Research On Laser Stable Transmission And Correlated Imaging In The Scattering Medium

In the process of optical communication and active illumination imaging,the influence of scattering medium such as rain and fog in atmosphere and underwater impurities is unavoidable.Due to the existence of scattering medium,the wavefront of the beam will be distorted with the increase of scattering intensity.This adverse effect brings great challenges to laser stable transmission,outdoor optical communication based on orbital angular momentum,correlated imaging and many other fields.It is difficult to clarify the influence law of scattering medium on laser transmission through experiments in actual environment.Therefore,establishing a reliable scattering model numerically is helpful to predict the effect of scattering medium on optical signal in actual environment.And it is of great significance to select the appropriate optimization scheme to eliminate the influence of scattering medium in practical optical communication and correlated imaging applications.The existing scattering models include Mie scattering theory to model a single scattering particle,Mont-Calo scattering model to simulate the motion of a large number of scattering particles,and transmission matrix to characterize the optical properties of scattering medium.However,the existing multiple scattering models lack the detailed characterization of the physical parameters of the scattering medium.In this paper,a scattering model integrating the physical parameters of the scattering medium is proposed,and the reliability of the model is verified by the laser transmission experiments affected by the scattering medium.At the same time,the influence of the scattering medium on the stable transmission of the orbital angular momentum beam and the correlated imaging quality is studied in detail based on the scattering model.The main work of this paper is summarized as follows:(1)A scattering model with comprehensive physical parameters is established based on Mie scattering theory and multiple phase screens method,and the influence of physical parameters of scattering medium on laser transmission is clarified.Firstly,the effect of a single-phase screen on laser transmission is studied,and corresponding experiments are carried out to verify the numerical results,which ensures the reliability of the scattering model.Then,the scattering model is extended to simulate the scattering medium with large thickness in real environment.Based on the extended scattering model,the empirical equation between the statistical properties of the multiple phase screens and the physical parameters of the scattering medium is summarized.At the same time,the laser transmission in the haze with different concentration is studied experimentally,so the scattering model with large thickness and empirical formula are verified.Next,in order to extend the application range of the model,the effect of rain and fog scattering of two particle sizes on laser in real environment is simulated numerically.The numerical results are verified by corresponding experiments,which shows that the scattering model is suitable for predicting the influence of rain and fog scattering environment.Finally,the influence of underwater scattering environment on Gaussian light and pseudo-thermal light is discussed numerically and experimentally,and the application range of the model is further extended.The scattering model in this chapter lays a reliable theoretical foundation for the study of stable transmission of orbital angular momentum beam and correlated imaging under the disturbance of the scattering medium.(2)Based on the scattering model,the influence law of scattering medium on the orbital angular momentum spectrum is clarified,and two schemes to suppress the scattering effect are proposed and verified to ensure the stable transmission of orbital angular momentum beam and improve the performance of orbital angular momentumbased optical communication.Firstly,the orbital angular momentum spectrum of the vortex light under the disturbance of the diffuser with different scattering intensity is measured experimentally,and the variation rule of the orbital angular momentum spectrum is preliminarily obtained.Then,the influence of scattering medium concentration,particle size and thickness on the single and superimposed states of orbital angular momentum is discussed.The results show that the crosstalk phenomenon of orbital angular momentum spectrum becomes more serious when the particle size,concentration and thickness of the scatter ing medium increase.The larger the topological charge is,the better the orbital angular momentum beam can resist the disturbance of the scattering medium.Finally,in view of the crosstalk phenomenon of the orbital angular momentum of the vortex light caused by the scattering medium,two schemes for optimizing beam quality are proposed.One is to select an ap propriate receiving aperture to suppress crosstalk.Second,an optimization scheme is proposed to extract the scattering phase by using the light intensity transmission equation combined with the amplitude gradient addition iterative algorithm.And the scattering phase is pre-compensated in the complex optical field of vortex light for transmission.The results show that both schemes can suppress the influence of scattering medium to a certain extent to ensure the stable transmission of laser.(3)Based on the scattering model and the correlated imaging theory,the correlated imaging model under the scattering medium disturbance is established,and the influence law of scattering medium on correlat ed imaging is clarified.A subtractive correlated imaging scheme is proposed to eliminate disturbance of scattering medium and the active role of scattering medium in the correlat ed imaging process is explored.Firstly,a numerical model of correlat ed imaging with scattering medium disturbance is established.Based on the reliable numerical model,the signal-to-noise ratio of the correlated imaging results of binary objects and gray objects is calculated when the scattering medium concentration,particle size and thickness change.Then,aiming at the influence of scattering medium on correlat ed imaging,a subtraction correlated imaging optimization scheme is proposed.By deducing the correlation function of intensity fluctuation under the disturbance of the scattering medium,it is found that the final correlated image consists of the image of scattering medium and the image of target object.Thus,the image of the target object can be recovered by subtracting the image of the scattering medium from the final correlated image.Finally,aiming at the problems of poor quality of binary quantization correlated imaging,it is proposed that adding a lower concentration of scattering medium in front of the target object can randomize the output signal and reduce the correlation between the error data and the effective data,so as to improve the quality of binary quantization correlat ed imaging.Then,for the problem that there are obstacles behind the object which limit the field of view of correlated imaging,it is proposed that adding a high concentration of scattering medium behind the obstacle can expand the field of view of correlat ed imaging.The range and conditions of scattering medium to expand the field of view are revealed.