Free-space optical communication (FSO) enables high-bandwidth data links that are difficult to detect, intercept, and jam. In this paper we provide an overview of a small form factor FSO prototype intended for UAVs called OCELOT (Optical Communication Efficient Low-profile Terminal). NRL designed, developed, and tested an OCELOT prototype, and demonstrated a 1 Gbps duplex link 16 km across the Chesapeake Bay. We will discuss the design decisions and tradeoffs, highlighting the low-SWaP FSO technologies used in the prototype.
In free-space optical communication (FSO) photodetectors are used for both data reception and positioning. For most FSO systems, these functions are performed by two separate photodetectors. The development of a new avalanche photodiode (APD) array has allowed positioning and data reception to be done on a single device. This device takes the form of concentric cells with four cells forming a circle and a fifth cell in the center. The change from a standard quadrant cell format to a concentric cell format affects the tracking algorithm an FSO system needs to use. Analytical characterization of the effects of a combinatorial sensor of this type on an FSO tracking algorithm has been done but it has not been verified experimentally. Here we test different tracking algorithms in an existing FSO system to determine the optimal way to use a combinatorial detector for position sensing.