Research On Light Focusing Through Scattering Media Based On Wavefront Shaping And Fluorescent Structure Detection

Detecting and imaging of tissue structure is an important research subject in biomedicine field.Compared with the traditional non-optical imaging methods such as X-ray computed tomography,magnetic resonance imaging,and ultrasonography,optical imaging methods(such as optical coherence tomography,laser scanning confocal microscopy,and photoacoustic tomography)possess the advantages of high resolution,high imaging speed,noninvasiveness,and inexpensiveness.Therefore,optical imaging methods have received widespread attention.The optical imaging methods in biomedicine field rely on the photons that propagate in tissue to acquire information.However,for the electromagnetic wave whose wavelength is in the spectrum range(i.e.,optical wave),the biological tissue is a kind of strongly scattering media.Light will be strongly scattered during the transport process,thus scramble its paths in extremely disordered way.Wavefront shaping technique is expected to solve this problem.Wavefront shaping technique uses a phase or amplitude modulator to modulate the wavefront of the incident light,thereby realizes the regulation of the path of the scattered light.However,before the wavefront shaping technique is used to detect the inner structure of tissue,several problems must be solved.These problems include low peak-to-background ratio,complicated and design-needful system setup,difficult preparation of sample.Aiming at solving these problems,this thesis has studied theoretically and experimentally the light-focusing technique based on wavefront shaping.Then based on the research results,this thesis has designed and installed the system of fluorescent-structure detection through scattering media.This system will provide a guidance for exploring the structure-detecting methods of tissue.The main innovations are summarized as follows:1.Fundamental research on light focusing based on wavefront shaping.(1)A new discriminant for light focusing based on DMD amplitude modulation is proposed,i.e.,SBR discriminant.We carried out light-focusing simulations with the SBR discriminant and achieved a relative enhancement value of 22.5%,which is higher than that with the traditional TPI discriminant(15.9%).Then this result was proved by the experiment.This discriminant is meaningful for obtaining a focus with high peak-to-background ratio.(2)Aiming at solving the problem of ambiguous reference definition,based on the study of speckle field intensity distribution,a new reference-defining standard that is suitable for all wavefront shaping experiments is proposed,i.e.,1/e2 criterion.With this criterion,we have carried out light-focusing experiments based on DMD amplitude modulation.The enhancements obtained from the repetitive experiments were all in the reasonable range,demonstrating its validity.This criterion will provide a comparison standard for different light-focusing experiments in wavefront shaping field.(3)The method of superpixel-based amplitude and phase modulation has been studied.Then it was applied to a DMD-based wavefront-shaping light-focusing system.With this setup,a light-focusing experiment based on genetic algorithm was carried out.A bright focus with a relative enhancement of 71.4%was achieved.2.Sample preparation method.In the aspect of scattering samples preparation,the scattering property of the scattering media was studied,and the ZnO diffuser samples with strong scattering property were prepared.Then according to the absorption and scattering properties of biological tissue,this thesis prepared the TiO2 biological phantoms with similar optical parameters.In the aspect of fluorescent-structure samples preparation,a photoetching system was installed to prepare fluorescent-structure samples with the size of 100 ?m-1 mim.Then fluorescent-structure samples of less than 100 ?m in size were prepared by spray coating method.Besides,the fluorescent-structure samples with strong scattering property were designed and prepared.3.Based on the wavefront-shaping light-focusing technique,optical memory effect,and laser scanning fluorescence imaging principle,we have designed the system of detecting the microscopic fluorescent structure through scattering media.Then the setups that based on DMD amplitude modulation,LC-SLM phase modulation,and superpixel-based phase and amplitude modulation were installed.With these setups,we have achieved imaging the microscopic fluorescent structure through scattering media.With the SSIM index,we assessed these setups and concluded that the light focus with high signal-to-background ratio is crucial for this fluorescent-structure-detection system.The designed system will provide a guidance for imaging of inner structure of tissue.