500 Gb/s PAM4 FSO-UWOC Convergent System With a R/G/B Five-Wavelength Polarization-Multiplexing Scheme

A 500-Gb/s four-level pulse amplitude modulation (PAM4) free-space optical (FSO)underwater wireless laser transmission (UWOC) convergent system over 100 m free-space transmission with either 10 m piped underwater link or 5 m turbid underwater link is established, employing a red/green/blue (R/G/B) five-wavelength polarization-multiplexing scheme as a demonstration for the first time. Integrating PAM4 modulation with five-wavelength polarization-multiplexing scheme, the channel capacity of FSOUWOC convergent systems is significantly increased with an aggregate data rate of 500 Gb/s [50 Gb/s PAM4/wavelength × 5 wavelengths × 2 polarizations (p- and s-polarizations)]. Results reveal that five R/G/B laser diode (LD) transmitters with two-stage light injection and optoelectronic feedback techniques are capably adopted for 500 Gb/s PAM4 signal transmission. Compared with prior FSO-UWOC convergence and visible light communication using polarization-multiplexing R/G/B LDs, it shows a prominent one with the benefits of high aggregate transmission rate and long-range optical wireless link. Excellent bit error rate performance and accepted PAM4 eye diagrams are attained over a 110-m/105-m FSO-UWOC link. This demonstrated five-wavelength polarization-multiplexing FSO-UWOC convergent system is promising because it not only integrates free-space backbone with underwater optical wireless feeder, but it also substantially multiplies total channel capacity.