Endoscopic Imaging Method Based On Transmission Matrix Of Scattering Medium

Scattering media are ubiquitous in our daily life,such as ground glasses,rough walls,and opaque biological tissues.On the one hand,the scattering medium is a hindrance in the imaging process,because it will change the direction of the light randomly,and then destroy the relationship between the object and the image;On the other hand,the scattering medium is also a good optical element,its strong scattering ability can expand the numerical aperture of the imaging system,and then improve the imaging resolution;At the same time,the scattering medium also has many applications in the industrial and medical fields,such as the multimode fiber,as a special scattering medium,has been used to explore ultra-fine endoscopes and portable spectrometers.Therefore,it has a lot of practical demands and application prospects to study the scattering imaging.Some techniques have already been widely used to image through the scattering medium,such as the focus point scanning,computational ghost imaging,speckle correlation,deep learning and transmission matrix.Among these techniques,the focus point scanning and computational ghost imaging are very time wasted,as cumbersome scanning operations are needed during the imaging process;the speckle correlation is only effective for special thin scattering media;and the imaging quality of deep learning heavily depends on the number and characteristics of training samples.Compared with the above methods,the transmission matrix has its unique advantages as no scanning operation is needed,suitable for thick scattering media,and robust to different target objects.Therefore,study the principle and technology of transmission matrix in scattering imaging is not only very necessary,but also has a lot of application prospects.Based on the ??imaging theory of transmission matrix,this paper has studied the optical field transmission responses of scattering media at first,constructed the optical field transmission matrix,and tested its imaging ability in different scattering environments;Then,to solve the defects of the optical field transmission matrix in some specific application areas,especially when applied to the optical fiber endoscopes,a flexible endoscopic imaging strategy based on the spectral transmission matrix of the scattering medium has been explored.The main work and the innovative results of this paper are as follows:(1)The imaging mechanism of optical field transmission matrix in scatteringenvironment is studied,and the key factors that affect the imaging quality are analyzed.Firstly,based on the photon random walk model and the optical field propagation theory,the complete propagation process of light passing through the scattering medium and then reaching at the imaging plane is analyzed,and then the optical field transmission matrix of the scattering medium has been constructed;Secondly,the relationship between the spatial information of the target object and its output speckle field is derived based on the linear time-invariant theory,and the theoretical feasibility of scattering imaging based on the optical field transmission matrix is verified by simulation experiments;Furthermore,the core problems in the optical field transmission matrix technology,such as the speckle field acquisition and the reconstruction algorithms are studied,and then the effects of optical field measurement error and detector resolution on the imaging quality have also been analyzed.(2)The imaging ability of the optical field transmission matrix technology to the intensity target is verified through experiments,a method of constructing the complex amplitude target is proposed,and the imaging ability to the complex amplitude target has also been further verified.Firstly,an imaging system where the object light and the reference light are separated is built,using the digital phase shift interference technology,the optical field transmission matrix of the scattering medium is obtained,and then the intensity target is reconstructed based on the transmission matrix;Secondly,a method of constructing the complex amplitude target based on liquid crystal spatial light modulator(LC-SLM)is proposed,and the corresponding imaging system where the object light and the reference light are merged is built.The imaging result shows that the optical field transmission matrix technology can both reconstruct the amplitude(intensity)and the phase(morphology)information of the target object.(3)The current optical field transmission matrix technology can only overcome the one-way distortion of the optical field,it can not do the endoscopic imaging where the target is hidden behind the scattering medium,so an active illumination endoscopic imaging strategy based on the round-trip optical field transmission matrix is proposed.The round-trip optical field transmission matrix not only has recorded the output transmission responses of the reflect light,but also has recorded the input illumination speckle optical field of the incident light.Therefore,it has the ability to overcome both the input and the output scattering at the same time.In the experiment,an active illumination endoscopic imaging system is built,and both the binary intensity target constructed by the digital micromirror device(DMD)and the USAF1951 resolution test board have been imaged to verify the feasibility of this method.(4)Since the effectiveness of the optical field transmission matrix technology highly depends on the stability of the imaging system,a flexible endoscopic imaging method based on the spectral transmission matrix of the scattering medium is proposed.This method uses a single-mode optical fiber to transmit the light,and uses a scattering medium to do the random space-spectrum encoding,has the potential to explore a new ultra-fine flexible endoscope.In the experiment,the spectral transmission matrix of the scattering medium is measured at first,and then the space information of the target is reconstructed directly from its spectral transmission response.At last,the robustness to the fiber deformation has also been verified.