Four-element division algorithm for focusing light through scattering medium

Abstract

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 information 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 0-2πup 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 further 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 the 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.