Abstract

Free-space optical communications (FSOCs) are gaining popularity as a promising solution to the capacity crunch of aerial and satellite backhaul networks for next-generation mobile communication systems. A major technical challenge associated with such FSOC systems in motion is the pointing, acquisition, and tracking (PAT). To facilitate PAT, it is absolutely necessary to steer the optical beam over a wide angle and also adjust the beam divergence angle adaptively to the link condition and the amount of pointing error. The conventional methods to steer the optical beam and/or to adjust the beam divergence angle rely on mechanical mechanisms. Recently, the optical beam steering technique using variable focus lenses has been proposed and demonstrated. By using an on-axis variable focus lens followed by two decentered variable focus lenses, we can steer the optical beam over a wide range in a non-mechanical fashion. In this paper, we explore the possibility of using variable focus lenses for the realization of beam steering and adaptive beam control for FSOCs. For this purpose, we employ three variable focus lenses, one on-axis lens for adaptive beam divergence control and two decentered lenses for two-dimensional beam steering. We demonstrate the simultaneous beam steering and adaptive beam control capability of this variable focus lens-based system over a 104-m long free-space optical link. We evaluate the maximum beam steering angle and beam quality as we vary the offset distance of decentered variable focus lenses. Also evaluated experimentally is the impact of gravity on the beam quality. Finally, we show that the beam steering and adaptive beam control techniques implemented by variable focus lenses improve the performance of 10-Gb/s FSOC system considerably in the presence of pointing errors.

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