Investigation On Imaging Through Scattering Media Based On Speckle Correlation

When a beam of light propagates through scattering media,such as fog,haze,turbid media,biological tissues,etc.,the intensity and phase of output field will be changed seriously because of photons' multi-scattering in the scattering media.Consequently,it will fail to image an object.However,the speckle patterns still carry main information of the object,such as the amplitude,phase and polarization.The information is encoded into the random scattered light.Some techniques have been proposed to solve the issue,such as adaptive optics techniques,optical gate techniques,wavefront shaping techniques and so on.It is important for many applications,including astronomy,remote sensing,ocean exploration and biomedical imaging.However,these techniques have some disadvantages of low efficiency,timeconsuming and complex system,which limit the practical application.Therefore,aiming at fast methods with no requirement of phase correction through scattering media,this thesis focused on the investigation of speckle correlation imaging through scattering media and accomplished the following main contents:1.The investigation of speckle-autocorrelation imaging through scattering media.The magnification,field of view and system's resolution are theoretically derived,and the influence of background on imaging quality is analyzed.Moreover,we simulate the influence of object structure,complexity,gray level and image signal-to-noise ratio on imaging quality,and propose a tunable output-frequency filter(TOF)algorithm to reconstruct the object from low image contrast and noisy experimental data under low-power partially coherent illumination.We employ Gaussian functions with different filter windows at different stages in iterative process.This reduces corruption from experimental noise to search for a global minimum in the reconstruction.In comparison with Fienup-type phase-retrieval algorithm,the proposed TOF algorithm is immunity to strong noise.Moreover,the spatial resolution and distinctive features are retained in the reconstruction since the filter is applied only to the region outside the object.2.We analyze the influence of different initial conditions on image quality.In Fienuptype phase-retrieval algorithm,the random function is employed as the initial input.But,the input of a random function is difficult to ensure the convergence of the algorithm.To retrieve reliable image,it is necessary to change the initial input to find the optimal initial function for convergence.However,if a distribution similar to the object function is used as the initial function,image can be retrieved only by selecting the initial value once.Therefore,a hybrid bispectrum-iteration algorithm for speckle-correlation imaging through scattering media is proposed.In iterative process,an approximate distribution of object obtained from a bispectrum-analysis method is employed as the initial input.This method needs one input of initial function and avoids several different runs performed with different random initial conditions.Moreover,it also reduces the number of sub-images in the bispectrum analysis algorithm,and the loss of image quality can be compensated by further iterative algorithm.Therefore,it balances the image quality and the reconstruction speed.3.Speckle crosscorrelation imaging method through scattering media is investigated and the parameters influencing the experimental results are analysed.We reconstruct the biological sample and moving sample through scattering medium.The speckle autocorrelation imaging is limited to the sparsity of the object.To solve the problem,a speckle crosscorrelation imaging method has been introduced.In this system,a reference point source is introduced and two speckle patterns are recorded.Finally,the object can be reconstructed by cross-correlation operation.This method has no limitation to the sparsity of the sample.Thus it overcomes the limitation of autocorrelation method for the reconstruction of binary sample.Moreover,we analyse the effect of experimental parameters,such as the diameter of pinhole,the pixels and spatial coherence of light source,on the results.Furthermore,the lens-like properties of the scattering medium is investigated based on crosscorrelation imaging method.We consider the scattering medium as an unconventional imaging lens and experimentally demonstrate its lens-like properties,such as tunable magnification,depth of field and working distance.4.Complex object retrieval through scattering media with a large field of view is proposed.Although researches have been done to reconstruct the hidden object,the techniques are limited field of view within the memory effect.Based on ptychography,we propose a novel configuration for the retrieval of complex object through scattering media with a large field of view of 2.64 mm beyond the memory effect range of 0.9mm.Meanwhile,the technique is free from the sparsity of object.Furthermore,by virtue of the imaging configuration,our proposed imaging technique is insensitive to dynamic scattering medium.These advantages will provide a potential application in biomedical imaging.