Three-stage tracking approach in development of a free-space laser communicator

ABSTRACT We report the concept and preliminary test results of development of a three-stage beam tracking system for demonstration of a Gbps free-space laser communicator. The development goal has been high-speed optical inter-satellite communications. The tracking system consists of three sub-systems: the coarse tracker (a 10.5 cm-diameter motorized Maksutov telescope with acquisition sensor); the intermediate electro-optical tracker (a voice-coil and a Si-based position detector), and the nonlinear optical fine tracker (a nonlinear cell with a liquid crystal). Three-stage tracking allows for efficient compensation of jitter of up to kHz while maintaining a sub-microradian pointing precision. A combination of the tracking system with a signal feed / modulation unit has been designed to demonstrate a 2.5-10 Gbps-level performance at distances of about 5-7 km. Keywords: Free-space laser communica tions, optical tracking 1. INTRODUCTION There has been remarkable growth in the area of free-space laser communications and, in particular, in optical intersatellite links (OISL) up until now due to numerous advantages that OISL offers over microwave links. A large variety of space missions such as geo-clusters, constellations, formation flyers and even planetary explorers could benefit from free-space laser communications.