Study On Focusing Laser Beams Through Strongly Scattering Media

When light passes through the inhomogeneous random materials, such as milk, paint and human tissue, light will be multiply scattered. Light propagating in such materials is diffuse.It is the interference of the multiple scattering light that results in a random speckle pattern. Light incident into scattering media will lost the original spatial coherence,the stability of the coherent light will be scattered to randomization of coherent light, and orderly images become random, which seriously affected the human’s ability to extract information.For this kind of wavefront aberration caused by strong scattering medium, adaptive optics technology is unable to effectively correct the wave aberration.Therefore, to overcome the strong scattering effect has become a research focus in various fields.Based on the strongly scattering medium as a research object, focusing laser beams through strongly scattering medium is studied.The main contents is as follows:1. We focus on investigation of laser beams focusing through scattering media theoretically. Based on angular spectrum diffraction theorem and circular Gaussian distribution model of scattering media, light focusing through scattering media is numerically simulated. A high contrast focus in the target area is produced by using a feedback optimization algorithm to shape the wavefront of the scattering light. By adjusting the target area, it is also possible to get one or multiple foci at will. The influence of the number of square segments of the spatial light modulation and the phase precision of a single segment on the intensity enhancement is discussed. The result shows that the light intensity at the target area increases linearly with the increasing number of square segments of the spatial light modulation, and becomes stronger with the increasing phase precision. Simulation results are consistent with theoretical analysis.2. We investigate focusing laser beams through opaque scattering mediaexperimentally. In scattering media, such as milk, biological tissue et al, multiple scattering makes laser beams diffuse and forms a volume speckle field. To focus light through scattering media is of great significance. We report experimentally that the continuous sequential feedback algorithm and spatial light modulator are used to construct a wavefront by phase modulation, light focusing through scattering media and a high contrast focus in the target area is produced. The influence of the number of square segments of the spatial light modulation and the phase precision of a single segment on the intensity enhancement is discussed. The experimental result shows that the light intensity at the target area increases with the increasing number of square segments of the spatial light modulation, and becomes stronger with the increasing phase precision.3. We studylight focusing through strongly scattering media by binary amplitude modulation. Based on angular spectrum method and circular Gaussian distribution model of scattering media, we numerically simulate light focusing through scattering media. A high contrast focus in the target area is produced by using feedback optimization algorithm with binary amplitude modulation. It is possible to form one or multiple foci at arbitrary areas. The influence of the number of square segments of spatial light modulation on the enhancement factor of intensity was discussed. Simulation results are found to be in good agreement with theoretical analysis for light refocusing.4. We researchlight focusing through scattering media by particle swarm optimization. We demonstrated light focusing through scattering media by introducing particle swarm optimization to modulate the phase wavefront. Light refocusing is simulated numerically based on angular spectrum method and circular Gaussian distribution model of the scattering media. Experimentally, spatial light modulator is used to control the phase of incident light. The shaped light is modified to make the scattered light converge to a focus. The influence of divided segments of input light and the number of iterations effect on light enhancement are investigated. Simulation results are found to be in good agreement with theoretical analysis for light refocusing.