Optical Networking Best Practices Handbook

7.4. SECURE FREE-SPACE OPTICAL COMMUNICATION

It is desirable in certain applications to establish bursty, high-speed, free-space optical links over distances of up to several kilometers between rapidly moving platforms, such as air or ground vehicles, while minimizing the probability that a link is detected or intercepted. In a collaboration between University of California, Berkeley, Stanford University, Princeton University, and Sensors Unlimited, researchers have undertaken work toward this goal [4].

There are several key elements in the researchers' approach to covert optical links. To minimize atmospheric scattering, they used a long transmission wavelength; 1.55 μm was chosen because of the availability of key transmitting and receiving components. Combining a high-power laser and a two-dimensional beam scanner employing micromirrors, researchers obtained a steerable transmitter with milliradian beam width and submillisecond aiming time. They combined a wide-angle lens and InGaAs photodiode array with a dual-mode readout integrated circuit (ROIC) capable of both imaging and high-speed data reception, obtaining an electronically steerable receiver with a wide FOV and angular resolution in the milliradian range [4].

Covertness is defeated most easily during the link acquisition phase, when at least one communicating party must perform a broad-field scan to acquire the position of the other party, and risks revealing their presence to an observer. The researchers adopted a ...