Laser communication with modulated retroreflectors - progress report

This report summarises this years´s work in the FOI project "Retrocommunication in tactical applications". We have been leading a cooperative programme within the Western European Armament Group named "Free space optical communications in tactical applications", EUCLID CEPA TA No. 108.077. The goal of the study was to find follow-up projects of common interest within the field. Free-space (wireless) optical communication as a tactical method for future operations, e.g. within the framework of European battlegroups, was evaluated. Six European countries participated. A main conclusion was that transfer of data from a tactical UAV is a highly prioritised application for the majority of the participating nations. A demonstration of a free-space high data rate link, using modulated retro reflectors from a UAV with small load capacity was a suggested follow-on project. The experimental work of the past year focused on the development of a new laser unit with the simultaneous ability to transmit beams of different wavelengths with the aim of studying correlation properties. An application is to guide a weak quantum channel carrying laser beam of a quantum key distribution link with a stronger laser beam. The degree of correlation between two laser beams of different wavelength under the influence of a turbulent atmosphere was measured. The influence from turbulence on the received signal was studied. We also carried out a pilot study on coding and modulation techniques suitable for retrocommunication systems. Finally, in cooperation with Acreo AB, we have developed and assembled a modulated retro reflector in the so called cat´s eye configuration, operating at 1550 nm with a modulation bandwidth of 10 MHz. The device was assembled at Acreo and the measurements were made at FOI.