Fast Auto-Align ment Underwater Wireless Optical Communications Employing Orbital Angular Momentum Modes

Underwater wireless communication plays an increasingly important role in more and more emerging ocean and sea activities. Traditional underwater acoustic communication is limited by low transmission capacity (tens of Kbit/s). Underwater wireless optical communication (UWOC) provides high bandwidth (GHz). Yet the challenge is not only its continuous capacity scaling, but its resistance to environment disturbance, which is an intrinsic weakness of wireless optical communications with high directivity. Here, we propose and demonstrate fast auto-alignment underwater wireless optical communications employing orbital angular momentum (OAM) modes. The OAM multiplexing, an alternative approach of space-division multiplexing, enables capacity scaling. The fast auto-alignment system, having two stages for thorough alignment, facilitates stable and reliable communication link against vibration. Two OAM modes multiplexing transmission link with 4-Gbit/s aggregate capacity is demonstrated in the experiment under four different vibration conditions assisted by the fast auto-alignment system with a response time of 244 Hz. The demonstrations may open up new perspective to robust stable underwater wireless optical communications exploiting spatial modes in practical environment.