Research On Pseudo-thermal Light Propagation And Correlated Imaging Under Turbulence Interference

Correlated imaging,namely,ghost imaging or quantum imaging,can nonlocally image objects by using two spatially separated but correlated light beams,which is an indirect imaging technique.One beam,called the signal light beam(object light beam),carries object information and its total intensity value is recorded by a single pixel(bucket)detector.While the other,never containing object information,is referred as the the reference light beam and the intensity distribution of the light field is captured by a multipixel detector.The image of the unknown object can be reconstructed in the reference path by measurting the correlation of the intensity fluctuations between the two light beams.In the past 30 years,this novel imaging technology has attracted continuous attention because of its excellent characteristic of high detection sensitivity,imaging at low light level and strong anti-interference.It has broad application prospects in biomedical,radar,remote sensing,military detection and other fields.However,the channel disturbances,including scattering,fading and turbulence is unavoidable in real environment.In particular,the negative effects such as light intensity fluctuation,beam wander and beam deformation because of the existence of turbulence can result in the imaging quality degradation of correlated imaging,and even imaging can not be achieved.Although turbulence has been studied extensively,repeatable and high confidence models need to be re-examined.Therefore,it is necessary to clarify the influence of turbulence on pseudo-thermal light propagation and correlated imaging based on the high confidence model,which is of great significance to promote the practicality of correlated imaging.In this thesis,the effects of turbulence on pseudo-thermal light propagation and correlated imaging are studied in detail.The main works of this thesis are summarized as follows:(1)The reliability of the optical propagation model under the influence of turbulence is verified.The influence law of turbulence effect on pseudo-thermal light propagation is revealed,and the equivalent relationship between atmospheric turbulence and water turbulence is established.Firstly,the effects of atmospheric turbulence on laser propagation are analyzed theoretically and numerically.The laser propagation experiments of 80 m,260 m and 500 m are carried out in atmospheric turbulence,and the reliability of the numerical model is verified by the scintillation index and beam wander.The result shows that the laser spot at the receiving surface experiences light intensity fluctuations,wander,and deformation.Through calculating the scintillation index or beam wander,atmospheric turbulence intensity can be obtained.The theoretical formula of beam wander is revised and the reliability of atmospheric turbulence intensity measurement is guaranteed.Then,the effect of water turbulence on laser propagation is studied experimentally.The results show that the larger the water flow velocity is,the larger the spot wander and scintillation are.Using the scintillation or wander theory formula of atmospheric turbulence,water turbulence intensity can be obtained.It is concluded that the influence of water turbulence on laser propagation is greater than that of atmospheric turbulence.At the same time,the equivalent relationship between water turbulence and atmospheric turbulence is established,which provides an alternative scheme for long-distance atmospheric turbulence experiment.Then,we experimentally study the influence of airflow on laser propagation,including airflow scale and wind speed,and compare the difference of beam quality caused by laser passing through the airflow zone from different directions.The results show that the peak fluctuation of light spot is sensitive to the airflow scale,the higher the wind speed is,the greater the influence on the beam quality is,and the influence on the beam quality is greater in the longitudinal airflow region than in the transverse airflow region.Finally,we study the influence of turbulence effect on the propagation of pseudo-thermal light based on the turbulence channel model with high confidence.Increasing the turbulence intensity,propagation distance and light source size,will lead to the degradation of the transmission quality of pseudo-thermal light,which provides a reliable theoretical basis for studying the effect of atmospheric turbulence on pseudo-thermal light correlated imaging.(2)The numerical model of correlated imaging through homogeneous atmospheric turbulence is established when the bucket detector size is limited,and the numerical model of correlated imaging through inhomogeneous atmospheric turbulence is also established.Based on these two models,the influence of the collection range of a bucket detector on the quality of correlated imaging is studied,and the difference of the influence of up-link turbulence and down-link turbulence on correlated imaging are studied.Firstly,the pixel number from the border of the imaging object to that of the light intensity from the imaging object is chosen as the evaluation criterion,which can be used to analyze the effect of the collection range on the quality of correlated imaging under different turbulent intensity.The numerical results show that the quality of correlated imaging increases with increment in the collection range of the bucket detector and with a decrease in turbulence intensity.Then,the correlated imaging experiment is carried out in the real atmospheric environment,and the results show that the imaging quality is low,and the imaging quality can be improved by increasing the number of measurements or by means of interval sampling correlation.At the same time,the effect of the distance between the imaging object and the receiving lens on the quality of correlated imaging is investigated.It is worth noting that because the aperture of the receiving lens is limited,the larger the distance between the imaging object and the receiving lens,the smaller the collection range of the bucket detector.The experimental results show that with the increase of the distance,the reconstructed image contains less information about the object,which well proves the reliability of our numerical results.Finally,for the actual needs of air-to-ground and ground-to-air satellite communication and remote sensing imaging,the numerical model of correlated imaging through atmospheric turbulence along a slant path is established,and the influence of atmospheric turbulence along an uplink path and a downlink path on imaging quality is studied.The results show that imaging quality decreases with the increase of zenith angle,and the effect of downlink turbulence on the quality of correlated imaging is less than that of uplink,which can be explained by the phase modulation effect.(3)The concept of critical resolution(the resolvable minimum-separation between two adjacent objects)is proposed,and a method to evaluate the resolution limit of correlated imaging system is provided.The quantitative relationship between the resolution limit and the speckle size of pseudo-thermal light is obtained,and the effect of atmospheric turbulence on the critical resolution of correlated imaging system is studied.Based on Fresnel diffraction integral,the theoretical imaging formula of lensless correlated imaging system is derived.This formula shows that the correlated imaging result is the convolution of the intensity function of object and the pointspread function,and the width of the point-spread function is equal to the speckle radius.We propose a concept of critical resolution,which describes how well two adjacent objects can be resolved in a correlated imaging system.Taking two circular holes as examples,we theoretically analyze the critical resolution of pseudo-thermal correlated imaging system,and provide an accurate judgment of the critical resolution,which can be quantified by an analytical formula.This formula shows that the critical resolution is determined by the speckle size,from which it can be deduced that the resolution limit of pseudo-thermal correlated imaging system is 1.98 times the speckle size.Then,we verify the reliability of the quantitative formula about the critical resolution through the corresponding experiment.Finally,based on Fresnel diffraction integral,the theoretical imaging formula of lensless correlated imaging through atmospheric turbulence is derived,and the influence of turbulence effect on the critical resolution is analyzed in detail.The results show that the stronger the turbulence intensity is,the greater the decrease of the critical resolution is.The larger the imaging distance is,the greater the effect of turbulence on the critical resolution is.In addition,the critical resolution cannot be improved infinitely by increasing the size of the light source under turbulence works.(4)An accurate numerical model of pseudo-thermal light is established,and the critical resolution and optimal signal-to-noise ratio of correlated imaging system are explored experimentally and numerically.A scheme to improve the critical resolution of correlated imaging system is proposed by using vortex beam.The vortex beam is applied to the correlated imaging system under the influence of atmospheric turbulence to suppress the influence of atmospheric turbulence on imaging resolution.Firstly,the comparison between the propagation effect for the Gaussian beam and vortex beam after passing through the rotating ground-glass.It is shown that the speckle sizes from the vortex beam are smaller than those of the Gaussian beam under the same propagation distance,and the larger the topological charge is,the smaller speckle size is.Then,the relation between speckle size and resolution as well as that between speckle size and signal-to-noise ratio in correlated imaging system is analyzed in detail.It is found that the critical resolution of correlated imaging system is approximately equal to the speckle diameter,and an optimum signal-to-noise ratio exists in correlated imaging system.Based on the above conclusions,we propose a scheme to enhance the critical resolution of correlated imaging system by using vortex beam and the enhancement ability under different topological charges is clearly presented,which can be quantized by a fitting formula.In addition,the comparison between the critical resolution and the Rayleigh diffraction limit is performed,and the results show that the critical resolution with the vortex beam can exceed the Rayleigh diffraction limit.Finally,we apply the vortex beam to the correlated imaging system under the influence of turbulence,and the results show that the vortex beam can suppress the influence of turbulence on the imaging resolution to a certain extent.