Infrared Data Link Using an MQW Modulator on a Retroreflector

A n infrared data link between a ground station and a small uninhabited helicopter in flight has been demonstrated in an effort to develop a type of system for free-space optical communication between (1) a larger and relatively stationary platform, and (2) a smaller and relatively mobile platform. In a system of this type, rather than using laser transmitters with their associated gimbaled telescopes and pointing/tracking subsystems on both platforms, one uses only a single such laser transmitter on the larger platform (in this case, the ground station). The single laser transmitter is capable of tracking the smaller platform (in this case, the helicopter) and transmitting data to the smaller platform in the conventional way via modulation of the outgoing laser beam. The field of view of the receiver on the smaller platform is wide enough to capture the laser beam, without need for a large receiving telescope and its aiming subsystem. For transmitting data from the smaller to the larger platform, a large fraction of the laser power incident on the smaller platform is modulated and retroreflected to the larger platform, by means of an InGaAs-based multiple-quantum-well (MQW) light modulator on a corner-cube retroreflector (see Figure 1).