Orbital Angular Momentum-based Space Division Multiplexing for High-capacity Underwater Optical Communications

TLDR

The study aims to boost underwater optical communication capacity by transmitting multiple orthogonal spatial beams, each carrying an independent data channel. The authors implement orbital angular momentum multiplexing by generating data at 1064 nm and 520 nm, imprinting OAM onto green beams with a metasurface phase mask, directly modulating a green laser diode for a 4‑Gbit/s link, and investigating the effects of scattering, turbidity, and thermal gradients. They achieve a 40‑Gbit/s link using four green OAM beams, find thermal gradients cause the most distortion and turbidity the most loss, and demonstrate that multi‑channel equalisation can mitigate thermal‑gradient‑induced crosstalk.

Abstract

Abstract To increase system capacity of underwater optical communications, we employ the spatial domain to simultaneously transmit multiple orthogonal spatial beams, each carrying an independent data channel. In this paper, we show up to a 40-Gbit/s link by multiplexing and transmitting four green orbital angular momentum (OAM) beams through a single aperture. Moreover, we investigate the degrading effects of scattering/turbidity, water current, and thermal gradient-induced turbulence, and we find that thermal gradients cause the most distortions and turbidity causes the most loss. We show systems results using two different data generation techniques, one at 1064 nm for 10-Gbit/s/beam and one at 520 nm for 1-Gbit/s/beam; we use both techniques since present data-modulation technologies are faster for infrared (IR) than for green. For the 40-Gbit/s link, data is modulated in the IR, and OAM imprinting is performed in the green using a specially-designed metasurface phase mask. For the 4-Gbit/s link, a green laser diode is directly modulated. Finally, we show that inter-channel crosstalk induced by thermal gradients can be mitigated using multi-channel equalisation processing.