Research On Feedback-based Wavefront Shaping Technology

With the rapid development of information technology,people's demand for communication capacity has also increased.In order to meet the demand for high-capacity optical communication,in recent years,multimode fiber has become a hot spot for researchers.However,when the beam is transmitted inside the multimode fiber,the output beam will have a certain phase delay due to the different phase velocities among the propagating modes,eventually forming a disordered speckle.Limited by this problem,it is difficult for researchers to extract valid information from the multimode fiber output light field.In addition,the intensity distribution of speckle produced by the laser passing through scattering medium is uneven.The glare in the background noise will mask the weak light carrying the information of sample,which limits many optical imaging and sensing applications.In order to solve these two problems,this paper proposes that the feedbackbased wavefront shaping technology is expected to realize the control of the scattered light field after transmission through scattering medium by modulating the wavefront of the incident light.In this paper,the Hadamard encoding algorithm(HEA)was used to focus the 1550 nm laser through multimode fiber.In addition,this paper also uses the improved HEA algorithm to achieve the suppression of glare.The specific work content is as follows:1.Based on the principle of wavefront shaping and HEA algorithm,the simulation of focusing light through scattering medium is carried out by using MATLAB software,which verifies the feasibility of HEA algorithm for focusing light through scattering medium.Then HEA algorithm is compared with the partitioning algorithm(PA)and the genetic algorithm(GA).It is proved that the HEA algorithm has faster initial light intensity growth rate and better signal-to-noise ratio.2.It is verified by experiment that the HEA algorithm can achieve the focusing of the 1550-nm light through multimode fiber,and the enhancement factor of the focal point is 690,which is 86% of the theoretical enhancement factor.At the same time,the experiment of focusing 1550 nm light through ground diffusers has obtained a focusing point with an enhancement factor of 620,reaching 77.3% of the theoretical value.The experiment of ground diffusers has fully verified the superior performance of the HEA algorithm.3.The HEA algorithm is improved.It is proved in theory and simulation that the improved HEA algorithm can be used for the suppression of glare in speckle.In the simulation,the light intensity of the target area drops down to 10-30 at around 56 iterations.The HEA is also compared with the GA,which fully demonstrates that the HEA has faster light intensity attenuation speed and better ability to suppress strong background noise.4.The feasibility of using HEA to reduce glare in the scattered light field is verified by experiments.The performance of HEA and GA for glare suppression is compared experimentally,which fully proves the superiority of HEA.Moreover,it is further verified by experiments that HEA has excellent multi-points suppression capability,which is of great significance for imaging through scattering medium.