Fiber bundle-based beam tracking approach for free space optical communications link optimization

A robust pointing, acquisition, and tracking approach is critical to closing and maintaining a free space optical communications link. Many fiber-coupled terminal architectures use a beamsplitter to direct a portion of the received light onto a quadrant detector and generate an error signal. A feedback control loop uses this error signal to adjust a fine steering element to maximize power into the collection fiber. However, this approach produces additional insertion loss due to transmission through a beamsplitter and tracking loss due to inevitable boresighting errors between the quadrant detector and the collection fiber. We present an alternative architecture that makes use of a hexagonally packed, seven-fiber bundle for data beam tracking. The outer fibers are inherently co-boresighted position sensors used to sense displacement of the received beam from the central data fiber. We present field test data directly comparing performance between a fiber bundle-based terminal and a quadrant detector-based terminal. Our results show nearly an order of magnitude of improvement in control loop tracking stability and a 2.8× improvement in tracking performance as seen in Strehl ratio for the fiber bundle system.