Optical Communications in Space
In summer 1977, ESA placed the first technological study contract in the domain of intersatellite optical links. Now, twenty years later, a major milestone has been reached with the SILEX laser terminals having been flight tested for integration with their host spacecraft. At the same time, ESA is preparing itself for a new challenge: the potential massive use of optical cross links in satellite constellations for mobile communications and global multimedia services. This is an opportune moment to look back at the past twenty years of ESA effort in laser communications, to take stock of the results achieved and to reflect on ways to face the challenges of the future.
Twenty years ago, in summer 1977, ESA placed a technological research contract for the assessment of modulators for high-data- rate laser links in space. This marked the beginning of a long and sustained ESA involvement in space optical communications. A large number of study contracts and preparatory hardware development followed, conducted under various ESA R&D and support technology programmes. In the mid- 1980 s, ESA took an ambitious step by embarking on the SILEX (Semiconductor laser Intersatellite Link Experiment) programme, to demonstrate a pre-operational optical link in space.
SILEX, which will be in operation in the year 2000, has put ESA in a world-leading position in civilian optical intersatellite links. While SILEX formed the backbone of ESA s optical communications activities in the recent past, additional R&D activities were undertaken to develop attractive second-generation systems, particularly for the commercial satellite market. Indeed, at the turn of the century, literally thousands of intersatellite links - radio-frequency (RF) and optical - are expected to be in operation in commercial multi-satellite constellations providing mobile communications, video conferencing and multimedia services. The race is on for the European laser communication industry to enter this lucrative market. Optical technology offers too many advantages in terms of mass, power, system flexibility and cost, to leave the field entirely to RF. With the heritage of twenty years of technological preparation, European industry is well positioned to face this burgeoning demand for commercial laser terminals. The early days when ESA started to consider optics for intersatellite communications, virtually no component technology was available to support space system development. The available laser sources were rather bulky and primarily laboratory devices. ESA selected the CO2 gas laser for its initial work. This laser was the most efficient and reliable laser available at the time and Europe had a considerable background in CO2 laser technology for industrial applications. ESA undertook a detailed design study of a CO2 laser communication terminal and proceeded with the breadboarding of all critical subsystems which were integrated and tested in a complete laboratory breadboard transceiver model.
This laboratory system breadboarding enabled ESA to get acquainted with the intricacies of coherent, free-space optical communication. However, it soon became evident that the 10 micron CO2 laser was not the winning technology for use in space because of weight, lifetime and operational problems. Towards the end of the 1970 s, semiconductor diode lasers operating at room temperature became available, providing a very promising transmitter source for optical intersatellite links. In 1980, therefore, ESA placed the first studies to explore the potential of using this new device for intersatellite links. At the same time, the French national space agency, CNES, started to look into a laser-diode-based optical data-relay system called Pastel. This line of development was consequently followed and resulted in the decision, in 1985, to embark on the SILEX pre-operational, in-orbit optical link experiment.