| When light passes through disordered media such as milk,dense fog,clouds,paint,biological tissues,it is difficult for us to see things clearly.Because when the light passes through these media with uneven refractive index,scattering effect will cause the original wavefront to be distorted.As a result,the original information of things is lost.This phenomenon of light scattering through disordered media is an urgent issue in many fields,such as satellite detection,space flight,navigational exploration,and biomedical imaging.The wavefront shaping technology developed in recent years is expected to solve this issue.Wavefront shaping is a promising light field control technology.It can modulate the amplitude or phase of the incident light wavefront to achieve refocusing or imaging after light propagation through the scattering medium.The greatest advantage of wavefront shaping technology is that it can be applied to thick and strong scattering media,and the light focus point obtained by algorithm optimization has ultra-diffraction limit resolution,up to nanometer level.So far,plenty of new applications of wavefront shaping technology have been explored by many scholars,such as optical trapping,super-resolution imaging,endoscope,cryptography,phototherapy,optogenetics,and optical communicationIn the field of wavefront shaping research,continuously improving light control performance is always the goal of researchers.Many efforts have been made on single-point focusing.In contrast,there are fewer studies on multi-point focusing.In fact,multi-point focusing is more significant than single-point focusing in some specific research fields.Especially in optogenetics(optogenetics is expected to solve nervous system diseases such as Parkinson’s disease,depression,epilepsy,and Alzheimer’s disease),the light beam can be controlled to generate any desired activation patterns through the biological tissue using the multi-point focusing technology.Combined with the photochemical probes,it can simultaneously activate multiple deep neuron cells,so as to control neuron connections and the expression of photosensitive protein genes.Multi-point focusing is actually a multi-objective optimization problem.Unlike single-point focusing,not only the intensity but also the uniformity of focus points needs to be taken into account on multi-point focusing.However,some existing multi-point focusing methods of feedback-based optimization are actually implemented by changing the fitness function of the single-objective optimization genetic algorithm.In this way,there are many defects that make these algorithms cannot achieve a perfect multi-point focus effect,i.e.,ensure that both the intensity and the uniformity of the focus point are good enough.Therefore,the main purpose of this paper is to develop a feedback-based optimization algorithm that is more suitable for multi-point light focusing to achieve perfect multi-point light focusing.In addition,a series of researches on the system of feedback-based wavefront shaping and the application of multi-point focusing are carried out.The main contents of this paper are as follows:1.The improvement of optimization speed of feedback-based wavefront shaping system is not only conducive to the research and development of the algorithm,but also to the promotion and application of the technology.Therefore,this paper uses MATLAB and Lab VIEW to build a feedback-based wavefront shaping automatic control closed-loop system,which mainly solves the three problems of mask display on screen,image acquisition and timing synchronization.The running speed of the system is increased from 5 Hz to 11 Hz,and the system also has the advantages of good visualization and simple operation.2.Enhancement is an important index to evaluate focusing effect.Higher enhancement means better focusing effect(higher SNR).Based on the single-point focusing genetic algorithm and the transmission matrix model,this paper analyzes the fundamental difference between phase optimization and amplitude optimization,and proposes to replace the TPI discriminant with the SBR discriminant as the fitness function of the genetic algorithm,thus increasing the theoretical value of the relative enhancement of amplitude optimization by 4～7 percentage points.Simulation results show that the improved discriminant can effectively improve the enhancement of single focus or multi focus based on amplitude optimization3.In the field of wavefront shaping,there is no real multi-objective optimization algorithm applied to multi-point focusing at present.Non dominated sorting genetic algorithm with elite strategy(NSGA2)is one of the best multi-objective optimization algorithms.In this paper,for the first time,a new NSGA2 based on a hybrid optimization scheme is proposed,it is called hybrid non dominated sorting genetic algorithm(NSGA2-H).The hybrid factor H and geometric average discriminant are introduced to ensure that all the focus points have acceptable uniformity,and the enhancement is as high as possible.The simulation and experimental results of uniform intensity focusing and customized intensity focusing show that NSGA2-H has powerful optical control capability in multi-point light focusing.4.Multi-color light focusing and three-dimensional space focusing are two important functional applications of multi-point light focusing.In this paper,we use NSGA2-H to complete the experimental study of multi-color focusing and three-dimensional space focusing,including the feasibility analysis and system transformation.On the one hand,the experimental results prove the feasibility of multi-point focusing in frequency domain and spatial domain,on the other hand,these results further reflect the excellent performance of NSGA2-H in multi-point focusing optimization. |
