Free space optical communication utilizing modulating retro-reflectors (MRR) can greatly reduce the complexity of a system in both pointing requirements as well as the necessity for a laser transmitter at both ends of the link. Retroreflectors are susceptible to the same atmospheric turbulence effects of scintillation and beam wander of any laser communication system. An MRR link using an array (N>1) of retroreflectors is affected by self-interference of the return beams. This self-interference can create additional fluctuation that compound to increase the apparent scintillation of the received signal. Data were collected over a 1km outdoor path on the interference pattern returned from a pair of 7mm and 12.5mm retro-reflectors, with multiple spacing distances, in varying turbulence regimes with a 1550nm and 1070nm laser. The interference data of the retroreflectors were correlated with Cn2 data collected simultaneously over the same 1km horizontal path. Under weak turbulence, the self-interference fringes matched diffraction theory, under stronger turbulence regimes the self-interference fringes were either visibly reduced or completely destroyed. We also analyze the contrast of the interference fringes as a function of wavelength for varying turbulence regimes as well as the ability to measure Fried’s parameter from the retroreflector spacing and the returned self-interference pattern.
This effort develops and tests algorithms and a user-portable optical system designed to autonomously optimize the laser communication wavelength in open and coastal oceans. In situ optical meteorology and oceanography (METOC) data gathered and analyzed as part of the auto-selection process can be stored and forwarded. The system performs closedloop optimization of three visible-band lasers within one minute by probing the water column via passive retroreflector and polarization optics, selecting the ideal wavelength, and enabling high-speed communication. Backscattered and stray light is selectively blocked by employing polarizers and wave plates, thus increasing the signal-to-noise ratio. As an advancement in instrumentation, we present autonomy software and portable hardware, and demonstrate this new system in two environments: ocean bay seawater and outdoor test pool freshwater. The next generation design is also presented. Once fully miniaturized, the optical payload and software will be ready for deployment on manned and unmanned platforms such as buoys and vehicles. Gathering timely and accurate ocean sensing data in situ will dramatically increase the knowledge base and capabilities for environmental sensing, defense, and industrial applications. Furthermore, communicating on the optimal channel increases transfer rates, propagation range, and mission length, all while reducing power consumption in undersea platforms.