Modulating Retro-Reflector Devices and Current Link Performance at the Naval Research Laboratory

Free-space optical communication is often desired between two nodes having different capabilities. Using a modulating retroreflector (MRR) shifts most of the power, weight, and pointing requirements to one end of the link, allowing the other end to be extremely small, low-power, and requiring only rough pointing (to within +/-15 degrees). For a 5 Mb/s link at 2 km, our entire MRR package including drive electronics weighs only 8.5 g and requires 60 mW. This same device has also been arrayed to further relax the pointing requirements. In the simplest MRR link, an unmodulated (CW) laser interrogates a MRR comprised of an absorptive modulator and a retroreflector. If the interrogation beam is within the retroreflector's field of view (FOV), the beam will return to the interrogator with data impressed on it. In this discussion, we present the range of MRRs developed since 1998 at the Naval Research Laboratory (NRL). NRL's MRRs include both corner cube retroreflector and cat's eye retroreflector types, both individually and in arrays. Each variation has unique characteristics which may be beneficial or disqualifying in different situations. Size, weight, FOV, power consumption, cost, speed × range product, modulator type, ruggedness, time-to-market, and requirement for angle division multiplexing are all determining factors in MRR design and retroreflector choice. We compare strengths, weaknesses, and give current link performance data for several different systems. Links demonstrated include shore-to-shore, boat-to-shore, and UAV-to-ground over a wide range of distances and data rates.