  Seafloor observation network can perform all-weather, long-term, continuous,
real-time, and in-situ observation of the ocean by combing various observation
methods including cabled seafloor nodes, self-contained nodes, as well as
mobile platforms, where reliable and long-term high-speed underwater wireless
communication becomes an essential demand. Recently, underwater wireless
optical communication (UWOC) has emerged as a highly promising solution and is
rapidly becoming a research hotspot for meeting this requirement. In this
article, we demonstrate the experiment and application of high-speed UWOC
system for deep sea seafloor observation network. To the best of our knowledge
this is the first long-term real-time deep-sea UWOC link with bitrate as high
as 125 Mbps. Between 30 m distance and at a depth of 1650 m, two-way Ethernet
UWOC links are realized with 125 Mbps direction-adjustable green light link and
6.25 Mbps non-line-of-sight (NLOS) blue light link. High quality video
transmission of 8K 30 FPS and 4K 120 FPS are realized through high-speed 125
Mbps green light link, with 100% peak signal-to-noise ratio (PSNR) agreement,
showing the capability of transmitting high-quality videos lossless. The 30-day
long-term measurement results show that the BER performance of both 125 Mbps
and 6.25 Mbps links is lower than 10-5, proving the stability and reliability
of this UWOC system at depth of 1650 m. The maximum transmission distance for
the green and blue light links are estimated to be 117.7 and 128.3 m with
considering the geometry loss, which can be extended to 231.6 and 337.5 m
without geometry loss. As the first long-term and real-time UWOC system in deep
sea, we believe this demonstration can provide valuable experience for further
UWOC studies and converged ocean observation networking with cabled and
cable-less observation platforms.
