A terahertz radar detection method and system based on photon technology

Abstract

The present invention discloses a terahertz radar detection method and system based on photon technology. The method uses a wavelength selection module to select two comb teeth, one of which is divided into upper and lower paths. The baseband signal performs suppressed-carrier single-sideband modulation on the upper optical path comb tooth to obtain a frequency-swept optical signal, and then combines it with the other comb tooth of the optical frequency comb to form a composite optical signal. A part of the composite optical signal is converted into a terahertz signal by a photodetector and then radiated into the target environment. After the receiving unit receives the target echo signal, it is down-converted by a harmonic mixer to obtain a baseband echo signal, and the baseband echo signal modulates the lower path comb tooth to obtain a received optical signal. The received optical signal and the other part of the composite optical signal are sent to a coherent receiving module to achieve coherent reception, obtaining an intermediate-frequency signal carrying target information, and the detection target information can be extracted through an algorithm. The present invention realizes the photon generation and real-time coherent reception of terahertz radar signals by synchronizing the optical frequency comb and the harmonic mixer with the same reference source, and the radar system parameters are flexibly adjustable and the anti-interference ability is strong.

Description

Technical Field

[0001] The present invention relates to a radar detection method, and in particular to a micro-terahertz radar detection method and system based on photon technology. Background Art

[0002] Multi-dimensional high-resolution radars are widely used in military and civilian fields. Based on the electromagnetic wave scattering characteristics and medium propagation characteristics, radars operating in different frequency bands have their own advantageous characteristics. The related technologies of microwave-band radars have been widely developed, while the development of terahertz-band radars is limited by terahertz devices, especially terahertz radar systems that cover a wide terahertz spectral space, have a flexible adjustable working band, and can perform real-time high-precision signal processing and analysis. Currently, terahertz radar systems are mainly realized based on the frequency multiplication and up-conversion of microwave baseband signals. However, when the current radio frequency amplification, frequency multiplication, mixing, and transmission links are used to perform functions such as the generation, sampling, and processing of broadband signals, there are potential problems such as intermodulation / harmonic interference, impedance matching, amplitude / phase nonlinearity, etc., which seriously affect the performance of terahertz radar systems (see [M.Caris, S.Stanko, S.Palm, et al. 300GHz radar for high resolution SAR and ISAR applications. 2015 16th International Radar Symposium, Dresden, 2015, 577-580.]). Thanks to the rapid development of microwave photonics technology, the optical-domain generation, transmission, and processing of microwave signals can solve problems that cannot be handled in the traditional electrical domain, such as photonic mixing, photonic frequency multiplication, photonic true time delay, photonic coherent reception, etc., which provide new technical support for overcoming the traditional radar electronic bottleneck problem and improving technical performance, and have become the key technologies for next-generation radars (see [Ghelfi P, Laghezza F, Scotti F, et al. A fully photonics-based coherent radar system[J]. Nature, 2014, 507(7492): 341-345.]). In particular, photonics-based technologies can better solve some bottleneck problems that limit the development of terahertz radars. Technologies such as the generation of broadband radar detection signals based on photonic frequency multiplication technology and the real-time reception and processing of broadband radar echo signals based on photonic mixing technology have been used in new radar reception technologies (see [Zhang F, Guo Q, Zhang Y, et al. Photonics-based real-time and high-resolution ISAR imaging of non-cooperative target[J]. Chinese Optics Letters, 2017, 15(11): 112801.]). However, the current terahertz radar signal generation scheme based on photonic frequency multiplication technology has the following problems: 1) The frequency multiplication factor realized by optical frequency multiplication based on a special modulator is limited and cannot be flexibly adjusted; 2) Due to the overlap of higher-order sidebands, it is difficult to generate a terahertz band signal without spurs.3) The signal generation method limits that most radar signal reception schemes are difficult to receive broadband target echo signals through coherent reception. Summary of the Invention

[0003] The technical problem to be solved by the present invention is: to overcome the deficiencies of the prior art, through the selection of optical frequency comb teeth, to realize the generation of terahertz radar transmitted signals with reconfigurable bands, and based on clock synchronization locking and photon coherent reception technology, to realize real-time orthogonal dechirping of terahertz radar echo signals. The system is flexible and adjustable, and has excellent anti-interference performance.

[0004] The present invention specifically adopts the following technical solutions to solve the above technical problems:

[0005] First, an optical frequency comb generator generates an optical frequency comb signal with a repetition frequency of f LO . The amplified optical frequency comb signal is sent into a wavelength selection module. The wavelength selection module selects two comb teeth with different frequencies from the optical frequency comb signal respectively. One of the comb teeth is divided into upper and lower paths. The baseband signal f LFM performs suppressed-carrier single-sideband modulation on the upper-path comb tooth optical signal to obtain a frequency-swept optical signal. The frequency-swept optical signal and another comb tooth optical signal are combined into a composite optical signal. After the composite optical signal is divided into two paths, one path of the composite optical signal is converted into a terahertz signal with a frequency of (M + N)f LO + f LFM by a radar transmitting unit and radiated into the target environment (M, N are integers). After the radar receiving unit receives the terahertz radar echo signal, the terahertz radar echo signal is down-converted to obtain a baseband received signal through a harmonic mixer. The baseband received signal is amplified by a low-noise amplifier and then modulates the lower-path comb tooth optical signal to obtain a received modulated optical signal. The received modulated optical signal and the other path of the composite optical signal are sent into a coherent reception module to perform coherent reception, and an intermediate-frequency signal carrying target information is obtained. The detection target information can be obtained by analyzing the intermediate-frequency signal through an algorithm.

[0006] Preferably, the optical frequency comb generator can be a mode-locked laser, a femtosecond laser, a micro-ring optical frequency comb generator or an externally modulated optical frequency comb generator; the fundamental frequency of the harmonic mixer is equal to the optical frequency comb repetition frequency f LO , and the optical frequency comb generator and the harmonic mixer are synchronized by the same reference signal.

[0007] Further, the wavelength selection module is specifically a device such as an optical beam shaper, dual-laser injection locking, an optical filter, etc.; by controlling the working state of the wavelength selection module, the positions of two comb teeth with different frequencies can be controlled, and then the working band of the terahertz signal can be coarsely adjusted, that is, the size of M + N in the terahertz signal (M + N)f LO + f LFM .

[0008] Further, the baseband signal fLFM The frequency of LO is not limited by the repetition frequency f of the optical frequency comb signal, that is, by controlling the frequency of the baseband signal f LFM the operating frequency and bandwidth of the terahertz signal can be finely tuned.

[0009] According to the same inventive concept, the following technical solutions can also be obtained:

[0010] A terahertz radar detection system based on photon technology, comprising:

[0011] An optical frequency comb generator for generating an optical frequency comb signal with a frequency interval of f LO ;

[0012] A reference signal source for generating reference signals for the optical frequency comb generator and the receiving unit;

[0013] An optical amplifier for amplifying the optical frequency comb signal generated by the optical frequency comb generator;

[0014] A wavelength selection module for separately selecting two comb tooth signals with different frequencies from the optical frequency comb signal and outputting them respectively;

[0015] A first optical coupler for splitting one comb tooth optical signal into upper and lower paths and sending them to a first electro-optic modulator and a second electro-optic modulator respectively;

[0016] A baseband signal source for generating a baseband chirp signal with a frequency of f LFM ;

[0017] A first electro-optic modulator for modulating the baseband signal onto the comb tooth optical signal input to the first electro-optic modulator to obtain a frequency-swept optical signal;

[0018] A second optical coupler for combining the frequency-swept optical signal with another comb tooth signal output by the wavelength selection module into a composite optical signal, and splitting the composite optical signal into upper and lower paths. The upper path composite optical signal is sent to the transmitting unit, and the lower path composite signal is sent as a reference optical signal to an input end of the coherent receiving module;

[0019] A transmitting unit for converting the composite optical signal into a terahertz signal and radiating it into the target environment;

[0020] A receiving unit for receiving the terahertz radar echo signal and down-converting the terahertz radar echo signal into a baseband received signal;

[0021] A low-noise amplifier for amplifying the baseband received signal;

[0022] A second electro-optic modulator, configured to modulate the baseband received signal amplified by the low-noise amplifier onto the comb optical signal input to the second electro-optic modulator to obtain a received modulated optical signal, and send it to another receiving end of the coherent receiving module;

[0023] A coherent receiving module, configured to perform coherent fusion detection on the reference optical signal and the received modulated optical signal in the optical domain to obtain two orthogonal intermediate-frequency signals carrying target information;

[0024] An acquisition and processing module, configured to perform analog-to-digital conversion on the two orthogonal intermediate-frequency signals, and perform radar digital signal processing to extract target information.

[0025] Further, the transmitting unit includes:

[0026] A uni-traveling-carrier photodetector, configured to convert the composite optical signal into a terahertz signal;

[0027] A terahertz transmitting amplifier, configured to amplify the terahertz signal output by the uni-traveling-carrier photodetector;

[0028] A terahertz transmitting antenna, configured to transmit the amplified terahertz signal to obtain a terahertz radar detection signal;

[0029] Further, the receiving unit includes:

[0030] A terahertz receiving antenna, configured to receive a terahertz radar echo signal;

[0031] A terahertz receiving amplifier, configured to amplify the terahertz radar echo signal received by the terahertz receiving antenna;

[0032] A harmonic mixer, configured to down-convert the terahertz radar echo signal into a baseband received signal;

[0033] Preferably, the optical frequency comb generator can be a mode-locked laser, a femtosecond laser, a micro-ring optical frequency comb generator, an externally modulated optical frequency comb generator, etc.; where the fundamental frequency of the harmonic mixer is equal to the optical frequency comb repetition frequency f LO and the optical frequency comb generator and the harmonic mixer are synchronized by the same reference signal.

[0034] Further, the wavelength selection module is specifically a device such as an optical beam shaper, dual-laser injection locking, an optical filter, etc.

[0035] Further, the first electro-optic modulator is a dual-parallel Mach-Zehnder modulator, and the second electro-optic modulator is a Mach-Zehnder modulator, or a phase modulator, or a dual-parallel modulator, etc.

[0036] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0037] 1) The signal generation part of the present invention adjusts the operating band of the terahertz radar signal based on optical frequency comb frequency selection. Based on the rich spectral characteristics of the optical frequency comb, it can flexibly achieve wide adjustment of the terahertz radar signal in the terahertz band, and the radar operating bandwidth is not limited by the repetition frequency of the optical frequency comb.

[0038] 2) The signal reception part of the present invention realizes the reception of the terahertz radar echo signal through a harmonic mixer combined with optical coherence technology. Coherent reception of broadband terahertz signals can be achieved in the optical domain, and system noise and image frequency interference signals can be effectively suppressed.

[0039] 3) The present invention provides a reference signal for both the optical frequency comb generator and the harmonic mixer based on a single high-performance reference signal, ensuring that the fundamental frequency signal participating in the down-conversion by the harmonic mixer is strictly coherent with the terahertz radar signal, so as to ensure strict coherent reception of the radar echo signal, and improve the stability and signal-to-noise ratio of the radar system. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a schematic diagram of the principle of the terahertz radar system of the present invention;

[0041] Figure 2 is a structural diagram of a specific embodiment of the terahertz radar of the present invention;

[0042] Figure 3 is Figure 2 the signal spectrum and signal schematic diagram generated at the corresponding nodes in the shown terahertz radar system;

[0043] Among them, A corresponds to the spectral distribution of the optical signal output by the mode-locked laser, B corresponds to the spectral distribution of the composite optical signal, C corresponds to the spectral distribution of the terahertz radar transmission signal, D corresponds to the spectral distribution of the baseband echo signal, E corresponds to the spectral distribution of the radar receiving optical signal, F corresponds to the spectrogram after the fusion of the reference composite optical signal and the radar receiving optical signal, and G corresponds to the spectrum of the intermediate frequency signal in complex form. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] Aiming at the deficiencies of the prior art, the idea of the present invention is based on the rich spectral components of the optical frequency comb, realizes the generation of a band-reconfigurable terahertz radar signal through wavelength selection technology, realizes the down-conversion of the terahertz radar echo signal based on a stable reference signal, and realizes the coherent reception of the radar echo signal based on the photon coherent reception method. The radar operating parameters of this solution are flexibly adjustable, the signal processing is real-time and efficient, and the anti-spurious ability is strong.

[0045] A terahertz radar detection system based on photon technology of the present invention specifically includes, as Figure 1As shown in the figure, it includes: an optical frequency comb generator, an optical amplifier, a wavelength selection module, a baseband signal source, a reference signal source, a first electro-optic modulator and a second electro-optic modulator, a 1×2 optical coupler (OC1), a 2×2 optical coupler (OC2), a transmitting unit, a receiving unit, a coherent receiving module, an acquisition and processing module, a low-noise amplifier (LNA), etc.

[0046] First, the optical frequency comb generator generates an optical frequency comb signal with a repetition frequency of f LO The optical frequency comb signal is amplified by the optical amplifier and then sent to the wavelength selection module. The wavelength selection module selects two comb tooth optical signals with different frequencies from the optical frequency comb signal respectively. One of the comb tooth optical signals is divided into upper and lower paths. The baseband signal f LFM generated by the baseband signal source realizes suppressed-carrier single-sideband modulation on the upper path comb tooth optical signal passing through the first electro-optic modulator to obtain a frequency-swept optical signal. The frequency-swept optical signal and another comb tooth optical signal are combined into a composite optical signal. After the composite optical signal is divided into two paths, one path of the composite optical signal is converted into a terahertz signal with a frequency of (M + N)f LO + f LFM by the radar transmitting unit and radiated into the target environment (M and N are integers). After the radar receiving unit receives the terahertz radar echo signal, the terahertz radar echo signal is down-converted by the harmonic mixer to obtain a baseband received signal. The baseband received signal is amplified by the low-noise amplifier and then modulated by the second electro-optic modulator on the lower path comb tooth optical signal to obtain a received modulated optical signal. The received modulated optical signal and the other path of the composite optical signal are sent to the coherent receiving module to achieve coherent reception, obtaining an intermediate-frequency signal carrying target information. The acquisition and processing module can extract target information from the intermediate-frequency signal through algorithms by collecting and processing the intermediate-frequency signal.

[0047] For the convenience of public understanding, the technical solution of the present invention will be further described in detail below through a specific embodiment.

[0048] It should be noted that the second electro-optic modulator can adopt various modulator types. Preferably, the Mach-Zehnder modulator scheme is selected in this embodiment. The optical frequency comb generator can adopt various devices. The mode-locked laser is selected in this embodiment. The wavelength selection module can adopt various devices. The beam shaper is preferably selected in this embodiment.

[0049] Such as Figure 2As shown in the figure, the terahertz radar system of this embodiment includes: 1 mode-locked laser, 1 optical amplifier, 1 beam shaper, 1 baseband signal source, 1 reference signal source, 1 dual-parallel Mach-Zehnder modulator (dual-parallel MZM), 1 Mach-Zehnder modulator (MZM), 1 1×2 optical coupler (OC1), 1 2×2 optical coupler (OC2), 1 uni-travelling-carrier photodetector (UTC-PD), 2 terahertz amplifiers (TTA, TRA), 2 terahertz antennas (Tx, Rx), 1 harmonic mixer, 1 low-noise amplifier (LNA), 1 coherent receiving module, and 1 acquisition and processing module.

[0050] In the terahertz radar detection system of the present invention, first, a reference signal with a frequency of f LO output by the reference signal source is input to the clock input terminal of the mode-locked laser to make the mode-locked laser operate in the mode-locked state. The mode-locked laser outputs an optical frequency comb signal with a frequency interval of f LO . Its spectrum is as shown in A of Figure 3 , where f C is the optical carrier. After the optical frequency comb signal is amplified by the optical amplifier, it is sent to the beam shaper. The filtering response curve of the beam shaper is set, and two comb tooth optical signals with frequencies of f C -Mf LO and f C +Nf LO are respectively output at the two output terminals of the beam shaper. One of the comb tooth optical signals is selected and sent to the 1×2 optical coupler to be divided into upper and lower paths. Here, the comb tooth f C +Nf LO is taken as an example. The upper path comb tooth signal is sent to the dual-parallel Mach-Zehnder modulator. The baseband linear frequency modulation signal with a frequency of f LFM =f 0 +kt (0≤t≤T) generated by the baseband signal source suppresses the carrier single-sideband modulation of the comb tooth optical signal through the dual-parallel Mach-Zehnder modulator, where f 0 is the starting frequency of the baseband linear frequency modulation signal, k is the frequency modulation slope, and T is the period, obtaining a swept optical signal. The swept optical signal is a positive first-order swept sideband or a negative first-order swept sideband. Here, the positive first-order swept sideband is taken as an example, and its instantaneous frequency is f C +Nf LO +f LFM . In the time domain, it can be expressed as:

[0051] S T (t)=Aexp[j2π(f C +Nf LO +f 0 +0.5kt)t] (0≤t≤T) (1)

[0052] where A is the amplitude of the signal electric field. The swept-frequency optical signal and the comb-shaped optical signal with a frequency of f C -M f LO are combined into one path through a 2×2 optical coupler to obtain a composite optical signal, and the spectrum of the composite optical signal is as shown in B of Figure 3 . The 2×2 optical coupler divides the composite optical signal into two paths. One path is used as the transmitted optical signal and sent to the transmitting unit, and the other path is used as the reference optical signal and sent to an optical input end of the coherent receiving module. After the transmitted optical signal sent to the transmitting unit is converted into a terahertz signal through a uni-traveling carrier photodetector, the terahertz signal is sent to a terahertz transmitting amplifier (TTA) for amplification, and the amplified terahertz signal is sent to a terahertz transmitting antenna (Tx) for transmission, and is radiated into the target environment as the terahertz radar transmitting signal. The spectrum diagram of the terahertz radar transmitting signal is as shown in C of Figure 3 . The terahertz radar transmitting signal reflected by the target generates a terahertz radar echo signal. The terahertz radar echo signal is received by the terahertz receiving antenna (Rx) of the receiving unit, amplified by a terahertz receiving amplifier (TRA) and sent to a harmonic mixer. Another reference signal output by the reference signal source is input to the fundamental frequency input port of the harmonic mixer of the receiving unit, and frequency doubling occurs in the harmonic mixer to obtain a harmonic terahertz signal with a frequency of (M + N)f LO . After the harmonic terahertz signal is mixed with the terahertz radar echo signal, a down-converted baseband receiving signal is obtained, and its spectrum is as shown in D of Figure 3 . In the time domain, it can be expressed as:

[0053] S Tr (t) = Bexp[j2π(f 0 (t - τ) + 0.5k(t - τ) 2 )] (0 ≤ t ≤ T) (2)

[0054] where B is the amplitude of the signal electric field, and τ is the delay of the terahertz radar echo signal relative to the terahertz radar transmitting signal. The baseband receiving signal is amplified by a low-noise amplifier and then adjusted by a Mach-Zehnder modulator to an instantaneous frequency of f C +Nf LO of the dropped comb-shaped optical signal to obtain a received modulated optical signal, and the spectrum diagram of the received modulated optical signal is as shown in E of Figure 3 . In the time domain, it can be expressed as:

[0055] S R (t) = A - 1 exp[j2π((f C +Nf LO -f 0 )(t - τ) - 0.5k(t - τ) 2 )] + A 0exp[j2π((f C +Nf LO )(t - τ))]

[0056] +A 1 exp[j2π((f C +Nf LO +f 0 )(t - τ)+0.5k(t - τ) 2 )(0 ≤ t ≤ T) (3) where A - 1 、A 0 and A 1 are the electric field amplitudes of the negative first - order, carrier, and positive first - order sideband signals. The received optical signal is sent to another optical receiving end of the coherent receiving module. The received optical signal and the reference optical signal are fused in the optical domain, and its positive first - order sideband coincides with the reference optical signal. Its spectrum is as shown in F of Figure 3 . The instantaneous frequency difference of the overlapping part is kτt. After the received optical signal and the reference optical signal are coherently received in the coherent receiving module, two orthogonal intermediate - frequency signals can be obtained at the two output ends of the coherent receiving module. The intermediate - frequency electrical signal can be expressed as:

[0057]

[0058] That is, the two orthogonal components S I (t), S Q (t) of the intermediate - frequency signal carrying the target information, where φ is the phase information of the intermediate - frequency signal. The corresponding complex - number form of the signal is:

[0059] S IF (t) = S I (t)+jS Q (t) = Cexp[j2πkτt + jφ] (0 ≤ t ≤ T) (5)

[0060] C is the amplitude of the complex intermediate - frequency signal. After the intermediate - frequency signal is converted by analog - to - digital conversion, information such as the target distance, speed, and scattering characteristics can be obtained based on the radar signal - processing algorithm. Its spectrum is as shown in G of Figure 2 .

[0061] In this scheme, by locking the optical frequency comb generator with the same clock source and providing the fundamental - frequency signal for the harmonic mixer, the good coherence of the radar system can be guaranteed; by selecting different teeth of the optical frequency comb, flexible adjustment of the working band of the terahertz radar can be realized; and based on the photon coherent receiving scheme, while ensuring that the radar system operates in the terahertz band, real - time coherent reception is achieved, and a complex intermediate - frequency signal is obtained. Compared with the real - number intermediate - frequency signal, it not only has one more dimension of information but also has a stronger ability to resist image - frequency interference. The overall signal - to - noise ratio of the receiver can also be greatly improved.

[0062] Finally, it should be noted that the specific embodiments listed above are only for the present invention. The present invention is not limited to the above embodiments and there can be many variations. All variations that can be directly derived or associated by those of ordinary skill in the art from the disclosed content of the present invention shall be considered as within the protection scope of the present invention.

Claims

1. A terahertz radar detection method based on photon technology, characterized in that, the method is specifically as follows: First, an optical frequency comb generator generates an optical frequency comb signal with a repetition frequency of f LO . Two comb tooth optical signals with different frequencies are selected from the optical frequency comb signal. One of the comb tooth optical signals is divided into upper and lower paths, and the baseband signal f LFM performs suppressed-carrier single-sideband modulation on the upper-path comb tooth optical signal to obtain a frequency-swept optical signal. The frequency-swept optical signal and another comb tooth optical signal are combined into a composite optical signal. After the composite optical signal is divided into two paths, one path of the composite optical signal is converted into a terahertz signal with a frequency of (M + N)f LO + f LFM by a radar transmitting unit and radiated into the target environment, where M and N are integers. After encountering the target, a terahertz radar echo signal is generated. After receiving the terahertz radar echo signal, the terahertz radar echo signal is down-converted by a harmonic mixer to obtain a baseband received signal. After the baseband received signal is amplified, it modulates the lower-path comb tooth optical signal to obtain a received modulated optical signal; Receiving a modulated optical signal and another composite optical signal to achieve coherent reception, obtaining an intermediate-frequency signal carrying target information, and analyzing the intermediate-frequency signal to obtain detection target information.

2. The method according to claim 1, characterized in that, The optical frequency comb generator is a mode-locked laser, a femtosecond laser, a micro-ring optical frequency comb generator or an externally modulated optical frequency comb generator; wherein the fundamental frequency of the harmonic mixer is equal to the optical frequency comb repetition frequency f LO and the optical frequency comb generator and the harmonic mixer are synchronized by the same reference signal.

3. The method according to claim 1, characterized in that, Selecting two optical comb signals with different frequencies from the optical frequency comb signal is specifically achieved by an optical beam shaper, dual-laser injection locking, or an optical filter to select wavelengths; by controlling the positions of the two optical combs with different frequencies, the working band of the terahertz signal is then roughly adjusted, that is, the terahertz signal (M + N)f LO + f LFM The magnitude of M + N in 4. The method according to claim 1, characterized in that, Fine-tune the operating frequency and bandwidth of the terahertz signal by controlling the frequency of the baseband signal f LFM ​ 5. A terahertz radar detection system based on photon technology, characterized in that, comprising: An optical frequency comb generator for generating an optical frequency comb signal with a repetition frequency of f LO ; A reference signal source for generating reference signals for an optical frequency comb generator and a receiving unit; An optical amplifier for amplifying the optical frequency comb signal generated by the optical frequency comb generator; A wavelength selection module for respectively selecting two comb tooth optical signals with different frequencies from the optical frequency comb signal and outputting them respectively; A first optical coupler for dividing one comb tooth optical signal into upper and lower paths and sending them to a first electro-optic modulator and a second electro-optic modulator respectively; A baseband signal source for generating a baseband signal with a frequency of f LFM ; A first electro-optic modulator for modulating a baseband signal onto the comb tooth optical signal input to the first electro-optic modulator to obtain a frequency-swept optical signal; A second optical coupler for combining the frequency-swept optical signal with another comb tooth signal output by the wavelength selection module into a composite optical signal, and dividing the composite optical signal into upper and lower paths. The upper composite optical signal is sent to a transmitting unit, and the lower composite signal is sent as a reference optical signal to an input end of a coherent receiving module; A transmitting unit for converting the composite optical signal into a terahertz signal and radiating it into a target environment; A receiving unit for receiving a terahertz radar echo signal and down-converting the terahertz radar echo signal into a baseband received signal; A low-noise amplifier for amplifying the baseband received signal; A second electro-optic modulator for modulating the baseband received signal amplified by the low-noise amplifier onto the comb tooth optical signal input to the second electro-optic modulator to obtain a received modulated optical signal and sending it to another receiving end of the coherent receiving module; A coherent receiving module for coherently fusing and detecting the reference optical signal and the received modulated optical signal in the optical domain to obtain two orthogonal intermediate-frequency signals carrying target information; An acquisition and processing module for performing analog-to-digital conversion on the two orthogonal intermediate-frequency signals, performing radar digital signal processing, and extracting target information.

6. The system according to claim 5, characterized in that, the transmitting unit includes: A uni-traveling-carrier photodetector for converting the composite optical signal into a terahertz signal; A terahertz transmitting amplifier for amplifying the terahertz signal output by the uni-traveling-carrier photodetector; A terahertz transmitting antenna for transmitting the amplified terahertz signal to obtain a terahertz radar detection signal; The receiving unit includes: A terahertz receiving antenna for receiving a terahertz radar echo signal; A terahertz receiving amplifier for amplifying the terahertz radar echo signal received by the terahertz receiving antenna; A harmonic mixer for down-converting the terahertz radar echo signal into a baseband received signal.

7. The system according to claim 5, characterized in that, The optical frequency comb generator is a mode-locked laser, a femtosecond laser, a microring optical frequency comb generator, or an externally modulated optical frequency comb generator; wherein the fundamental frequency of the harmonic mixer is equal to the optical frequency comb repetition frequency f LO and the optical frequency comb generator and the harmonic mixer are synchronized by the same reference signal.

8. The system according to claim 5, characterized in that, The wavelength selection module is an optical beam shaper, a dual laser, or an optical filter.

9. The system according to claim 5, wherein, the first electro-optic modulator is a dual-parallel Mach-Zehnder modulator, and the second electro-optic modulator is a Mach-Zehnder modulator, a phase modulator, or a dual-parallel modulator.

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