We have, so far as we know, proposed and demonstrated the first 30 Gb/s four-level pulse amplitude modulation (PAM4) underwater wireless laser transmission (UWLT) system with an optical beam reducer/expander over 12.5-m piped underwater channel/2.5-m high-turbidity harbour underwater channel. In piped underwater links, the performances of PAM4 UWLT systems get better with beam reduction given a small amount of light absorbed by the piped water. In highly turbid harbour underwater links, the performances of PAM4 UWLT systems get better with beam expansion given a large amount of scattered light received by the optical receiver. The effect of high-turbidity harbour water that induces scattering angle (beam divergence) on beam diameter is analyzed and optimised to enhance the transmission performances. This proposed PAM4 UWLT system, which uses an optical beam reducer/expander, provides a practical choice for high transmission capacity and considerably develops clarity and high-turbidity scenarios. It presents promising features for affording a high-transmission-rate underwater optical wireless transmission and opening an access to accelerate wide applications of UWLT systems.

A wavelength-division-multiplexing (WDM) fourlevel pulse amplitude modulation (PAM4) free-space optical (FSO)-underwater wireless optical communication (UWOC) integrated system with a channel capacity of 100 Gb/s is proposed and attainably demonstrated. Analytic results reveal that 1.8-GHz 405-nm blue-violet-light and 1.7-GHz 450-nm blue-light laser diodes (LDs) with two-stage light injection and optoelectronic feedback techniques are competently adopted for 100 Gb/s PAM4 signal transmission through a 500-m free-space transmission with 5-m clear ocean underwater link. Combining dual-wavelength WDM scenario with PAM4 modulation, the channel capacity of FSO-UWOC integrated systems is significantly enhanced with an aggregate transmission rate of 100 Gb/s (25 Gbaud PAM4/wavelength × 2 wavelengths). With doublet lenses in FSO, laser beam reducer and transmissive spatial light modulator in UWOC, a sufficiently low bit error rate of 10 −9 and acceptable PAM4 eye diagrams are acquired. This demonstrated 100 Gb/s PAM4 FSO-UWOC integrated system with a WDM scenario is advantageous for the enhancement of a high-speed optical wireless link with long-reach transmission.

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A polarization-division-multiplexing (PDM)-based bi-directional fibre-free-space optical (FSO) integration with two reflective semiconductor optical amplifiers (RSOAs) scheme to efficiently wipe off the modulated data for upstream modulation is proposed and successfully demonstrated. For downstream modulation, a high-speed 128 Gb/s vestigial sideband (VSB)-four-level pulse amplitude modulation (PAM4) fibre-FSO integration is feasibly established. The transmission capacity is increased up to four times through PDM operation and VSB-PAM4 modulation. For uplink transmission, a 10 Gb/s non-return-to-zero fibre-FSO integration with two RSOAs scheme to effectually erase the downstream modulated data is practically constructed. The upstream performance exhibits noticeable enhancement by using of two RSOAs scheme to wipe off the modulated data clearly. Such illustrated PDM-based bi-directional 128 Gb/s (downstream)/10 Gb/s (upstream) fibre-FSO integration is shown to be prominent not only due to its enhancement in the convergence of fibre backhaul and optical wireless reach extender but also because of its benefit in bi-directional transmission for affording high transmission capacity with long-reach optical wireless link and improved upstream performance.

A 448-Gb/s four-level pulse amplitude modulation (PAM4) free-space optical (FSO) communication through 600 m free-space link was constructed, utilizing polarization-multiplexing injection-locked vertical-cavity surface-emitting lasers (VCSELs) for presentation. When uniting fourwavelength polarization-multiplexing and PAM4 modulation schemes, the transmission capacity of FSO communications is substantially multiplied, with an aggregate transmission rate of 448 Gb/s [56 Gb/s PAM4/wavelength × 4 wavelengths × 2 polarizations (x-and y-polarizations)]. The results show that four 1.55 µm VCSEL transmitters with injection locking technique are sufficiently powerful for 448 Gb/s PAM4 signal transmission. Adopting a tunable optical band-pass filter and a polarization beam splitter for wavelength filtering and polarization de-multiplexing, the polarized wavelengths are effectually filtered and de-multiplexed in each polarized state. As the polarization state is well preserved in the scenario over 600 m free-space transmission, a sophisticated polarization tracker is not required in this proposed FSO communication. High bit error rate performance and accepted PAM4 eye diagrams are attained through 600 m free-space transmission. The four-wavelength polarization-multiplexing PAM4 FSO communication established here reveals the prominent benefits of high aggregate transmission capacity and long-distance free-space link.