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]
Summary
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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