Underwater wireless-to-plastic optical fiber communication systems with a passive front end

In this paper, we first propose and then experimentally demonstrate underwater wireless-to-plastic optical fiber (POF) communication system with a fully passive optical lens installed at the front end of the POF. A 2-m low-cost POF is used as an underwater extender to efficiently increase the transmission distance and easily deliver the laser beam to the desired place. A fiber-pigtailed green laser diode (520 nm) with 68 mA current was used to transmit the laser beam. A high coupling efficiency of 88% was measured in our system, which is significantly higher than the coupling efficiency from 2-m calm water to single mode fiber (39%). The coupling loss of underwater wireless optical communication (UWOC) to POF was 0.11 dB. We obtained coupling efficiency of 86% and 90% from 7-m free-space to 2-m POF (numerical aperture (NA) 0.3–0.5), respectively, while considering coupling lenses with a NA of 0.125 or 0.30. Our achieved results demonstrate that the NA of the lens and that of the fiber should be close for high coupling efficiency.