Terahertz radar imaging system based on four-inch spin emission source

Abstract

The invention relates to a terahertz radar imaging system based on four-inch spinning, which belongs to the technical field of terahertz wave imaging and comprises a femtosecond laser light source, a beam splitter, a laser pumping terahertz transmitting module, a detection laser transmission module and a terahertz detection module which are arranged along the laser incidence direction. An efficient, large-size and ultra-wideband terahertz radiation source and a radar imaging device are fused, high-signal-to-noise-ratio and high-resolution terahertz imaging of a large-size target is achieved, and the system is low in cost, easy to construct, free of complex parabolic mirror combination, firm and reliable in performance and suitable for large-size target imaging. And the requirements of various engineering application scenes on synthetic aperture and nondestructive detection imaging can be practically met.

Description

Technical Field

[0001] The invention relates to the technical field of terahertz wave imaging, and in particular to a terahertz radar imaging system based on four-inch spin. Background Art

[0002] The terahertz band is located in the frequency range of 0.1-10THz, in the transition band between low-energy electronics and high-energy photonics, and has excellent electromagnetic properties. Terahertz imaging is a key technical application of terahertz science. People's research on terahertz imaging systems has important value and broad development prospects in many fields such as national defense security, medical diagnosis, and material testing. Compared with lower electromagnetic frequency bands, the wavelength of terahertz is in the submillimeter order and has a wider frequency band. It has the advantages of strong information transmission capability and high resolution characteristics when detecting and imaging, and has important application value in high-resolution imaging and fine identification of targets. Compared with higher frequency bands, terahertz waves have good penetration ability for media such as smoke and floating dust, are insensitive to aero-optical effects and thermal environment effects, have a wider field of view during imaging, better search capabilities, can work all day, and can be used for complex environments and high-speed moving target detection in space. Terahertz waves also have the advantages of anti-stealth and anti-interference, which makes terahertz radar imaging have great application potential in modern large-size target or cluster target detection scenarios.

[0003] Terahertz imaging has been industrialized in the field of non-destructive testing, but with the diversification of application scenarios, the small size of terahertz radiation waves limits its imaging capabilities in large-scale complex target scenes. To achieve large-scale terahertz radar imaging, the key is to obtain large-scale terahertz electromagnetic waves and a low-cost and easy-to-integrate system.

[0004] The reported research is based on the tilted wavefront technology, using lithium niobate crystals pumped by femtosecond laser amplifiers as the terahertz emission source, and combining time-domain spectroscopy technology to form a nearly single-station terahertz radar system, which realizes non-scanning direct detection imaging of a 10cm-sized model. However, due to the limited volume of bulk crystals, a system composed of parabolic mirrors is required to expand the terahertz wave to obtain a terahertz quasi-plane wave with a spot diameter of 73mm at half maximum. The system is complex, costly and difficult to implement. Another method is to use a photoconductive dipole antenna to transmit and receive terahertz signals, radiating terahertz waves with a frequency of 0.1-1.3THz and a spot diameter of 120mm at half maximum. Compared with previous studies, the bandwidth is improved and there is no need to use additional devices to adjust the wavefront direction, but multiple sets of parabolic mirrors are still required to control the size of the terahertz wave to achieve beam expansion.

[0005] In summary, the existing technology still faces problems such as lack of high-power, large-size terahertz radiation sources, high complexity of radar imaging technology, and high cost. It has not been able to realize a terahertz radar imaging system with the advantages of high signal-to-noise ratio, large size, ultra-wideband, low cost and easy construction. Summary of the invention

[0006] In view of the above problems, the present invention provides a terahertz radar imaging system based on a four-inch spin emission source. The present invention integrates a high-efficiency, large-size, ultra-wideband terahertz radiation source with a radar imaging device to achieve high signal-to-noise ratio and high-resolution terahertz imaging of large-size targets. Moreover, the present invention has low cost, simple construction, no need for a complex parabolic mirror combination, and has firm and reliable performance, and can practically meet the needs of various engineering application scenarios for synthetic aperture and non-destructive testing imaging.

[0007] The present invention provides a terahertz radar imaging system based on a four-inch spin emission source, comprising: a femtosecond laser light source 1, a beam splitter 2, a laser pumped terahertz emission module, a detection laser transmission module, and a terahertz detection module;

[0008] The laser pumped terahertz emission module comprises: a chopper 3, a concave lens 4 and a convex lens 5 arranged along the laser incident direction;

[0009] In one embodiment of the present invention, the working magnetic field of the chopper 3 is 500 Hz, which suppresses noise and improves the signal-to-noise ratio of the system;

[0010] The terahertz detection module includes a spin terahertz emission source 6 arranged along the laser incident direction, a one-dimensional rotating platform 7, an off-axis parabolic mirror 8, a high-density polyethylene plate 9 and an indium tin oxide glass 12;

[0011] The spin terahertz emission source 6 is arranged on the side of the convex lens 5 outputting the laser, and generates a highly efficient, ultra-broadband four-inch terahertz wave under the pumping of the femtosecond laser;

[0012] The one-dimensional rotating stage 7 is used to set the target to be measured; the high-density polyethylene plate 9 is used to transmit the terahertz wave while blocking the laser propagation; the indium tin oxide glass 12 is used to reflect the terahertz wave into the terahertz detection module;

[0013] The detection laser transmission module includes a one-dimensional delay line 10, a detection light focusing lens 11 and an indium tin oxide glass 12 arranged along the laser incident direction; the one-dimensional delay line 10 is used to control the optical path of the detection laser and the terahertz wave to ensure that the detection laser and the terahertz wave coincide in time in the terahertz detection module; the indium tin oxide glass 12 is used to transmit the detection laser into the terahertz detection module;

[0014] The terahertz detection module includes an electro-optical crystal zinc telluride 13 arranged along the laser incident direction, a detection light collimating lens 14, a 1 / 4 wave plate 15, a Wollaston prism 16 and a balanced detector 17;

[0015] Preferably, the spin terahertz emission source 6 comprises a substrate and a one-dimensional photonic crystal structure and a nano-ferromagnetic heterojunction thin film layer connected to the substrate in sequence;

[0016] Preferably, the substrate is made of silicon;

[0017] Preferably, the nano-ferromagnetic heterojunction film layer is a W / CoFeB / Pt heterojunction film;

[0018] Furthermore, the diameter of the nano-ferromagnetic heterojunction thin film layer is 4 inches;

[0019] Exemplarily, a W film, a CoFeB film, and a Pt film are sequentially grown on a substrate to form a W / CoFeB / Pt heterojunction film;

[0020] The nano-ferromagnetic heterojunction thin film layer is prepared on a silicon substrate by magnetron sputtering technology; the nano-ferromagnetic heterojunction thin film layer can emit broadband terahertz waves with an effective bandwidth greater than 4 THz under a pump laser, significantly improving the high-resolution imaging performance of the radar system.

[0021] Preferably, a multilayer one-dimensional photonic crystal structure is designed between the nano-ferromagnetic heterojunction thin film layer and the substrate;

[0022] Exemplarily, SiO2 thin films and HfO2 thin films are periodically arranged alternately to form a multilayer one-dimensional photonic crystal structure;

[0023] The relationship between the thickness h1 of the SiO2 film, the thickness h2 of the HfO2 film, and the thickness h3 of the multilayer film one-dimensional photonic crystal structure is expressed as follows:

[0024] (h1+h2)*40=h3

[0025] Furthermore, the thickness h1 of the SiO2 film is 136 nm; the thickness h2 of the HfO2 film is 92 nm; the thickness h3 of the multilayer film one-dimensional photonic crystal structure is about 9 μm;

[0026] In the present invention, 40 groups of SiO2 with a thickness of 136nm and HfO2 with a thickness of 92nm are periodically and alternately arranged between the silicon substrate and the heavy metal W to form a multilayer film one-dimensional photonic crystal structure with a total thickness of about 9μm. This can improve the sample's utilization rate of pump laser energy, achieve terahertz wave coherent enhancement, increase the field intensity of terahertz radiation, and thus improve the signal-to-noise ratio of the radar system.

[0027] In the present invention, based on the inverse spin Hall effect, a W / CoFeB / Pt heterojunction film saturated with an external magnetic field generates a spin current under femtosecond laser pumping and converts it into a charge current, thereby generating a strong field terahertz wave.

[0028] The diameter of the W / CoFeB / Pt heterojunction film in the present invention is 4 inches. Under the pumping of a laser with a diameter of 4 inches, large-scale terahertz wave radiation with a diameter of 4 inches is directly generated. There is no need to use an off-axis parabolic mirror combination to expand the terahertz wave. While achieving high-efficiency and large-scale terahertz emission, the complexity of the system is reduced, making it easier to integrate.

[0029] Another object of the present invention is to provide a method for using a terahertz radar imaging system based on a four-inch spinning terahertz emission source, comprising:

[0030] The femtosecond laser output by the femtosecond laser light source 1 passes through the beam splitter 2 and the pump laser transmission module, and is injected into the spin terahertz emission source 6 to generate a terahertz wave, which continues to propagate to the target to be detected by the terahertz emission module and is scattered to obtain a scattered terahertz signal;

[0031] The scattered terahertz signal overlaps with the detection laser transmitted through the detection laser transmission module in the terahertz detection module, and the polarization state of the detection laser is modulated by the electric field of the terahertz signal and then processed and detected by the terahertz detection module, outputting the time domain waveform of the terahertz signal.

[0032] Exemplarily, the femtosecond laser output by the femtosecond laser light source 1 is injected into the spin terahertz emission source 6 through the beam splitter 2 and the pump laser transmission module to generate a terahertz wave, which continues to propagate to the target to be detected by the terahertz emission module and is scattered to obtain a scattered terahertz signal;

[0033] The scattered terahertz signal overlaps with the detection laser transmitted through the detection laser transmission module at the electro-optical crystal zinc telluride 13, and the polarization state of the detection laser is modulated by the electric field of the terahertz signal and processed by the terahertz detection module and detected by the balanced detector 17, which characterizes and outputs the time domain waveform of the terahertz signal.

[0034] Exemplarily, a computer program is used to control the one-dimensional rotating stage 7 and the one-dimensional delay line 10 in the system to perform two-dimensional scanning, so as to obtain terahertz wave signals scattered by the target to be measured on the one-dimensional rotating stage 7 at different angles.

[0035] Preferably, by adjusting the focal length and spacing of the concave lens 4 and the convex lens 5, a collimated laser beam with a diameter of 1 inch can be expanded into a collimated laser beam with a diameter of 4 inches;

[0036] Preferably, the one-dimensional delay line 10 has a movable range of 20 cm, so as to achieve complete detection of scattered signals of large-sized targets to be detected;

[0037] The one-dimensional rotating stage 7 realizes automatic scanning without manual operation under various imaging functions through the whole process computer control capability;

[0038] In one embodiment of the present invention, a femtosecond laser 1 enters the imaging system from one side and passes through a beam splitter 2 to obtain a pump laser and a detection laser;

[0039] After the pump laser passes through the chopper 3, it passes through the concave lens 4 and the convex lens 5, and irradiates the spin terahertz emission source 6 to generate a terahertz wave with a diameter of 4 inches; the terahertz wave with a diameter of 4 inches is incident on the surface of the target to be measured placed on the one-dimensional rotating table 7 to emit and scatter, and obtain a terahertz wave scattering signal; the terahertz wave scattering signal is collected by the off-axis parabolic mirror 8, and is reflected by the high-density polyethylene plate 9 and the indium tin oxide glass 12, and then focused to the electro-optical crystal zinc telluride 13;

[0040] The detection laser is focused by the detection light focusing lens 11 after passing through the one-dimensional delay line 1 and a plurality of reflectors, and coincides with the terahertz wave scattered signal in the electro-optical zinc telluride crystal 13; the electric field of the terahertz signal modulates the polarization state of the detection laser in the electro-optical zinc telluride crystal 13 to obtain the detection laser carrying the terahertz electric field information;

[0041] The detection laser carrying the terahertz electric field information is collimated by the detection light collimating lens 14 to obtain a collimated laser; the collimated laser is adjusted by the 1 / 4 wave plate 15 and the Wollaston prism 16 respectively to obtain an adjusted laser 1 and an adjusted laser 2, which are transmitted to the balanced detector 17, and the adjusted laser 1 and the adjusted laser 2 are processed by a computer to output a terahertz time domain waveform;

[0042] The terahertz time domain waveforms scattered by the target to be measured at various angles of the one-dimensional rotating stage 7 are collected and processed by a back-projection algorithm for terahertz radar imaging.

[0043] In one embodiment of the present invention, a femtosecond laser 1 with a diameter of 0.5 inches enters the imaging system from one side, and is divided into a pump path and a detection path by a beam splitter 2. The femtosecond laser with 90% of the energy enters the pump path to obtain a pump laser, and the femtosecond laser with 10% of the energy enters the detection path to obtain a detection laser.

[0044] A chopper 3 connected and synchronized with the laser pulse is placed in the pump circuit, and the working magnetic field of the chopper 3 is 500Hz, which suppresses noise and improves the signal-to-noise ratio of the system;

[0045] The pump laser passes through a laser beam expansion system composed of a concave lens 4 with a focal length of -50mm and a convex lens 5 with a focal length of 500mm, and is irradiated on a 4-inch spin terahertz emission source 6 to generate a large-size ultra-wideband strong-field terahertz wave with a diameter of 4 inches; the terahertz wave is incident on the surface of the target to be measured placed on a one-dimensional rotating table 7 and emits and scatters, and the scattered signal is collected by an off-axis parabolic mirror 8 with a focal length of 203.2mm, and is focused to the electro-optical crystal zinc telluride 13 through a high-density polyethylene plate 9; the function of the high-density polyethylene plate 9 is to block the propagation of the laser while transmitting the terahertz wave;

[0046] The detection laser is focused by a detection light focusing lens 11 with a focal length of 150 mm after passing through a one-dimensional delay line 1 and a plurality of reflectors, and overlaps with the terahertz wave in the electro-optical crystal zinc telluride 13 to obtain a detection laser carrying terahertz electric field information;

[0047] The detection laser carrying the terahertz electric field information is collimated by the detection light collimating lens 14 to obtain a collimated laser; the collimated laser is adjusted by the 1 / 4 wave plate 15 and the Wollaston prism 16 respectively to obtain an adjusted laser 1 and an adjusted laser 2, which are transmitted to the balanced detector 17, and the adjusted laser 1 and the adjusted laser 2 are processed by a computer to output a terahertz time domain waveform;

[0048] The terahertz time domain waveforms scattered by the target to be measured at various angles of the one-dimensional rotating stage 7 are collected and processed by a back-projection algorithm for terahertz radar imaging.

[0049] In the present invention, based on the electro-optic effect, the electric field of the terahertz wave changes the refractive index of the electro-optic crystal zinc telluride 13 and then changes the polarization state of the detection laser, which is collimated again by the detection light collimating lens 14; a 1 / 4 wave plate 15 and a Wollaston prism 16 are used to adjust the two orthogonal polarization components of the detection laser, and a balanced detector 17 is used to collect them respectively, and the terahertz waveform signal is drawn in the self-compiled software after being processed by the phase-locked amplifier.

[0050] The present invention also includes computer-controlled one-dimensional delay line 10 movement to change the optical path of the detection laser, which is used to map the electric field intensity of the terahertz wave at each moment and draw the shape of the terahertz pulse;

[0051] In one embodiment of the present invention, the optical path lengths of the pump laser and the detection laser are 2380-2420 mm.

[0052] In one embodiment of the present invention, the terahertz waves are propagated at a height of 145-155 mm above the optical platform to avoid reflection caused by the desktop.

[0053] Compared with the prior art, the present invention has at least the following beneficial effects:

[0054] (1) The imaging effect of the present invention is good. The 4-inch spin terahertz emission source has the advantages of high emission efficiency, wide radiation bandwidth, controllable polarization, stable and reliable, simple structure, etc. It can emit high-intensity, short-pulse width terahertz signals, and can achieve higher signal-to-noise ratio and higher resolution imaging compared with traditional methods;

[0055] (2) The present invention does not require the use of parabolic mirror combinations and expensive terahertz cameras and other equipment, greatly reducing the cost of device construction, use and maintenance;

[0056] (3) The present invention has a simple structure, a wide applicable signal frequency band, a strong and reliable performance, can be miniaturized, and can practically meet various requirements for polarization adjustment;

[0057] (4) In addition to reflective radar imaging, the present invention can also perform nondestructive testing on metal objects covered with a variety of materials, which can meet various needs of scientific research and production practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] The drawings are only for the purpose of illustrating particular embodiments and are not to be construed as limiting the invention.

[0059] Figure 1 is a schematic diagram of a terahertz radar imaging system in an embodiment of the present invention;

[0060] Figure 2 Schematic diagram of the structure of a four-inch spin terahertz emission source in an embodiment of the present invention.

[0061] Reference numerals:

[0062] 1-Femtosecond laser light source, 2-Beam splitter, 3-Chopper, 4-Concave lens, 5-Convex lens, 6-Spin terahertz emission source, 7-One-dimensional rotating stage, 8-Off-axis parabolic mirror, 9-High-density polyethylene plate, 10-One-dimensional delay line, 11-Detection light focusing lens, 12-Indium tin oxide glass, 13-Electro-optic crystal zinc telluride, 14-Detection light collimating lens, 15-1 / 4 wave plate, 16-Wollaston prism, 17-Balanced detector. DETAILED DESCRIPTION

[0063] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. In addition, the present invention can also be implemented in other ways different from those described herein, and therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0064] A specific embodiment of the present invention, as Figure 1-2, discloses a terahertz radar imaging system based on a four-inch spin terahertz emission source. In order to illustrate the effectiveness of the method proposed in the present invention, the above technical solution of the present invention is described in detail through a specific embodiment below. The specific implementation steps are as follows:

[0065] The present invention provides a terahertz radar imaging system based on a four-inch spin emission source, comprising: a femtosecond laser light source 1, a beam splitter 2, a laser pumped terahertz emission module, a detection laser transmission module, and a terahertz detection module;

[0066] The laser pumped terahertz emission module comprises: a chopper 3, a concave lens 4 and a convex lens 5 arranged along the laser incident direction;

[0067] In one embodiment of the present invention, the working magnetic field of the chopper 3 is 500 Hz, which suppresses noise and improves the signal-to-noise ratio of the system;

[0068] The terahertz detection module includes a spin terahertz emission source 6 arranged along the laser incident direction, a one-dimensional rotating platform 7, an off-axis parabolic mirror 8, a high-density polyethylene plate 9 and an indium tin oxide glass 12;

[0069] The spin terahertz emission source 6 is arranged on the side of the convex lens 5 outputting the laser, and generates a highly efficient, ultra-broadband four-inch terahertz wave under the pumping of the femtosecond laser;

[0070] The one-dimensional rotating stage 7 is used to set the target to be measured; the high-density polyethylene plate 9 is used to transmit the terahertz wave while blocking the laser propagation; the indium tin oxide glass 12 is used to reflect the terahertz wave into the terahertz detection module;

[0071] The detection laser transmission module includes a one-dimensional delay line 10, a detection light focusing lens 11 and an indium tin oxide glass 12 arranged along the laser incident direction; the one-dimensional delay line 10 is used to control the optical path of the detection laser and the terahertz wave to ensure that the detection laser and the terahertz wave coincide in time in the terahertz detection module; the indium tin oxide glass 12 is used to transmit the detection laser into the terahertz detection module;

[0072] The terahertz detection module includes an electro-optical crystal zinc telluride 13 arranged along the laser incident direction, a detection light collimating lens 14, a 1 / 4 wave plate 15, a Wollaston prism 16 and a balanced detector 17;

[0073] Preferably, the spin terahertz emission source 6 comprises a substrate and a one-dimensional photonic crystal structure and a nano-ferromagnetic heterojunction thin film layer connected to the substrate in sequence;

[0074] Preferably, the substrate is made of silicon;

[0075] Preferably, the nano-ferromagnetic heterojunction film layer is a W / CoFeB / Pt heterojunction film;

[0076] Furthermore, the diameter of the nano-ferromagnetic heterojunction thin film layer is 4 inches;

[0077] Exemplarily, a W film, a CoFeB film, and a Pt film are sequentially grown on a substrate to form a W / CoFeB / Pt heterojunction film;

[0078] The nano-ferromagnetic heterojunction thin film layer is prepared on a silicon substrate by magnetron sputtering technology; the nano-ferromagnetic heterojunction thin film layer can emit broadband terahertz waves with an effective bandwidth greater than 4 THz under a pump laser, significantly improving the high-resolution imaging performance of the radar system.

[0079] Preferably, a multilayer one-dimensional photonic crystal structure is designed between the nano-ferromagnetic heterojunction thin film layer and the substrate;

[0080] Exemplarily, SiO2 thin films and HfO2 thin films are periodically arranged alternately to form a multilayer one-dimensional photonic crystal structure;

[0081] The relationship between the thickness h1 of the SiO2 film, the thickness h2 of the HfO2 film, and the thickness h3 of the multilayer film one-dimensional photonic crystal structure is expressed as follows:

[0082] (h1+h2)*40=h3

[0083] Furthermore, the thickness h1 of the SiO2 film is 136 nm; the thickness h2 of the HfO2 film is 92 nm; the thickness h3 of the multilayer film one-dimensional photonic crystal structure is about 9 μm;

[0084] In the present invention, 40 groups of SiO2 with a thickness of 136nm and HfO2 with a thickness of 92nm are periodically and alternately arranged between the silicon substrate and the heavy metal W to form a multilayer film one-dimensional photonic crystal structure with a total thickness of about 9μm. This can improve the sample's utilization rate of pump laser energy, achieve terahertz wave coherent enhancement, increase the field intensity of terahertz radiation, and thus improve the signal-to-noise ratio of the radar system.

[0085] In the present invention, based on the inverse spin Hall effect, a W / CoFeB / Pt heterojunction film saturated with an external magnetic field generates a spin current under femtosecond laser pumping and converts it into a charge current, thereby generating a strong field terahertz wave.

[0086] The diameter of the W / CoFeB / Pt heterojunction film in the present invention is 4 inches. Under the pumping of a laser with a diameter of 4 inches, large-scale terahertz wave radiation with a diameter of 4 inches is directly generated. There is no need to use an off-axis parabolic mirror combination to expand the terahertz wave. While achieving high-efficiency and large-scale terahertz emission, the complexity of the system is reduced, making it easier to integrate.

[0087] Another object of the present invention is to provide a method for using a terahertz radar imaging system based on a four-inch spinning terahertz emission source, comprising:

[0088] The femtosecond laser output by the femtosecond laser light source 1 passes through the beam splitter 2 and the pump laser transmission module, and is injected into the spin terahertz emission source 6 to generate a terahertz wave, which continues to propagate to the target to be detected by the terahertz emission module and is scattered to obtain a scattered terahertz signal;

[0089] The scattered terahertz signal overlaps with the detection laser transmitted through the detection laser transmission module in the terahertz detection module, and the polarization state of the detection laser is modulated by the electric field of the terahertz signal and then processed and detected by the terahertz detection module, outputting the time domain waveform of the terahertz signal.

[0090] Exemplarily, the femtosecond laser output by the femtosecond laser light source 1 is injected into the spin terahertz emission source 6 through the beam splitter 2 and the pump laser transmission module to generate a terahertz wave, which continues to propagate to the target to be detected by the terahertz emission module and is scattered to obtain a scattered terahertz signal;

[0091] The scattered terahertz signal overlaps with the detection laser transmitted through the detection laser transmission module at the electro-optical crystal zinc telluride 13, and the polarization state of the detection laser is modulated by the electric field of the terahertz signal and processed by the terahertz detection module and detected by the balanced detector 17, which characterizes and outputs the time domain waveform of the terahertz signal.

[0092] Exemplarily, a computer program is used to control the one-dimensional rotating stage 7 and the one-dimensional delay line 10 in the system to perform two-dimensional scanning, so as to obtain terahertz wave signals scattered by the target to be measured on the one-dimensional rotating stage 7 at different angles.

[0093] Preferably, by adjusting the focal length and spacing of the concave lens 4 and the convex lens 5, a collimated laser beam with a diameter of 1 inch can be expanded into a collimated laser beam with a diameter of 4 inches;

[0094] Preferably, the one-dimensional delay line 10 has a movable range of 20 cm, so as to achieve complete detection of scattered signals of large-sized targets to be detected;

[0095] The one-dimensional rotating stage 7 realizes automatic scanning without manual operation under various imaging functions through the whole process computer control capability;

[0096] In one embodiment of the present invention, a femtosecond laser 1 enters the imaging system from one side and passes through a beam splitter 2 to obtain a pump laser and a detection laser;

[0097] After the pump laser passes through the chopper 3, it passes through the concave lens 4 and the convex lens 5, and irradiates the spin terahertz emission source 6 to generate a terahertz wave with a diameter of 4 inches; the terahertz wave with a diameter of 4 inches is incident on the surface of the target to be measured placed on the one-dimensional rotating table 7 to emit and scatter, and obtain a terahertz wave scattering signal; the terahertz wave scattering signal is collected by the off-axis parabolic mirror 8, and is reflected by the high-density polyethylene plate 9 and the indium tin oxide glass 12, and then focused to the electro-optical crystal zinc telluride 13;

[0098] The detection laser is focused by the detection light focusing lens 11 after passing through the one-dimensional delay line 1 and a plurality of reflectors, and coincides with the terahertz wave scattered signal in the electro-optical zinc telluride crystal 13; the electric field of the terahertz signal modulates the polarization state of the detection laser in the electro-optical zinc telluride crystal 13 to obtain the detection laser carrying the terahertz electric field information;

[0099] The detection laser carrying the terahertz electric field information is collimated by the detection light collimating lens 14 to obtain a collimated laser; the collimated laser is adjusted by the 1 / 4 wave plate 15 and the Wollaston prism 16 respectively to obtain an adjusted laser 1 and an adjusted laser 2, which are transmitted to the balanced detector 17, and the adjusted laser 1 and the adjusted laser 2 are processed by a computer to output a terahertz time domain waveform;

[0100] The terahertz time domain waveforms scattered by the target to be measured at various angles of the one-dimensional rotating stage 7 are collected and processed by a back-projection algorithm for terahertz radar imaging.

[0101] In one embodiment of the present invention, a femtosecond laser 1 with a diameter of 0.5 inches enters the imaging system from one side, and is divided into a pump path and a detection path by a beam splitter 2. The femtosecond laser with 90% of the energy enters the pump path to obtain a pump laser, and the femtosecond laser with 10% of the energy enters the detection path to obtain a detection laser.

[0102] A chopper 3 connected and synchronized with the laser pulse is placed in the pump circuit, and the working magnetic field of the chopper 3 is 500Hz, which suppresses noise and improves the signal-to-noise ratio of the system;

[0103] The pump laser passes through a laser beam expansion system composed of a concave lens 4 with a focal length of -50mm and a convex lens 5 with a focal length of 500mm, and is irradiated on a 4-inch spin terahertz emission source 6 to generate a large-size ultra-wideband strong-field terahertz wave with a diameter of 4 inches; the terahertz wave is incident on the surface of the target to be measured placed on a one-dimensional rotating table 7 and emits and scatters, and the scattered signal is collected by an off-axis parabolic mirror 8 with a focal length of 203.2mm, and is focused to the electro-optical crystal zinc telluride 13 through a high-density polyethylene plate 9; the function of the high-density polyethylene plate 9 is to block the propagation of the laser while transmitting the terahertz wave;

[0104] The detection laser is focused by a detection light focusing lens 11 with a focal length of 150 mm after passing through a one-dimensional delay line 1 and a plurality of reflectors, and overlaps with the terahertz wave in the electro-optical crystal zinc telluride 13 to obtain a detection laser carrying terahertz electric field information;

[0105] The detection laser carrying the terahertz electric field information is collimated by the detection light collimating lens 14 to obtain a collimated laser; the collimated laser is adjusted by the 1 / 4 wave plate 15 and the Wollaston prism 16 respectively to obtain an adjusted laser 1 and an adjusted laser 2, which are transmitted to the balanced detector 17, and the adjusted laser 1 and the adjusted laser 2 are processed by a computer to output a terahertz time domain waveform;

[0106] The terahertz time domain waveforms scattered by the target to be measured at various angles of the one-dimensional rotating stage 7 are collected and processed by a back-projection algorithm for terahertz radar imaging.

[0107] In the present invention, based on the electro-optic effect, the electric field of the terahertz wave changes the refractive index of the electro-optic crystal zinc telluride 13 and then changes the polarization state of the detection laser, which is collimated again by the detection light collimating lens 14; a 1 / 4 wave plate 15 and a Wollaston prism 16 are used to adjust the two orthogonal polarization components of the detection laser, and a balanced detector 17 is used to collect them respectively, and the terahertz waveform signal is drawn in the self-compiled software after being processed by the phase-locked amplifier.

[0108] The present invention also includes computer-controlled one-dimensional delay line 10 movement to change the optical path of the detection laser, which is used to map the electric field intensity of the terahertz wave at each moment and draw the shape of the terahertz pulse;

[0109] In one embodiment of the present invention, the optical path lengths of the pump laser and the detection laser are 2380-2420 mm.

[0110] In one embodiment of the present invention, the terahertz waves are propagated at a height of 145-155 mm above the optical platform to avoid reflection caused by the desktop.

[0111] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A terahertz radar imaging system based on a four-inch spinning terahertz emission source, characterized in that: include: A femtosecond laser light source (1), a beam splitter (2), a laser pumping terahertz emission module, a detection laser transmission module and a terahertz detection module are arranged along the laser incident direction; The terahertz detection module comprises a spin terahertz emission source (6), a one-dimensional rotating platform (7), an off-axis parabolic mirror (8) and a high-density polyethylene plate (9) which are sequentially arranged along the laser incident direction; the high-density polyethylene plate (9) is used for transmitting terahertz waves while blocking laser propagation.

2. The terahertz radar imaging system based on a four-inch spin terahertz emission source according to claim 1, characterized in that: The laser pumped terahertz emission module comprises a chopper (3), a concave lens (4) and a convex lens (5) arranged along the laser incident direction.

3. The terahertz radar imaging system based on a four-inch spin terahertz emission source according to claim 2, characterized in that: The spin terahertz emission source (6) is arranged on the side of the convex lens (5) outputting the laser, and generates a highly efficient, ultra-broadband four-inch terahertz wave under the pumping of a femtosecond laser; The one-dimensional rotating platform (7) is provided with an object to be measured.

4. The terahertz radar imaging system based on a four-inch spin terahertz emission source according to claim 3 is characterized in that: The detection laser transmission module comprises a one-dimensional delay line (10) arranged along the laser incident direction, a detection light focusing lens (11) and an indium tin oxide glass (12); the indium tin oxide glass (12) is used to reflect the terahertz wave into the terahertz detection module; The one-dimensional delay line (10) is used to control the optical path of the detection laser and the terahertz wave, so as to ensure that the detection laser and the terahertz wave coincide in time in the terahertz detection module; The indium tin oxide glass (12) is also used to transmit the detection laser into the terahertz detection module.

5. The terahertz radar imaging system based on a four-inch spin terahertz emission source according to claim 4, characterized in that: The terahertz detection module also includes an electro-optical crystal zinc telluride (13) arranged along the laser incident direction, a detection light collimating lens (14), a quarter wave plate (15), a Wollaston prism (16) and a balanced detector (17).

6. The terahertz radar imaging system based on a four-inch spin terahertz emission source according to claim 2, characterized in that: The spin terahertz emission source (6) comprises a substrate and a multi-layer one-dimensional photonic crystal structure and a nano-ferromagnetic heterojunction thin film layer which are sequentially connected to the substrate.

7. The terahertz radar imaging system based on a four-inch spin terahertz emission source according to claim 6, characterized in that: The nano ferromagnetic heterojunction film layer is a W / CoFeB / Pt heterojunction film.

8. The terahertz radar imaging system based on a four-inch spin terahertz emission source according to claim 6, characterized in that: The diameter of the nano-ferromagnetic heterojunction thin film layer is 4 inches.

9. The terahertz radar imaging system based on a four-inch spin terahertz emission source according to claim 6, characterized in that: The multi-layer one-dimensional photonic crystal structure comprises SiO2 thin films and HfO2 thin films which are periodically and alternately arranged.

10. A method for using a terahertz radar imaging system based on a four-inch spin terahertz emission source according to any one of claims 1 to 9, characterized in that: include: The femtosecond laser output by the femtosecond laser light source (1) is injected into the spin terahertz emission source (6) through the beam splitter (2) and the pump laser transmission module to generate a terahertz wave, which continues to propagate to the target to be detected by the terahertz emission module and is scattered to obtain a scattered terahertz signal; The scattered terahertz signal overlaps with the detection laser transmitted through the detection laser transmission module in the terahertz detection module, and the polarization state of the detection laser is modulated by the electric field of the terahertz signal and then processed and detected by the terahertz detection module, outputting the time domain waveform of the terahertz signal.

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