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Optimum beam setting for near-field free-space optical communication system with bidirectional beacon tracking

This paper reports a new beam setting for single-mode-fiber coupled free-space laser communication terminals with bidirectional beacon tracking. To maintain stable and robust laser beam transmission over 0.2-2km terrestrial horizontal link, optimum beam setting is found to be not a collimated beam but a symmetric confocal beam between the two terminals. The paper also describes the detail of optics, such as optical antenna and fiber coupling lens used in the compact free-space optical terminals.