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

TL;DR

This paper demonstrates the use of orbital angular momentum beams for high-capacity underwater optical communication, analyzing environmental effects and proposing mitigation techniques for crosstalk.

Contribution

It introduces multiplexing four OAM beams in underwater communication and investigates environmental impacts, proposing a multi-channel equalization method to reduce crosstalk.

Findings

Thermal gradients cause significant beam distortions.

Turbidity results in notable signal loss.

Multi-channel equalization mitigates crosstalk effectively.

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 multiplex and transmit 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 higher-rate link, data is modulated in the IR, and OAM imprinting is performed in the green using a specially-designed metasurface phase mask. For the lower rates, 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.