truncation ratio and the irradiance level, a represents an adjustment factor, c represents the speed of light, and a represents the frequency-dependent absorption coefficient.

[0019] In a possible implementation, the deconvolution operation is performed on the time-domain signal i(x, y) and the time-domain form PSF(x, y) of the point spread function to obtain a reconstructed image signal, and the image reconstruction signal is:

[0020] o(x,y) = i(x,y) * PSF(x,y) -1 PSF(x,y);

[0021] where (x,y) represents the lateral coordinates of the sample sampling plane orthogonal to the z-axis, -1 denotes a deconvolution operator.

[0022] In one possible implementation, the time of flight of the terahertz pulse wave is determined according to the positional relationship among the transmitting antenna, the sample, and the receiving antenna of the terahertz time-domain spectroscopy system.

[0023] In a second aspect, an embodiment of the present application provides a lateral high-resolution terahertz time-domain spectroscopy imaging system, comprising:

[0024] An acquisition module is configured to acquire a terahertz time-domain pulse sampling signal.

[0025] A processing module is configured to perform Fourier transform on the terahertz time-domain pulse sampling signal to obtain a frequency spectrum, perform inverse Fourier transform on the frequency spectrum after high-pass filtering and low-pass filtering, obtain a time-domain signal, establish a point spread function, perform deconvolution operation on the time-domain signal and the established point spread function, and obtain an image reconstruction signal.

[0026] An imaging module is configured to determine a lateral high-resolution terahertz time-domain spectroscopy image according to the image reconstruction signal of each sampling point in a scanning region and the time of flight of the terahertz pulse wave.

[0027] In a third aspect, an embodiment of the present application provides a computer device, comprising a processor, a memory, and a bus, the memory stores machine readable instructions executable by the processor, when the computer device is running, the processor and the memory communicate through the bus, and the machine readable instructions are executed by the processor to perform the steps of the lateral high-resolution terahertz time-domain spectroscopy imaging method as described in the first aspect and any possible implementation of the first aspect.

[0028] In a fourth aspect, an embodiment of the present application provides a computer readable storage medium, the computer readable storage medium stores a computer program, and the computer program is executed by the processor to perform the steps of the lateral high-resolution terahertz time-domain spectroscopy imaging method as described in the first aspect and any possible implementation of the first aspect.

[0029] The above one or more technical solutions have the following beneficial effects:

[0030] The application provides a transverse high-resolution terahertz time-domain spectroscopy imaging method, which comprises the following steps: comprehensively utilizing a point spread function of a terahertz time-domain spectroscopy imaging system, a terahertz time-domain processing method and a frequency-domain processing method, and acquiring a terahertz time-domain pulse sampling signal; performing Fourier transform on the terahertz time-domain pulse sampling signal to obtain a frequency spectrum; performing inverse Fourier transform on the frequency spectrum after high-pass filtering and low-pass filtering to obtain a time-domain signal, so as to suppress low-frequency and high-frequency noise interference; establishing a point spread function, and performing deconvolution operation on the time-domain signal and a time-domain form of the established point spread function to obtain an image reconstruction signal; and determining a transverse high-resolution terahertz time-domain spectroscopy image according to the image reconstruction signal of each sampling point in a scanning area and a time of flight of a terahertz pulse wave, so that image blurring caused by terahertz beam distortion can be better eliminated, and the transverse resolution of the terahertz imaging system and the quality of the reconstructed image can be improved.

[0031] In addition, the terahertz time-domain spectroscopy system performs multiple measurements on each sampling point of a sample, and then takes an average to significantly reduce the noise floor of the terahertz pulse sampling signal, improve the signal-to-noise ratio of the signal, and reduce image noise. BRIEF DESCRIPTION OF DRAWINGS

[0032] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application and are incorporated in and constitute a part of this application. The embodiments of the application illustrate the embodiments of the application and, together with the description, serve to explain the principles of the application.

[0033] Figure 1 FIG. 1 is a structural schematic diagram of a terahertz time-domain spectroscopy instrument structure provided by an embodiment of the application;

[0034] Figure 2 FIG. 2 is a flowchart of a transverse high-resolution terahertz time-domain spectroscopy imaging method provided by an embodiment of the application. DETAILED DESCRIPTION

[0035] The application will be further described below in conjunction with the drawings and embodiments.

[0036] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the application belongs.

[0037] It should be noted that the terms used herein are only intended to describe specific embodiments, and are not intended to limit the exemplary embodiments according to the application. As used herein, the singular form is intended to include the plural form unless the context clearly indicates otherwise, and furthermore, it should be understood that when the terms "comprise" and / or "include" are used in the specification, there is a feature, step, operation, device, component and / or combination thereof.

[0038] Embodiment one

[0039] A typical terahertz time-domain spectroscopy system is composed of an ultrafast pulsed laser, a terahertz emitter, a terahertz detector and a time delay controller. The system emits terahertz pulses to interact with a sample, measures the curve of the terahertz electric field intensity after the interaction, and calculates the terahertz intensity and phase information of the sample. Different detection modes such as transmission, reflection, differential and elliptical can be used according to different samples, different testing requirements and different interaction modes of terahertz waves and samples.

[0040] The structural diagram of the terahertz time-domain spectroscopy instrument is shown in Figure 1 The terahertz time-domain spectroscopy instrument includes an ultrafast laser, a beam splitter, a mirror, an optical delay device, a photoconductive emitter antenna, a photoconductive receiver antenna and a processor. The ultrafast laser emits an ultra-short light pulse, which is divided into two beams by the beam splitter. One of the beams is converted into a terahertz pulse wave by the photoconductive emitter antenna and emitted to the surface of the measured object, and the other beam enters the optical delay device through the mirror. The processor can control the optical delay device to realize step delay, so that the output signal of the optical delay device and the terahertz reflection echo received by the photoconductive receiver antenna are scanned coherently along the time axis, and then the terahertz reflection echo digital signal is obtained by using the equivalent sampling principle. In this embodiment, the effective frequency band of the terahertz time-domain pulse is 60GHz-3THz, and the average output power is 65mW.

[0041] The existing algorithm-based terahertz imaging system resolution improvement method is a data post-processing method, which does not fully utilize the imaging characteristics of the terahertz imaging system itself; the optical-based terahertz imaging system resolution improvement method needs to increase additional optical devices, which increases the system complexity and reduces the signal-to-noise ratio in many cases, which is a method of doing twice the work at half the effort.

[0042] Therefore, in the embodiments of the present application, a transverse high-resolution terahertz time-domain spectroscopy imaging method is provided, which comprises the following steps:

[0043] S101: Obtain a terahertz time-domain pulse sampling signal.

[0044] In specific implementation, the terahertz time-domain pulse sampling signal is obtained by using a terahertz time-domain spectroscopy instrument. Optionally, the sample is scanned by the terahertz time-domain spectroscopy system, and a plurality of sets of terahertz time-domain pulse data are collected at each sampling point, and the terahertz time-domain pulse sampling signal is obtained by averaging. The repetition frequency of the terahertz pulse wave emitted by the terahertz time-domain spectroscopy system can usually reach more than 100MHz, and even if the number of repeated collections at each sampling point reaches several hundred or several thousand times, the imaging speed is not greatly affected. Multiple measurements and averaging can significantly reduce the noise floor of the terahertz pulse sampling signal, improve the signal-to-noise ratio, and reduce image noise.

[0045] S102: Fourier transform is performed on the terahertz time-domain pulse sampling signal to obtain a frequency spectrum, the frequency spectrum is processed by high-pass filtering and low-pass filtering, and inverse Fourier transform is performed to obtain a time-domain signal; a point spread function is established, and the time-domain signal is deconvoluted with a time-domain form of the established point spread function to obtain an image reconstruction signal.

[0046] In a specific implementation, first, the terahertz time-domain pulse sampling signal is subjected to frequency domain high-pass filtering processing. Specifically, the terahertz time-domain pulse sampling signal of each sampling point is first subjected to Fourier transform to obtain frequency domain data, and then a frequency domain high-pass filter is used to filter out low-frequency data of the frequency spectrum, so that the adverse effects of low-frequency noise on imaging can be eliminated.

[0047] Further, the terahertz time-domain pulse sampling signal is subjected to frequency domain low-pass filtering processing for suppressing high-frequency noise. Here, the high-pass filtering turning frequency and the low-pass filtering turning frequency need to cooperate with each other, so that the high-pass filtering turning frequency is lower than the low-pass filtering turning frequency, and the working bandwidth can cover most of the effective components of the signal. The working frequency band of a common existing terahertz time-domain spectrometer is usually 0.1-2.0 THz. The closer to the low-frequency limit and the high-frequency limit, the more serious the noise. Therefore, the noise proportion in the frequency spectrum close to 0.1 THz and 2.0 THz is large. In addition, in order to reflect the advantage of a large bandwidth of the terahertz time-domain spectrometer, the frequency spectrum data of a frequency width of at least greater than 0.5 THz is usually retained. Optionally, the high-pass filtering cutoff frequency is usually selected to be 0.3 THz, and the low-pass filtering cutoff frequency is usually selected to be 1.5 THz.

[0048] A point spread function model is established for the terahertz time-domain spectrum system, and the expression of the point spread function model PSF(z,f) is as follows:

[0049]

[0050] wherein z represents a position coordinate in the radial direction of the optical axis (the direction of terahertz wave propagation), f represents frequency, I ref represents reference beam intensity, p represents beam radius in the z plane, NA represents numerical aperture, k represents a factor related to the truncation ratio and the irradiance level, a represents an adjustment factor, c represents the speed of light, and a represents the frequency-dependent absorption coefficient. ref , NA, p, and k are optical parameters of the terahertz time-domain spectrum system, which are mainly determined by the physical parameters and relative positions of the transmitting antenna, the sample, and the receiving antenna, and a and a are test parameters of the terahertz time-domain spectrum system, which can be obtained by experimental measurement.

[0051] The filtered frequency domain sampling signal is inversely Fourier transformed to obtain a time domain signal i(x, y) again, and then is deconvoluted with a time domain form PSF(x, y) of the point spread function model to obtain a reconstructed image signal. The time domain form PSF(x, y) of the point spread function model is obtained by inversely Fourier transforming the frequency domain form PSF(z, f), and the image reconstruction signal is:

[0052] o(x, y) = i(x, y) * PSF(x, y) -1 .

[0053] Wherein (x, y) represents a lateral coordinate of a sample sampling plane orthogonal to the z axis, and -1 represents a deconvolution operator.

[0054] S103: determining a lateral high-resolution terahertz time-domain spectrum image according to the image reconstruction signal of each sampling point in the scanning area and the time of flight of the terahertz pulse wave.

[0055] In a specific implementation, all sampling points of the terahertz time-domain spectrum system are processed by the above steps to obtain a complete image reconstruction signal. The time of flight is obtained according to the positional relationship among the terahertz time-domain spectrum system transmitting antenna, the sample, and the receiving antenna. Based on the complete image reconstruction signal and the time of flight, a lateral high-resolution terahertz time-domain spectrum image is obtained.

[0056] The method flowchart proposed in the embodiment of the present application is shown in Figure 2 As shown in the figure, by comprehensively utilizing the point spread function of the terahertz time-domain spectrum imaging system, the terahertz time-domain and frequency-domain processing method, each sampling point of the sample is measured multiple times, then the signal noise is reduced by averaging, low-pass and high-pass filtering processing is performed in the frequency domain, low-frequency and high-frequency noise interference is suppressed, and deconvolution operation is performed with the point spread function established based on the physical parameters of the terahertz time-domain spectrum system, so that the image blur caused by the terahertz beam distortion can be better eliminated, and the lateral resolution and the reconstructed image quality of the terahertz imaging system can be improved.

[0057] Embodiment two

[0058] The embodiment of the present application also provides a lateral high-resolution terahertz time-domain spectrum imaging system, which comprises:

[0059] An acquisition module is configured to acquire a terahertz time-domain pulse sampling signal;

[0060] A processing module is configured to perform Fourier transform on the terahertz time-domain pulse sampling signal to obtain a frequency spectrum, inversely Fourier transform the frequency spectrum after high-pass filtering and low-pass filtering processing to obtain a time domain signal, establish a point spread function, and perform deconvolution operation on the time domain signal and the established point spread function to obtain an image reconstruction signal.

[0061] an imaging module configured to determine a lateral high-resolution terahertz time-domain spectroscopy image according to the image reconstruction signals of the sampling points in the scanning region and the time of flight of the terahertz pulse wave.

[0062] The lateral high-resolution terahertz time-domain spectroscopy imaging system provided by the embodiment is used to implement the lateral high-resolution terahertz time-domain spectroscopy imaging method, and therefore the specific implementation of the lateral high-resolution terahertz time-domain spectroscopy imaging system can be seen from the embodiment part of the lateral high-resolution terahertz time-domain spectroscopy imaging method, which will not be described here again.

[0063] Embodiment three

[0064] The embodiment of the present application further provides a computer device, which comprises a processor, a memory and a bus.

[0065] The memory stores machine readable instructions executable by the processor, and when the computer device is running, the processor and the memory communicate through the bus. When the machine readable instructions are executed by the processor, the steps of the lateral high-resolution terahertz time-domain spectroscopy imaging method in the method embodiment can be implemented, and the specific implementation can be referred to the method embodiment, which will not be described here again. Figure 2

[0066] Embodiment four

[0067] Based on the same inventive concept, the embodiment of the present application further provides a computer readable storage medium, which stores a computer program. When the computer program is run by a processor, the steps of the lateral high-resolution terahertz time-domain spectroscopy imaging method in the method embodiment are executed.

[0068] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by a computer program instructing related hardware. The program can be stored in a computer readable storage medium, and when the program is executed, the processes of the above-mentioned embodiments can be included. The storage medium can be a magnetic disc, an optical disc, a read-only memory (ROM) or a random access memory (RAM).

[0069] The above only describes the preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.​

Claims

1. A transverse high-resolution terahertz time-domain spectral imaging method, characterized in that, include: Acquire terahertz time-domain pulse sampling signals; The terahertz time-domain pulse sampling signal is subjected to Fourier transform to obtain the spectrum. The spectrum is then subjected to high-pass filtering and low-pass filtering, followed by inverse Fourier transform to obtain the time-domain signal. A point spread function is established, and the time-domain signal is deconvolved with the time-domain form of the established point spread function to obtain the image reconstruction signal; Based on the image reconstruction signal of each sampling point in the scanning area and the flight time of the terahertz pulse wave, the transverse high-resolution terahertz time-domain spectral image is determined; The point spread function is expressed as: ； in, Indicates the radial position coordinates of the optical axis. Indicates frequency, Indicates the reference beam strength. express beam radius of a plane Indicates numerical aperture. Factors related to cutoff ratio and irradiance level Indicates the adjustment factor. Represents the speed of light. This represents the absorption coefficient related to frequency.

2. The lateral high-resolution terahertz time-domain spectral imaging method as described in claim 1, characterized in that, The sample is scanned by a terahertz time-domain spectroscopy system, and multiple sets of terahertz time-domain pulse data are collected at each sampling point. The average of these data is then used to obtain the terahertz time-domain pulse sampling signal.

3. The lateral high-resolution terahertz time-domain spectral imaging method as described in claim 2, characterized in that, A Fourier transform is performed on the terahertz time-domain pulse sampling signal at each sampling point to obtain the spectrum; a frequency-domain high-pass filter is used to filter out the low-frequency data of the spectrum, and a frequency-domain low-pass filter is used to filter out the high-frequency data of the spectrum, and then an inverse Fourier transform is performed to obtain the time-domain signal; wherein, the corner frequency of the high-pass filter is lower than the corner frequency of the low-pass filter.

4. The transverse high-resolution terahertz time-domain spectral imaging method as described in claim 3, characterized in that, The high-pass filter cutoff frequency is 0.3THz, and the low-pass filter cutoff frequency is 1.5THz.

5. The lateral high-resolution terahertz time-domain spectral imaging method as described in claim 1, characterized in that, Time domain signal Time-domain form of the point spread function Perform deconvolution operation to obtain the reconstructed image signal, which is: ； in, Indicates and The horizontal coordinates of the sample sampling plane orthogonal to the axis. This represents the deconvolution operator.

6. The lateral high-resolution terahertz time-domain spectral imaging method as described in claim 2, characterized in that, The flight time of the terahertz pulse wave is determined based on the relative positions of the transmitting antenna, the sample, and the receiving antenna of the terahertz time-domain spectroscopy system.

7. A transverse high-resolution terahertz time-domain spectral imaging system, characterized in that, include: The acquisition module is used to acquire terahertz time-domain pulse sampling signals; The processing module is used to perform a Fourier transform on the terahertz time-domain pulse sampling signal to obtain a spectrum. The spectrum is then subjected to high-pass and low-pass filtering followed by an inverse Fourier transform to obtain a time-domain signal. A point spread function is established, and the time-domain signal is deconvolved with the established point spread function to obtain an image reconstruction signal. The point spread function is expressed as: ； in, Indicates the radial position coordinates of the optical axis. Indicates frequency, Indicates the reference beam strength. express beam radius of a plane Indicates numerical aperture. Factors related to cutoff ratio and irradiance level Indicates the adjustment factor. Represents the speed of light. This represents the frequency-dependent absorption coefficient; The imaging module is used to determine a transverse high-resolution terahertz time-domain spectral image based on the image reconstruction signal of each sampling point in the scanning area and the flight time of the terahertz pulse wave.

8. A computer device, characterized in that, include: The computer device includes a processor, a memory, and a bus. The memory stores machine-readable instructions executable by the processor. When the computer device is running, the processor communicates with the memory via the bus. When the machine-readable instructions are executed by the processor, they perform the steps of the lateral high-resolution terahertz time-domain spectroscopic imaging method as described in any one of claims 1 to 6.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by a processor, performs the steps of the lateral high-resolution terahertz time-domain spectral imaging method as described in any one of claims 1 to 6.

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