Stokes polarization detection pulse compression correlation imaging method and system

Abstract

The invention discloses a Stokes polarization detection pulse compression correlated imaging method, which comprises the following steps of: obtaining reference light field full-polarization information from a correlated imaging reference light field by utilizing polarization state difference between an environment background and a moving target echo light field, and calculating to obtain a Stokes vector reference light field; echo light field full polarization information is obtained from the moving target echo light field, and a Stokes vector echo light field is obtained through calculation; performing frequency mixing on the echo light and the local oscillation light to obtain a Stokes vector orthogonal difference frequency signal, and performing fast Fourier transform to obtain frequency spectrum information of the difference frequency signal so as to obtain distance and speed information of the moving target; second-order intensity correlation operation is carried out on the Stokes vector orthogonal difference frequency signal and a reference light field signal to obtain a Stokes vector image of a moving target, and target polarization degree and polarization angle are introduced to improve correlation image contrast and signal-to-noise ratio. According to the invention, the image contrast and the imaging quality of the moving target can be greatly improved.

Description

Technical Field

[0001] The present invention belongs to the field of optical remote sensing imaging. More specifically, it uses the pulse compression correlation imaging technology based on Stokes full polarization information to achieve ranging, velocity measurement, and imaging contrast enhancement for distant targets. Background Art

[0002] Pulse compression correlation imaging is an imaging technology for ranging and velocity measurement of distant targets. The core of this technology lies in two aspects: pulse compression and correlation imaging. On the one hand, the system emits a chirped amplitude-modulated long pulse train to the surface of a moving target. After the echo signal received by the bucket detector is mixed with the local oscillator optical signal, the distance and velocity information of the moving target are extracted, and the long pulse signal is compressed into a short pulse at the receiving end, thereby improving the ranging resolution and signal-to-noise ratio at a longer detection distance. On the other hand, the difference frequency signal after mixing is subjected to a second-order intensity correlation operation with the reference optical field signal, thereby calculating and reconstructing the characteristic image of the target. By combining pulse compression and correlation imaging, the ranging, velocity measurement, and imaging functions of fast-moving targets in a dynamic scene are realized.

[0003] However, traditional pulse compression correlation imaging detects the intensity information in the reflected light field of a moving target. In a complex outdoor environment, it is difficult to effectively distinguish the environmental background and target features only through the difference in reflectivity, thus limiting the target image contrast and application scenarios of pulse compression correlation imaging. Summary of the Invention

[0004] The present invention aims to overcome the above-mentioned deficiencies of the prior art and provides a Stokes polarization detection pulse compression correlation imaging method. It can distinguish different surface features of the background and the target through polarization information in a complex outdoor environment, effectively suppress environmental noise and interference signals, and greatly improve the image contrast and imaging quality of moving targets.

[0005] The present invention adopts the following technical solutions to achieve the invention purpose:

[0006] The characteristics of the Stokes polarization detection pulse compression correlation imaging method of the present invention are as follows: By utilizing the characteristic that the return light fields of the environmental background and moving targets have different polarization states, first obtain the full polarization information of the reference light field from the reference light field of correlation imaging; calculate the corresponding Stokes vector reference light field based on the full polarization information of the reference light field; then obtain the full polarization information of the return light field from the return light field of the moving target, and calculate the Stokes vector return light field based on the full polarization information of the return light field; finally, mix the return light with the local oscillator light to obtain the Stokes vector orthogonal difference frequency signal, perform a fast Fourier transform on the Stokes vector orthogonal difference frequency signal to obtain the spectral information of the difference frequency signal, and further obtain the distance and velocity information of the moving target; perform a second-order intensity correlation operation on the Stokes vector orthogonal difference frequency signal and the Stokes vector reference light field signal to obtain the Stokes vector image of the moving target, and introduce the calculation of the degree of polarization and polarization angle of the target to improve the contrast and signal-to-noise ratio of the correlation image.

[0007] The characteristics of the Stokes polarization detection pulse compression correlation imaging system of the present invention are as follows: The system is used to implement the Stokes polarization detection pulse compression correlation imaging method, and the system composition includes:

[0008] A chirped amplitude modulated infrared light source module, a first fiber optic beam splitter, a fiber optic amplifier, a fiber optic collimator, a spatial light modulator, a spatial light collimator, a spatial light beam splitter, a 2f system, a full polarization detection camera, a light emission module, a synchronous trigger module, a fiber optic coupled detection module, a second fiber optic beam splitter, an optical polarization modulation module, a Stokes detection module, a coherent detection module, a bandpass filter module, and a correlation operation module;

[0009] The chirped amplitude modulated infrared light source module is used to emit a chirped modulated laser long pulse train. The laser long pulse train is split into four local oscillator lights and one signal light by the first fiber optic beam splitter. The local oscillator lights are subsequently mixed with the Stokes vector echo beam to obtain the distance and velocity of the target; the signal light sequentially passes through a fiber optic amplifier, a fiber optic collimator, a spatial light modulator, and a spatial light collimator to generate a spatially fluctuating random pseudo-thermal speckle field E s (x s ,t);

[0010] The random pseudo-thermal speckle field is split into the same first optical path and second optical path by the spatial light beam splitter. The first optical path passes through a 2f system built by Fourier lenses and is imaged onto the full polarization detection camera to form the camera target surface light field E CCD, the reference light field with full polarization states of 0°, 90°, 45°, and right-handed circular is acquired by the full polarization detection camera, and the corresponding Stokes vector reference light field intensities S0, S1, S2, and S3 are obtained through calculation; the second optical path is emitted by the Galilean telescope type light emission module to the surface of the target to form the light field distribution E of the target surface o (x o , t);

[0011] The Newtonian reflector type fiber optic coupled detection module is used to receive the echo light field reflected by the target to form the echo light field distribution E r (x r , t), the echo light field is divided into six identical optical paths by the second fiber optic beam splitter, and the six optical paths are respectively adjusted by the optical polarization adjustment module for polarization adjustment operations of 0°, 45°, 90°, 135°, left-handed circular, and right-handed circular, and then four Stokes vector echo signals S0′, S1′, S2′, and S3′ are obtained through modulation by the Stokes detection module;

[0012] The echo signals S0′, S1′, S2′, and S3′ are mixed with the four local oscillator optical signals from the first fiber optic beam splitter in the coherent detection module to obtain the orthogonal Stokes vector difference frequency signals i I (x r , t) and i U (x r , t);

[0013] For a stationary target, the difference frequency signal is subjected to fast Fourier transform to obtain the spectral peak intensity and its corresponding frequency position; the frequency position represents the target distance information, and the spectral peak intensity represents the echo signal intensity;

[0014] For a moving target, the difference frequency signal is subjected to fast Fourier transform, the positive and negative Doppler frequency shifts carry the target velocity information, the spectral peak intensity represents the echo signal intensity, and the phase of the orthogonal difference frequency signal output by the coherent detection module represents the instantaneous distance and velocity information of the object;

[0015] The Stokes vector echo signal of the stationary target or the moving target is subjected to second-order intensity correlation operation with the Stokes vector reference light field intensity signal to reconstruct the target image G a ; By calculating the polarization degree p and polarization angle θ information of the target, different surface features of the background and the target are distinguished, environmental noise and interference signals are suppressed, the stability of the imaging system in a complex environment is improved, and the image contrast and imaging quality of the target are improved.

[0016] The characteristics of the Stokes polarization detection pulse compression correlation imaging system of the present invention also lie in that: the system calculates and obtains the image information, polarization degree and polarization angle information, as well as velocity and distance information of the target according to the following method:

[0017] The chirped amplitude modulation infrared light source module emits a chirped modulated light beam, and the chirp frequency f(t0) is obtained by calculation according to Equation (1):

[0018]

[0019] where: f0 is the center frequency; B is the chirp modulation bandwidth;

[0020] T0 is the chirp modulation period; t0 is the chirp signal modulation time, and t0 is between 0 and T0;

[0021] The chirp modulation signal s(t0) is obtained by calculation according to Equation (2):

[0022]

[0023] where: φ is the initial random phase;

[0024] The four local oscillator (LO) optical fields are obtained by calculation according to Equation (3):

[0025] E LO (x r ,t) = [1 + m·s(t - n·T)]P(t - n·T)A LO φ LO (3)

[0026] where: t represents the duration; A LO represents the amplitude of the local oscillator optical field; φ LO represents the phase of the local oscillator optical field;

[0027] n represents the nth long pulse; m is the chirp modulation depth;

[0028] x r is the spatial coordinate of the plane where the Newtonian reflector telescope type fiber optic coupling detection module is located;

[0029] T is the long pulse period; s(t - n·T) represents the chirp signal at the moment of t - n·T; P(t - n·T) represents the long pulse train;

[0030] And there is Equation (4):

[0031]

[0032] The random pseudo-thermal speckle field E s (x s ,t) is obtained by calculation according to Equation (5):

[0033] E s (x s ,t) = [1 + m·s(t - n·T)]·P(t - n·T)·Es (x s ) (5)

[0034] Where: x s represents the plane coordinates of the spatial light modulator;

[0035] E s (x s ) represents the amplitude and phase at different coordinates on the plane of the spatial light modulator;

[0036] The light field E on the camera target surface CCD is obtained by calculating with Equation (6):

[0037]

[0038] Where: represents the Fourier transform;

[0039] The full polarization detection camera respectively obtains the measured intensities I 0° ′, I 90° ′, I 45° ′, I R ′ of the 0°, 90°, 45°, and right-handed circular polarization states of the speckle field through its pixel array;

[0040] The theoretical intensities of the six full polarization states of 0°, 90°, 45°, 135°, left-handed circular, and right-handed circular of the speckle field are obtained by calculating with Equation (7-1), Equation (7-2), Equation (7-3), Equation (7-4), Equation (7-5), and Equation (7-6):

[0041]

[0042] Where:

[0043] I 0° , I 90° , I 45° and I 135° correspondingly represent the intensities of the 0°, 90°, 45°, and 135° linearly polarized components of the pseudo-thermal speckle field; I R and I L respectively represent the right-handed circular polarization intensity and left-handed circular polarization intensity of the pseudo-thermal speckle field;

[0044] t TM and t TE respectively represent the transmission coefficients of the linear polarization filters for TM light and TE light;

[0045] t R and t L respectively represent the transmission coefficients of the circular polarization filters for right-handed circular polarization and left-handed circular polarization light;

[0046] Accordingly, the Stokes vector reference optical field intensity signals S0, S1, S2, and S3 are calculated as follows:

[0047] S0 = I 0° + I 90° (8 - 1)

[0048] S1 = I 0° - I 90° (8 - 2)

[0049] S2 = I 45° - I 135° (8 - 3)

[0050] S3 = I R - I L (8 - 4)

[0051] The optical field distribution E o (x o , t) on the surface of the target is calculated by Equation (9):

[0052]

[0053] where: k is the wave number; d is the distance from the Galilean telescope emission system to the target;

[0054] c is the speed of light; x o is the spatial coordinate of the target plane;

[0055] The backscattered optical field distribution E r (x r , t) is calculated by Equation (10):

[0056]

[0057] where: R(x o ) is the target reflection complex amplitude;

[0058] Then, the backscattered light intensity Q(t) is calculated by Equation (11):

[0059]

[0060] where: ρ is the optical coupling efficiency of the Newtonian reflector type fiber optic coupling detection module;

[0061] Then, the four-channel Stokes vector backscattered light intensity signals S0′, S1′, S2′, and S3′ are respectively:

[0062] S0′ = Q 0· + Q 90· (12 - 1)

[0063] S1′ = Q 0·-Q 90· (12 - 2)

[0064] S2′ = Q 45° -Q 135° (12 - 3)

[0065] S3′ = Q R -Q L (12 - 4)

[0066] where: Q 0° 、Q 45° 、Q 90° 、Q 135° 、Q L 、Q R correspond to the light intensities of 0°, 45°, 90°, 135°, left - hand circular and right - hand circular polarization states respectively;

[0067] The orthogonal Stokes vector difference - frequency signals i I (x r , t) and i U (x r , t) are characterized by equations (13 - 1) and (13 - 2):

[0068]

[0069] where: the phase α is:

[0070] The spectral peak frequency f x and peak spectral intensity are obtained by Fourier transform of equations (13 - 1) and (13 - 2),

[0071] For a stationary target, according to the spectral peak frequency f x , the distance d from the Galileo telescope emission system to the target is calculated by equation (14):

[0072]

[0073] For a uniformly moving target, according to the frequencies of the three spectral lines in the spectrum being f x 、(f x - f d ) and (f x + f d ), the Doppler frequency is f d = [(f x + f d ) - (f x - f d )] / 2, and the moving speed v x is calculated by equation (15):

[0074]

[0075] Using the orthogonal Stokes vector difference frequency signal i I (x r , t) and i U (x r , t), the instantaneous distance and velocity information of the object are obtained by calculating through Equations (16) and (17):

[0076]

[0077] Wherein:

[0078] Indicates the complex current combination after band-pass filtering of i I (x r , t) and i U (x r , t);

[0079] φ i (x r , t) represents the phase of the complex current signal, and BPF[·] represents band-pass filtering;

[0080] The target image G a Is obtained by calculating through Equation (18):

[0081]

[0082] Wherein: a = 0, 1, 2, 3, and N represents the number of long pulses;

[0083] The polarization degree p and polarization angle θ are obtained by calculating through Equations (19) and (20):

[0084]

[0085] Wherein: arctan[·] represents the arctangent operation.

[0086] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0087] 1. The method of the present invention applies the Stokes polarization correlation imaging technology, utilizes the change in the polarization state of the reflected light field caused by the difference in the surface characteristics between the environmental background and the moving target, calculates the Stokes vector reference light field signal and the difference frequency signal through the full polarization detection camera and the coherent detection module, and reconstructs the Stokes vector image of the target through the second-order intensity correlation operation. In a complex outdoor environment, the surface characteristics of the background and the target are effectively distinguished through polarization information, environmental noise and interference signals are suppressed, and the image contrast and imaging quality of the target are significantly improved.

[0088] 2. The method of the present invention emits a chirped amplitude - modulated long laser pulse train, mixes the echo signal and the local oscillator signal to achieve pulse compression, improves the ranging and velocity measurement resolutions, and overcomes the contradiction between the distance and velocity resolutions and the single - pulse energy in the traditional narrow - pulse laser ranging technology.

[0089] 3. The method of the present invention breaks through the technical bottleneck of traditional pulse - compression correlation imaging that only utilizes the intensity information in the echo light field, overcomes the problem that it is difficult to distinguish the environmental background and target features only through the difference in reflectivity, and provides an effective technical approach for further improving the contrast of optical remote - sensing imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0090] Figure 1 It is a block diagram of the Stokes polarization - detection pulse - compression correlation imaging system of the present invention;

[0091] Figure 2 It is a schematic diagram of the composition of the metasurface pixels of the full - polarization detection camera in the Stokes polarization - detection pulse - compression correlation imaging system of the present invention.

[0092] Reference numerals in the figure: 1 chirped amplitude - modulated infrared light source module, 2 first fiber optic beam splitter, 3 fiber optic amplifier, 4 fiber optic collimator, 5 spatial light modulator, 6 spatial light collimator, 7 spatial light beam splitter, 8 is a 2f system, 9 full - polarization detection camera, 10 optical emission module, 11 synchronous trigger module, 12 fiber optic coupling detection module, 13 second fiber optic beam splitter, 14 optical polarization adjustment module, 15 Stokes detection module, 16 coherent detection module, 17 band - pass filter module, 18 correlation operation module, a is a 0° linear polarization filter, b is a 90° linear polarization filter, c is a 45° linear polarization filter, d is a Z - shaped right - hand circular polarization light filter. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0093] In the retroreflected light field, different characteristics of the environmental background and the surface of the moving target cause differences in the polarization state of the retroreflected light field. In this embodiment, the Stokes polarization detection pulse compression correlation imaging method is as follows: Utilizing the characteristic that the retroreflected light fields of the environmental background and the moving target have polarization state differences, first obtain the full polarization information of the reference light field from the reference light field of correlation imaging; calculate the corresponding Stokes vector reference light field based on the full polarization information of the reference light field; then obtain the full polarization information of the retroreflected light field from the retroreflected light field of the moving target, and calculate the Stokes vector retroreflected light field based on the full polarization information of the retroreflected light field; finally, mix the retroreflected light with the local oscillator light to obtain the Stokes vector quadrature difference frequency signal, perform a fast Fourier transform on the Stokes vector quadrature difference frequency signal to obtain the spectral information of the difference frequency signal, and further obtain the distance and velocity information of the moving target; perform a second-order intensity correlation operation on the Stokes vector quadrature difference frequency signal and the Stokes vector reference light field signal to obtain the Stokes vector image of the moving target, and introduce the calculation of the polarization degree and polarization angle of the target to improve the contrast and signal-to-noise ratio of the correlation image.

[0094] To implement the Stokes polarization detection pulse compression correlation imaging method in this embodiment, this embodiment provides Figure 1 the Stokes polarization detection pulse compression correlation imaging system shown in the figure. The system composition includes:

[0095] A chirped amplitude-modulated infrared light source module 1, a first fiber optic beam splitter 2, a fiber optic amplifier 3, a fiber optic collimator 4, a spatial light modulator 5, a spatial light collimator 6, a spatial light beam splitter 7, a 2f system 8, a full polarization detection camera 9, an optical emission module 10, a synchronous trigger module 11, a fiber optic coupled detection module 12, a second fiber optic beam splitter 13, an optical polarization adjustment module 14, a Stokes detection module 15, a coherent detection module 16, a band-pass filter module 17, and a correlation operation module 18;

[0096] The chirped amplitude-modulated infrared light source module 1 is used to emit a chirped-modulated laser long pulse train. The laser long pulse train is split into four local oscillator lights and one signal light by the first fiber optic beam splitter 2. The local oscillator lights are subsequently mixed with the Stokes vector echo beam to obtain the distance and velocity of the target; the signal light sequentially passes through the fiber optic amplifier 3, the fiber optic collimator 4, the spatial light modulator 5, and the spatial light collimator 6 to generate a spatially fluctuating random pseudo-thermal speckle field E s (x s , t);

[0097] The random pseudo-thermal speckle field is split into the same first optical path and second optical path by the spatial light beam splitter 7. The first optical path passes through the 2f system 8 built by Fourier lenses and is imaged onto the Stokes full polarization camera 9 to form the camera target surface light field E CCD, the reference light field with full polarization states of 0°, 90°, 45°, and right-handed circular is obtained by the Stokes full polarization camera 9, and the corresponding Stokes vector reference light field intensities S0, S1, S2, and S3 are obtained through calculation; the second optical path is emitted by the Galilean telescope type light emission module 10 to the surface of the target to form the light field distribution E of the target surface o (x o , t);

[0098] The Newtonian reflector type fiber optic coupled detection module 12 is used to receive the echo light field reflected by the target to form the echo light field distribution E r (x r , t), the echo light field is divided into six identical optical paths by the second fiber optic beam splitter 13, and the optical polarization modulation module 14 performs polarization modulation operations of 0°, 45°, 90°, 135°, left-handed circular, and right-handed circular on the six optical paths one by one, and then four Stokes vector echo signals S0′, S1′, S2′, and S3′ are obtained through modulation by the Stokes detection module 15;

[0099] The echo signals S0′, S1′, S2′, and S3′ are mixed with the four local oscillator light signals from the first fiber optic beam splitter 2 in the coherent detection module to obtain the orthogonal Stokes vector difference frequency signals i I (x r , t) and i U (x r , t);

[0100] For a stationary target, the difference frequency signal is subjected to a fast Fourier transform to obtain the spectral peak intensity and its corresponding frequency position; the frequency position characterizes the target distance information, and the spectral peak intensity characterizes the echo signal intensity;

[0101] For a moving target, the difference frequency signal is subjected to a fast Fourier transform, the positive and negative Doppler frequency shifts carry the target velocity information, the spectral peak intensity characterizes the echo signal intensity, and the phase of the orthogonal difference frequency signal output by the coherent detection module characterizes the instantaneous distance and velocity information of the object;

[0102] The Stokes vector echo signal of the stationary target or the moving target is subjected to a second-order intensity correlation operation with the Stokes vector reference light field intensity signal to reconstruct the target image G a ; by calculating the polarization degree p and polarization angle θ information of the target, different surface features of the background and the target are distinguished, environmental noise and interference signals are suppressed, the stability of the imaging system in a complex environment is improved, and the image contrast and imaging quality of the target are improved.

[0103] In a specific implementation, the system calculates and obtains the image information, polarization degree, polarization angle information, and velocity distance information of the target according to the following method:

[0104] The chirped amplitude modulation infrared light source module emits a chirped modulated light beam, and the chirp frequency f(t0) is obtained by calculation according to Equation (1):

[0105]

[0106] where: f0 is the center frequency; B is the chirp modulation bandwidth;

[0107] T0 is the chirp modulation period; t0 is the chirp signal modulation time, and t0 is between 0 and T0;

[0108] The chirp modulation signal s(t0) is obtained by calculation according to Equation (2):

[0109]

[0110] where: φ is the initial random phase;

[0111] The chirped amplitude modulation light beam is divided into five identical light beams by the first fiber optic splitter 2: Four of them, the local oscillator (LO) light fields, are obtained by calculation according to Equation (3):

[0112] E LO (x r ,t) = [1 + m·s(t - n·T)]P(t - n·T)A LO φ LO (3)

[0113] where: t represents the duration; A LO represents the local oscillator light field amplitude; φ LO represents the local oscillator light field phase; n represents the nth long pulse; m is the chirp modulation depth; x r is the spatial coordinate of the plane where the Newtonian reflector telescope type fiber optic coupling detection module is located; T is the long pulse period; s(t - n·T) represents the chirp signal at the moment of t - n·T; P(t - n·T) represents the long pulse train, and there is Equation (4):

[0114]

[0115] The other light beam is amplified by the fiber optic amplifier 3 to the power requirement suitable for remote detection, and then collimated into a free space light beam by the fiber optic collimator 4 and incident on the target surface of the spatial light modulator 5 to generate a pseudo-thermal speckle field E s (x s ,t), which can be obtained by calculation according to Equation (5):

[0116] E s (x s ,t) = [1 + m·s(t - n·T)]·P(t - n·T)·E s (x s ) (5)

[0117] Where: xs represents the plane coordinates of the spatial light modulator; E s (x s ) represents the amplitude and phase at different coordinates on the plane of the spatial light modulator;

[0118] The pseudo-thermal speckle field E s (x s , t) After being collimated by the spatial light collimator 6, it is divided into the same first optical path and second optical path by the spatial light beam splitter 7: Among them, the first optical path passes through the 2f system composed of the Fourier lens 8 and is imaged on the full-polarization detection camera 9. The light field E CCD on the camera target surface is obtained by calculating from Equation (6):

[0119]

[0120] Where: represents the Fourier transform;

[0121] The full-polarization detection camera 9 is composed of a all-dielectric metasurface and a CCD pixel array. The metasurface is composed of a pixel array. Each 2×2 pixel sub-array is composed as shown in Figure 2 shown, Figure 2 In the figure, regions a, b, and c are nanowire gratings oriented in three different directions. The directions of their electric field vectors are oriented at 90°, 0°, and 45° relative to the x-axis respectively, and can transmit linearly polarized light for transmission at 90°, 0°, and 45°. Region d has a simple Z-shaped planar pattern and can transmit right-handed circularly polarized light and block left-handed circularly polarized light. Therefore, each 2×2 pixel sub-array can obtain the measured intensities I 0° ′, I 90° ′, I 45° ′, I R ′ of the speckle field in (0°, 90°, 45°, right-handed circle). Using the measured intensities of the four polarization states, the theoretical intensities of the six full-polarization states of 0°, 90°, 45°, 135°, left-handed circle, and right-handed circle of the speckle field are calculated from Equations (7-1), (7-2), (7-3), (7-4), (7-5), and (7-6):

[0122]

[0123] Where:

[0124] I 0° 、I 90° 、I 45° and I 135° correspond one-to-one to represent the intensities of the 0°, 90°, 45°, and 135° linearly polarized components of the pseudo-thermal speckle field;

[0125] I Rand I L respectively represent the right - hand circular polarization intensity and the left - hand circular polarization intensity of the pseudo - thermal speckle field;

[0126] t TM and t TE respectively represent the transmission coefficients of the linear polarization filters for TM light and TE light;

[0127] t R and t L respectively represent the transmission coefficients of the circular polarization filters for right - hand circularly polarized light and left - hand circularly polarized light;

[0128] Accordingly, the Stokes vector reference optical field intensity signals S0, S1, S2, S3 are calculated as follows:

[0129] S0 = I 0° +I 90° (8 - 1)

[0130] S1 = I 0° -I 90° (8 - 2)

[0131] S2 = I 45° -I 135° (8 - 3)

[0132] S3 = I R -I L (8 - 4)

[0133] In addition, the second optical path is emitted to the target surface through the Galilean telescope emission system 10, and the optical field distribution E o (x o , t) on the target surface is calculated by Equation (9):

[0134]

[0135] where: k is the wave number; d is the distance from the Galilean telescope emission system to the target; c is the speed of light; x o is the spatial coordinate of the target plane;

[0136] The optical field E o (x o , t) on the target surface is reflected and then propagates freely to the Newtonian reflector - type fiber - optic coupling detection module 12. Since the emission module and the reception module are located at the same position, the back - reflected optical field distribution E r (x r , t) is calculated by Equation (10):

[0137]

[0138] where: R(x o ) is the target reflection complex amplitude;

[0139] Then, the intensity Q(t) of the backward scattered light is obtained by calculation from Equation (11):

[0140]

[0141] where ρ is the optical coupling efficiency of the Newtonian reflector type fiber optic coupling detection module;

[0142] The backward scattered light is divided into six paths in the second fiber optic beam splitter. The optical polarization adjustment module 14 modulates the six paths of light into polarization states of 0°, 45°, 90°, 135°, left-handed circular, and right-handed circular respectively, and then passes through the Stokes detection module 15 for modulation to obtain four Stokes vector backward scattered light intensity signals S0′, S1′, S2′, and S3′ respectively as follows:

[0143] S0′ = Q 0° +Q 90° (12 - 1)

[0144] S1′ = Q 0° -Q 90° (12 - 2)

[0145] S2′ = Q 45° -Q 135° (12 - 3)

[0146] S3′ = Q R -Q L (12 - 4)

[0147] where: Q 0° , Q 45° , Q 90° , Q 135° , Q L , Q R correspondingly represent the light intensities of polarization states of 0°, 45°, 90°, 135°, left-handed circular, and right-handed circular respectively;

[0148] The four modulated Stokes vector backward scattered light signals and the four local oscillator light signals are respectively input into the coherent detection module 16 for mixing, and the process of de-chirping yields orthogonal Stokes vector difference frequency signals i I (x r , t) and i U (x r , t) characterized by Equation (13 - 1) and Equation (13 - 2):

[0149]

[0150] where: The phase α is:

[0151] The spectral peak frequency f is obtained by Fourier transform from equations (13-1) and (13-2). x and the peak spectral intensity

[0152] When analyzing a stationary target, based on the spectral peak frequency f x , the distance d from the Galilean telescope emission system to the target is calculated by equation (14):

[0153]

[0154] When analyzing the motion state of the target, if the target is moving at a constant speed, a spectral peak and positive and negative Doppler frequency shifts can be observed in the spectrum of the difference frequency signal. This spectral peak contains the distance information of the target, and the positive and negative Doppler frequency shifts carry the motion information of the object. In this case, the frequencies of the three spectral lines in the spectrum are fx, (f x -f d ) and (f x +f d ), and the Doppler frequency can be obtained as f d = [(f x +f d ) - (f x -f d )] / 2, and the motion speed v x is calculated by equation (15):

[0155]

[0156] Using the orthogonal Stokes vector difference frequency signals i I (x r ,t) and i U (x r ,t), the instantaneous distance and speed information of the object are calculated through equations (16) and (17):

[0157]

[0158] Where:

[0159] represents the complex current combination after band-pass filtering of i I (x r ,t) and i U (x r ,t);

[0160] φ i (x r ,t) represents the phase of the complex current signal, and BPF[·] represents band-pass filtering;

[0161] Subsequently, the four-channel Stokes vector echo signals S0′, S1′, S2′, S3′ and the Stokes vector reference optical field intensities S0, S1, S2, S3 synchronously collected by the synchronization trigger module 11 are correlated and reconstructed by the correlation operation module 18 to obtain the Stokes vector image G of the target a , which is calculated by Equation (18):

[0162]

[0163] where: a = 0, 1, 2, 3, N represents the number of long pulses; <·> represents taking the statistical average;

[0164] According to the Stokes vector image G of the target reconstructed by correlation a , the degree of polarization p and the polarization angle θ are calculated by Equation (19) and Equation (20):

[0165]

[0166] where: arctan[·] represents the arctangent operation.

[0167] Under harsh environmental conditions, by calculating the degree of polarization and the polarization angle, the clarity and contrast of the image can be significantly improved, and the interference of background noise and light scattering can be effectively reduced. The degree of polarization reflects the polarization intensity of light, which helps to extract the global polarization characteristics of the target, so as to accurately distinguish the target in a complex background. The polarization angle provides the polarization direction information of light, reveals the reflection characteristics of the object surface, and helps to distinguish the characteristics of different materials and shapes of objects. In extreme environments such as strong light interference, severe light scattering, and haze, the introduction of polarization information not only significantly enhances the image recognition ability, improves the image contrast, but also improves the accuracy of target detection.

[0168] The present invention combines the Stokes polarization detection technology and pulse compression correlation operation, making up for the deficiency of the traditional pulse compression correlation imaging method that only relies on the reflectivity difference for correlation operation in a complex outdoor environment, overcoming the defects of the traditional method such as low signal-to-noise ratio of the reconstructed image and susceptibility to interference signals due to insufficient reflectivity contrast, providing a high-contrast imaging solution suitable for complex scenes, and providing important technical support for target detection, motion tracking, and high-contrast imaging in complex outdoor scenes.

Claims

1. A Stokes polarization detection pulse compression correlation imaging method, characterized in that: Taking advantage of the characteristic that the echo light field of the environmental background and the moving target has a difference in polarization state, firstly, the full polarization information of the reference light field is obtained from the reference light field of the associated imaging; the corresponding Stokes vector reference light field is calculated based on the full polarization information of the reference light field; then the full polarization information of the echo light field is obtained from the echo light field of the moving target, and the Stokes vector echo light field is calculated based on the full polarization information of the echo light field; finally, the echo light is mixed with the local oscillator light to obtain the Stokes vector orthogonal difference frequency signal, and the Stokes vector orthogonal difference frequency signal is subjected to fast Fourier transform to obtain the spectrum information of the difference frequency signal, and then the distance and speed information of the moving target are obtained; the Stokes vector orthogonal difference frequency signal is subjected to a second-order intensity correlation operation with the Stokes vector reference light field signal to obtain the Stokes vector image of the moving target, and the calculation of the polarization degree and polarization angle of the target is introduced to improve the contrast and signal-to-noise ratio of the associated image.

2. A Stokes polarization detection pulse compression correlation imaging system, characterized in that: The system is used to implement the Stokes polarization detection pulse compression correlation imaging method according to claim 1, and the system composition includes: Chirp amplitude modulated infrared light source module (1), a first optical fiber beam splitter (2), an optical fiber amplifier (3), an optical fiber collimator (4), a spatial light modulator (5), a spatial light collimator (6), a spatial light beam splitter (7), a 2f system (8), a full polarization detection camera (9), an optical emission module (10), a synchronous trigger module (11), an optical fiber coupling detection module (12), a second optical fiber beam splitter (13), an optical deflection adjustment module (14), a Stokes detection module (15), a coherent detection module (16), a bandpass filter module (17) and a correlation operation module (18); The chirp amplitude modulated infrared light source module (1) is used to emit a chirp modulated long laser pulse train, the long laser pulse train is split into four local oscillator lights and one signal light after passing through a first optical fiber beam splitter (2), the local oscillator lights are subsequently mixed with a Stokes vector echo beam to obtain the distance and speed of the target; the signal light is sequentially passed through an optical fiber amplifier (3), an optical fiber collimator (4), a spatial light modulator (5) and a spatial light collimator (6) to generate a random pseudothermal speckle field E with spatial fluctuations. s (x s ,t); The random pseudo-thermal speckle field is split into the same first light path and second light path by a spatial light beam splitter (7), and the first light path is imaged onto a full polarization detection camera (9) through a 2f system (8) constructed by a Fourier lens to form a camera target surface light field E CCD The full polarization detection camera (9) acquires a reference light field having full polarization states of 0°, 90°, 45° and right-handed circular polarization, and obtains corresponding Stokes vector reference light field intensities S0, S1, S2, and S3 by calculation; the second light path is emitted to the surface of the target by the Galilean telescope light emitting module (10), forming a light field distribution E on the surface of the target. o (x o ,t); The Newtonian reflecting telescope type optical fiber coupling detection module (12) is used to receive the echo light field reflected by the target object to form an echo light field distribution E r (x r ,t), the echo light field is divided into six identical optical paths by a second optical fiber beam splitter (13), the optical deflection adjustment module (14) performs deflection adjustment operations of 0°, 45°, 90°, 135°, left-handed circular and right-handed circular on the six optical paths one by one, and then modulated by a Stokes detection module (15) to obtain four Stokes vector echo signals S0′, S1′, S2′, S3′; The echo signals S0′, S1′, S2′, S3′ are mixed with four local oscillator optical signals from the first optical fiber beam splitter (2) in a coherent detection module to obtain orthogonal Stokes vector difference frequency signals i I (x r ,t) and i U (x r ,t); For stationary targets, the difference frequency signal is subjected to fast Fourier transform to obtain the spectrum peak intensity and its corresponding frequency position; the frequency position represents the target distance information, and the spectrum peak intensity represents the echo signal strength; For moving targets, the difference frequency signal is subjected to fast Fourier transform, the positive and negative Doppler frequency shifts carry the target speed information, the spectrum peak intensity represents the echo signal strength, and the phase of the orthogonal difference frequency signal output by the coherent detection module represents the instantaneous distance and speed information of the object; The Stokes vector echo signal of the stationary target or the moving target is subjected to a second-order intensity correlation operation with the Stokes vector reference light field intensity signal to reconstruct the target image G a ; By calculating the target's polarization degree p and polarization angle θ information, the different surface features of the background and the target can be distinguished, environmental noise and interference signals can be suppressed, the stability of the imaging system in complex environments can be improved, and the image contrast and imaging quality of the target can be improved.

3. The Stokes polarization detection pulse compression correlation imaging system according to claim 2, characterized in that: The system calculates and obtains the image information, polarization degree and polarization angle information, as well as the speed and distance information of the target in the following manner: The chirped amplitude modulated infrared light source module emits a chirped modulated light beam, and the chirp frequency f(t0) is calculated by formula (1): Where: f0 is the center frequency; B is the chirp modulation bandwidth; T0 is the chirp modulation period; t0 is the chirp signal modulation time, t0 is between 0 and T0; The chirp modulation signal s(t0) is calculated by equation (2): Where: φ is the initial random phase; The four-way local oscillator (LO) light field is calculated by formula (3): E LO (x r ,t)=[1+m·s(t-n·T)]P(t-n·T)A LO φ LO (3) Where: t represents the duration; A LO Indicates the amplitude of the local oscillator light field; φ LO represents the phase of the local oscillator light field; n represents the nth long pulse; m is the chirp modulation depth; x r is the spatial coordinate of the plane where the Newtonian reflecting telescope type optical fiber coupled detection module is located; T is the long pulse period; s(tn·T) represents the chirp signal at time tn·T; P(tn·T) represents the long pulse train; And there is formula (4): The random pseudo thermal speckle field E s (x s ,t) is calculated by formula (5): E s (x s ,t)=[1+m·s(t-n·T)]·P(t-n·T)·E s (x s ) (5) Where: x s represents the plane coordinates of the spatial light modulator; E s (x s ) represents the amplitude and phase at different coordinates of the spatial light modulator plane; The camera target light field E CCD Calculated by formula (6): in: represents Fourier transform; The full polarization detection camera obtains the measured intensity I of the four polarization states of 0°, 90°, 45° and right-hand circular of the speckle field through its pixel array. 0° ′、I 90° ′、I 45° ′、I R ′; The theoretical intensities of the six full polarization states of the speckle field, 0°, 90°, 45°, 135°, left-handed circular and right-handed circular, are calculated by equations (7-1), (7-2), (7-3), (7-4), (7-5) and (7-6): in: I 0° ,I 90° ,I 45° and I 135° One-to-one correspondence represents the intensity of the 0°, 90°, 45°, and 135° linear polarization components of the pseudothermal speckle field; I R and I L represent the right-hand circular polarization intensity and left-hand circular polarization intensity of the pseudothermal speckle field, respectively; t TM and t TE represent the transmission coefficients of the linear polarization filters for TM light and TE light, respectively; t R and t L represent the transmission coefficient of the circular polarization filter for right-handed circularly polarized and left-handed circularly polarized light, respectively; Accordingly, the Stokes vector reference light field intensity signals S0, S1, S2, and S3 are calculated as follows: S0=I 0° +I 90° (8-1) S1=I 0° -IN 90° (8-2) S2=I 45° -I 135° (8-3) S3=I R -I L (8-4) The light field distribution E on the surface of the target o (x o ,t) is calculated by formula (9): Where: k is the wave number; d is the distance from the Galileo telescope launch system to the target; c is the speed of light; x o is the spatial coordinate of the target plane; The echo light field distribution E r (x r ,t) is calculated by formula (10): Where: R(x o ) is the target reflection complex amplitude; Then, the echo light intensity Q(t) is calculated by formula (11): Where: ρ is the optical coupling efficiency of the Newtonian reflecting telescope type fiber-coupled detection module; Then, the four-way Stokes vector echo intensity signals S0′, S1′, S2′ and S3′ are respectively: S0′=Q 0° +Q 90° (12-1) S1′=Q 0° -Q 90° (12-2) S2′=Q 45° -Q 135° (12-3) S3′=Q R -Q L (12-4) Where: Q 0° , Q 45° , Q 90° , Q 135° , Q L , Q R One-to-one correspondence is expressed as 0°, 45°, 90°, 135°, left-hand circular and right-hand circular polarization state intensity; The orthogonal Stokes vector difference frequency signal i I (x r ,t) and i U (x r ,t) is represented by formula (13-1) and formula (13-2): Where: Phase α is: The spectrum peak frequency f is obtained by Fourier transform from equations (13-1) and (13-2): x and peak intensity, For a stationary target, according to the spectrum peak frequency f x , the distance d from the Galileo telescope launch system to the target is calculated by formula (14): For a target moving at a uniform speed, the frequencies of the three spectral lines in the spectrum are f x 、(f x -f d ) and (f x +f d ), we can get the Püller frequency as f d =[(f x +f d )-(f x -f d )] / 2, the motion speed v is calculated by formula (15) x : Using the orthogonal Stokes vector difference frequency signal i I (x r ,t) and i U (x r ,t), the instantaneous distance and speed information of the object are calculated by equations (16) and (17): in: Indicates that i I (x r ,t) and i U (x r ,t) complex current combination after bandpass filtering; φ i (x r ,t) represents the phase of complex current signal, BPF[·] represents bandpass filtering; The target image G a Calculated by formula (18): Where: a=0,1,2,3, N represents the number of long pulses; The polarization degree p and polarization angle θ are calculated by equation (19) and equation (20): Where: arctan[·] represents the inverse tangent operation.

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