Doppler tomography laser radar imaging device based on carrier phase noise compensation and optimization method

Abstract

The invention relates to the technical field of coherent laser radar detection and imaging, in particular to a Doppler tomography laser radar imaging device based on carrier phase noise compensation and an optimization method. Phase noise is accurately estimated through a double-reference channel structure, an image quality evaluation index is adopted as a parameter optimization basis and a compensation effect judgment standard, target echo phase noise is accurately compensated, and therefore high-resolution Doppler tomography of a high-speed spinning target at the rotating speed of 30 Hz is achieved under the condition that the optical path difference exceeds the distance of 80 times of the laser coherence length. The method is designed for the broadband characteristic of the echo signal of the Doppler tomography laser radar, and has the characteristics of high compensation precision, wide distance dynamic range, low manufacturing cost and the like.

Description

Technical Field

[0001] This invention relates to the field of coherent lidar detection and imaging technology, and in particular to a Doppler tomography lidar imaging device and optimization method based on carrier phase noise compensation. Background Technology

[0002] Doppler tomography lidar is a single-frequency continuous-wave lidar based on coherent laser detection. It acquires the spatial spectrum of high-speed spinning targets (such as UAV rotors and wind turbine blades) by incoherently superimposing Doppler projection information from multiple observation angles. Then, it uses a filtered back-projection algorithm to invert the target's scattering characteristics, thereby reconstructing the target's two-dimensional velocity distribution or spatial structure image. Doppler tomography lidar has a simple structure and low hardware requirements. It does not require complex modulation modules to generate high-repetition-rate, high-linearity, and high-bandwidth transmitted pulses, yet still achieves a high received signal-to-noise ratio. This technology enables rapid, high-resolution imaging of distant, high-speed rotating targets in resource-constrained target perception scenarios, thus possessing significant application value in non-cooperative target identification and spatial situational awareness.

[0003] However, in long-range detection scenarios, the performance of Doppler tomography lidar is severely limited by laser carrier phase noise. According to the principle of coherent detection, its effective detection range is limited to the coherent length determined by the laser phase noise. As the target distance increases, the optical path difference between the echo signal and the local oscillator increases, and the accumulated laser phase noise leads to broadening of the beat frequency signal spectrum and a decrease in the signal-to-noise ratio, resulting in blurred reconstructed images and loss of detail. This limits the spatial resolution and effective range of the Doppler lidar, making effective target identification impossible and directly impacting the system's moving target detection performance. Furthermore, current laser technology struggles to achieve a low-cost balance between narrow linewidth and high power; the high-power laser signal required for long-range detection also introduces additional noise, affecting radar performance.

[0004] Currently, existing laser carrier phase noise compensation technologies can be divided into two directions: hardware and algorithms. Hardware compensation technologies can reduce the phase noise of the laser output signal at the source, such as external cavity feedback technology, linewidth narrowing technology, and phase-locked loops (PLLs). However, the system structure is complex and the overall cost is high, which is not conducive to the miniaturization and low-cost requirements of equipment. In addition, using time-delayed fiber to physically compensate for optical path difference can improve the temporal coherence of the signal, but it is not suitable for long-distance moving non-cooperative targets and cannot flexibly match a large distance dynamic range.

[0005] On the other hand, digital domain phase noise compensation algorithms can reduce hardware requirements through increased computing power, offering high flexibility and controllable costs. Currently, the implementation and application of phase noise compensation algorithms are mostly concentrated in fields such as frequency-modulated continuous wave lidar (e.g., inverse synthetic aperture lidar) or fiber optic sensing (e.g., optical frequency domain reflectometers). These include algorithms such as digital phase delay and generalized inverse of the observation matrix. These techniques typically use a single auxiliary interferometer to acquire phase noise and then compensate it through digital signal processing, thereby reducing the impact of laser carrier phase noise on detection performance, improving the effective detection range of the system, and ensuring that the theoretical resolution determined by the frequency modulation bandwidth is achieved.

[0006] However, besides facing their own challenges such as model complexity and phase discontinuity, the aforementioned methods also share a common drawback: the algorithm optimization process and performance evaluation heavily rely on the sharpness of the signal spectrum. Since the echo signal in a Doppler tomography system contains Doppler modulation information, it exhibits broadband characteristics, making traditional signal layer evaluation metrics (such as sharpness functions) difficult to apply. Furthermore, phase noise compensation devices based on a single auxiliary interferometer perform well in scenarios where the transmitted signal optical path difference is an integer multiple of the auxiliary interferometer's delay difference. However, in scenarios where the path difference is not an integer multiple, residual phase error estimation becomes difficult, limiting the dynamic range of distance and making them unsuitable for imaging moving, non-cooperative targets in real-world scenarios.

[0007] In summary, existing phase noise compensation techniques based on frequency-modulated continuous wave (FM-CW) systems have limitations in application scenarios and lack a high-precision phase noise compensation scheme aimed at achieving the final image quality under single-frequency CW systems. Therefore, there is an urgent need to research a carrier phase noise compensation method suitable for high-speed spin target Doppler imaging systems. Summary of the Invention

[0008] The purpose of this invention is to provide a Doppler tomography lidar imaging device and optimization method based on carrier phase noise compensation.

[0009] The objective of this invention can be achieved through the following technical solutions: A Doppler tomography lidar imaging device based on carrier phase noise compensation includes a laser, a first optical beamsplitter, an acousto-optic frequency shifter, a second optical beamsplitter, a second collimating lens, a third optical beamsplitter, a first optical coupler, a second optical coupler, and a signal acquisition and processing unit. The input end of the first optical beamsplitter is connected to the output end of the laser, the first output end is connected to the input end of the third optical beamsplitter, the second output end is connected to the input end of the acousto-optic frequency shifter, the input end of the second optical beamsplitter is connected to the output end of the acousto-optic frequency shifter, the first output end illuminates the target through the first collimating lens, the second output end is connected to the signal path input end of the second optical coupler through a first delay fiber, the first output end of the third optical beamsplitter is connected to the local oscillator input end of the first optical coupler, the second output end is connected to the local oscillator input end of the second optical coupler, the received signal reflected by the target is input to the signal path input end of the first optical coupler through the second collimating lens, the output end of the first optical coupler is connected to the signal acquisition and processing unit through a first balanced detector, and the output end of the second optical coupler is connected to the signal acquisition and processing unit through a second balanced detector. The imaging device further includes a second delay fiber, a third optical coupler, and a third balanced detector. Both the second and third optical beam splitters are three-output beam splitters. The third output of the second optical beam splitter is connected to the signal input of the third optical coupler, and the third output of the third optical beam splitter is connected to the local oscillator input of the third optical coupler. The optical path lengths of the first and second delay fibers are not equal.

[0010] The optical path length of the first delay fiber is 3 to 10 times that of the second delay fiber.

[0011] Preferably, the optical path length of the first delay fiber is 5 times that of the second delay fiber.

[0012] The signal acquisition and processing unit is configured to perform the following steps: Differential phase noise terms are extracted from the heterodyne reference signals output by the second and third balanced detectors, and further reconstructed as noise estimates that match the phase noise in the target mixed electrical signal output by the first balanced detector. The noise estimate is used to cancel the target mixed electrical signal to obtain the compensated target Doppler signal; Image reconstruction based on the compensated target Doppler signal.

[0013] Differential phase noise term extracted based on the heterodyne reference signal output by the second balanced detector. for: in: For laser carrier phase noise, for Delay in time The subsequent phase, A fixed time delay is introduced for the first delay fiber; Differential phase noise term extracted based on the heterodyne reference signal output by the third balanced detector. for: in: for Delay in time The subsequent phase, A fixed time delay is introduced for the second delay fiber.

[0014] The noise estimate is obtained as follows: based on the constructed digital delay adaptive filter, and using the differential phase noise term for digital delay, an estimate matching the phase noise in the target mixing electrical signal is reconstructed as the noise estimate. in: This is a noise estimate. For the first adaptive filter, This is the second adaptive filter.

[0015] First adaptive filter for: Second adaptive filter for: in: The delay of the received signal reflected by the target. For the closest integers, unit impulse signal After fixed delay The delayed version For the closest integers, For the target distance, At the speed of light, β for and The ratio of .

[0016] The compensated target Doppler signal is: in: For the compensated target Doppler signal, The target is the mixing electrical signal.

[0017] The image reconstructed based on the compensated target Doppler signal includes: Based on the compensated target Doppler signal, a short-time Fourier transform is performed on the one-dimensional time-domain signal to obtain its intensity image. Transform the intensity image from the time-frequency domain to the sinusoidal domain; Image reconstruction is performed using a filtered backprojection algorithm based on the intensity image transformed to the sinusoidal domain.

[0018] A method for optimizing the parameters of an imaging device as described above, comprising: Step A1: Initialize the optical path length of the first and second delay fibers; Step A2: Obtain the reconstructed image output by the signal acquisition and processing unit; Step A3: Calculate the image quality evaluation index: in: As a quality evaluation metric, P represents the image height, and Q represents the image width. This represents the grayscale value of the pixel with x+1 as the x-coordinate and y as the y-coordinate. This represents the grayscale value of the pixel with x-coordinate and y-coordinate. The grayscale value of the pixel with x-coordinate and y+1; Step A4: Change the optical path length of the first delay fiber and the second delay fiber, and repeat step A3 multiple times. The optical path length of the first delay fiber and the second delay fiber corresponding to the image with the best quality evaluation index is taken as the optimization result.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. High compensation accuracy and wide dynamic range: By setting two reference channels with different delay fiber lengths, differential phase noise terms are extracted and estimated, and the integer multiples of the delay of the long fiber in the target channel are used for coarse compensation and the short fiber is used for fine compensation.

[0020] 2. Highly targeted and innovative evaluation mechanism: Specifically designed for single-frequency continuous wave Doppler tomography lidar, especially suitable for the broadband characteristics of Doppler echo signals. It abandons the traditional signal spectrum evaluation method and innovatively introduces image quality evaluation indicators into the closed-loop optimization process of phase noise compensation as the basis for parameter optimization and the evaluation standard for compensation effect. This ensures that the compensation effect directly serves the final imaging quality improvement and solves the problem of imaging distance limitation caused by its unique phase noise.

[0021] 3. Significant performance improvement: The proposed method can effectively compensate for laser carrier phase noise and reduce the laser linewidth parameter requirements. The system can still achieve high-quality target imaging even under extreme conditions where the target optical path difference exceeds 80 times the laser coherence length, which greatly extends the effective working distance and scene adaptability of the system.

[0022] 4. High practicality: The device structure can be realized by adding dual reference channels to the basic principle prototype. It requires little modification, is easy to implement, and has good prospects for engineering applications. Attached Figure Description

[0023] Figure 1 This is a structural block diagram of the present invention; Figure 2 A flowchart illustrating the method implemented according to the present invention; Figure 3 This is a schematic diagram illustrating the key principles of the innovation. Figure 4 A physical image of a spin target according to a preferred embodiment of the present invention; Figure 5 These are imaging results of a spin dot target under different phase noise compensation methods according to a preferred embodiment of the present invention. Figure 6 This is a schematic diagram illustrating the optimization of the spectrum and image parameters of the target channel echo signal according to a preferred embodiment of the present invention. Figure 7 The images show the imaging results of a spin-extended target under different phase noise compensation methods according to a preferred embodiment of the present invention.

[0024] The components include: 1. Laser modulation and emission unit; 2. Laser; 3. First optical beam splitter; 4. Acousto-optic frequency shifter; 5. Second optical beam splitter; 6. First collimating lens; 7. Second collimating lens; 8. Target detection unit; 9. Third optical beam splitter; 10. First optical coupler; 11. First balanced detector; 12. Reference channel unit; 13. First delay fiber; 14. Second delay fiber; 15. Second optical coupler; 16. Third optical coupler; 17. Second balanced detector; 18. Third balanced detector; and 19. Signal acquisition and processing unit. Detailed Implementation

[0025] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0026] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0027] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "proximal," "distal," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Physical quantities in formulas, unless otherwise specified, should be understood as basic quantities in the International System of Units (SI), or derived quantities derived from basic quantities through mathematical operations such as multiplication, division, differentiation, or integration.

[0028] Furthermore, terms such as "horizontal," "vertical," and "sag" do not imply that components must be absolutely horizontal or suspended, but rather that they can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0029] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0030] A Doppler tomography lidar imaging device based on carrier phase noise compensation, such as Figure 1As shown, the system includes a laser 2, a first optical beam splitter 3, an acousto-optic frequency shifter 4, a second optical beam splitter 5, a third optical beam splitter 9, a first optical coupler 10, a second optical coupler 15, and a signal acquisition and processing unit 19. The input of the first optical beam splitter 3 is connected to the output of the laser 2, its first output is connected to the input of the third optical beam splitter 9, its second output is connected to the input of the acousto-optic frequency shifter 4, and the input of the second optical beam splitter 5 is connected to the output of the acousto-optic frequency shifter 4. The first output illuminates the target through a first collimating lens 6, and the second output is connected through a first delay fiber. 13 is connected to the signal input terminal of the second optical coupler 15. The first output terminal of the third optical beam splitter 9 is connected to the local oscillator input terminal of the first optical coupler 10. The second output terminal is connected to the local oscillator input terminal of the second optical coupler 15. The received signal reflected by the target is input to the signal input terminal of the first optical coupler 10 through the second collimating lens 7. The output terminal of the first optical coupler 10 is connected to the signal acquisition and processing unit 19 through the first balanced detector 11. The output terminal of the second optical coupler 15 is connected to the signal acquisition and processing unit 19 through the second balanced detector 17. The imaging device also includes a second delay fiber 14, a third optical coupler 16, and a third balanced detector 18. The second optical beam splitter 5 and the third optical beam splitter 9 are both three-output beam splitters. The third output end of the second optical beam splitter 5 is connected to the signal input end of the third optical coupler 16, and the third output end of the third optical beam splitter 9 is connected to the local oscillator input end of the third optical coupler 16. The optical paths of the first delay fiber 13 and the second delay fiber 14 are not equal.

[0031] The basic implementation process of this application is as follows: Figure 2 As shown, the laser modulation and emission unit 1 is used to generate a single-frequency continuous wave laser signal and output it to the target detection unit 8 and the reference channel unit 12. In the laser modulation and emission unit 1, the laser 2 generates a carrier laser, which is then split into two beams by the first optical beam splitter 3. One beam is input to the acousto-optic frequency shifter 4, and the other beam is input to the input end of the third optical beam splitter 9. Generally, for example, in this embodiment, the two beams split by the first optical beam splitter 3 are equally divided.

[0032] The main function of the acousto-optic frequency shifter 4 is to modulate the modulation optical signal into the carrier laser, and then split it into three beams by the second optical beam splitter 5. One of the beams is irradiated onto the target through the first collimating lens 6, and the other two are respectively input to the signal input terminals of the second optical coupler 15 and the third optical coupler 16 through delay optical fibers with different optical paths. The three beams split by the third optical beam splitter are respectively input to the local oscillator input terminals of the three optical couplers. For the first optical coupler 10, its signal input terminal comes from the beam reflected by the target.

[0033] Generally, the third optical beam splitter 9 and the second optical beam splitter 5 also split the light equally to ensure that the three outputs are the same.

[0034] Generally, the optical path length of the first delay fiber 13 is 3 to 10 times that of the second delay fiber 14. The optical path length of the first delay fiber 13 should be set to be equal to the coherence length of the laser. Specifically, the optical path length is the product of the physical length of the fiber and its refractive index.

[0035] In this embodiment, the optical path of the first delay fiber 13 is 5 times that of the second delay fiber 14.

[0036] like Figure 3 As shown, in existing technologies, a single-auxiliary interferometer scheme is typically used, which is a single reference channel. In this case, if the optical path of the delay fiber is too long, such as... Figure 3 As shown in S5-2, there is a relatively large mismatch region. Although reducing the optical path length of the delay fiber, i.e., as... Figure 3 As shown in S5-3, the mismatch area can be reduced, but this will increase the number of pieces, thus introducing more errors.

[0037] Based on this, such as Figure 3 As shown in S5-4, by adopting the scheme of this application, the size of the adaptation area can be reduced while ensuring a smaller number of data through dual reference channels.

[0038] In this embodiment, the signal acquisition and processing unit 19 is configured to perform the following steps: Differential phase noise terms are extracted from the heterodyne reference signals output by the second balanced detector 17 and the third balanced detector 18, and further reconstructed as noise estimates that match the phase noise in the target mixed electrical signal output by the first balanced detector 11. The noise estimate is used to cancel the target mixed electrical signal to obtain the compensated target Doppler signal; Image reconstruction based on the compensated target Doppler signal.

[0039] Differential phase noise term extracted based on the heterodyne reference signal output by the second balanced detector 17. for: in: For laser carrier phase noise, for Delay in time The subsequent phase, A fixed time delay is introduced for the first delay fiber. A fixed time delay is introduced for the first delay fiber. The refractive index of the optical fiber. This represents the physical length of the first delay fiber.

[0040] Differential phase noise term extracted based on the heterodyne reference signal output by the third balanced detector 18. for: in: for Delay in time The subsequent phase, A fixed time delay is introduced for the second delay fiber.

[0041] The noise estimate is obtained as follows: based on the constructed digital delay adaptive filter, and using the differential phase noise term for digital delay, an estimate matching the phase noise in the target mixing electrical signal is reconstructed as the noise estimate. in: This is a noise estimate. For the first adaptive filter, This is the second adaptive filter.

[0042] First adaptive filter for: Second adaptive filter for: in: The delay of the received signal reflected by the target. For the closest integers, unit impulse signal After fixed delay The delayed version For the closest integers, For the target distance, At the speed of light, β for and The ratio of .

[0043] Based on the above design, two digital delay adaptive filters are used, and the two differential phase noise terms of the reference channel unit are used for digital delay to correspond to the integer multiples of the long fiber delay and the residual phase error in the target channel. This achieves coarse compensation and fine compensation for the phase noise of the target channel, respectively. The compensated target Doppler signal is: in: For the compensated target Doppler signal, The target mixing electrical signal is, specifically: in: For signal amplitude, Modulate the frequency for the acousto-optic frequency shifter. For laser carrier phase noise, The delay of the received signal reflected by the target. For the target distance, At the speed of light, The time-varying phase carries Doppler information from a high-speed spin target.

[0044] The image reconstructed based on the compensated target Doppler signal includes: Based on the compensated target Doppler signal, a short-time Fourier transform is performed on the one-dimensional time-domain signal to obtain its intensity image. : Transform the intensity image from the time-frequency domain to the sinusoidal domain: in: To convert to an intensity image in the sinusoidal domain, For the angle of observation, For the target spin angular velocity, For Doppler frequency, The center wavelength of the laser used. These are the projected coordinates.

[0045] Image reconstruction based on filtered backprojection algorithm using intensity images transformed to the sinusoidal domain: in: To reconstruct the image, This is the filtering function (convolution kernel).

[0046] Furthermore, this application also provides a method for optimizing the parameters of the imaging device as described above, comprising: Step A1: Initialize the optical path length of the first delay fiber 13 and the second delay fiber 14; Step A2: Obtain the reconstructed image output by the signal acquisition and processing unit 19; Step A3: Calculate the image quality evaluation index: in: As a quality evaluation metric, P represents the image height, and Q represents the image width. This represents the grayscale value of the pixel with x+1 as the x-coordinate and y as the y-coordinate. This represents the grayscale value of the pixel with x-coordinate and y-coordinate. The grayscale value of the pixel with x-coordinate and y+1; Step A4: Change the optical path length of the first delay fiber 13 and the second delay fiber 14, and repeat step A3 multiple times. The optical path length of the first delay fiber 13 and the second delay fiber 14 corresponding to the image with the best quality evaluation index is taken as the optimization result.

[0047] like Figure 3 As shown, S5-1 represents the phase noise to be compensated in the target channel echo signal, and the delay is determined by the target distance. Related. S5-2 represents the differential phase estimate when a relatively long delay fiber is selected in the traditional phase noise compensation method based on a single auxiliary interferometer. It can be seen that in this scenario, after an N-order digital phase delay, a large "phase mismatch zone" still exists between the estimated value and the target value to be compensated. This makes the method perform well only when the target optical path difference is an integer multiple of the reference optical path. It performs poorly in scenarios involving targets or moving objects at distances that do not meet this requirement, thus limiting the system's dynamic range of distance detection. S5-2 represents the case where a single auxiliary interferometer composed of short-delay fibers obtains the phase estimate. While this shortens the mismatch zone, the phase cascade order increases proportionally, leading to an accumulation of estimation errors and thus poor performance.

[0048] To shorten the mismatch region while controlling the cascade order and ensuring compensation accuracy, the compensation method proposed in this application sets up two reference channels with different delay fiber lengths in step S5, combining the advantages of both, as shown in the schematic diagram. Figure 3 As shown in S5-1.

[0049] To make the description of the embodiments of the present invention clearer, the following description and explanation are given in conjunction with preferred embodiments.

[0050] A preferred embodiment of the present invention provides a long-range Doppler tomography lidar imaging device and method, the purpose of which is to compensate for laser carrier phase noise and achieve high-resolution image reconstruction of high-speed spinning targets. The structural block diagram is shown below. Figure 1 As shown.

[0051] Optionally, laser 2 can be a narrow-linewidth laser, which has low phase noise and good performance for coherent detection. Preferably, to improve subsequent phase noise compensation and imaging effects, a 3.4MHz large-linewidth laser is used as the light source, corresponding to a coherence length of 28 meters and a physical length of 19.17 meters for an optical fiber with a refractive index of 1.466. The laser's operating wavelength is 1550nm. Preferably, the first delay fiber is 20 meters long and the second delay fiber is 4 meters long.

[0052] Optionally, an acousto-optic frequency shifter with an operating frequency of 50MHz is used to prevent the coherent detection signal from being detected at low frequencies. Noise interference. The output optical power of the first collimating lens is 300mW, and the sampling rate of the analog-to-digital converter (ADC) in the signal acquisition and processing unit is 500MS / s.

[0053] Optionally, the spin target material is a high-reflectivity bright strip, and dot targets and extended targets are selected as follows: Figure 4 As shown, the rotation speed is set to 30Hz, and the distance to the transceiver optical system is 50 meters.

[0054] Laser phase noise follows a random walk model, with respect to time phase difference. The differential phase noise has the following variance: in: The variance of the differential phase noise. Given the coherence time determined by the laser characteristics, after compensating for phase noise using dual reference channels with different delays, the target channel phase noise is reduced to: in: The target channel phase noise suppression ratio.

[0055] In this preferred embodiment, ∠0 = 0.3333 ∠1, ∠1 = 0.0977 ∠2, ∠2 = 0.0195 ∠3. At this point, ∠1 = 17, ∠2 = 3, ∠3 = 5. Without considering the delay introduced by other components in the fiber optic path, theoretically, the variance is reduced to 1 / 3 of its original value. .

[0056] Figure 5 The imaging results of a 30Hz spinning dot target under different phase noise compensation methods are shown. Figure 5 Part (a) serves as the control group, where a narrow-linewidth laser was selected as the light source for direct Doppler tomography, resulting in good reconstruction and a high signal-to-noise ratio. Figure 5Part (b) is the reconstructed image using a 3.4MHz linewidth laser without phase noise compensation, which completely fails to identify the original appearance of the target. Figure 5 Parts (c) and (d) are the reconstructed images using a conventional single auxiliary interferometer and the dual-reference channel method proposed in this invention to compensate for phase noise, respectively. Figure 5 The signal-to-noise ratio and reconstruction effect of part (d) are significantly better than those of the previous part. Figure 5 In part (c), the proposed method also shows better noise suppression in the central region.

[0057] Figure 6 Part (a) shows the spectral characteristics of the target channel echo signal. Since both the laser carrier phase noise and the spin target Doppler frequency shift exhibit spectral broadening effects, it is impossible to evaluate the compensation effect and parameter optimization at the signal level using spectral sharpness. Traditional phase noise compensation algorithms need to be adapted and optimized when ported to Doppler tomography lidar.

[0058] Therefore, this invention innovatively uses image evaluation metrics as criteria for compensation effect and parameter optimization. Figure 6 Part (b) shows the optimization results of the phase noise compensation image based on the average gradient, which can quickly and accurately estimate the phase digital delay order N1 and the delay fiber length FL1, verifying the feasibility of the method proposed in this invention.

[0059] A star-shaped extended target was used as the detection target for the Doppler tomography lidar. Figure 7 Parts (a) and (b) show the direct image reconstruction results using a narrow-linewidth laser and a 3.4MHz large-linewidth laser, respectively. The latter suffers from severe distortion due to lack of phase noise compensation. After phase noise compensation using a conventional single-auxiliary interferometer and the dual-reference channel method proposed in this invention, the reconstructed image is as follows: Figure 7 As shown in parts (c) and (d), it can be seen that the method proposed in this invention performs far better than the former for the extended target.

[0060] like Figure 1 As shown, a long-distance delay fiber is added between the second optical beam splitter 5 and the first collimating lens 6 in the optical fiber path to simulate a longer optical path difference and verify the effectiveness of the compensation method. Long-distance delay fibers of 650 meters and 1460 meters are selected and added to the optical path respectively. Figure 7 Sections (e) and (f) of this application disclose reconstructed images after phase noise compensation by the method proposed in this application, demonstrating the scene adaptability of the proposed method. Figure 7 In part (f), the optical path difference of the target channel exceeds 80 times the coherence length of the laser used, yet high-resolution Doppler tomography can still be achieved, verifying the effectiveness of the device and method disclosed in this invention.

[0061] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this invention, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

Claims

1. A Doppler tomography lidar imaging device based on carrier phase noise compensation, comprising a laser (2), a first optical beam splitter (3), an acousto-optic frequency shifter (4), a second optical beam splitter (5), a third optical beam splitter (9), a first optical coupler (10), a second optical coupler (15), and a signal acquisition and processing unit (19). The input end of the first optical beam splitter (3) is connected to the output end of the laser (2), the first output end is connected to the input end of the third optical beam splitter (9), the second output end is connected to the input end of the acousto-optic frequency shifter (4), the input end of the second optical beam splitter (5) is connected to the output end of the acousto-optic frequency shifter (4), and the first output end illuminates the target through a first collimating lens (6). The second output terminal is connected to the signal input terminal of the second optical coupler (15) via the first delay fiber (13). The first output terminal of the third optical beam splitter (9) is connected to the local oscillator input terminal of the first optical coupler (10), and the second output terminal is connected to the local oscillator input terminal of the second optical coupler (15). The received signal reflected by the target is input to the signal input terminal of the first optical coupler (10) via the second collimating lens (7). The output terminal of the first optical coupler (10) is connected to the signal acquisition and processing unit (19) via the first balanced detector (11), and the output terminal of the second optical coupler (15) is connected to the signal acquisition and processing unit (19) via the second balanced detector (17). Its features are, The imaging device further includes a second delay fiber (14), a third optical coupler (16), and a third balanced detector (18). The second optical beam splitter (5) and the third optical beam splitter (9) are both three-output beam splitters. The third output end of the second optical beam splitter (5) is connected to the signal input end of the third optical coupler (16), and the third output end of the third optical beam splitter (9) is connected to the local oscillator input end of the third optical coupler (16). The optical paths of the first delay fiber (13) and the second delay fiber (14) are not equal.

2. The carrier phase noise compensation method for Doppler tomography lidar according to claim 1, characterized in that, The optical path length of the first delay fiber (13) is 3 to 10 times that of the second delay fiber (14).

3. The carrier phase noise compensation method for Doppler tomography lidar according to claim 2, characterized in that, The optical path length of the first delay fiber (13) is 5 times that of the second delay fiber (14).

4. The carrier phase noise compensation method for Doppler tomography lidar according to claim 1, characterized in that, The signal acquisition and processing unit (19) is configured to perform the following steps: Differential phase noise terms are extracted from the heterodyne reference signals output by the second balanced detector (17) and the third balanced detector (18), and further reconstructed as noise estimates that match the phase noise in the target mixed electrical signal output by the first balanced detector (11). The noise estimate is used to cancel the target mixed electrical signal to obtain the compensated target Doppler signal; Image reconstruction based on the compensated target Doppler signal.

5. A carrier phase noise compensation method for Doppler tomography lidar according to claim 4, characterized in that, Differential phase noise term extracted based on the heterodyne reference signal output by the second balanced detector (17) for: in: For laser carrier phase noise, for Delay in time The subsequent phase, A fixed time delay is introduced for the first delay fiber; Differential phase noise term extracted based on the heterodyne reference signal output by the third balanced detector (18) for: in: for Delay in time The subsequent phase, A fixed time delay is introduced for the second delay fiber.

6. A carrier phase noise compensation method for Doppler tomography lidar according to claim 4, characterized in that, The noise estimate is obtained as follows: based on the constructed digital delay adaptive filter, and using the differential phase noise term for digital delay, an estimate matching the phase noise in the target mixing electrical signal is reconstructed as the noise estimate. in: This is a noise estimate. For the first adaptive filter, This is the second adaptive filter.

7. A carrier phase noise compensation method for Doppler tomography lidar according to claim 6, characterized in that, First adaptive filter for: Second adaptive filter for: in: The delay of the received signal reflected by the target. For the closest integers, unit impulse signal After fixed delay The delayed version For the closest integers, For the target distance, At the speed of light, β for and The ratio of .

8. A carrier phase noise compensation method for Doppler tomography lidar according to claim 6, characterized in that, The compensated target Doppler signal is: in: For the compensated target Doppler signal, The target is the mixing electrical signal.

9. A carrier phase noise compensation method for Doppler tomography lidar according to claim 8, characterized in that, The image reconstructed based on the compensated target Doppler signal includes: Based on the compensated target Doppler signal, a short-time Fourier transform is performed on the one-dimensional time-domain signal to obtain its intensity image. Transform the intensity image from the time-frequency domain to the sinusoidal domain; Image reconstruction is performed using a filtered backprojection algorithm based on the intensity image transformed to the sinusoidal domain.

10. A method for optimizing the parameters of an imaging device as described in any one of claims 1-9, characterized in that, include: Step A1: Initialize the optical path length of the first delay fiber (13) and the second delay fiber (14); Step A2: Obtain the reconstructed image output by the signal acquisition and processing unit (19); Step A3: Calculate the image quality evaluation index: in: As a quality evaluation metric, P represents the image height, and Q represents the image width. This represents the grayscale value of the pixel with x+1 as the x-coordinate and y as the y-coordinate. This represents the grayscale value of the pixel with x-coordinate and y-coordinate. The grayscale value of the pixel with x-coordinate and y+1; Step A4: Change the optical path length of the first delay fiber (13) and the second delay fiber (14), and repeat step A3 multiple times. The optical path length of the first delay fiber (13) and the second delay fiber (14) corresponding to the image with the best quality evaluation index is taken as the optimization result.

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