Fast auto-alignment underwater wireless optical communications employing orbital angular momentum modes

Abstract

Underwater wireless communication plays an increasingly important role in more and more emerging ocean activities. Underwater wireless optical communication is a potential underwater communication technology with the advantages of high communication rate, large information capacity, and high bandwidth, but its ability to resist environmental turbulence is challenged. Different from free space optical communication, in underwater optical communication, turbulence not only directly affects the beam transmission, but also influences the transceiver, causing severe mechanical vibration of the transceivers, thus interfering with the optical communication link. At present, the research on mechanical vibration underwater has not received much attention. Hence, we propose and demonstrate fast auto-alignment underwater wireless optical communications employing orbital angular momentum (OAM) modes. The fast auto-alignment system is used to against the mechanical 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. After the fast auto-alignment system, the vibrations under for conditions are all greatly reduced to maintain a stable link. The demonstrations may open up new perspective to robust stable underwater wireless optical communications exploiting spatial modes in practical environment.