Optical Field Manipulation Of Laser Beams Through Scattering Media

In recent years,optical field manipulation of laser beams passing through the scattering medium attracts a considerable attention.Owing to the laser beams through scattering media,such as paper,paint powders and biological tissues,the original spatial coherence of the incident light will lost.Because of the scattering effect,the original information is disordered generating a random speckle with disordered intensity distribution,which has seriously affected our ability to obtain information from the optical beams that through the scattering media.Therefore,it is a significant research to study focusing of light beams through scattering medium or to reduce speckle.Among the parameters characterizing the light field,phase modulation is the most effective and direct method to modulate the light field.Based on this conception,the ideal focusing field is obtained by modulating the phase of the incident light.Moreover,the speckle suppression by partial coherent light is also studied.The creative contents of this thesis mainly include:1.We investigate the longitudinal intensity distribution near the focus produced by He-Ne laser beam through scattering media.When the light propagates through scattering media,the light beam is scattered in all directions and can not achieve a focus.To focus light through scattering media is of great significance.By using the spatial light modulators(SLMs)to modulate the incident beams,a desired bright focal spot can be generated in our experiment.The influence of different scattering media,the number of the square segments of the spatial light modulation,and the phase precision of a single segment on the longitudinal focusing depth is discussed.The experimental result shows that the longitudinal focusing depth of the focus only dependent on the scattering medium.2.We focus on investigation of speckle reduction by partial coherent light.As we all know,the laser beam is scattered into a number of small emitting beams,leading to coherent superposition,which forms a speckle pattern.This seriously degrades the display quality of the image.In the experiment,we study the coherence of the laser has an influence on the speckle by reducing the degree of coherence of the incident light.It is found that the speckle effect caused by the high coherence of the laser can be well suppressed by partially coherent light.3.We investigate the influence of partially coherent light on the focusing of He-Ne laser beam passing through the scattering medium.To the best of our knowledge,most researchers study focusing of completely coherent light through scattering medium.However few papers focus on the focusing of partially coherent light passing through scattering medium.Moreover we study the variation of the spatial coherence of the incident light on focusing through the scattering medium.There is a significant result shows that the intensity of the focal spot decreases with the decreasing coherence of the beam and the spot size is also reduced.