Optical Equalization in SiPM-Based Underwater Optical Communication System

In this article, a spatial optical equalization method based on the Engineered Diffuser (ED) is proposed to mitigate the nonlinearity degradation caused by the dead time of a silicon photomultiplier (SiPM). The ED can generate a uniform light spot and prevent part of the pixels in SiPM from saturation, allowing them to operate in the linear range. Simulation results demonstrate that optical equalization can reduce the count loss caused by the dead time and enhance the dynamic range of communication. Experiments in a water tank are also conducted to verify the feasibility and effectiveness of optical equalization. The SiPM-based underwater wireless optical communication system with the proposed method can achieve a data rate of 70 Mbps with a received power of -52 dBm while the system cannot work without optical equalization. This method is also qualitatively proved to have a better anti-interference ability against scintillation and misalignment caused by turbulence. The proposed method in this article is a new solution to the nonlinearity caused by the dead time of SiPM in optical domain with low complexity, which is an interesting and effective supplement to the nonlinear equalization in electrical domain.