Optical Networking Best Practices Handbook

7.5. THE MINIMIZATION OF ACQUISITION TIME

Free-space optical communication can be made less susceptible to unwanted detection than radio-frequency communication because it is possible to concentrate an optical transmission in a narrow beam aimed toward the intended recipient. Hence free-space optical transmission is an attractive option for covert communication between moving platforms, such as aircraft or ground vehicles. However, the desired covertness may be easily defeated during the acquisition phase of the communication sequence, when at least one party has to perform a broad-field search to acquire the position of the other party, thereby revealing his presence. Moreover, because the optical beam is typically narrow, when the communicating parties are in rapid motion, it may be difficult to maintain a communication link for a significant time interval. Under these conditions, it may be necessary to perform link acquisition repeatedly, thus increasing the risk of detection. To maximize covertness, it is desirable to achieve acquisition and data transfer in the shortest possible time and for the parties to emit no light until the start of another transmission sequence. Thus, this section addresses the issue of minimizing the acquisition time in short-range links (ranges of the order of 1 km) between rapidly moving platforms. Here, researchers show how to minimize this time by the choice of raster scan pattern and by optimization of the beam divergence and scan speed subject ...