四元裂解位相调制实现相干光通过散射介质聚焦

Four-element division algorithm for focusing light through scattering medium

光在不均匀介质中传播会受到散射的干扰,在这些散射材料中,例如粉末、生物组织、亚波长颗粒对入射光多次散射使得出射光无法聚焦,从而在接收平面形成散斑.本文提出四元裂解位相调制方法对入射相干光场进行调制,使其通过散射介质聚焦.此方法利用入射光场全场调制,充分考虑光场单元之间的干涉作用,从整个空间光调制器的调制面开始,逐层进行四元裂解及位相优化.运用此方法在实验中实现了相干光的前向散射和后向散射有效聚焦,这为生物医学领域中通过散射介质成像提供了新的思路和方法.

Light transport in complex disordered medium, such as white paint, milk, is a fundamental physical phenomenon, and it plays an important role in numerous applications including imaging through turbid layers, and quantum informa- tion processes. However, all spatial coherence is lost due to the distorted incident wavefront caused by repeated scattering and interference. Incident coherent light diffuses through the medium and cannot form a geometric focus but a volume speckle field on the imaging plane. In this paper, we propose a four-element division algorithm and experimentally demonstrate that using this algorithm to modulate the incident light, the shaped wavefront can focus through disordered material. At the beginning, we start with four segments on spatial light modulator （SLM）, changing the phase of each segment from 0-2Tt to search for the optimal phase in terms of the maximal output intensity at a certain field. After the optimal phase of these four segments is found, each of all segments is divided fllrther into four subsegments, so 16 subsegments are formed on the SLM. Just like the first step, the optimal phase is found by cycling the phases of these 16 subsegments. Sequentially, this procedure is repeated several times, so more and more subsegments are obtained. As a result, the modulated input light from SLM can be focused after it has passed through the turbid scattering medium. By employing this approach in the forward scattered experiment, the total pixels of spatial light modulator are divided into 4 4096 segments to shape the incident light. After separately searching for all tile optimal phase distributions, we can see that a sharp focusing is gradually achieved. Likewise, in backscattered experiment, 4-1024 segments are used to focus the incident light after passing through the diffuse material. In comparison with stepwise sequential algorithm, the main advantage of our method is that the interference effect of all segments on SLM is taken into consideration, which means that the modulated and the modulating segments are connected with each other. In this way, the signal-to-noise ratio is higher and no iteration is needed. All this experiment shows that the four-element division algorithm can be employed to focus the incident light passing through a disorder material efficiently, which maybe provide a new idea and method in the field of biomedical imaging through scattering medium.