Underwater Laser Communication Using an OFDM-modulated 520-nm Laser Diode

In this paper, we propose and experimentally demonstrate an underwater wireless optical communication system based on a simple and cost-effective TO56 transversal multi-mode green-light laser diode (LD) and a low-cost 150-MHz positive-intrinsic-negative detector. We use QAM-OFDM modulation, with fine frequency granularity and high spectral efficiency, to tackle the challenging issue imposed by the ragged and uneven frequency response of the green LD within a limited bandwidth. We have demonstrated that a bit rate of 1.118 Gb/s (net bit rate: 0.927 Gb/s) can be achieved at a BER of 2.98 ×10-3, with the assistance of bit loading (using both 16-QAM and 256-QAM signals). A spectral efficiency as high as 6.18 bits/Hz is also obtained in the proposed UWOC system.