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

In this letter, 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 quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing 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 bit-error-rate 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 bit/s/Hz is also obtained in the proposed underwater wireless optical communication system.