Wavefront Shaping For Light Refocusing Through A Highly Scattering Turbid Medium

When laser light was reflected from a surface or transmitted through stationary diffusers such as white paint,walls,paper,frosted glass,biological tissue and so on,a high-contrast,fine-scale granular pattern and fine scale fluctuations of the intensity in space would be seen by an observer looking at the scattering screen due to destructive or constructive interference between scattering subwaves with different optical paths.this type of granularity is well-known as speckle.The speckle caused by random medium is one of the main obstacles in many optical technologies.In optical fibers,due to scattering of fibers to light the signal or energy transmitted by optical fiber will be attenuated and the transmission distance will be greatly shortened.In particle manipulation it becomes more difficult that one manipulates particles by optical tweezer due to light scattering by particles.The scattering will result in the broadening of the pulse width and the change of the spatial and temporal distribution of the energy.In biomedical photonics,the strong scattering of light by biological tissue leads to poor depth and resolution of optical imaging and also reduces the effect of photothermal therapy.Obviously,it is critical in optics to control the transmission characteristics of beam in random scattering medium precisely and reduce the influence of scattering on various optical technologies.The theoretical model of light propagation in random scattering medium is studied based on the diffraction theory in classical optics,and the process of focusing light in random scattering optical system has been simulated.We built an experimental system based on the transmitted spatial light modulator(SLM)which can control the propagation of light in random scattering medium and focusing of the light again.Additionally,the feedback algorithms used to modulate wavefront phase has been investigated in the dissertation.The main work is as follows:(1)The theoretical model of light propagation in scattering medium originated from Fresnel diffraction and Fourier optics has been presented and the transmitted matrix is used to model light propagation in scattering media.Light propagation in scattering media and speckles have been simulated.(2)We have discussed principles of three different optimization algorithms,that is,Continuous Sequential Algorithm,Genetic Algorithm and Particle Swarm Optimization Algorithm.All these algorithms have been used to simulate the phase modulation of input light of scattering media.Simulation shows that focusing and imaging through scattering medium with different algorithms have occurred and the different performances of focusing have been investigated and analyzed.(3)We have designed and built an experimental setup to control the focusing of light through random scattering medium based on transmitted-type spatial light modulator by wavefront shaping.In experiments the spatial light modulator is divided into many regions in which there were many pixels.The experimental results have been measured using three algorithms and intensities of focused spot,optimization time and with different divided number of regions have been discussed and analyzed.The influence of the number of square segments of spatial light modulation on the enhancement factor of intensity was discussed.We have a conclusion that the more the number of square segments of spatial light modulation,the better the quality of the focused spot.Meanwhile,the relationship between the optimal optimization time and the focus quality has also been obtained: the quality of the focused spot is better,but the time of optimization is longer and the convergence speed is slower,when CSAs which control phase with single optimization strategy is used;the quality of the focused spot is stable and the convergence speed is accelerated when we use global optimization strategy similarly genetic algorithm and particle swarm algorithm which can simultaneously optimize all mesh phases.