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. Introduction 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. SILEX SILEX is a free-space optical communication system which consists of two optical communication payloads to be embarked on the ESA Artemis (Advanced Relay and TEchnology MIssion Satellite) spacecraft and on the French Earth-observation spacecraft SPOT-4. It will allow data transmission at 50Mbps from low Earth orbit (LEO) to geostationary orbit (GEO) using GaAlAs laser-diodes and direct detection. The SILEX Phase A and B studies were conducted around 1985, followed by technology breadboarding and predevelopment of the main critical elements which were tested on the so-called 'System Test Bed' to verify the feasibility of SILEX. A detailed design phase was carried out in parallel with the System Test Bed activities up to July 1989. At that time, the development of SPOT-4 Phase C/D was agreed with an optical terminal as passenger. This was an important decision since it made a suitable partner satellite available for the ESA data-relay satellite project; the stage was therefore set to start the main SILEX development effort in October 1989. In March 1997, a major milestone was reached in the SILEX programme: both terminals underwent a stringent environmental test programme and are now ready for integration with their host spacecraft. However, due to the agreed SPOT-4 and Artemis launch dates, it is likely that the in-orbit demonstration of the overall system will not start before mid-2000. Consequently, the GEO terminal will need to be stored after the completion of the spacecraft testing. The first host spacecraft (SPOT-4) is planned for launch in February 1998. The launch of Artemis on a Japanese H2A is delayed for non-technical reasons until February 2000. Apart from launching Artemis, Japan is participating in the SILEX programme with its own laser terminal, LUCE (Laser Utilizing Communications Equipment), to be carried onboard the Japanese OICETS satellite (Optical Inter-orbit Communications Engin-eering Test Satellite), set for launch in summer 2000. Optical ground station on Tenerife As part of the SILEX in-orbit check-out programme, ESA started to construct an optical ground station on the Canary Islands in 1993 (Fig. 2). This station, which will be completed by the end of 1997, simulates a LEO optical terminal using a 1 m telescope, allowing the performances of the GEO optical terminal on Artemis to be verified. The optical ground station will receive and evaluate the data transmitted from Artemis and will simultaneously transmit data at optical wavelengths towards Artemis. In addition to its primary objective as the SILEX in-orbit check-out facility, the optical ground station will also be used for space-debris tracking, lidar monitoring of the atmosphere and astronomical observations