  Future spacecraft will require a paradigm shift in the way the information is
transmitted due to the continuous increase in the amount of data requiring
space links. Current radiofrequency-based communication systems impose a
bottleneck in the volume of data that can be transmitted back to Earth due to
technological as well as regulatory reasons. Free-space optical communication
has finally emerged as a key technology for solving the increasing bandwidth
limitations for space communication while reducing the size, weight and power
of satellite communication systems, and taking advantage of a license-free
spectrum. In the last few years, many missions have demonstrated in orbit the
fundamental principles of this technology proving to be ready for operational
deployment, and we are now witnessing the emergence of an increasing number of
projects oriented to exploit space laser communication (lasercom) in scientific
and commercial applications. This chapter describes the basic principles and
current trends of this new technology.
