Research On Reflection-matrix-method-based Optical Coherence Tomography

Optical coherence tomography has already been widely adopted in many kinds of fundamental researches and clinical applications,because of its advantages including high resolution,non-intrusive and real-time imaging.However,due to the inhomogeneous refractive index,the existence of multiple scattering when light passes through disordered biological tissue limits the imaging depth of OCT to 1?2 mm.In recent years,with the demonstration of using a spatial light modulator to optimize the wavefront of the incident light,the measurement of sample's reflection or transmission matrix,and the time reversal operate,scientists have proposed many new technologies to image through,focus through,and even focus inside a highly scattering medium.Typical newly proposed methods are wavefront shaping,optical transmission matrix measurement,optical memory effect,optical phase conjugate,neuro-network-based deep learning technology,and so on.To overcome the aberration caused by the light scattering and image through highly scattering medium,I have combined the theories of image and focus through scattering medium with OCT technology in this thesis.In aspects of achieving deeper imaging depth,improving the sampling speed of the system,increasing the speed of wavefront shaping,studying the transmission characteristics of the beam inside the medium,and realizing the focus of the beam inside the medium,I have done the following four parts of work:1.By combining the measurement of optical reflection matrix,singular value decomposition algorithm,and optical-heterodyne-detection-based OCT technology,I successfully extracted the single scattered photons,which obtain the useful information of image plane,from the dominant multiple scattering photons.With the help of these photons,I show that our proposed method has the ability to image through highly scattering medium.In terms of sampling speed,I used lock-in detection instead of the previous four-step phase shift to speed up the process of obtaining the sample's complex light field.The sampling time for each scanning point has been improved from 4.15 s to0.37 s.The imaging depth of our OCT system is 15.2 times the scattering mean free path,while the traditional OCT is about 6 to 7 times the scattering mean free path.2.This part of the work is firstly based on a high phase-sensitivity interferometer to reconstruct the complex light field.Then,by proposing a single in-&-out light field analysis,I can get the optimal wavefront in a very short time.Finally,with the help of a spatial light modulator,the whole time consuming to realize light focusing is 113 ms.Either to focus light through highly scattering medium or to achieve aberration-free imaging,the demonstration of wavefront shaping technology proves that light scattering can be overcome and compensated so that one can get a focused beam after a highly scattering medium.However,the conventional wavefront shaping method,either found the optimum field using the iterative optimization algorithm or first measured transmission matrix elements and then calculated the ideal field,both of them are time-consuming.Therefore,it is urgent to improve the finding speed of the optimized wavefront.Then,this technology can be applied to different kinds of optical imaging methods to improve the image resolution or increase the imaging depth.Meanwhile,for all the optical imaging systems that base on the point-by-point scanning strategy,achieving focus beams at the imaging depth deep into the sample is always the priority.To this end,our proposed method has shortened the time-consuming from the original several minutes to several hundred milliseconds.It may pave the way to break through the imaging limitation in the near future.3.With the accurate measurement of sample's reflection matrix and based on time reversal operation,I present a time reversal operation to measure the resolution matrix located at different axial and lateral positions of the medium.Different from the theoretical lateral and axial resolution I used to describe the performance of an OCT system,this is the first time,that the actual spatial resolution of an OCT imaging at different locations inside the sample has been measured.I believe the measurement of the actual spatial resolution can help people better understand the microstructure of the medium.I also put forward a concept,imaging contribution,to describe why the OCT imaging quality decline with the increasing of imaging depth.4.Here,I propose a method that does not depend on the guide-stars to focus light inside the medium.Therefore,one of the advantages of our proposed method is the general practicality.The energy redistribution of the beam along the propagation direction is a very complex process.With the help of time reversal operation,I obtain the energy distribution of the beam at different imaging depths.The significance of this step is that the reversal results act as inserting a camera inside the medium,which enables the observation of the beam within the medium.Then,the optimal wave is found by first measuring the reflection matrix of the sample and then taking the inverse operation of it.A spatial light modulator is subsequent used to reshape the incident light according to the optimal wavefront.Finally,I repeat the time reversal operation in the first step to observe the degree of focus of the beam after wavefront shaping.In our experiment,I successfully focus the beam when the penetration depth is 200 ?m.Although I did not get the perfect focusing spot at a depth of 300 ?m,the degree of focus is improved as well as the relative energy is increased by an order.