Research On Multi-dimensional Physical Extended Computational Imaging Through Scattering Media

When light passes through scattering media,such as ground glass,biological tissue,smoke and dust,etc.,due to the inhomogeneous distribution of the refractive index inside or on the surface of the medium,the incident photons will randomly scatter and thus distorts the wavefront and form speckle patterns.Lens-based imaging method is no longer able to decode the information of the hidden target directly from the speckle.Moreover,due to the scattering phenomenon,the number of ballistic photons carrying target information decreases exponentially with the increase of penetration depth,which makes imaging through the scattering medium more challenging.Recently,thanks to the enormous progress in light field modulation devices,incorporated with the exploration of mesoscopic physics and random matrix theory,new technologies based on wavefront shaping,transmission matrix inversion,phase conjugation,speckle correlation,scattering holography,deep learning and so forth,have provided well-posed solutions under certain boundary conditions for the speckle inverse problem.Despite these ingenious approaches have enabled to achieve color,three-dimensional and dynamic imaging through scattering media,there are problems lie in spatial,spectral,temporal and detection limitation to be solved:i)Due to the correlation range of the scattering medium,the existing convolution scattering imaging model has a limited imaging field-of-view,that is,the limitation of the optical memory effect regime.It is still intractable to achieve wide-field multi-target or extend target imaging through scattering media.ii)For simplicity’s sake,the existing scattering imaging methods need to restrict the source illumination bandwidth in exchange for high contrast speckle pattern and simple model interpretation.However,the use of spectral filters dramatically reduces the flux of light sources and at the same time,the imaging applications,especially in passive scattering sensing,the applicability of broad-spectrum illumination is particularly important.iii)It is still unclear for the interaction between the ultrafast lasers and the scattering medium as well as the temporal optical memory effect,making it difficult to achieve deep penetration depth and information extraction.iv)The single-shot speckle correlation method,based on the high contrast autocorrelation of the system PSF,is inherently incompatible with the extremely low signal-to-noise ratio of the imaging condition.It is difficult to achieve accurate object estimation and the detection limitation needs to be improved.Aiming to solve the shortcomings of the existing scattering imaging methods,the dissertation mainly focuses on the following research work:(1)To expand the effective imaging field-of-view,a multi-shot large field-of-view of multitarget speckle de-aliasing imaging technology based on random attenuation illumination combined with independent component analysis is proposed.In this method,the lateral fieldof-view is expanded by 5.56 times,and the longitudinal depth-of-field is expanded to 7 times comparing to the original parameters.The proposed strategy can be easily implemented in other related scattering imaging applications.Based on this method,a single-shot multitarget demixing technique rely on non-negative matrix decomposition and a single-shot color speckle demixing imaging technique based on color crosstalk are presented and demonstrated.As for the extended object imaging,a multi-distance light field reconstruction technology based on round-trip transmission is further studied.By combining with the spatial shower curtain effect of the scattering medium,the extended object with both amplitude and phase information can be reconstructed.(2)Due to the multi-spectral speckle aliasing and speckle contrast degradation,the broadband speckle correlation theory is established and derivated in detail.The experiment verification about the weak correlation phenomenon is demonstrated.Here,we propose and demonstrate two general technologies that enable single-shot lensless scattering imaging under broadband illumination in both non-invasive and invasive modes.Based on the two approaches,high-fidelity reconstruction results can be obtained under broadband illumination with its spectral full-width-at-half-maximum spans more than 280 nm at the visible region.At the same time,an anisotropic correlation phenomenon of broadband speckle is observed through theoretical analysis,numerical simulation as well as experiment verification.Moreover,the broadband spectral response and the correlation degradation characteristics of the scattering medium are investigated.Finally,the imaging performance under narrowband and broadband illumination are compared experimentally and intriguingly the broadband lighting performs similar with the narrowband case.(3)To probe the degradation of the scattering field in the temporal domain,the propagation characteristics of ultrafast pulsed lasers such as femtosecond and picosecond pulses in the scattering medium are studied.The time domain scattering imaging models of both single scattering and volume scattering are given.A forward light cone model modified by the scattering phase function is proposed.Based on the convex optimization technique,threedimensional imaging through scattering medium is realized.Furthermore,in the experiment,we demonstrate imaging through a single layer ground glass diffuser with a 5m distance between the hidden object and the diffuser.The achieved lateral imaging resolution is about10 cm and the vertical resolution can reach 1cm.For thick scattering media such as thick foam and biological tissue,an improved diffusion transport equation is proposed and the imaging under 20 mean free paths with a "sandwich" framework in transmission is verified.(4)Aiming to explore the detection limit of speckle correlation imaging,speckles stained by different noise levels are investigated.A speckle autocorrelation refinement technology is proposed and demonstrated.High-fidelity reconstruction results under a detection signal-tobackground ratio(SBR)=-22.23 d B are observed experimentally without introducing any additional optical elements.We demonstrate a method for extracting a weak object’s autocorrelation signals from a strong background noise condition using a Zernike-based background fitting method and a modified low rank and sparse decomposition(LRSD)strategy.Compared with the original speckle correlation imaging,the detection limit is increased by 6.66 d B,laboratory and outdoor scattering imaging experiments are performed.Multi-dimensional physical extended computational imaging technology is expected to be applied in mobile phone photography,road detection,unmanned driving,underwater rescue and many other fields,providing new solutions for laboratory scattering imaging and outdoor applications.