Scrambled and Unscrambled Optical Speckle with Multiple Scattering Layers: Applications to Optical Wireless Communication

When a beam of incident light waves is sent onto a scattering material, the waves are scattered within the material and the output is a scrambled speckle pattern. By placing a slab near a receiver, relevant properties arise for applications in encryption or as physically unclonable functions and keys, that take advantage of the randomness and complexity of the speckle pattern. Here, we extend this light-scattering system to a situation with \textit{two} scattering slabs, one near a sending device and the second near a receiving device, and address the question: does the intermediate speckle pattern observed in between the slabs have correlations with the pattern observed at the receiver, after two slabs? To this end, we send spatially shaped wavefronts from the sender that are designed to arrive at the receiver in one of two spatial regions of interest, considered to be states 0 or 1. We then collect the intermediate speckle patterns corresponding to either of the two states and study their properties. We characterize the correlation distribution between speckles using the Kolmogorov-Smirnov (K-S) test, and unsupervised classification algorithms. We found no correlation between the intermediate speckle and the message. Thus, if data are sent as binary 0s and 1s into the final speckle pattern, an observer is not able to infer it from the intermediate speckle pattern. Consequently, we propose a communication link for optical wireless communication (OWC) with a new encoding scheme based on our findings.