A scanning-free terahertz imaging system fusing single-cavity dual-optical comb and dispersion fourier transform

Abstract

This invention discloses a scanning-free terahertz imaging system integrating single-cavity dual-comb and dispersive Fourier transform, belonging to the terahertz time-domain domain. It solves the problems of poor real-time performance and the mutual constraint between time scanning and spatial encoding in existing terahertz time-domain imaging systems. The system includes a single-cavity dual-comb erbium-doped fiber mode-locked laser module. This module outputs a pump comb and a probe comb. The pump comb is input to a terahertz emission and spatial encoding module and outputs an encoded terahertz light field. The probe comb is input to a terahertz detection and data acquisition module. The encoded terahertz light field illuminates the target sample, carrying the sample information, and is then injected into the terahertz detection and data acquisition module to output a terahertz time-domain waveform. The terahertz time-domain waveform and the probe comb are jointly input into a dispersion compensation fiber to output spectral interference fringes. These spectral interference fringes are injected into a data acquisition board to reconstruct a two-dimensional spatial image. This invention is applied to terahertz time-domain imaging.

Description

Technical Field

[0001] This invention relates to the field of terahertz time-domain imaging technology, specifically to a scanning-free terahertz imaging system that integrates single-cavity dual optical comb and dispersive Fourier transform. Background Technology

[0002] Terahertz time-domain imaging (THz-TDI) has shown broad application prospects in non-destructive testing, biomedical diagnostics, and safety screening due to the unique advantages of terahertz waves' penetrability to nonpolar materials, low photon energy, and the ability to simultaneously acquire multi-dimensional information such as sample amplitude, phase, and absorption coefficient. A typical terahertz time-domain imaging system usually uses a femtosecond laser as the core light source, dividing the laser pulse into a pump beam and a probe beam: the pump beam generates terahertz pulses through optical rectification or an optical guide antenna, illuminating the sample and carrying sample information; the probe beam adjusts the time delay between itself and the pump beam through a mechanical delay line or asynchronous optical sampling, and is then received by electro-optic sampling or an optical guide antenna, thereby reconstructing the terahertz time-domain waveform point by point. Simultaneously, spatial image acquisition typically relies on a two-dimensional mechanical displacement platform to scan the sample point by point, or uses spatial light modulators such as digital micromirror devices (DMDs) combined with single-pixel detection principles to achieve spatially encoded imaging.

[0003] However, the aforementioned technical approaches face insurmountable bottlenecks in both the temporal and spatial dimensions. In the temporal dimension, both mechanical delay lines and asynchronous optical sampling essentially rely on cumulative scanning across multiple pulse cycles—each time delay point requires multiple averagings to improve the signal-to-noise ratio, resulting in acquisition times for complete time-domain waveforms typically exceeding milliseconds, making it difficult to meet the real-time measurement requirements of transient events or rapid dynamic processes. Simultaneously, asynchronous optical sampling systems typically require two independent mode-locked femtosecond lasers equipped with precise phase-locked frequency stabilization circuits, leading to system complexity, high cost, and environmental sensitivity. Specifically, at the detection end, traditional optical guide antenna receiving schemes require femtosecond probe light pulses to be injected into the antenna for coherent detection; the probe light and pump light must maintain femtosecond-level precise synchronization. This constraint forcibly binds the probe light path to the receiving antenna, preventing independent optimization. In the spatial dimension, even with DMD replacing two-dimensional mechanical scanning, existing time sampling mechanisms still require a complete time waveform scan to be completed for each frame of spatial coding pattern switching. The temporal and spatial measurements are mutually constrained, making true parallel acquisition impossible. Furthermore, traditional imaging methods based on time-pulse envelope fitting have an inherent problem—when the probe pulse and the reference pulse are too close, their waveforms severely overlap and become indistinguishable. Therefore, there is an urgent need for a novel terahertz time-domain imaging method that can achieve single-shot, non-cumulative measurements in the time dimension, overcome the limitation of forced bonding between the probe light and the receiving antenna, and support truly parallel temporal-spatial acquisition. Summary of the Invention

[0004] To address the technical problems in existing terahertz time-domain imaging systems, such as the poor real-time performance caused by the reliance on cumulative sampling for time-domain waveform acquisition, and the mutual constraints between the probe optical path and the receiving antenna, as well as the time scanning and spatial coding, this invention proposes a scanning-free terahertz imaging system that integrates a single-cavity dual optical comb and dispersive Fourier transform.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a scanning-free terahertz imaging system integrating single-cavity dual-optical-comb and dispersive Fourier transform, including a single-cavity dual-optical-comb erbium-doped fiber mode-locked laser module. The single-cavity dual-optical-comb erbium-doped fiber mode-locked laser module outputs two optical frequency combs. One of them is used as a pump optical comb and input to the terahertz emission and spatial coding module, where it is split into a first pump light and a second pump light. The other is used as a probe optical comb and input to the terahertz detection and data acquisition module.

[0006] The first pump light is incident on the digital micromirror device of the terahertz emission and spatial coding module and, after spatial modulation, is projected onto the high-resistivity silicon wafer of the terahertz emission and spatial coding module. At the same time, the second pump light is injected into the first optical guide transmitting antenna of the terahertz emission and spatial coding module to generate a terahertz pulse sequence. The terahertz pulse sequence is transmitted through the high-resistivity silicon wafer, so that the terahertz pulse sequence carries DMD pattern coding information and forms a coded terahertz light field.

[0007] The coded terahertz light field is irradiated onto the target sample. After the coded terahertz light field carrying the target sample information is injected into the second optical guide receiving antenna of the terahertz detection and data acquisition module, a first electrical signal is output. The first electrical signal generates a terahertz time-domain waveform carrying DMD pattern coding information through an electro-optic modulator. The terahertz time-domain waveform carrying DMD pattern coding information is loaded onto the detector optical comb to obtain a modulated detector optical comb. The modulated detector optical comb is injected into a dispersion compensation fiber to complete the dispersion Fourier transform under large dispersion conditions and output spectral interference fringes.

[0008] The spectral interference fringes are sequentially injected into the photodetector and data acquisition board of the terahertz detection and data acquisition module to reconstruct the terahertz time-domain waveform and two-dimensional spatial image.

[0009] Furthermore, the single-cavity dual-comb erbium-doped fiber mode-locked laser module includes a pump source, a wavelength division multiplexer, an erbium-doped fiber, a first coupler, a polarization-maintaining fiber, and a first polarization controller connected in sequence via optical paths. The output of the first polarization controller is optically connected to the input of the wavelength division multiplexer to form a closed ring laser resonant cavity. The output of the first coupler is also connected to the input of the circulator. The two outputs of the circulator are respectively connected to the input of the terahertz emission and spatial coding module and the input of the terahertz detection and data acquisition module.

[0010] Furthermore, an isolator and a single-walled carbon nanotube saturable absorber are connected in series between the erbium-doped fiber and the first coupler.

[0011] Furthermore, a fiber Bragg grating is optically connected between the circulator and the terahertz detection and data acquisition module.

[0012] Furthermore, the terahertz emission and spatial coding module includes a first erbium-doped fiber amplifier. The input end of the first erbium-doped fiber amplifier is connected to the output optical path of the single-cavity dual-comb erbium-doped fiber mode-locked laser module. The output optical path of the first erbium-doped fiber amplifier is connected to a second coupler. The second coupler splits the received pump light comb into a first pump light and a second pump light.

[0013] The first pump light is sequentially injected into the collimator, double freeform lens and digital micromirror device of the terahertz emission and spatial coding module to complete spatial modulation. The spatially modulated first pump light is then projected onto the high-resistivity silicon wafer. At the same time, the second pump light is sequentially injected into the second polarization controller and the first optical guide transmitting antenna of the terahertz emission and spatial coding module to output a terahertz pulse sequence. The terahertz pulse sequence is transmitted through the high-resistivity silicon wafer to form a coded terahertz light field.

[0014] Furthermore, the terahertz detection and data acquisition module includes a second erbium-doped fiber amplifier. The input end of the second erbium-doped fiber amplifier is optically connected to the output end of the single-cavity dual-comb erbium-doped fiber mode-locked laser module. The output end of the second erbium-doped fiber amplifier is sequentially optically connected to a third polarization controller, a transimpedance amplifier, an electro-optic modulator, a dispersion compensation fiber, a photodetector, and a data acquisition board.

[0015] The electro-optic modulator is also connected to the output of the second optical guide receiving antenna via a transimpedance amplifier.

[0016] Furthermore, when the modulated probe light pulse passes through a dispersion-compensated fiber of length L and group velocity dispersion coefficient β2, the time-domain waveform of the output modulated probe light comb pulse, which is stretched by dispersion, is shown. The expression is:

[0017] ;

[0018] In the formula, The imaginary unit, ;

[0019] The group velocity dispersion coefficient of the dispersion-compensating fiber;

[0020] The length of the fiber used for dispersion compensation;

[0021] It is a time variable;

[0022] The spectrum of the modulated probe light pulse;

[0023] This is the complex envelope of the modulated probe light pulse.

[0024] Furthermore, the frequency f of the spectral interference fringes is:

[0025] ;

[0026] In the formula, τ is the time delay between two femtosecond pulses in the probe optical comb pulse;

[0027] L is the length of the dispersion compensation fiber;

[0028] β2 is the group velocity dispersion coefficient of the dispersion-compensated fiber.

[0029] Furthermore, the data acquisition board integrates a signal processing unit, which is configured as follows:

[0030] Based on the principle of dispersive Fourier transform, the complete terahertz time-domain waveform is extracted from the acquired spectral interference fringes.

[0031] Two-dimensional spatial images of the sample are reconstructed based on the DMD pattern encoding information using inverse Hadamard transform or compressed sensing algorithms.

[0032] Furthermore, the laser output from the pump source is in the 976nm~980nm wavelength range.

[0033] The advantages of this invention over the prior art are as follows:

[0034] 1. This invention employs a single-cavity, dual-comb, erbium-doped fiber mode-locked laser that simultaneously outputs a pump comb and a probe comb, replacing two independent lasers. A dispersive Fourier transform is introduced at the probe end to map the terahertz time-domain waveform into spectral interference fringes of a single pulse, achieving non-cumulative single-shot time waveform capture and fundamentally replacing asynchronous optical sampling that relies on multi-pulse scanning. Simultaneously, a digital micromirror device is used to spatially encode the pump light, and the terahertz wave transmittance is modulated by the photoinduced carrier effect of a high-resistivity silicon plate, achieving mechanical-free scanning in the spatial dimension. Compared to traditional terahertz time-domain imaging systems, this invention's system, through the coordinated operation of single-shot measurement in the time dimension and parallel encoding in the spatial dimension, eliminates the need for traditional mechanical delay lines, two-dimensional mechanical scanning platforms, and complex dual-laser frequency stabilization and phase-locked loop structures, greatly simplifying the system structure, reducing costs, and significantly improving imaging speed and system stability.

[0035] 2. The single-cavity dual-comb erbium-doped fiber mode-locked laser module of the present invention simultaneously outputs a pump comb and a probe comb, eliminating the need for dual lasers and phase-locked circuits. It features a compact structure, low cost, and strong common-mode noise suppression capability.

[0036] 3. The terahertz emission and spatial coding module and the terahertz detection and data acquisition module of this invention work together to map the terahertz time-domain waveform into the spectral interference fringes of a single pulse through dispersive Fourier transform, thereby achieving single-pulse waveform acquisition without the need for mechanical delay lines and asynchronous sampling. The time-domain waveform acquisition time is reduced from the millisecond level to the microsecond level.

[0037] 4. In this invention, the second optical receiving antenna of the terahertz detection and data acquisition module only serves as the front end for terahertz-to-electrical signal conversion, and the detection optical comb independently enters the electro-optic modulator, eliminating the femtosecond-level synchronization requirement and reducing system complexity.

[0038] 5. This invention replaces two-dimensional mechanical scanning with DMD image encoding and combines the single-shot measurement capability of dispersive Fourier transform. Each frame of DMD pattern encoding information corresponds to a single time-domain waveform capture, realizing time-space parallel acquisition and effectively improving imaging speed. Attached Figure Description

[0039] The present invention will be further described below with reference to the accompanying drawings:

[0040] Fig. 1 This is a schematic diagram of the system structure of the present invention;

[0041] Fig. 2 This is a schematic diagram of the process of generating spectral interference fringes by a probe optical comb modulated by an encoded terahertz optical field according to an embodiment of the present invention; wherein, Figure (a) shows two femtosecond pulses of the modulated probe optical comb output after being modulated by an electro-optic modulator; Figure (b) shows the waveforms of the two femtosecond pulses of the probe optical comb being widened and overlapping after passing through a 20km dispersion compensation fiber; Figure (c) shows the electrical signal output by the photodetector.

[0042] In the diagram: 1 is the pump source, 2 is the wavelength division multiplexer, 3 is the erbium-doped fiber, 4 isolator, 5 is the single-walled carbon nanotube saturable absorber, 6 is the first coupler, 7 is the polarization-maintaining fiber, 8 is the first polarization controller, 9 is the circulator, 10 is the fiber Bragg grating, 11 is the first erbium-doped fiber amplifier, 12 is the second coupler, 13 is the second polarization controller, 14 is the collimator, 15 is the double freeform lens, 16 is the digital micromirror device, and 17 is the first photoconductor. 18 is a high-resistivity silicon wafer, 19 is a sample, 20 is a second optical guide receiving antenna, 21 is a second erbium-doped fiber amplifier, 22 is a third polarization controller, 23 is a transimpedance amplifier, 24 is an electro-optic modulator, 25 is a dispersion compensation fiber, 26 is a photodetector, 27 is a data acquisition board, 101 is a single-cavity dual-comb erbium-doped fiber mode-locked laser module, 102 is a terahertz emission and spatial coding module, and 103 is a terahertz detection and data acquisition module. Detailed Implementation

[0043] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate relative orientations or positional relationships and are used only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0044] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0045] like Figs. 1-2As shown, the present invention provides a scanning-free terahertz imaging system integrating single-cavity dual-comb and dispersive Fourier transform, including a single-cavity dual-comb erbium-doped fiber mode-locked laser module 101. The single-cavity dual-comb erbium-doped fiber mode-locked laser module 101 outputs two coherent high-coherence optical frequency combs with a small repetition frequency difference (e.g., 200~800Hz). One of them is used as a pump comb and input to the terahertz emission and spatial coding module 102, where it is split into a first pump beam and a second pump beam; the other is used as a probe comb and input to the terahertz detection and data acquisition module 103.

[0046] Specifically, the single-cavity dual-comb erbium-doped fiber mode-locked laser module 101 includes a pump source 1, a wavelength division multiplexer 2, an erbium-doped fiber 3, a first coupler 6, a polarization-maintaining fiber 7, and a first polarization controller 8, which are connected in sequence via optical paths. The output of the first polarization controller 8 is optically connected to the input of the wavelength division multiplexer 2 to form a closed ring laser resonant cavity. The output of the first coupler 6 is also connected to the input of a circulator 9. The two outputs of the circulator 9 are respectively connected to the inputs of the terahertz emission and spatial coding module 102 and the terahertz detection and data acquisition module 103. More specifically, an isolator 4 and a single-walled carbon nanotube saturable absorber 5 are connected in series between the erbium-doped fiber 3 and the first coupler 6; a fiber Bragg grating 10 is also optically connected between the circulator 9 and the terahertz detection and data acquisition module 103.

[0047] Furthermore, in the optical frequency comb output by the first coupler 6, one channel with a power ratio of 10% serves as the probe optical comb, while the remaining 90% is processed by the circulator 9 and the fiber Bragg grating 10 and then used as the pump optical comb. In a specific embodiment, the pump source 1 outputs a 976nm~980nm (preferably 980nm) laser, which is coupled into a closed ring laser resonator via a wavelength division multiplexer 2, and the erbium-doped fiber 3 generates a broadband gain spectrum. Simultaneously, a single-walled carbon nanotube saturable absorber 5 is used as the mode-locking element, and the birefringence and nonlinear effects introduced by the polarization-maintaining fiber 7 within the closed ring laser resonator are utilized to achieve a self-starting, highly stable mode-locked optical pulse output to the first coupler 6. The first port of circulator 9 receives mode-locked optical pulses from the first coupler 6 with a beam splitting ratio of 10:90. The second port of circulator 9 is connected to the optical path of fiber Bragg grating 10. Fiber Bragg grating 10 acts as a wavelength-selective reflector, reflecting mode-locked optical pulses that satisfy the Bragg condition back to circulator 9. The remaining mode-locked optical pulses are transmitted and output. The reflected mode-locked optical pulses are output through the third port of circulator 9. That is, the third port of fiber Bragg grating 10 and circulator 9 respectively output two coherent optical frequency combs with a small repetition frequency difference. The two optical frequency combs serve as the probe comb and pump comb, respectively. This structure eliminates the need for two independent lasers and complex active frequency stabilization phase-locked circuits, has strong common-mode noise suppression capability within the resonant cavity, and features a compact system structure and high stability. The first pump light is incident on the digital micromirror device 16 of the terahertz emission and spatial coding module 102 and, after spatial modulation, is projected onto the high-resistivity silicon wafer 18 of the terahertz emission and spatial coding module 102. Simultaneously, the second pump light is injected into the first optical guide transmitting antenna 17 of the terahertz emission and spatial coding module 102 to generate a terahertz pulse sequence. The terahertz pulse sequence is transmitted through the high-resistivity silicon wafer 18, so that the terahertz pulse sequence carries DMD pattern coding information to form a coded terahertz light field.

[0048] Specifically, the terahertz emission and spatial coding module 102 includes a first erbium-doped fiber amplifier 11. The input end of the first erbium-doped fiber amplifier 11 is optically connected to the output end of the single-cavity dual-comb erbium-doped fiber mode-locked laser module 101. The output end of the first erbium-doped fiber amplifier 11 is optically connected to a second coupler 12 with a splitting ratio of 20:80. The second coupler 12 splits the received pump comb into a first pump light and a second pump light according to the splitting ratio of 20:80. The first pump light is sequentially injected into the collimator 14, the double freeform lens 15, and the digital micromirror device 16 of the terahertz emission and spatial coding module 102 to complete spatial modulation. The spatially modulated first pump light is then projected onto the high-resistivity silicon wafer 18. Simultaneously, the second pump light is sequentially injected into the second polarization controller 13 and the first optical guide transmitting antenna 17 of the terahertz emission and spatial coding module 102 to output a terahertz pulse sequence. The terahertz pulse sequence is transmitted through the high-resistivity silicon wafer 18 to form a coded terahertz light field.

[0049] The working mechanism is as follows: The pump light comb is amplified by the first erbium-doped fiber amplifier 11 and split into a first pump light and a second pump light by the second coupler 12. The first pump light is injected into the collimator 14 and collimated into spatial light. The spatial light is homogenized into a flat-top beam by the double freeform lens 15 and then incident on the digital micromirror device 16. By loading a series of orthogonal spatial coding patterns (such as Hadamard matrix patterns), the digital micromirror device 16 achieves spatial modulation of the pump light comb. The pump light comb modulated by the digital micromirror device 16 is projected onto the high-resistivity silicon wafer 18, forming a local light-controlled region on the high-resistivity silicon wafer 18 corresponding to the coding pattern. The pump light comb modulated by the digital micromirror device 16 is near-infrared light (such as 800nm). The near-infrared light induces a photoinduced carrier effect in the high-resistivity silicon wafer 18, causing a spatially selective change in the transmittance of its terahertz waves—from a high-transmission state to a low-transmission / blocking state, thereby forming a spatially coded transmission mask on the silicon wafer surface. At the same time, the second pump light directly drives the first optical guide transmitting antenna 17 to generate a terahertz pulse sequence. This terahertz pulse sequence is transmitted through the high-resistivity silicon wafer 18, and its transmittance varies spatially with the light-controlled area, so that the terahertz wave carries spatial coding information (DMD pattern coding information) corresponding to the pattern of the digital micromirror device 16, forming a coded terahertz light field, that is, a terahertz light field carrying DMD pattern coding information.

[0050] An encoded terahertz light field is irradiated onto the target sample 19. The encoded terahertz light field carrying information about the target sample 19 is injected into the second optical guide receiving antenna 20 of the terahertz detection and data acquisition module 103 and outputs a first electrical signal. The first electrical signal generates a terahertz time-domain waveform carrying DMD pattern encoding information through the electro-optic modulator 24. The terahertz time-domain waveform carrying DMD pattern encoding information is loaded onto the probe optical comb, so that the probe optical comb pulse carries the terahertz time-domain waveform information carrying DMD pattern encoding information, resulting in a modulated probe optical comb. The modulated probe optical comb is injected into the dispersion compensation fiber 25, and a dispersion Fourier transform is performed under large dispersion conditions. The modulated probe optical comb pulse is stretched and transformed to output spectral interference fringes. The spectral interference fringes are sequentially injected into the photodetector 26 and the data acquisition board 27 of the terahertz detection and data acquisition module 103 to reconstruct the terahertz time-domain waveform and two-dimensional spatial image.

[0051] The first electrical signal is a weak current signal (in the nanoampere to microampere range) that is proportional to the intensity of the electric field component in the terahertz light field.

[0052] Specifically, the terahertz detection and data acquisition module 103 includes a second erbium-doped fiber amplifier 21. The input end of the second erbium-doped fiber amplifier 21 is optically connected to the output end of the single-cavity dual-comb erbium-doped fiber mode-locked laser module 101. The output end of the second erbium-doped fiber amplifier 21 is optically connected in sequence to a third polarization controller 22, an electro-optic modulator 24, a dispersion compensation fiber 25, a photodetector 26, and a data acquisition board 27. The radio frequency port of the electro-optic modulator 24 is also connected to the output end of the second optical guide receiving antenna 20 through a transimpedance amplifier 23.

[0053] Specifically, the data acquisition board 27 integrates a signal processing unit, which is configured as follows:

[0054] Based on the principle of dispersive Fourier transform, the complete terahertz time-domain waveform is extracted from the acquired spectral interference fringes through Fourier analysis.

[0055] The two-dimensional spatial image of sample 19 is reconstructed based on the DMD pattern encoding information using inverse Hadamard transform or compressed sensing algorithms.

[0056] The working mechanism is as follows: The coded terahertz light field modulated by the target sample 19 is directly received by the second optical guide receiving antenna 20 (without the need for a probe optical comb input). The second optical guide receiving antenna 20 outputs a weak current signal proportional to the intensity of the electric field component in the terahertz light field. This current signal is amplified by the transimpedance amplifier 23 and then input to the radio frequency port of the electro-optic modulator 24, generating a terahertz time-domain waveform carrying DMD pattern coding information. Simultaneously, the probe optical comb is amplified by the second erbium-doped fiber amplifier 21 and its polarization state is adjusted by the third polarization controller 22 before being input to the optical port of the electro-optic modulator 24. Driven by the radio frequency electrical signal, the probe optical comb is modulated by the terahertz time-domain waveform carrying DMD pattern coding information to obtain a modulated probe optical comb. The modulated probe light pulse enters the dispersion compensation fiber 25, undergoes a dispersion Fourier transform, and maps the terahertz time-domain waveform into spectral interference fringes of a single pulse. The spectral interference fringes are converted into a second electrical signal by the high-speed photodetector 26, acquired by the data acquisition board 27, and transmitted to the computer for further processing.

[0057] The essence of the dispersive Fourier transform is to stretch the optical pulse in the time domain using a highly dispersive medium, so that its spectral envelope is directly mapped to a time-domain waveform. When the modulated probe optical pulse passes through a dispersion-compensated fiber 25 of length L and group velocity dispersion coefficient β2, the output time-domain waveform of the dispersion-stretched modulated probe optical comb pulse is... The expression is:

[0058] ;

[0059] In the formula, The imaginary unit, ;

[0060] The group velocity dispersion coefficient of dispersion-compensating fiber 25;

[0061] The length of the dispersion compensation fiber 25;

[0062] It is a time variable;

[0063] The spectrum of the modulated probe light pulse;

[0064] This is the complex envelope of the modulated probe light pulse.

[0065] This equation shows that the time-domain envelope of the modulated probe optical comb pulse stretched by the dispersion-compensating fiber 25 has the same shape as the spectral envelope of the modulated probe optical pulse, and the time variable... With frequency coordinates The relationship is linear. When two probe light pulses modulated by the electro-optic modulator 24, each with a small time delay τ (4ns~20ns), enter the dispersion compensation fiber 25, a frequency of [frequency missing] will be generated on the photodetector 24. The spectral interference fringes are thus observed. Therefore, by measuring the frequency of the spectral interference fringes, the delay between the two pulses of the probe light pulse modulated by the electro-optic modulator 24 can be deduced, thereby obtaining the terahertz time-domain waveform.

[0066] like Fig. 2 As shown, Figure (a) shows the two femtosecond pulses of the modulated probe optical comb output after modulation by the electro-optic modulator 24. Figure (b) shows that after passing through a 20km dispersion-compensating fiber 25, the two femtosecond pulses of the modulated probe optical comb are widened and overlapped, but the delay τ of the two femtosecond pulses of the modulated probe optical comb remains unchanged compared to Figure (a). Figure (c) shows the second electrical signal output by the photodetector 24, where the time-domain oscillation frequency f of the second electrical signal is... i The time delay τ between the two femtosecond pulses of the original modulated probe optical comb and the time delay τ are linearly related. ,in, It is the total dispersion of the optical fiber.

[0067] In the system of the present invention, after the probe optical comb is modulated by the terahertz time-domain waveform, its spectrum carries the mutual information of the terahertz electric field. After the modulated probe optical comb is stretched by the dispersion compensation fiber 25, the frequency of the time-domain spectral interference fringes recorded by the high-speed photodetector 26 corresponds one-to-one with the time delay of the terahertz time-domain waveform, thereby realizing single-shot waveform acquisition.

[0068] The signal processing unit first extracts the complete terahertz time-domain waveform from the acquired single-channel spectral interference fringes using Fourier analysis based on the dispersive Fourier transform principle, achieving single-shot, scan-free detection in the time dimension. Secondly, according to the orthogonal spatial coding pattern sequence loaded on the digital micromirror device 16, the terahertz time-domain waveform characteristic values ​​(such as peak values ​​or integral values) corresponding to each DMD pattern coding information are correlated with the DMD pattern coding information. The two-dimensional spatial image of sample 19 is reconstructed using inverse Hadamard transform or compressed sensing algorithms, achieving mechanically scan-free imaging in the spatial dimension. Since the dispersive Fourier transform can complete time-domain waveform capture within a single pulse, measurements in both time and spatial dimensions can be performed in parallel without sequential waiting, thus truly achieving dual-dimensional, fully scan-free terahertz time-domain imaging.

[0069] Regarding the specific structure of this invention, it should be noted that the connection relationships between the various component modules used in this invention are definite and achievable. Except as specifically described in the embodiments, their specific connection relationships can bring about corresponding technical effects and solve the technical problems proposed by this invention without relying on the execution of corresponding software programs. The models of the components, modules, and specific components appearing in this invention, the connection methods between them, and the conventional usage methods and expected technical effects brought about by the above technical features, unless specifically described, are all publicly disclosed content in patents, journal articles, technical manuals, technical dictionaries, and textbooks that can be obtained by those skilled in the art before the application date, or belong to conventional technology, common knowledge, and other existing technologies in this field. There is no need to elaborate, which makes the technical solution provided in this case clear, complete, and achievable, and can reproduce or obtain corresponding physical products based on this technical means.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform, characterized in that, The system includes a single-cavity dual-comb erbium-doped fiber mode-locked laser module (101). The single-cavity dual-comb erbium-doped fiber mode-locked laser module (101) outputs two optical frequency combs. One of them is used as a pump comb and input to the terahertz emission and spatial coding module (102) to split the beam into a first pump beam and a second pump beam. The other of them is used as a probe comb and input to the terahertz detection and data acquisition module (103). The first pump light is incident on the digital micromirror device (16) of the terahertz emission and spatial coding module (102) and after spatial modulation, it is projected onto the high-resistivity silicon wafer (18) of the terahertz emission and spatial coding module (102); at the same time, the second pump light is injected into the first optical guide transmitting antenna (17) of the terahertz emission and spatial coding module (102) to generate a terahertz pulse sequence. The terahertz pulse sequence is transmitted through the high-resistivity silicon wafer (18) so that the terahertz pulse sequence carries DMD pattern coding information and forms a coded terahertz light field. The encoded terahertz light field is irradiated onto the target sample (19). The encoded terahertz light field carrying the information of the target sample (19) is injected into the second optical guide receiving antenna (20) of the terahertz detection and data acquisition module (103) and outputs a first electrical signal. The first electrical signal generates a terahertz time-domain waveform carrying DMD pattern encoding information through the electro-optic modulator (24). The terahertz time-domain waveform carrying DMD pattern encoding information is loaded onto the detection optical comb to obtain the modulated detection optical comb. The modulated probe comb is injected into the dispersion compensation fiber (25) to complete the dispersion Fourier transform under large dispersion conditions and output spectral interference fringes; The spectral interference fringes are sequentially injected into the photodetector (26) and data acquisition board (27) of the terahertz detection and data acquisition module (103) to reconstruct the terahertz time-domain waveform and two-dimensional spatial image.

2. The non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform according to claim 1, characterized in that, The single-cavity dual-comb erbium-doped fiber mode-locked laser module (101) includes a pump source (1), a wavelength division multiplexer (2), an erbium-doped fiber (3), a first coupler (6), a polarization-maintaining fiber (7), and a first polarization controller (8) connected in sequence. The output end of the first polarization controller (8) is connected to the input end of the wavelength division multiplexer (2) to form a closed ring laser resonant cavity. The output end of the first coupler (6) is also connected to the input end of the circulator (9). The two output ends of the circulator (9) are connected to the input ends of the terahertz emission and spatial coding module (102) and the terahertz detection and data acquisition module (103), respectively.

3. The non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform according to claim 2, characterized in that, An isolator (4) and a single-walled carbon nanotube saturated absorber (5) are connected in series between the erbium-doped fiber (3) and the first coupler (6).

4. The non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform according to claim 2, characterized in that, A fiber Bragg grating (10) is also optically connected between the circulator (9) and the terahertz detection and data acquisition module (103).

5. The non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform according to claim 1, characterized in that, The terahertz emission and spatial coding module (102) includes a first erbium-doped fiber amplifier (11). The input end of the first erbium-doped fiber amplifier (11) is optically connected to the output end of the single-cavity dual-comb erbium-doped fiber mode-locked laser module (101). The output end of the first erbium-doped fiber amplifier (11) is optically connected to a second coupler (12). The second coupler (12) splits the received pump comb into a first pump light and a second pump light. The first pump light is sequentially injected into the collimator (14), double freeform lens (15), and digital micromirror device (16) of the terahertz emission and spatial coding module (102) to complete spatial modulation. The spatially modulated first pump light is projected onto the high-resistivity silicon wafer (18). At the same time, the second pump light is sequentially injected into the second polarization controller (13) and the first optical guide transmitting antenna (17) of the terahertz emission and spatial coding module (102) to output a terahertz pulse sequence. The terahertz pulse sequence is transmitted through the high-resistivity silicon wafer (18) to form a coded terahertz light field.

6. The non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform according to claim 1, characterized in that, The terahertz detection and data acquisition module (103) includes a second erbium-doped fiber amplifier (21). The input end of the second erbium-doped fiber amplifier (21) is optically connected to the output end of the single-cavity dual-comb erbium-doped fiber mode-locked laser module (101). The output end of the second erbium-doped fiber amplifier (21) is optically connected in sequence to a third polarization controller (22), a transimpedance amplifier (23), an electro-optic modulator (24), a dispersion compensation fiber (25), a photodetector (26), and a data acquisition board (27). The electro-optic modulator (24) is also connected to the output of the second optical receiving antenna (20) via a transimpedance amplifier (23).

7. The non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform according to claim 6, characterized in that, When the modulated probe optical pulse passes through a dispersion-compensated fiber (25) of length L and group velocity dispersion coefficient β2, the time-domain waveform of the output modulated probe optical comb pulse stretched by dispersion is shown. The expression is: ； In the formula, The imaginary unit, ; The group velocity dispersion coefficient of the dispersion-compensating fiber (25); The length of the dispersion compensation fiber (25); It is a time variable; The spectrum of the modulated probe light pulse; This is the complex envelope of the modulated probe light pulse.

8. The non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform according to claim 6, characterized in that, The frequency f of the spectral interference f is: ； In the formula, τ is the time delay between two femtosecond pulses in the probe optical comb pulse; L is the length of the dispersion compensation fiber (25); β2 is the group velocity dispersion coefficient of the dispersion-compensated fiber (25).

9. The non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform according to claim 1, characterized in that, The data acquisition board (27) integrates a signal processing unit, which is configured as follows: Based on the principle of dispersive Fourier transform, the complete terahertz time-domain waveform is extracted from the acquired spectral interference fringes. The two-dimensional spatial image of the sample (19) is reconstructed based on the DMD pattern encoding information using inverse Hadamard transform or compressed sensing algorithm.

10. The non-scanning terahertz imaging system integrating single-cavity dual optical comb and dispersive Fourier transform according to claim 2, characterized in that, The laser output from the pump source (1) is in the 976nm~980nm band.

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