| Ghost imaging(GI)lidar is a novel imaging lidar mechanism based on the principle of intensity-correlation imaging.GI lidar irradiates the target with multiple spatially modulated partial coherent photons and collects the reflected or scattered photons by a point detector with no resolution capability,and the target’s spatial distribution information can be reconstructed by calculating the fluctuation correlation between the prebulit reference light field and the echo signal light.Due to the unique nonlocal characteristics and the receiving and transmitting mechanisms of area array transmission and point reception,GI lidar presents high detection sensitivity and high information acquisition efficiency,and has sparked great interest and continuous development in the fields of telemetry and remote sensing,early warning and reconnaissance,and target recognition.Traditional GI lidar can achieve staring imaging with a large field of view(FOV),but the divergence angle will expand sharply with the distance,resulting in a continuous decline in the luminous flux of the illumination field.The fewer photons reflected or scattered by the target result in weak light detection,which affects the imaging quality seriously,and the target even cannot be imaged.Obviously,the expansion of GI lidar to a longer range is greatly restricted.In addition,it is difficult to achieve a longer detection distance by increasing the laser power simply due to the limitations of the size and security of the radar platform.Therefore,it is significant to explore an efficient scheme to improve the effective range and information flux to promote the practicality of GI lidar.In this thesis,a novel scheme of GI lidar from the perspective of system design is proposed,and the feasibility of the scheme and the image reconstruction of the target under turbulence disturbance are analyzed and verified in detail.The main works are summarized as follows:(1)The high-confidence relationship between the effective range of GI lidar and echo power is established,which provides a theoretical basis for the design of a correlated imaging radar system for long-distance applications.Firstly,the basic model of optical transmission with and without turbulence disturbance is established based on the paraxial equation,Fresnel approximation,and split-step Fourier transform method.Taking a Gaussian beam as an example,the reliability of the numerical model of optical transmission with and without turbulence disturbance is verified,and the influence law is also analyzed.Then,the correlated imaging model of complex optical systems is derived by combining Collins formula.Finally,the equation of GI lidar is derived based on the application scenario.The system parameters such as transmit light power,radar cross section(RCS)of the target,and imaging FOV that affect the echo optical power and effective range are analyzed,which provides a theoretical basis for long-range GI lidar system design.(2)A correlated imaging system based on laser lidar is designed to address the problem that the imaging distance of traditional correlated imaging lidar is limited.The theoretical and experimental results demonstrated that the collimated illumination field of the system has several orders of magnitude of advantage over the traditional divergent illumination field in the effective range.Firstly,the problem limiting the detected range caused by the large divergence angle in traditional GI lidar based on the Gaussian imaging formula is analyzed theoretically and experimentally.Inspired by the collimation characteristics of laser radar,a novel scheme of GI lidar with a set of lens assemblies is proposed.The formulas for correlated imaging and FOV of the proposed lidar system are derived based on the Collins formula,optical transfer matrix theory,geometric optics theory,and transmission effect,respectively.In comparison to the conventional divergent system,the evaluation method of the echo optical power and effective imaging distance is presented,and the effect of system deviation on imaging performance is analyzed and confirmed.Finally,experiments demonstrate that the collimated illumination field in the imaging FOV can achieve a greater luminous flux per unit area than the divergent illumination field when the output light power and detection capability are determined.This provides a reliable answer to the problem of the existing correlated imaging radar’s limited effective range.Moreover,a larger illuminating FOV to image comprehensively can also be obtained just by adjusting the lens assembly in the object path when the target is close to the receiver,which is suitable for the application of short-range imaging.(3)The influence of turbulence disturbance on the signal light beam and correlated imaging of the proposed lidar system is obtained.In a long-distance and low-light environment,the impact of detection noise-signal-ratio(NSR)on the imaging of the proposed correlated imaging radar system is assessed.This evaluation forms the basis for the establishment of conditions for the development of information processing methods at the rear end of the system.Firstly,the numerical calculation model of signal light in correlated imaging is clarified.Based on the high-confidence turbulent channel light transmission model,the effects of atmospheric turbulence intensity,the size of the light source,and wind speed related to turbulence disturbance on the signal light beam are studied.The results show that the stronger the turbulence intensity,the larger the light source,and the wind speed,the greater the decrease in beam quality.Then,the influence of turbulence disturbances,including atmospheric turbulence and airflow environments,on correlated imaging is also studied.Correlated imaging experiments with a signal beam transmitting through the air flow horizontally and vertically are conducted in the actual wind tunnel environment.The results show that horizontal transmission has a greater impact on imaging than vertical transmission,and the greater the wind speed,the greater the impact on imaging.Finally,an evaluation method to analyze the impact of turbulence disturbance,power attenuation,and other factors on imaging by quantifying the detection NSR under weak light detection is studied,and the simulative and actual atmospheric experimental results have verified the influence law.(4)For the ill-posed problem of the correlated imaging matrix equation in a noisy setting and undersampling,a total variation(TV)regularization-based solution strategy is suggested.The imaging results confirm the method’s effectiveness in improving the quality of reconstructed images.Firstly,the degree of undersampling of the target image obtained by correlated imaging is evaluated in accordance with the sparse properties of the target,and the ill-posed problem caused by undersampling in correlated imaging in a noisy environment is analyzed based on the characteristics of the random intensity distribution of the reference speckle field,and the conditions to ensure the optimal solution are clarified.Next,the regularization method is introduced to overcome the ill-posed problem of the inverse solving of the correlated imaging matrix equation.It is found that Tikhonov regularization in correlated imaging presents the drawbacks of excessive smoothing and long computational time in image reconstruction.To avoid this problem,according to the analysis of the spatial gradient sparsity of common natural objects,a correlated imaging method based on TV regularization is proposed.Finally,the simulation and experimental results demonstrate that the proposed method can obtain higher image quality and more clear edge features and validate the imaging efficacy under the condition of a high NSR when compared to the image reconstruction method based on compressed sensing. |
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