Optimized Coding and Protocols for Free-Space Optical Communications Links

High-speed laser communications in adverse maritime conditions are necessary to transfer large amounts of data needed for command-and-control, target selection, or other intelligence. Atmospheric turbulence causes scintillation, variation of signal strength, and an increase in the bit-error rate (BER). Obscurations such as debris or fog cause scattering and a decrease in the signal and the information it contains. Conventional adaptive optics can be used to mitigate some of the turbulence effects, but data rates still need to remain high. In collaboration with our partner, the University of North Carolina at Charlotte (UNCC), we have recently finished some theoretical developments and laboratory demonstrations that will provide a novel optimized coding for the optical communications beam. Our coding scheme is based upon modulating a beam with an optical vortex with different vortex “charge” applied to each pulse. The pulse is then detected and the optical charge is determined. We can vary the vortex charge and thereby code each pulse with various values to increase the final data rate. The vortices placed on the beam are known to maintain their charge as the beam propagates through atmospheric turbulence, around small particulates (dust), and even fog.