An intensity-correlated micro-Doppler imaging device and imaging method

Abstract

An intensity-correlated micro-Doppler imaging device and method are disclosed. The device includes: a narrow-linewidth laser, an acousto-optic modulator, a waveform generator, a spatial light modulator, a beam splitter, a lens, an array detector, a target under test, a coupling receiver, a photodetector, and an image reconstruction computer. The acousto-optic modulator, waveform generator, spatial light modulator, array detector, and photodetector are synchronously triggered and operate simultaneously by a synchronization signal generator. Spectral analysis is performed on the current signal output from the photodetector to extract the intensity fluctuations in the micro-Doppler spectrum of the target under test. These intensity fluctuations are then correlated with the spatial distribution of the reference light of the array detector to obtain the cross-correlation intensity distribution information of the detection signal and the modulation signal, i.e., the micro-Doppler image generated by the target's micro-vibration.

Description

Technical Field

[0001] This invention belongs to the field of optical imaging, specifically an intensity-correlated micro-Doppler imaging device and imaging method. Background Technology

[0002] Micro-Doppler features generated by target micro-vibrations contain information such as the target's fine structure and motion details. These features can be used to infer the physical structure and local motion parameters of related target components, which is of great significance in applications such as target detection and identification, target feature extraction, building vibration monitoring, and life detection. Existing micro-Doppler radars can achieve long-range target detection and high-resolution micro-Doppler feature extraction and testing, classifying targets based on differences in their micro-Doppler frequencies. For example, the US Army's AMOR missile optical test system is used for long-range target detection and high-resolution micro-Doppler feature extraction and testing; Lie-Svendsen et al. used the DiMuRa bistatic radar system to detect micro-Doppler signals from helicopter main rotor and tail rotor, obtaining rotor speed and blade number; the 23rd Research Institute of the Second Academy of China Aerospace Science and Industry Corporation studied the micro-Doppler characteristics of warheads and decoys, utilizing the target's micro-Doppler characteristic values ​​(period, intensity changes, etc.) for target identification.

[0003] However, existing micro-Doppler radars are limited to the detection and extraction of micro-Doppler features of one-dimensional time signals, lacking the ability to perceive the spatial location information of micro-vibrating targets in a scene. This makes it difficult to meet the application requirements of local precision surveillance, accurate strikes, and high-accuracy identification of true and false targets. Therefore, there is an urgent need to develop new detection and imaging technologies that can simultaneously acquire target micro-vibration characteristics, target micro-vibration image information, and spatial location information. Summary of the Invention

[0004] To address the problems existing in target micro-Doppler detection, this invention provides an intensity-correlated micro-Doppler imaging device. The target detection optical path only requires a point detector to record the target echo signal. The micro-Doppler signal is extracted through the object optical path and then correlated with the spatial modulation signal to obtain the micro-Doppler image generated by the target's micro-vibration.

[0005] To solve the above problems, the technical solution of the present invention is as follows:

[0006] An intensity-correlated micro-Doppler imaging device is characterized by comprising: a narrow-linewidth laser, the laser beam of which is split into two paths: a probe beam and a local oscillator beam; the local oscillator beam is input to an acousto-optic modulator, the output of which is connected to the local oscillator input of a photodetector; a spatial light modulator is positioned in the direction of the probe beam to perform spatial random coding modulation of the light field; a beam splitter is positioned in the direction of the modulated beam to split the light field into reflected light (as a reference beam) and transmitted light (as the probe beam); a lens and an array detector are sequentially arranged along the direction of the reference beam; a target in the far field and a coupling receiver in the reflection direction of the target are sequentially arranged in the direction of the probe beam, the coupling receiver being connected to the signal input of the photodetector; a waveform generator is connected to the acousto-optic modulator; and the data output of the array detector and the output of the photodetector are respectively connected to an image reconstruction computer. The acousto-optic modulator, spatial light modulator, array detector, and photodetector are synchronously triggered and operate simultaneously by a synchronization signal generator.

[0007] The working process of the device of the present invention includes:

[0008] (1) The pulsed laser emitted by the narrow linewidth laser is split into two beams, which are used as the local oscillator input acousto-optic modulator and as the probe light irradiation spatial light modulator, respectively.

[0009] (2) The waveform generator produces a fixed frequency f if The signal is input to the acousto-optic modulator, which shifts the local oscillator frequency by f. if Then it is input to the local oscillator input port of the photodetector;

[0010] (3) The spatial light modulator randomly encodes and modulates the spatial distribution of the probe light irradiating its micromirror array surface. A beam splitter is set in the output direction of the modulated light to divide the optical path into a reference optical path in the beam splitter reflection direction and a probe optical path in the transmission direction.

[0011] (4) In the reference optical path, the spatial light modulator, lens and array detector constitute a 2f system, and the array detector records the far-field spatial distribution of the light field.

[0012] (5) In the detection optical path, the detection light is emitted to the target to be measured in the far field, and the reflected light from the target to be measured in the far field is received by the coupling receiving mirror; the coupling receiving mirror inputs the received light to the signal light input port of the photodetector; the photodetector performs coherent detection on the local oscillator light and the detection light, and outputs an intermediate frequency current signal containing target information;

[0013] (6) The system emits a series of pulsed lasers. The computer performs intensity correlation calculations on the signal recorded by the photodetector and the spatial distribution of the light field recorded by the array detector, and can obtain the micro-vibration image of the target, that is, the spatial distribution of the target's vibration frequency and amplitude.

[0014] The computer-based intensity correlation calculation method is as follows: perform spectral analysis on the intermediate frequency signal output corresponding to a certain pulse to obtain the micro-Doppler frequency distribution of the signal; integrate the energy spectrum of the spectral range exceeding the threshold to obtain the intensity fluctuation of the micro-moving target; perform correlation calculation on the intensity fluctuation and the spatial modulation signal of the corresponding pulse to obtain the composite intensity correlation distribution; and then statistically average the composite intensity correlation distributions of different pulses to obtain the cross-correlation intensity distribution information of the detection signal and the modulation signal, i.e., the micro-Doppler image generated by the micro-vibration of the target.

[0015] Compared with the prior art, the present invention has the following technical effects:

[0016] 1. The detection optical path only requires a single "point" detector to obtain the target vibration image, without the need for an area array detector or scanning structure.

[0017] 2. The imaging system is simple and low in cost. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the intensity-correlated micro-Doppler imaging device.

[0019] Wherein: 1 is a narrow linewidth laser, 2 is an acousto-optic modulator, 3 is a waveform generator, 4 is a spatial light modulator, 5 is a beam splitter, 6 is a lens, 7 is an area array detector, 8 is the target under test, 9 is a coupling receiver, 10 is a photodetector, and 11 is an image reconstruction computer. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0021] An intensity-correlated micro-Doppler imaging device is characterized by comprising: a narrow-linewidth laser 1, the laser beam of which is split into two paths: a probe beam and a local oscillator beam; the local oscillator beam is input to an acousto-optic modulator 2, the output of which is connected to the local oscillator input of a photodetector 10; a spatial light modulator 4 is configured to perform spatial random coding modulation of the light field in the direction of the probe beam, and a beam splitter 5 is configured in the direction of the modulated beam to divide the light field into a reference light path and a probe light path; the reflection direction of the beam splitter 5 is the reference light path, and a lens 6 and an array detector 7 are sequentially configured thereon; the transmission direction of the beam splitter 6 is the probe light path, and a target 8 located in the far field and a coupling receiver 9 located in the reflection direction of the target 8 are sequentially configured thereon, the coupling receiver 9 being connected to the signal input of the photodetector 10; a waveform generator 3 is connected to the acousto-optic modulator 2; and the data output of the array detector 2 and the output of the photodetector 10 are connected to an image reconstruction computer 11. The acousto-optic modulator 2, waveform generator 3, spatial light modulator 4, array detector 7, and photodetector 10 are all started and operated simultaneously by a synchronization signal generator.

[0022] The working process of the device of the present invention is as follows:

[0023] The acousto-optic modulator 2, waveform generator 3, spatial light modulator 4, array detector 7, and photodetector 10 are all started and operated simultaneously by a synchronization signal generator.

[0024] (1) The narrow linewidth laser 1 emits pulsed laser light, which is divided into two beams: one is used as the local oscillator light input to the acousto-optic modulator 2, and the other is used as the probe light to irradiate the spatial light modulator 4.

[0025] (2) Waveform generator 3 generates a signal f at a fixed frequency. if The input is fed into acousto-optic modulator 2, which shifts the local oscillator frequency by f. if Then, the light is input to the local oscillator input port of the photodetector 10;

[0026] (3) The spatial light modulator 4 randomly encodes and modulates the spatial distribution of the probe light irradiated by it. A beam splitter 5 is set in the output direction of the modulated light to divide the optical path into a reference optical path along the reflection direction of the beam splitter 5 and a probe optical path along the transmission direction.

[0027] (4) In the reference optical path, the spatial light modulator 4, lens 6, and array detector 7 constitute a 2f system, and the array detector 7 records the spatial distribution of the light field.

[0028] (5) In the detection optical path, the spatial light modulator 4 emits light that irradiates the target 8 located in the far field, and the coupled receiving mirror 9 receives the reflected light from the target in the far field; the coupled receiving mirror 9 inputs the received light into the signal light input port of the photodetector 10; the photodetector 10 performs coherent detection on the local oscillator light and the detection light, and outputs an intermediate frequency current signal containing target information.

[0029] (6) The device emits a series of pulsed lasers. The image reconstruction computer 11 performs intensity correlation image reconstruction calculation on the light field spatial distribution recorded by the output signal of the photodetector 10 and the spatial modulation signal 4, and can obtain the micro-Doppler image generated by the target micro-vibration.

[0030] The computer image reconstruction method described herein is as follows:

[0031] The spectrum I is obtained by performing spectrum analysis on the current signal output by photodetector 10. t (x t f), the intensity fluctuation I of the micro-moving target is obtained by integrating the intensity in the micro-Doppler frequency band. t (x t ), change I t (x t ) and array detector 7 record the spatial distribution of the light field I r (x rPerform the association operation according to the following formula:

[0032] ΔG (2,2) (x r ;x t )= r (x r )I t (x t )>- r (x r )> t (x t (2)

[0033] The cross-correlation intensity distribution information of the detection signal and the modulation signal is obtained, which is the micro-vibration image of the target.

[0034] The physical principle underlying intensity-correlated microDoppler imaging can be described by the following process:

[0035] exist Figure 1 In the apparatus shown, the transfer function from the light source surface to the target surface is denoted as exp[jkz]. tr H tr (x,x s The transfer function from the target surface to the probe surface is exp[jkz]. rc H rc (x t ,x), where z tr z rc x represents the axial distance from the light source to the target and from the target to the detection surface. s ,x,x t The coordinates of the entrance pupil plane of the spatial light modulator 4, the far-field target 8, and the coupling receiver 9 are respectively, H. tr (x,x s H rc (x t Let E(x) be the impulse response function of the transmitting and receiving optical paths. The optical field of the laser after time-varying modulation is denoted as E(x). s (x t According to the extended Huygens-Fresnel principle, the signal transmitted to the target is:

[0036] E o (x o ,t)=exp[jkz tr (t)]∫dx s E s [x s ,tz tr (t) / c]H tr (x,x s (3)

[0037] ​​​Let the spatial reflectance distribution function of a single quasi-plane target be o(x,y), and the target surface be optically rough, with first-order statistical properties satisfying:

[0038] <o(x)o * (x′)>=O(x)δ(xx′). (4)

[0039] Where O(x) is the intensity reflectance distribution function of the target to be measured.

[0040] The reflected light from the target propagates onto the coupling receiver 9, and the light field at the entrance pupil plane of the coupling receiver 9 is as follows:

[0041]

[0042] Consider the target's vibration motion mode along the axis: z(t) = z0 + Acos(2πf v t), where z0 is the target distance at the initial time, A, f v These are the vibration amplitude and frequency, respectively. The device satisfies the following assumptions: (1) the transmitted signal is spatiotemporally separable, i.e., E s (x s ,t)=s(t)E s (x s (2) The spatial light field distribution in equation (5) is relatively stable during the target's micro-vibration process, and the changes in phase terms above the second order can be ignored. Therefore, except for exp[j2kz(t)], z(t)≈z0 in equation (5); (3) The transmitting and receiving systems are very close to each other, therefore z tr ≈z tr = z(t). Based on the above conditions, equation (5) yields:

[0043]

[0044] in The local oscillator signal is generated by frequency shifting the laser output by f. if E is obtained. LO (x t ,t)=A LO exp{j[2π(ν-f if )t+φ LO Then, the coherent detection output current signal of the photodetector is:

[0045]

[0046] Expanding the above equation using the first-order K-th order Bessel function, we get:

[0047]

[0048] in:

[0049]

[0050] The Fourier transform of equation (8) yields:

[0051]

[0052] Equation (9) shows that the baseband photocurrent output by coherent detection contains the micro-vibration information of the target, i.e., the micro-Doppler frequency, and its spectral distribution is determined by the intermediate frequency f. if Centered at f v The spectral lines are composed of [a certain number of lines]. Let the range of the micro-Doppler spectrum distribution be denoted as Δf. md Integrating the intensity spectrum within this region yields the intensity fluctuation information, i.e.

[0053]

[0054] The spatial distribution of the reference optical field recorded by array detector 7 is as follows:

[0055] I n (x r )∝|∫dx s E s (x s )exp[j2πx r x s λf r ]| 2 (11)

[0056] Among them, f r The focal length is 6 for the lens.

[0057] Substituting equations (10) and (11) into (2), according to the principle of coherent detection-based correlation imaging (see: Deng C, Gong W, Han S. Pulse-compression ghost imaging lidar via coherent detection[J]. OptExpress, 2016, 24(23):25983-25994.), we can obtain:

[0058]

[0059] Among them, D s Let be the diameter of the light spot on the spatial light modulator. The result of equation (12) contains the spatial distribution information of the vibrating target, i.e., the angular resolution Δθ = λ / D. sThe real space image. According to equation (12), when the number of measurement pulses reaches the condition of ensemble averaging by using a "point" detector, the vibration image information of the target can be completely reconstructed. In order to ensure a certain imaging speed, or to minimize the imaging time for moving targets, the number of measurements should be minimized. Multiple coherent point detectors can be used to independently detect the speckle field. Finally, the intensity of the output results of each detector is superimposed, which can greatly improve the signal-to-noise ratio and imaging speed of the detector system.

Claims

1. An intensity-correlated microDoppler imaging device, characterized in that, The device includes: a narrow linewidth laser (1), the laser beam emitted from which is divided into two paths, one being a probe beam and the other a local oscillator beam; the local oscillator beam is input to an acousto-optic modulator (2), the output of which is connected to the local oscillator input of a photodetector (10); a spatial light modulator (4) is set in the direction of the probe beam to perform spatial random coding modulation on the light field; a beam splitter (5) is set in the direction of the modulated light emission to divide the light field into reflected light as a reference beam and transmitted light as a probe beam; a lens (6) and an array detector (7) are set sequentially along the direction of the reference beam; and a position sensor (6) is set sequentially along the direction of the probe beam. The target to be measured (8) in the far field and the coupling receiving mirror (9) located in the reflection direction of the target to be measured are connected to the signal input terminal of the photodetector (10); the waveform generator (3) is connected to the acousto-optic modulator (2); the data output terminal of the array detector (7) and the output terminal of the photodetector (10) are respectively connected to the input terminal of the image reconstruction computer (11); the acousto-optic modulator (2), the waveform generator (3), the spatial light modulator (4), the array detector (7), and the photodetector (10) are synchronously triggered and work simultaneously by a synchronous signal generator.

2. An intensity-correlated micro-Doppler imaging method using an intensity-correlated micro-Doppler imaging device as described in claim 1, characterized in that, The method includes the following steps: The spectrum of the current signal output by the photodetector (10) is obtained by spectrum analysis. ; The spectrum The intensity fluctuations of the target (8) are obtained by intensity integration in the micro-Doppler frequency band. ; Intensity fluctuations The far-field spatial distribution of the light field recorded by array detector 7 Perform the association operation according to the following formula: The cross-correlation intensity distribution information of the detection signal and the modulation signal is obtained, which is the micro-Doppler image generated by the micro-vibration of the target under test.

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