Directly modulated green-light diode-pumped solid-state laser for underwater wireless optical communication

It is widely known that a diode-pumped solid-state laser (DPSSL) has very limited modulation bandwidth. Recently, we directed our attention toward the opportunities for directly modulating a DPSSL to generate high-speed green-light signals, with high power and superior beam quality, which are highly desirable in underwater wireless optical communication. The constraint imposed by the limited modulation bandwidth of a DPSSL is circumvented with the strategy of orthogonal frequency-division multiplexing and power loading. With a compact DPSSL dismantled from a low-cost laser pointer, we achieve net bit rates of 108.55 Mb/s for the 64 quadrature amplitude modulation (QAM) signal at a bit error rate (BER) of 6.42×10^-4 and 89.55 Mb/s for the 32 QAM signal at a BER of 4.81×10^-4, respectively, over a 2 m underwater channel. When the underwater transmission distance is increased to 6 m, the BERs are still below the forward error correction (FEC) limit of 3.8×10^-3.