Research On Through-scattering Imaging Method Based On Multimode Optical Fibe

With the constant development of interventional medicine,people also put forward higher requirements for endoscopy.The traditional endoscope has approached the limit of resolution,detection aperture and detection depth,which can’t meet the need in observation of some narrow cavities,neurons and even cell layers in modern medicine.Therefore,it is urgent to develop a fiber optic endoscope with higher resolution,smaller probe size and higher flexibility.Multimode fiber with core diameter in micron scale and the ability to transmit multiple independent spatial channels in parallel is a suitable substitute for endoscope.However,limited by the effect of mode dispersion and mode coupling,a single multimode fiber can’t be directly used for imaging.Fortunately,based on the recent research on imaging techniques of light penetrating through scattering media,multimode fibers can be regarded as strong scattering media,and the coupling of light waves in multimode fibers can be controlled by using wavefront shaping and transmission matrix techniques.The transmission matrix technology obtains the speckle recovery solution by calibrating the nonlinear relation between the input and output of the MMF imaging system.Once the system calibration is completed,the single-frame imaging can be achieved.Wave-front shaping technology is based on iterative optimization algorithms to find the specific wave-front phase to compensate the scattering effect,and further establish the loss function to achieve focusing or imaging through the scattering medium.However,neither transmission matrix technology or wave-front shaping technology,the core of imaging is to reduce the time of calibration and iterative optimization,and further improve the quality and stability of imaging,which will have greatly theoretical and practical value for medical imaging such as deep tissue imaging,singlefiber endoscope imaging and scanning imaging.Based on this research purpose,this paper is organized as follows:1.Based on Maxwell’s equations and Green’s function,the propagation model of light wave in uniform media and randomly scattered media is derived.Based on these theories,the mode distribution and optical transmission characteristics of MMF are analyzed,and a mathematical model based on the optical field distribution at the input and output ends of MMF is established,which provides a theoretical support for the application of transmission matrix and wave-front shaping technology.2.The transmission matrix measurement methods of full-field phase control and binary control using liquid crystal spatial optical phase modulator(LC-SLM)and digital micro-mirror device(DMD)are studied respectively,and an optical experimental platform was built to analyze the imaging results,imaging quality,and further verify the feasibility of the imaging method.3.For wave-front shaping,the Modified squirrel search algorithm is proposed as the wavefront optimization algorithm and the gradient structural similarity is proposed as the cost function to realize single-point and multi-point imaging through MMF.The algorithm adopts the search strategy in the process of squirrel foraging and introduces crossover and adaptive mutation improvement strategies.Combining with the feature factor of seasonal variation and Levy Flight,this algorithm can effectively alleviate the problem that traditional algorithms are prone to fall into locally optimal solution by introducing the seasonal variation feature factor and Levy Fight.Simulation and experiment results show that this algorithm has a faster convergence speed and anti-noise performance.