Abstract

A 400-Gb/s wavelength-division-multiplexing (WDM) four-level pulse amplitude modulation (PAM4) optical wireless communication (OWC) system through a 200-m free-space transmission with either an 8.8-m piped water–air–piped water link or a 6.5-m turbid water–air–turbid water link is successfully constructed. Incorporating PAM4 modulation with an 8-wavelength WDM scheme greatly increases the total transmission rate of the WDM-PAM4 OWC system to 400 Gb/s (50 Gb/s/λ × 8 λs). By adopting doublet lenses in free-space transmission, a laser beam reducer/expander and a reflective spatial light modulator (SLM) with an angle expander through the water–air–water link, good bit error rate performance and acceptable PAM4 eye diagrams are obtained. Using a reflective SLM with an angle expander not only adaptively adjusts the laser beam, but also effectively solves the oceanic engineering problems. This demonstrates WDM-PAM4 OWC system outperforms existing OWC systems through the free-space transmission with an air–water–air link because it can solve the practical engineering problems in actual oceanic environments.

Highlights

  • Optical wireless communication (OWC) is a form of optical communication using laser beam or light-emitting diode light to deliver optical signals in an unguided environment

  • For a Underwater wireless laser transmission (UWLT) system passing through the piped water–air–piped water link, the main factor is absorption, and the low scattering coefficient prevents the laser beam from diverging

  • Using a laser beam reducer to decrease the beam size will improve the performance of the UWLT system passing through the piped water–air–piped water link because the pipe water absorbs less laser ­light[32]

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Summary

Introduction

Optical wireless communication (OWC) is a form of optical communication using laser beam or light-emitting diode light to deliver optical signals in an unguided environment. A 400-Gb/s wavelength-division-multiplexing (WDM) PAM4 OWC system passing through a 200-m free-space transmission with either an 8.8-m piped water–air–piped water link or a 6.5-m turbid water–air–turbid water link is proposed and constructed. By utilizing a pair of doublet lenses in free-space ­transmission[17,18], a laser beam reducer/expander and a reflective spatial light modulator (SLM) with an angle expander in water–air–water ­link[19,20,21], good low bit error rate (BER) and acceptable PAM4 eye pattern are obtained over a 200-m free-space transmission with either an 8.8-m piped water–air–piped water link or a 6.5-m turbid water–air–turbid water link. Compared with previous OWC systems through the free-space transmission with an air–water–air ­link[6,7,8,9,10,11], it presents an excellent system with the advantages of high accumulative transmission rate, long-distance optical wireless transmission, and high reliability

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