Correlation imaging radar system based on coherent detection and its imaging method

Abstract

A correlation imaging radar system based on coherent detection and an imaging method thereof, the system comprising a chirp modulated light source system, an optical fiber beam splitting system, an optical fiber amplification system, a collimation system, a spatial modulation system, a spatial light splitting system, a reference optical path system, a transmitting optical system, a receiving optical system, a post-processing system, a clock system and a main control and acquisition system. The system is based on coherent detection, combines the theory of compressed sensing with the theory of laser correlation imaging, and has the ability of three-dimensional imaging super-resolution imaging under low compression ratio and the ability to instantaneously propose motion or vibration. The present invention utilizes long modulation duration to increase the energy of a single pulse, pulse compression to increase the detection signal-to-noise ratio, and chirp modulation and demodulation technology to achieve high ranging resolution under low acquisition bandwidth and reduce the amount of data acquisition, and utilizes the motion and vibration characteristics of the target to fully utilize the high sensitivity characteristics and phase detection capabilities of optical coherent detection, and has the ability to image the target distance dimension and motion or vibration dimension separately.

Description

Technical Field

[0001] The invention relates to a compressed sensing laser correlation imaging radar system, in particular to a correlation imaging radar system based on coherent detection and an imaging method thereof. Background Art

[0002] Laser correlation imaging technology is a new type of laser imaging technology that emerged at the beginning of this century. It divides the light field with spatiotemporal fluctuations into two parts. One part uses a detector with spatial resolution to record the intensity distribution of the spatiotemporal fluctuations, which is the reference light path; the other part is imaged onto the target surface and a detector without spatial resolution is used to receive the echo signal, which is the object light path. By utilizing the fluctuation correlation properties of the reference light path and the object light path, and combining the compressed sensing algorithm, a correlated reconstructed target image based on compressed sensing can be obtained. However, in this technology, the echo signal of the object light path is generally very weak and the detection technology used is direct intensity detection. For pulsed laser signals, the minimum detectable power of direct intensity detection technology can be as low as 10 -9 W.

[0003] Optical coherence detection technology is a new type of coherence detection technology that has only emerged in the 1990s. It divides a beam of amplitude or phase modulated continuous narrow linewidth optical signal into two parts: one part is retained locally as a local oscillator optical signal, and the other part is emitted to the target and obtains a weaker target echo signal. The local oscillator optical signal with strong optical power and excellent coherence interferes with the weak echo signal to obtain the difference frequency signal between the local oscillator optical signal and the echo optical signal. By identifying the frequency of the difference frequency signal, the grayscale and distance information of the target can be obtained. It should be pointed out that compared with direct intensity detection technology, optical coherence detection technology has a detection sensitivity advantage of up to several orders of magnitude. In addition, this technology has phase detection capability, which can retain and record the Doppler frequency shift caused by the relative motion between the target and the platform. Therefore, this technology can also realize the extraction of motion or vibration dimension information of the target.

[0004] Laser correlation imaging radar is a new type of laser imaging radar that was proposed in the past 10 years. Its mechanism is fundamentally different from previous laser imaging radars. Since its mechanism exists in the possibility of combining with compressed sensing theory, laser correlation imaging radar based on compressed sensing was proposed by Han Shensheng's team at the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences in 2012 and was first verified by field tests. Afterwards, the team successively verified the high information extraction efficiency and super-resolution capability of laser correlation imaging radar based on compressed sensing. Therefore, laser correlation imaging radar based on compressed sensing has received more and more attention. Despite this, the detection system of this type of laser correlation imaging radar is still based on direct intensity detection, and the detection sensitivity is not high enough and there is no phase detection capability. For moving targets, although the reconstructed image of the target can be obtained according to the post-processing algorithm, this method can only be used for targets moving at a uniform speed. For targets with variable speed, this method is very easy to fail. In addition, the system also has the disadvantages of weak anti-background noise ability and large amount of data to be processed. Summary of the invention

[0005] The purpose of the present invention is to propose a correlation imaging radar system based on coherent detection to achieve low peak power, high ranging resolution, high noise resistance, and high sensitivity to extract target information in four dimensions, namely, three-dimensional target space and motion or vibration dimension, and to achieve natural decoupling of distance and Doppler velocity.

[0006] The chirp modulated light source emits broadband chirp modulated signal light, which is divided into two parts after passing through the fiber beam splitter. One part remains locally as the local oscillator light signal; the other part is used as the signal light to bombard the rotating frosted glass after passing through the fiber amplifier and collimator, and a pseudothermal light field is generated on the short focal field mirror behind the frosted glass. The pseudothermal light field is divided into two parts by the spatial optical beam splitter. One part is imaged on the CCD by the reference imaging mirror, and the CCD records the intensity distribution of the light field; the other part is imaged on the target surface by the optical emission mirror. The signal light returned after passing through the target is received by the optical receiving system and coupled into the receiving fiber. The local oscillator light signal and the receiving fiber introduce the echo light signal together into the integrated 2*4 90° balanced detector, where optical bridging and de-skewing mixing processes occur and two orthogonal low-frequency electrical signals are output. The frequency of the low-frequency electrical signal is the difference frequency between the local oscillator light signal and the echo light signal, and the amplitude is proportional to the amplitude of the local oscillator light signal and the echo light signal. The out-of-band noise of the low-frequency electrical signal is filtered out by the filtering system, and a relatively pure low-frequency electrical signal can be obtained. The analog-to-digital converter converts the low-frequency electrical signal into a digital signal and stores it by the main controller. The corresponding reference light field intensity distribution recorded by the CCD is also stored by the main controller. After multiple measurements, multiple low-frequency electrical signals and corresponding reference light field intensity distributions can be obtained. For stationary targets, the low-frequency electrical signal is fast Fourier transformed to obtain the spectrum peak and the corresponding intensity. The spectrum peak position represents the distance information, and the spectrum intensity is the corresponding detection signal intensity. If it is a moving or vibrating target, the low-frequency electrical signal is processed by short-time Fourier transform. The spectrum of the signal will show a distance spectrum and a motion or vibration spectrum. For the distance spectrum, the center position of the spectrum can represent the average position of the target in a measurement, and the detection signal intensity can be the sum of all spectrum intensities within the distance spectrum interval; for the motion or vibration spectrum, the target's motion or vibration speed can be calculated from this, the instantaneous position of the spectrum is the target's speed information, and the sum of the intensities within the spectrum interval is the detection signal intensity. The three-dimensional image of the target can be obtained according to the detection signal intensity of the distance spectrum and the motion or vibration spectrum and the reference light field in combination with the compressed sensing algorithm.

[0007] A typical system structure of the present invention includes: a chirp modulated light source system, a fiber beam splitting system, a fiber amplification system, a collimation system, a spatial modulation system, a spatial light splitting system, a reference optical path system, an optical transmitting system, an optical receiving system, a post-processing system, a clock system and a main control and acquisition system.

[0008] The chirp modulated light source system is used to generate a broadband modulated narrow linewidth modulated optical signal.

[0009] The optical fiber splitting system is used to split the modulated optical signal into a local oscillator optical signal and a modulated optical signal.

[0010] The optical fiber amplification system is used to amplify the modulated optical signal to meet the optical power requirement for long-distance detection.

[0011] The collimation system is used to collimate the modulated light signal output by the optical fiber and output a light spot of suitable size to the spatial modulation system.

[0012] The spatial modulation system is used to perform transverse spatial modulation on the light field of the modulated light signal to achieve the spatial fluctuation requirement of the light field and form a speckle field on the short-focus field mirror therein.

[0013] The spatial light splitting system is used to divide the light field after the spatial modulation system into two parts, one part goes to the reference light path system, and the other part goes to the optical emission system.

[0014] The reference optical path system is used to record the light field modulated by the spatial modulation system for reconstructing the target image.

[0015] The emission optical system is used to emit the light field modulated by the spatial modulation system to the target surface with smaller aberration.

[0016] The receiving optical system is used to converge the signal light returned from the target surface and couple it into the receiving optical fiber.

[0017] The post-processing system is used to mix the return optical signal with the local oscillator optical signal to output two orthogonal IQ low-frequency electrical signals. In this process, the noise signal is filtered out to a large extent, and the amplitude and phase information of the return optical signal are recorded. That is, the grayscale information and motion or vibration information of the target are all recorded.

[0018] The clock system is used to generate a synchronous control signal to synchronize the chirp modulated light source, the reference CCD, and the analog-to-digital conversion system.

[0019] The main control and acquisition system is used to control the clock system, store reference CCD light field information, collect and store IQ low-frequency electrical signals, and reconstruct output target images and motion or vibration information.

[0020] Technical effects of the present invention:

[0021] 1. Restore the three-dimensional spatial image data and one-dimensional motion or vibration data of the target. Perform compressed sensing reconstruction under uncorrelated speckle fields, and use the sparse prior information of the target itself to achieve high information extraction or even super-resolution extraction of the target. The system converts the high ranging resolution that originally requires high acquisition bandwidth to a modulated broadband optical signal, so that high ranging resolution can be achieved by low-bandwidth acquisition during the acquisition stage.

[0022] 2. Due to the use of coherent detection, the phase information of the light field can be extracted instantaneously. The motion or vibration information of the target is retained in the system in the form of phase changes. According to the change of the phase, the spectrum of the target can be identified according to the motion or vibration characteristics of the target in the changing spectrum of the short-time Fourier transform. In addition, due to the high sensitivity and noise resistance of coherent detection, the system also has weak signal detection capabilities and strong robustness.

[0023] 3. In the receiving optical system, the receiving fiber is placed on the Fourier plane of the optical condenser, and a single receiving fiber can collect all the information of the target. A single integrated 2*4 90° balanced detector can achieve good imaging quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a structural block diagram of Example 1 of the coherent detection correlation imaging radar system of the present invention.

[0025] The following are marked in the figure:

[0026] 1-Chirp modulated light source 2-Fiber beam splitting system 3-Fiber amplification system 4-Collimation system 5-Spatial modulation system 6-Spatial optical splitter 7-Reference optical path system 8-Transmitting optical system 9-Receiving optical system 10-Post-processing system 11-Clock system 12-Main controller

[0027] ①- Short focal field mirror surface ②- Reference CCD surface ③- Target surface ④- Condenser Fourier surface DETAILED DESCRIPTION

[0028] Combine the following Figure 1 This paper describes how the compressed sensing laser correlation imaging radar system based on coherent detection can obtain the three-dimensional surface image and motion or vibration information of an object. Figure 1 As shown, the system comprises a chirp modulated light source system 1, a fiber beam splitting system 2, a fiber amplification system 3, a collimation system 4, a spatial modulation system 5, a spatial light splitting system 6, a reference optical path system 7, a transmitting optical system 8, a receiving optical system 9, a post-processing system 10, a clock system 11, and a main control and acquisition system 12. The short-focus field mirror surface ① is conjugated with the reference CCD surface ② with respect to the reference imaging mirror, and the short-focus field mirror surface ① is also conjugated with the target surface ③ with respect to the transmitting optical system. The receiving optical fiber in the receiving optical system 9 is located at the Fourier surface ④ of the condenser.

[0029] The chirp modulated light source system 1 emits a broadband modulated chirp optical signal with a continuous narrow linewidth. The optical signal is divided into two parts by the optical fiber splitting system 2. One part is retained locally as the local oscillator optical signal LO and injected into the post-processing system 10; the other part is a modulated optical signal, which is emitted into the free space by the collimation system 4 after passing through the optical fiber amplification system 3 and bombards the spatial modulation system 5, and forms a pseudo-thermal speckle field with time-space fluctuations on the short-focus field mirror ① in the spatial modulation system. The speckle field is divided into two parts after passing through the spatial splitting system 6. One part is imaged by the reference optical path system onto the reference CCD surface ② and its light intensity distribution is recorded. The other part is imaged onto the target surface ③ through the transmitting optical system 8. The laser mirror in the receiving optical system 9 receives the echo of the target surface ③, and the receiving optical fiber receives it and injects it into the post-processing system 10. In the post-processing system 10, the local oscillator optical signal LO and the echo optical signal SIG undergo an optical bridge and mixing process, and after filtering, two orthogonal IQ low-frequency electrical signals are output. The IQ two-way signal is converted into a digital signal by the main control and acquisition system 12 and stored as I 1 , Q 1 At the same time, the main control and acquisition system 12 will refer to the light intensity distribution recorded by the CCD After M measurements, the reference measurement data is obtained. And IQ two-way signal data {I i} and {Q i}, where i is the number of measurements, and i = 1, 2, ..., M.

[0030] Next, the main control and acquisition system 12 completes the detection signal extraction, speed or vibration information extraction and target space three-dimensional information reconstruction process based on the reference measurement data and the IQ two-way signal data. First, the IQ two-way signal complex number is added to obtain the complex signal C i =I i +jQ i . i By performing a short-time Fourier transform, it can be observed that the target's spectrum is mainly divided into two parts: the motion or vibration spectrum and the distance spectrum. If the target moves at a uniform speed, a narrow spectrum peak of the target's distance and velocity coupling will be obtained in the distance spectrum, and a narrow spectrum peak will also be obtained in the motion or vibration spectrum; if the target moves at a non-uniform speed, then the distance spectrum interval broadened by motion or vibration can be observed, and a spectrum curve that changes with time will appear in the motion or vibration spectrum. According to the spectral position of the motion or vibration spectrum, the instantaneous motion or vibration speed v of the target can be calculated by the following formula:

[0031]

[0032] Where λ is the central wavelength of the light wave, f dis the corresponding instantaneous spectrum position. By adding the spectrum corresponding to the instantaneous spectrum position within a single measurement time, the detection signal intensity corresponding to the motion or vibration spectrum during a single measurement can be obtained. For the range spectrum interval, the target distance can be calculated based on the instantaneous motion or vibration spectrum position f at the corresponding moment. d Solved by the following formula:

[0033]

[0034] Where c is the speed of light, T is the duration of chirp modulation, B is the bandwidth of chirp modulation, and f R is the instantaneous spectrum position corresponding to the target and f R =f R+d -f d , f R+d f d The corresponding instantaneous distance spectrum peak position. As with the motion or vibration spectrum, the spectrum intensity radar class on the corresponding distance spectrum within a single measurement time can obtain the corresponding detection signal intensity.

[0035] In addition, reference will be made to the measured data Stretch it into a column and make it have the following form Where N = P × Q. When solving the image, the target image is reconstructed using the association reconstruction algorithm based on compressed sensing.

[0036] The target image corresponding to the motion or vibration spectrum and the distance spectrum can be reconstructed by the following formula:

[0037]

[0038] Represents the spatial image information corresponding to the object on the motion or vibration spectrum line d.

[0039] For the target image corresponding to the distance spectrum, it can be reconstructed by the following formula:

[0040]

[0041] Represents the spatial image information corresponding to the object on the distance spectrum line d.

[0042] In this way, it is possible to respectively calculate the three-dimensional information of the target space from the motion or vibration spectrum and the target distance spectrum, and extract the motion or vibration dimension information of the target.

Claims

1. A correlation imaging radar system based on coherent detection, characterized in that: It includes a chirp modulated light source system (1), an optical fiber splitting system (2), an optical fiber amplification system (3), a collimation system (4), a spatial modulation system (5), a spatial light splitting system (6), a reference optical path system (7), a transmitting optical system (8), a receiving optical system (9), a post-processing system (10), a clock system (11), and a main control and acquisition system (12); The chirp modulated light source system is used to generate a broadband modulated narrow linewidth modulated optical signal; The optical fiber splitting system is used to split the narrow linewidth modulated optical signal into a local oscillator optical signal and a modulated optical signal; The optical fiber amplification system is used to amplify the modulated optical signal to meet the optical power requirements for long-distance detection; The collimation system is used to collimate the modulated light signal output by the optical fiber and output a light spot of suitable size to the spatial modulation system; The spatial modulation system is used to perform transverse spatial modulation of the light field of the modulated light signal to achieve the spatial fluctuation requirement of the light field and form a speckle field on the short-focus field mirror therein; The spatial light splitting system is used to split the light field after the spatial modulation system into two parts, one part goes to the reference light path system, and the other part goes to the emission optical system; The reference optical path system is used to record the light field modulated by the spatial modulation system for reconstructing the target image; The transmitting optical system is used to transmit the light field modulated by the spatial modulation system to the target surface with a small aberration; The receiving optical system is used to converge the signal light returned from the target surface and couple it into the receiving optical fiber; The post-processing system is used to mix the return optical signal with the local oscillation optical signal to output two orthogonal IQ low-frequency electrical signals; in this process, the noise signal is filtered out to a large extent, and the amplitude and phase information of the return optical signal are recorded; that is, the grayscale information and motion or vibration information of the target are recorded; The clock system is used to generate a synchronous control signal to synchronize the chirp modulated light source, the reference optical path system, the main control and acquisition system; The main control and acquisition system is used to control the clock system, store the light field information of the reference light path system, collect and store the IQ low-frequency electrical signals, and reconstruct the output target image and motion or vibration information; The chirp modulated light source system (1) emits a broadband modulated chirp light signal with a continuous narrow line width, which is divided into two parts by an optical fiber splitting system (2). One part is injected into a post-processing system (10) as a local oscillator light signal (LO), and the other part is injected into a free space and bombards a spatial modulation system (5) as a modulated light signal after sequentially passing through an optical fiber amplification system (3) and a collimation system (4), thereby forming a pseudo-thermal speckle field with time-space fluctuations. The speckle field is divided into two parts by a spatial light splitting system (6). One part is imaged and the light intensity distribution is recorded by a reference optical path system (7) and output to a main control and acquisition system (12), and the other part is imaged onto a target surface (③) by a transmitting optical system (8); a receiving optical system (9) receives an echo light signal (SIG) of the target surface (③) and injects it into the post-processing system (10); After optical bridging and mixing of the local oscillator optical signal (LO) and the echo optical signal (SIG) in the post-processing system (10), two orthogonal IQ low-frequency electrical signals are output and converted into digital signals through the main control and acquisition system (12).

2. The correlation imaging radar system of coherent detection according to claim 1, characterized in that: The chirp modulated light source system (1) is a chirp amplitude or chirp phase modulation system.

3. The correlation imaging radar system of coherent detection according to claim 1, characterized in that: The optical fiber splitting system (2) is composed of an optical fiber splitter and an optical fiber attenuator. The optical fiber attenuator is used to adjust the optical power output to the post-processing system to a suitable range.

4. The correlation imaging radar system of coherent detection according to claim 1, characterized in that: The optical fiber amplification system (3) is composed of a single-frequency optical fiber amplifier.

5. The correlation imaging radar system of coherent detection according to claim 1, characterized in that: The collimation system is composed of a fiber collimator, which is used for collimating and adjusting the size of the output collimated light spot.

6. The correlation imaging radar system of coherent detection according to claim 1, characterized in that: The spatial modulation system (5) is a rotating frosted glass (5-2) and a short focal field lens (5-1) driven by a motor.

7. The correlation imaging radar system of coherent detection according to claim 1, characterized in that: The spatial light splitting system (6) is a light splitter, a light splitting prism or a light splitting wedge.

8. The correlation imaging radar system of coherent detection according to claim 1, characterized in that: The reference optical path system (7) comprises a reference imaging mirror (7-1) and a reference CCD (7-2).

9. The correlation imaging radar system of coherent detection according to claim 1, characterized in that: The receiving optical system (9) comprises a condenser (9-1) and a receiving optical fiber (9-2), and the receiving optical fiber (9-2) is located on the Fourier plane of the condenser (9-1).

10. A method for realizing imaging using the correlation imaging radar system of coherent detection according to any one of claims 1 to 9, characterized in that: The method comprises the following steps: Step 1. The main control and acquisition system (12) obtains the light intensity distribution data recorded by the reference CCD (7-2) for M measurements. and two IQ low frequency electrical signal data {I i } and {Q i }, where i is the number of measurements, and i = 1, 2, ..., M; Step 2. The main control and acquisition system (12) converts the IQ two-way signal data {I i } and {Q i } Complex numbers are added to obtain the complex signal C i =I i +jQ i , for C i Perform short-time Fourier transform to obtain the target spectrum, including motion or vibration spectrum and distance spectrum; Step 3. Calculate the instantaneous motion or vibration velocity v of the target using the following formula: Among them, λ is the central wavelength of the light wave, f d is the corresponding instantaneous spectrum position; by adding the spectrum corresponding to the instantaneous spectrum position within a single measurement time, the detection signal intensity corresponding to the motion or vibration spectrum during a single measurement can be obtained. For the range spectrum interval, the target distance can be calculated based on the instantaneous motion or vibration spectrum position f at the corresponding moment. d Solved by the following formula: Where c is the speed of light, T is the duration of chirp modulation, B is the bandwidth of chirp modulation, and f R is the instantaneous spectrum position corresponding to the target and f R =f R+d -f d , f R+d f d The corresponding instantaneous distance spectrum peak position; the spectrum intensity radar section on the corresponding distance spectrum within a single measurement time obtains the corresponding detection signal strength Step 4. Reference measurement data Stretched into a column and having the following form Where N = P × Q; Step 5. Reconstruct the target image using the association reconstruction algorithm based on compressed sensing: For the target image corresponding to the motion or vibration spectrum and the distance spectrum, it is reconstructed by the following formula: Indicates the spatial image information corresponding to the object on the motion or vibration spectrum line d, For the target image corresponding to the distance spectrum, it is reconstructed by the following formula: Represents the spatial image information corresponding to the object on the distance spectrum line d, In this way, it is possible to respectively calculate the target space three-dimensional information from the motion or vibration spectrum and the target distance spectrum, and simultaneously extract the target's instantaneous motion or vibration dimension information.

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